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Local and non-local deficits in amblyopia: Acuity and spatial interactions
Abstract
Amblyopic vision is thought to be limited by abnormal long-range spatial interactions, but their exact mode of action and relationship to the main amblyopic deficit in visual acuity is largely unknown. We studied this relationship in a group (N=59) of anisometropic (N=21) and strabismic (or combined, N=38) subjects, using (1) a single and multi-pattern (crowded) computerized static Tumbling-E test with scaled spacing of two pattern widths (TeVA), in addition to an optotype (ETDRS chart) acuity test (VA) and (2) contrast detection of Gabor patches with lateral flankers (lateral masking) along the horizontal and vertical axes as well as in collinear and parallel configurations. By correlating the different measures of visual acuity and contrast suppression, we found that (1) the VA of the strabismic subjects could be decomposed into two uncorrelated components measured in TeVA: acuity for isolated patterns and acuity reduction due to flanking patterns. The latter comprised over 60% of the VA magnitude, on the average and accounted for over 50% of its variance. In contrast, a slight reduction in acuity was found in the anisometropic subjects, and the acuity for a single pattern could account for 70% of the VA variance. (2) The lateral suppression (contrast threshold elevation) in a parallel configuration along the horizontal axis was correlated with the VA (R2=0.7), as well as with the crowding effect (TeVA elevation, R2=0.5) for the strabismic group. Some correlation with the VA was also found for the collinear configuration in the anisometropic group, but less suppression and no correlation were found for all the vertical configurations in all the groups. The results indicate the existence of a specific non-local component of the strabismic deficit, in addition to the local acuity deficit in all amblyopia types. This deficit might reflect long-range lateral inhibition, or alternatively, an inaccurate and scattered top-down attentional selection mechanism.
... PL consists in a performance improvement obtained after repeated practice in a visual task, associated with neural plasticity outside the critical period (see Sagi, 2011 for a review). Treatment of monocular functional amblyopia through PL has been carried out using several training tasks: identification of letters (Chung, Li, & Levi, 2006, 2008Levi, 2005); discrimination of relative position of line segments (Levi & Polat, 1996;Levi, Polat, & Hu, 1997) and detection of contrast defined stimuli either isolated (Huang, Lu, & Zhou, 2009;Huang, Zhou, & Lu, 2008;Zhou et al., 2006) or presented together with collinear flankers in a lateral masking configuration (Barollo et al., 2017;Chen & Tyler, 2008;Polat, et al., 2004;Polat, Ma-Naim, & Spierer, 2009), or even associated to neurostimulation (Campana, Camilleri, Pavan, Veronese, & Lo Giudice, 2014). ...
... To study PL effect on lateral interactions, individual thresholds obtained at 2, 3 and 4λ were normalized by the threshold obtained at 8λ. Indeed, previous studies indicate that when the target is presented in fovea, flankers do not affect target contrast detection when they are presented at a distance of 8λ (Polat et al., 2004;Polat & Sagi, 2006). Data were normalized according to the following formula: log 10 (Threshold 2-3-4λ /Threshold 8λ ), where values lower than 0 indicate facilitation compared to the 8λ condition. ...
... This work aimed to probe if bilateral organic albinistic amblyopia due to OCA can be treated. More specifically, the question addressed was if ABA can be treated using the same PL protocol used for functional amblyopia (Barollo et al., 2017;Chen & Tyler, 2008;Polat, et al., 2004;Polat, et al., 2009). ...
Background:
Several visual functions are impaired in patients with oculocutaneous albinism (OCA) associated to albinistic bilateral amblyopia (ABA).
Objective:
In this study, we aimed at exploring whether perceptual learning (PL) can improve visual functions in albinism.
Method:
Six patients and six normal sighted controls, were trained in a contrast detection task with lateral masking. Participants were asked to choose which of the two intervals contained a foveally presented low-contrast Gabor patch. Targets were presented between higher contrast collinear flankers with equal spatial frequency. When increasing target-to-flanker distance, lateral interactions effect normally switches from inhibition to facilitation, up to no effect.
Results:
Our findings showed that before PL, only controls showed facilitation. After PL, results suggest that facilitatory lateral interactions are found both in controls as well as in albino patients. These results suggest that PL could induce higher processing efficiency at early cortical level. Moreover, PL positive effect seems to transfer to higher-level visual functions, but results were not very consistent among tasks (visual acuity, contrast sensitivity function, hyperacuity and foveal crowding).
Conclusions:
Although a small sample size was tested, our findings suggest a rehabilitative potential of PL in improving visual functions in albinism.
... Specifically, they investigated lateral interactions in the amblyopic striate cortex monocularly, since V1 is largely accepted as the main site for mediating the lateral masking phenomena observed behaviourally [32][33][34][35] and its basis may be the long-range horizontal intrinsic connections that have been observed in visual cortex 36 . Polat & Sagi and their collaborators showed reduced lateral excitatory interactions in anisometropic amblyopia 20,37,38 , which was also reported in other results 39,40 . Further abnormalities in the interactions were also found for second-order stimuli, lateral interactions and texture patterns 41,42 . ...
... Lateral interactions in low-level visual processing have been related to specific structures and connectivity within early visual areas 63 . Psychophysical studies have demonstrated the presence of abnormal spatial interactions in anisometropic amblyopes e.g., refs 19-21, 38, 39, 64, with the idea that some psychophysical measures directly probe inhibitory/excitatory lateral interaction connections 4,20,38,41 . Our use of the centre-surround tilt illusion, which was interpreted almost 50 years ago as one instantiation of lateral inhibitory interactions between orientation sensitive neurons 45,65 , as a probe was based on cumulative knowledge from physiology, behaviour and computational work that permitted modelling the supposed underlying computational structure creating the effect 44,50,51 . ...
... This line of thought is also supported by the findings that there seems to be a stronger contrast gain control from the fellow eye to both (1) the signal in the AE and (2) the contrast gain control signal from the AE 73,74 . By contrast, our results show that affected amblyopes' eyes exhibiting stronger lateral inhibition could occur at a monocular level before the fellow eye acts on the AE (see also the discussion of Bonneh et al. 38 for such a proposition among other explanations). Therefore, another possibility can be put forward that the observed binocular suppression could naturally follow from monocular inhibition without any assumption of asymmetric binocular interactions 74 . ...
Human vision relies on correct information processing from the eye to various visual areas. Disturbances in the visual perception of simple features are believed to come from low-level network (e.g., V1) disruptions. In the present study, we modelled monocular losses in spatial vision through plausible multiple network modifications in early visual coding. We investigated perceptual deficits in anisometropic amblyopia and used the monocular tilt illusion as a probe of primary visual cortex orientation coding and inhibitory interactions. The psychophysical results showed that orientation misperception was higher in amblyopic eyes (AE) than in the fellow and neurotypical eyes and was correlated with the subject’s AE peak contrast sensitivity. The model fitted to the experimental results allowed to split these observations between different network characteristics by showing that these observations were explained by broader orientation tuning widths in AEs and stronger lateral inhibition in abnormal amblyopic system that had strong contrast sensitivity losses. Through psychophysics measures and computational modelling of V1, our study links multiple perceptual changes with localized modifications in the primary visual cortex.
... Another difference lies in using large (one-letter) target-flanker spacing (e.g., Strasburger et al., 1991), as opposed to small (less than a half-letter) spacing (e.g., Lev, Yehezkel, et al., 2014;Simmers, Gray, McGraw, & Winn, 1999), resulting in different crowding effects for otherwise similar parameters. The flanking stimuli themselves are also not consistent between studies, that is, flanking bars (e.g., Siderov et al., 2013), as opposed to letters (e.g., Bonneh, Sagi, & Polat, 2004Danilova & Bondarko, 2007;Lev, Yehezkel, et al., 2014), as well as the spacing calculation, that is, centerto-center separation measurements (e.g., Levi & Carney, 2011;Pelli et al., 2004) as opposed to edge-to-edge spacing (e.g., Danilova & Bondarko, 2007). Consequently, conclusions regarding target-flanker critical spacing dependency on the target size are controversial. ...
... The paradigm used here is very similar to the one we used before (Bonneh et al., 2004(Bonneh et al., , 2007Lev, Ludwig, et al., 2014;Lev, Yehezkel, et al., 2014). Each block measured the minimal detectable target size for a single duration and spacing using a staircase method, which is an adaptive procedure in which the pattern size and spacing were modified according to performance (chance level of 25%; the size increased for each mistake and decreased for two correct responses in a row). ...
... Moreover, interestingly, 0.6 logMar is the acuity of a normally sighted young subject at an eccentricity of 18-28, depending on the stimulus contrast (18 for 10% contrast and 28 for 45% contrast; Abdelnour & Kalloniatis, 2001). The same rules are applicable when relating to foveal acuity of anisometropic amblyopes (Bonneh et al., 2004). It is important that in presbyopia, in addition to the larger size of the target's detection threshold, there is an additional significant factor of blurring that merits extensive systematic investigation, with combined controlled manipulation of artificial blurring and several different spacings. ...
Spatial crowding decreases object recognition and conscious visual perception in clutter. In a previous study we used brief presentation times to reveal the effects of a crowded presentation in the fovea. Here we aimed to test the relationships between varying visual acuity (VA) and crowding in the fovea, under the assumption that in uncorrected presbyopia, the processing is relatively normal, whereas the retinal input is blurred. We tested whether normal participants whose near VA is gradually reduced due to age-related deterioration (presbyopia, or "aging eye") will show an acuity-dependent increase in foveal crowding. We used brief presentations and acuity-threshold letter targets in order to magnify the crowding-effect amplitude in the fovea. A total of 195 participants with an age range of 20-68 years and an average of 44.3 ± 11.7 years (M ± SD) were divided into four age groups, all without any optical correction for the near distance. Our findings show that crowding is proportional to VA. This proportionality is affected by VA-age dependency, with a nonlinear S-shaped pattern: A steep VA reduction begins to develop, which is compatible with the normal onset age of presbyopia symptoms and a saturation in the VA-age dependency in the oldest age group, for which we propose a VA-eccentricity account. Finally, there is a high variance in the crowding amplitude in the young, even before the onset age of presbyopia symptoms, suggesting crowding conditions with limited presentation times as a highly sensitive measure of VA, which predicts visual performance in complex tasks, such as reading.
... Amblyopia impairs both detection and identification of a target in clutter (Bonneh, Sagi, & Polat, 2004;Ellemberg, Hess, & Arsenault, 2002;Hess & Jacobs, 1979;Levi & Carney, 2011;Levi, Hariharan, & Klein, 2002a;Polat, Bonneh, Ma-Naim, Belkin, & Sagi, 2005). Bonneh et al. (2004) compared flanked and unflanked letter acuities in a large cohort of amblyopes, finding that flanked acuity is highly correlated with unflanked acuity in purely anisometropic amblyopia but not in strabismic amblyopia. ...
... Amblyopia impairs both detection and identification of a target in clutter (Bonneh, Sagi, & Polat, 2004;Ellemberg, Hess, & Arsenault, 2002;Hess & Jacobs, 1979;Levi & Carney, 2011;Levi, Hariharan, & Klein, 2002a;Polat, Bonneh, Ma-Naim, Belkin, & Sagi, 2005). Bonneh et al. (2004) compared flanked and unflanked letter acuities in a large cohort of amblyopes, finding that flanked acuity is highly correlated with unflanked acuity in purely anisometropic amblyopia but not in strabismic amblyopia. Flanker effects in amblyopic central vision are not yet well understood and are explored systematically here. ...
... A number of studies have reported large flanker effects in strabismic amblyopes, even in those who have only mildly impaired unflanked visual acuity (Bonneh et al., 2004;Hess & Jacobs, 1979;Levi, Hariharan, et al., 2002a). In the current study, superimposing the regression lines for normal vision (from Figure 4) on Figure 5b shows that the strabismic amblyopes' regression line (solid) lies above the blurred-normal regression line (dotted), i.e., for any given acuity, strabismic amblyopes have larger threshold spacing than do the normals with added blur. ...
Here, we systematically explore the size and spacing requirements for identifying a letter among other letters. We measure acuity for flanked and unflanked letters, centrally and peripherally, in normals and amblyopes. We find that acuity, overlap masking, and crowding each demand a minimum size or spacing for readable text. Just measuring flanked and unflanked acuity is enough for our proposed model to predict the observer's threshold size and spacing for letters at any eccentricity. We also find that amblyopia in adults retains the character of the childhood condition that caused it. Amblyopia is a developmental neural deficit that can occur as a result of either strabismus or anisometropia in childhood. Peripheral viewing during childhood due to strabismus results in amblyopia that is crowding limited, like peripheral vision. Optical blur of one eye during childhood due to anisometropia without strabismus results in amblyopia that is acuity limited, like blurred vision. Furthermore, we find that the spacing:acuity ratio of flanked and unflanked acuity can distinguish strabismic amblyopia from purely anisometropic amblyopia in nearly perfect agreement with lack of stereopsis. A scatter diagram of threshold spacing versus acuity, one point per patient, for several diagnostic groups, reveals the diagnostic power of flanked acuity testing. These results and two demonstrations indicate that the sensitivity of visual screening tests can be improved by using flankers that are more tightly spaced and letter like. Finally, in concert with Strappini, Pelli, Di Pace, and Martelli (submitted), we jointly report a double dissociation between acuity and crowding. Two clinical conditions-anisometropic amblyopia and apperceptive agnosia-each selectively impair either acuity A or the spacing:acuity ratio S/A, not both. Furthermore, when we specifically estimate crowding, we find a double dissociation between acuity and crowding. Models of human object recognition will need to accommodate this newly discovered independence of acuity and crowding.
... Amblyopia is a visual disorder manifested by monocular (anisometropia and strabismus) or binocular (binocular deprivation due to congenital cataract) reduction of visual acuity following abnormal binocular visual experience during the 'critical period' (Daw, 1998;Horton & Hocking, 1997;Hubel & Wiesel, 1970). It is characterized by several spatial vision abnormalities in the amblyopic eye (for reviews, see Ciuffreda, Levi & Selenow, 1991;Hess, Field & Watt, 1990;Levi, 1991;Levi & Carkeet, 1993) including reductions in visual acuity, contrast sensitivity, abnormal suppressive and facilitatory spatial interactions (Ellemberg et al., 2002b;Levi, Hariharan & Klein, 2002;Lewis, Ellemberg, Maurer, Wilkinson, Wilson, Dirks & Brent, 2002;Polat, Ma-Naim, Belkin & Sagi, 2004), as well as impaired contour detection (Hess, McIlhagga & Field, 1997;Kovacs, Polat, Pennefather, Chandna & Norcia, 2000) possibly due to the lack of collinear facilitation (Bonneh, Sagi & Polat, 2004;Ellemberg, Hess & Arsenault, 2002a;Polat, 2008;Polat et al., 2004). ...
... '20/80') which is four times worse than the standard adult vision of 0 LogMAR ('20/20'); LG's right-left eye Snellen equivalents: 6/19-2/24 m; 20/63/-20/80 ft) which could not be attributed to optical refractive error. Moreover, a significant part of the acuity deficit was due to visual crowding (~0.3 log units, twice as poor as standard adult vision), as measured with crowded and uncrowded displays of tumbling E patterns (Bonneh et al., 2004). Two tests suggested abnormal early and mid-level integration mechanisms. ...
... The crowding effect was measured in an identical way to that described previously (Bonneh et al., 2004). In short, VA was measured by a 'Tumbling-E patterns test' which is a LogMAR chart equivalent, monitor-based paradigm (see Figure 2A). ...
Long-term deprivation of normal visual inputs can cause perceptual impairments at various levels of visual function, from basic visual acuity deficits, through mid-level deficits such as contour integration and motion coherence, to high-level face and object agnosia. Yet it is unclear whether training during adulthood, at a post-developmental stage of the adult visual system, can overcome such developmental impairments. Here, we visually trained LG, a developmental object and face agnosic individual. Prior to training, at the age of 20, LG's basic and mid-level visual functions such as visual acuity, crowding effects, and contour integration were underdeveloped relative to normal adult vision, corresponding to or poorer than those of 5–6 year olds (Gilaie-Dotan, Perry, Bonneh, Malach & Bentin, ). Intensive visual training, based on lateral interactions, was applied for a period of 9 months. LG's directly trained but also untrained visual functions such as visual acuity, crowding, binocular stereopsis and also mid-level contour integration improved significantly and reached near-age-level performance, with long-term (over 4 years) persistence. Moreover, mid-level functions that were tested post-training were found to be normal in LG. Some possible subtle improvement was observed in LG's higher-order visual functions such as object recognition and part integration, while LG's face perception skills have not improved thus far. These results suggest that corrective training at a post-developmental stage, even in the adult visual system, can prove effective, and its enduring effects are the basis for a revival of a developmental cascade that can lead to reduced perceptual impairments.
... A recent study has shown that the PF's size in children is larger than in adults up to age 6, the age of maturation of visual processing 44 , hence, children have more crowding. Study (Bonneh, Sagi & Polat) 52 shows a large crowding effect in the fovea of strabismic amblyopia, a condition that is modeled as the periphery of normal vision 52 . Thus, we suggest that individuals with abnormal and underdeveloped visual systems might experience arrested development, leading to increased PF's size and, consequently, a more pronounced crowding effect under both low and high luminance conditions. ...
... A recent study has shown that the PF's size in children is larger than in adults up to age 6, the age of maturation of visual processing 44 , hence, children have more crowding. Study (Bonneh, Sagi & Polat) 52 shows a large crowding effect in the fovea of strabismic amblyopia, a condition that is modeled as the periphery of normal vision 52 . Thus, we suggest that individuals with abnormal and underdeveloped visual systems might experience arrested development, leading to increased PF's size and, consequently, a more pronounced crowding effect under both low and high luminance conditions. ...
Crowding is a phenomenon in which the ability to recognize an object in a clutter deteriorates. It is, therefore, a fundamental aspect of object recognition and crucial in deciphering resolution. For visually impaired individuals, deficiency in crowding has a tremendous effect on vision and may reflect and predict the amount of deterioration in vision. It is well established that albinos suffer much more from crowding than normally sighted individuals under daylight luminance conditions. However, to our knowledge, this study is the first to investigate crowding in albino participants under low light conditions. In this study, we explored the crowding effect in a group of albino participants (n = 9) and a control group of normally sighted participants (n = 9). Crowding was conducted under daylight (photopic vision) and low light (scotopic vision). We measured the visual acuity threshold under crowding in three-letter spacing (0.5, 1, and 1.5) and compared it to a single target. Results indicate that albino participants experienced stronger crowding than the control under the photopic condition, while crowding under the scotopic condition was apparent in the albino but abolished for the control group. These findings highlight the importance of considering luminance when discussing the visually impaired population in general. In particular, it suggests that crowding in albinism is based on a peripheral-like mechanism and may indicate a cessation in visual development.
... The prevalence of amblyopia is approximately 1-3% in humans [3]. Amblyopia is associated with reduced spatiotemporal vision that affects visual acuity [4,5], vernier acuity [6], contrast sensitivity [7], stereopsis [8], and abnormal spatial interactions [9]. ...
... The interactions between group and frequency, between group and brain region, and between frequency and brain region were not significant (F (2,207) = 1.57, P>0.05; F (2,207) = 0.67, P>0.05; F (4,207) = 0.19, P>0.05). The interaction among group, frequency and brain region was also not significant (F (4,207) = 0.80, P>0.05). ...
Cognition is very important in our daily life. However, amblyopia has abnormal visual cognition. Physiological changes of the brain during processes of cognition could be reflected with ERPs. So the purpose of this study was to investigate the speed and the capacity of resource allocation in visual cognitive processing in orientation discrimination task during monocular and binocular viewing conditions of amblyopia and normal control as well as the corresponding eyes of the two groups with ERPs. We also sought to investigate whether the speed and the capacity of resource allocation in visual cognitive processing vary with target stimuli at different spatial frequencies (3, 6 and 9 cpd) in amblyopia and normal control as well as between the corresponding eyes of the two groups. Fifteen mild to moderate anisometropic amblyopes and ten normal controls were recruited. Three-stimulus oddball paradigms of three different spatial frequency orientation discrimination tasks were used in monocular and binocular conditions in amblyopes and normal controls to elicit event-related potentials (ERPs). Accuracy (ACC), reaction time (RT), the latency of novelty P300 and P3b, and the amplitude of novelty P300 and P3b were measured. Results showed that RT was longer in the amblyopic eye than in both eyes of amblyopia and non-dominant eye in control. Novelty P300 amplitude was largest in the amblyopic eye, followed by the fellow eye, and smallest in both eyes of amblyopia. Novelty P300 amplitude was larger in the amblyopic eye than non-dominant eye and was larger in fellow eye than dominant eye. P3b latency was longer in the amblyopic eye than in the fellow eye, both eyes of amblyopia and non-dominant eye of control. P3b latency was not associated with RT in amblyopia. Neural responses of the amblyopic eye are abnormal at the middle and late stages of cognitive processing, indicating that the amblyopic eye needs to spend more time or integrate more resources to process the same visual task. Fellow eye and both eyes in amblyopia are slightly different from the dominant eye and both eyes in normal control at the middle and late stages of cognitive processing. Meanwhile, abnormal extents of amblyopic eye do not vary with three different spatial frequencies used in our study.
... The results show that patients with amblyopia do not show the typical collinear facilitation, rather their spatial interaction is dominated by suppression [1,[57][58][59][60][61][62][63][64][65][66][67][68]. This effect is accompanied with a high degree of visual crowding [69,70] that is one of the hallmarks of amblyopia [71]. ...
... This PL clinical therapy was designed to improve the abnormal spatial interactions in amblyopia [1,2,13]. The PL therapy was tailored and specifically designed for each patient's unique deficiencies [57,69,70]. The impaired spatial interactions were significantly improved, reaching the normal level after training [1,2]. ...
Until recent years, there was low awareness that the brain’s ability to adapt to changes in sensory input through the modification of neural pathways and processing functions, i.e., neuroplasticity, remains after late adolescent development. Over the past few decades, an extensive body of evidence has established that neuroplasticity not only endures in the adult neuronal processing system, but can also be augmented using advanced perceptual learning training, resulting in improved vision and image processing speed. It is well-established that during the early period of life, termed as critical period, the sensory system displays high sensitivity to environmental stimuli and develops in a particular way owing to adapt to the visual input that is received from the eye. The process of balancing the refraction of the anterior ocular segment and the axial length of the eye to produce a focused image on the retina, i.e., emmetropization, occurs during the first few years of life. Emmetropization involves fine-tuning of the refractive state by altering the refractive components of the eye toward zero refraction. Recent studies have shown that compromised cortical processing, either due to abnormal development, e.g., amblyopia, or due to uncorrected and blurred visual input from the eye, e.g., myopia or presbyopia, can be remarkably strengthened resulting in improved visual performance functions independent of emmetropization. Therefore, advanced perceptual training can be of significant benefit to patients who undergo refractive surgery. In these patients, the correction of the optical input is suddenly altered changing the coherence between the input delivered from the eye and the steady-state cortical processing. This abrupt effect may pose the need for the image processing functions of the visual cortex to readapt to the modified visual input. Boosting cortical processing using perceptual learning may provide enhanced vision benefits for these patients and increase their satisfaction. Future perceptual learning techniques are being modified to serve as a complementary vision performance enhancement program to noninvasive procedures or stand-alone. The new programs will allow clinicians the opportunity to offer patients personalized perceptual learning programs to improve a variety of visual functions that are not currently addressed by conventional treatments or procedures.
... 5,6 The sensory deficits associated with amblyopia have been researched extensively. For example, people with amblyopia exhibit deficits in spatial localization as a result of positional uncertainty, 4,7-9 increased spatial and temporal crowding, 10,11 and deficits in global motion perception. [12][13][14] These deficits have been attributed to increased noise in the amblyopic visual system, [15][16][17] spatial undersampling of visual neurons in the striate cortex, [18][19][20] and spatial distortions. ...
... During the plateau phase of adaptation, there was no significant difference between controls (1.368 6 1.28) and participants with amblyopia (1.368 6 1.08; t [16] ¼ 0.015, adjusted P ¼ 0.99). Additionally, among participants with amblyopia, there was a significantly greater squared mean of residuals during the rapid error correction phase (2.998 6 1.218) compared to the plateau phase (1.368 6 1.08; t [6] ¼ 3.6, adjusted P ¼ 0.02), but no such difference was observed in controls (rapid error correction, 0.888 6 1.028; plateau, 1.368 6 1.28; t [10] ¼ À1.4, adjusted P ¼ 0.25). ...
Using visual feedback to modify sensorimotor output in response to changes in the external environment is essential for daily function. Prism adaptation is a well-established experimental paradigm to quantify sensorimotor adaptation; that is, how the sensorimotor system adapts to an optically-altered visuospatial environment. Amblyopia is a neurodevelopmental disorder characterized by spatiotemporal deficits in vision that impacts manual and oculomotor function. This study explored the effects of anisometropic amblyopia on prism adaptation.
Eight participants with anisometropic amblyopia and 11 visually-normal adults, all right-handed, were tested. Participants pointed to visual targets and were presented with feedback of hand position near the terminus of limb movement in three blocks: baseline, adaptation, and deadaptation. Adaptation was induced by viewing with binocular 11.4° (20 prism diopter [PD]) left-shifting prisms. All tasks were performed during binocular viewing.
Participants with anisometropic amblyopia required significantly more trials (i.e., increased time constant) to adapt to prismatic optical displacement than visually-normal controls. During the rapid error correction phase of adaptation, people with anisometropic amblyopia also exhibited greater variance in motor output than visually-normal controls.
Amblyopia impacts on the ability to adapt the sensorimotor system to an optically-displaced visual environment. The increased time constant and greater variance in motor output during the rapid error correction phase of adaptation may indicate deficits in processing of visual information as a result of degraded spatiotemporal vision in amblyopia.
... In Figure 3c we present the results obtained from young participants with normal vision (N 5 18, average age 25.4 6 0.77). Here we used the adaptive method (staircase) that we used previously to measure the crowding effect in the fovea of controls and amblyopic subjects 3,21 . The method is similar to that of Flom et al., 1963 4 whereby the contour interaction is calculated as the difference between the thresholds of a recognized isolated target letter E and the target is embedded in a matrix of E letters. ...
... In cases of presbyopia we applied here the same paradigm that we had used before 3,21 in order to investigate the crowding at different presentation times. It consists of a LogMAR chart equivalent, monitor-based paradigm that used E-patterns presented for presentation times ranging from 30-240 msec. ...
Visual crowding, as context modulation, reduce the ability to recognize objects in clutter, sets a fundamental limit on visual perception and object recognition. It's considered that crowding does not exist in the fovea and extensive efforts explored crowding in the periphery revealed various models that consider several aspects of spatial processing. Studies showed that spatial and temporal crowding are correlated, suggesting a tradeoff between spatial and temporal processing of crowding. We hypothesized that limiting stimulus availability should decrease object recognition in clutter. Here we show, for the first time, that robust contour interactions exist in the fovea for much larger target-flanker spacing than reported previously: participants overcome crowded conditions for long presentations times but exhibit contour interaction effects for short presentation times. Thus, by enabling enough processing time in the fovea, contour interactions can be overcome, enabling object recognition. Our results suggest that contemporary models of context modulation should include both time and spatial processing.
... More than half of the children in our study (Worth 4-dot test) did not have simultaneous vision, and none of them had stereopsis vision prior to treatment. Strong evidence suggests that patients with strabismic amblyopia have a much lower probability of improvement with binocular training compared to anisometropic amblyopia [27,[31][32][33] . Another reason was that patching itself may lead to reduced binocular vision and stereopsis. ...
Purpose
Prism has been used as a non-surgical treatment for certain types of strabismus; however, its role as the primary intervention for amblyopia combined with esotropia has been rarely studied. The objective of this study was to compare the effectiveness of prismatic correction with traditional occlusion therapy in treating amblyopia combined with esotropia.
Methods
This retrospective case-control study included children aged 3 to 16 years with amblyopia combined with esotropia. After refractive correction, the patients were divided into two groups. The control group received traditional occlusion therapy combined with binocular treatment, while the prism group received full correct prisms combined with binocular treatment. Clinical characteristics, visual acuity, binocular functions, and the angle of esodeviation were assessed at each follow-up examination. The treatment outcomes after 12 months were evaluated to determine the more effective protocol.
Results
At baseline, there were no significant differences in age, gender ratio, best-corrected visual acuity (BCVA) of the amblyopic eye, deviation amount (△), and binocular vision function between the two groups (all P > 0.05). After treatment, both groups showed improvement in amblyopic eye visual acuity. The difference in visual acuity improvement between the two groups was not statistically significant at 6 months (0.18±0.17 vs 0.14±0.11, P > 0.05), but it was statistically significant at 12 months (0.35±0.20 vs 0.25±0.18, P < 0.05). The prism group demonstrated a significant improvement in binocular vision function, whereas there was no significant change in the control group at 6 months or 12 months after treatment. The deviation amount (△) remained stable in both groups, with no significant difference between the two groups after treatment.
Conclusion
Prismatic treatment combined with binocular treatment was more effective in improving amblyopic eye visual acuity and binocular vision compared to traditional occlusion therapy combined with binocular treatment in children with amblyopia combined with esotropia. Prismatic correction could be considered as the first-line treatment for amblyopia combined with esotropia.
... These results resemble the larger foveal crowding that was observed in strabismic amblyopia 51,59,85 but without being attributed to manifest strabismus. Note that amblyopia results from a lack of normal maturation of the visual system, which leads to reduced visual acuity 49,101,102 , diminished contrast sensitivity 101,103,104 , and especially at high spatial frequencies 81,101,103 , impaired spatial interactions 92 and a slower reading speed. Recently, it was shown that the crowding strength and extent are larger in the immature visual system of infants and that only with normal development will the crowding decrease to reach its standard effect at 5-6 years old 52,53 . ...
Heterophoria is a common type of binocular fusion disorder that consists of a latent eye misalignment with potential consequences on daily activities such as reading or working on a computer (with CVS). Crowding, a type of contextual modulation, can also impair reading. Our recent studies found an abnormal pattern of low-level visual processing with larger perceptive fields (PF) in heterophoria. The PF is the fundamental processing unit of human vision and both masking and crowding depend on its size. We investigated how heterophoria would impact the PF’s size via a lateral masking experiment and consequently affect the foveal crowding at different letter-spacings (the crowding zone). More specifically, we explored the relationship between crowding, lateral masking, the PF’s size, and the amount of heterophoria. The binocular horizontal PF’s size was larger with heterophoric subjects, in agreement with our previous study. We found a stronger crowding and an extended crowding zone associated with slower response times; this shows that the processing of letter identification under both crowded and uncrowded conditions requires more processing effort in heterophoric individuals. In agreement with previous studies, we found a correlation between the crowding zone and the PF’s size; each was strongly correlated with the amount of phoria. These findings resemble those involving the PF size and the extended crowding found at the fovea in amblyopia and young children. We suggest that these findings could help explain the inter-observers’ variability found in the masking literature, and the reading difficulties often encountered in subjects with high heterophoria.
... Contrast sensitivity. Advanced computerized tests to examine the spatial visual perception were performed based on a procedure previously described by our group 48,[100][101][102][103][104] . Briefly, the tests are based on a 'PSY-psychophysical tool' software including contrast sensitivity and backward masking (BM) 105 . ...
Studies have shown that Perceptual Learning (PL) can lead to enhancement of spatial visual functions in amblyopic subjects. Here we aimed to determine whether a simple flickering stimulus can be utilized in PL to enhance temporal function performance and whether enhancement will transfer to spatial functions in amblyopic subjects. Six adult amblyopic and six normally sighted subjects underwent an evaluation of their performance of baseline psychophysics spatial functions (Visual acuity (VA), contrast sensitivity (CS), temporal functions (critical fusion frequency (CFF) test), as well as a static and flickering stereopsis test, and an electrophysiological evaluation (VEP). The subjects then underwent 5 training sessions (on average, a total of 150 min over 2.5 weeks), which included a task similar to the CFF test using the method of constant stimuli. After completing the training sessions, subjects repeated the initial performance evaluation tasks. All amblyopic subjects showed improved temporal visual performance (CFF) in the amblyopic eye (on average, 17%, p << 0.01) following temporal PL. Generalization to spatial, spatio-temporal, and binocular tasks was also found: VA increased by 0.12 logMAR (p = 0.004), CS in backward masking significantly increased (by up to 19%, p = 0.003), and flickering stereopsis increased by 85 arcsec (p = 0.048). These results were further electrophysiologically manifested by an increase in VEP amplitude (by 43%, p = 0.03), increased Signal-to-Noise ratio (SNR) (by 39%, p = 0.024) to levels not different from normally sighted subjects, along with an improvement in inter-ocular delay (by 5.8 ms, p = 0.003). In contrast, no significant effect of training was found in the normally sighted group. These results highlight the potential of PL based on a temporal stimulus to improve the temporal and spatial visual performance in amblyopes. Future work is needed to optimize this method for clinical applications.
... The tests were performed using a procedure described by our group. [51][52][53][54] The tests are based on a 'PSY -psychophysical tool' software including contrast sensitivity and backward masking (BM). Gabor patches at various spatial frequencies as well as contrast and duration times (as is detailed for each test) were used as the visual stimuli. ...
Integration of information over the central nervous system is an important neural process that affects our ability to perceive and react to the environment. The visual system is required to continuously integrate information arriving from two different sources (the eyes) to create a coherent percept with high spatiotemporal precision. Although this neural integration of information is assumed to be critical for visual performance, it can be impaired under some pathological or developmental conditions. Here we took advantage of a unique developmental condition, amblyopia (“lazy eye”), which is characterized by an impaired temporal synchronization between the two eyes, to meticulously study the effect of synchronization on the integration of binocular visual information. We measured the eyes’ asynchrony and compensated for it (with millisecond temporal resolution) by providing time-shifted stimuli to the eyes. We found that re-synchronization of the ocular input elicited a significant improvement in visual functions, and binocular functions, such as binocular summation and stereopsis, were regained. This phenomenon was also evident in neurophysiological measures. Our results can shed light on other neural processing aspects and might also have translational relevance for the field of training, rehabilitation, and perceptual learning.
... However, after the RevitalVision treatment, it has been found that the amount of suppression was significantly reduced. [25][26][27][28] The concept of RevitalVision treatment technology is based on research conducted by Polat and colleagues in seventyseven amblyopic patients (aged 955 years). A test requiring the identification of low contrast (difficult to detect) or high contrast (easy to detect) Gabor stimulus, with or without other Gabors flanking it, was given to groups of individuals. ...
SIGNIFICANCE
Perceptual based vision training program improves visual acuity in adult amblyopic individuals beyond occlusion.
PURPOSE
To evaluate the efficacy of RevitalVision’s perceptual learning vision training software program
(Talshir Guy Medical Technologies Ltd, ISRAEL.)
in improving best-corrected visual acuity in
occlusion therapy-resistant adult amblyopic
subjects
METHODS
This prospective study was carried out at a tertiary
eye care center in northeast India. 45 subjects
were considered for the study. All subjects
underwent at least six months of part-time
occlusion therapy with no visual improvement
before starting the RevitalVision therapy. The
main outcome assessed in the study was the
improvement in distance best-corrected visual
acuity (BCVA).
RESULTS
The mean ± SD age of the subjects was 17.2 ±
10.2 years (range, 8-48 years). Mean distance
best corrected visual acuity improved significantly
from 0.54 LogMAR to 0.32 LogMAR (p<.001).
CONCLUSION
Preliminary evidence suggests RevitalVision
treatment improves best corrected visual acuity
in amblyopic subjects not responding to parttime occlusion therapy.
... Amblyopia is characterized by a large range of behavioral, neural, perceptual, oculomotor, and clinical abnormalities (for reviews, see 100,101 ). Among them, an abnormal pattern of lateral interactions was observed in functional strabismic amblyopia 85,102,103 : facilitation at 3λ was either markedly lower or replaced by inhibition. We www.nature.com/scientificreports/ ...
In an era of increasing screen consumption, the requirement for binocular vision is demanding, leading to the emergence of syndromes such as the computer vision syndrome (CVS) or visual discomfort reported by virtual reality (VR) users. Heterophoria (phoria) is a latent eye misalignment (with a prevalence up to 35%) that appears in conditions that disrupt binocular vision and may affect the quality of binocular fusion. Collinear facilitation (CF), the mechanism for grouping contour elements, is a process that reveals lateral interactions by improving the visibility of a target by flankers placed collinearly. An abnormal pattern of CF has been observed in strabismic amblyopia. We hypothesize that phoria may affect CF in the horizontal meridian (HM) due to latent eye misalignment and its impact on binocular fusion. Fully corrected participants (phoria group and controls) completed a standard CF experiment for horizontal and vertical meridians during binocular and monocular viewing. Phoric observers exhibited (1) an asymmetry and an abnormal pattern of CF only for the HM, during both monocular and binocular viewing, (2) poor binocular summation between the monocular inputs, and (3) no binocular advantage of the CF. Phoria affects the CF in a way that is reminiscent of meridional amblyopia without being attributed to abnormal refraction. The abnormal pattern of CF in monocular viewing suggests that phoria could be a binocular developmental disorder that affects monocular spatial interactions. We suggest that the results could contribute to explain the visual discomfort experienced with VR users or symptoms when presenting CVS.
... Thus, the crowding effect may not influence our conclusions. The crowding effect is found more in amblyopia with strabismus (Hess et al., 2001;Hariharan et al., 2005) but less in pure anisometropic amblyopia (Bonneh et al., 2004;Greenwood et al., 2012). In this study, our patients were, or used to be anisometropic amblyopia, implying that the crowding effect could be quite minimal. ...
Purpose
To develop a novel dichoptic visual acuity chart that measures the impact of interocular suppression on the visual acuity of each eye when two eyes are open.
Methods
Fifty-four subjects (19 anisometropic amblyopia, 20 treated amblyopia, and 15 normal children) participated in this study. The visual acuity that was tested under dichoptic-optotypes condition (i.e., presented optotypes to the untested eye) was compared with that under monocular condition (i.e., cover the untested eye with opaque patch). Visual acuity differences between these two conditions were compared among the three groups. The correlations between visual acuity differences and the depth of interocular suppression were then computed. Some participants performed the visual acuity test under dichoptic-luminance condition (i.e., presented mean luminance to the untested eye), and the test-retest reliability was established.
Results
A reduced visual acuity of the non-dominant eye was found in the dichoptic-optotypes condition for the amblyopia group (P < 0.001) and the treated group (P = 0.001); the difference in the treated group was less than that in the amblyopia group (P < 0.001) but more than that in the normal group (P = 0.026). A significant correlation was found between the visual acuity differences and the depth of suppression, which was tested with a binocular phase combination task (P = 0.005). No change was found in the dichoptic-luminance condition.
Conclusion
The amblyopic eye and the previous amblyopic eye seem to suffer from a reduced visual acuity when two eyes are open due to suppression. This was successfully captured by our novel and reliable dichoptic-optotypes visual acuity chart.
... Patients with amblyopia not only face decreased visual acuity but also other functional deficits such as decreased contrast sensitivity, low accommodation, crowding, suppression, abnormal spatial localization, and abnormal interaction of spatial and temporal function. [7,8] Conventional methods like patching are of limited use beyond the critical period of development, though levodopa [9] and citicoline [10] are tried. Recent data attributes the pathophysiology in anisometropic amblyopia to be due to difference in contrast signaling between the eyes, wherein the eye with higher refractive error carrying weaker contrast is suppressed by fellow normal eye with good contrast leading to amblyopia. ...
Purpose:
Refractive correction and patching is the timetested mainstay of treatment for anisometropic amblyopia within the critical period of visual development. Binocular therapies using dichoptic training which overcome suppression by balancing the contrast between two eyes has been increasingly gaining ground. We evaluated the efficacy of dichoptic training in the adult population with anisometropic amblyopia. This study aims to evaluate the effectiveness of dichopticbased active vision therapy, using "VisuoPrime" software as primary intervention, in adults with anisometropic amblyopes.
Materials and methods:
A prospective interventional study in adults (18-40 years) with anisometropic amblyopia was conducted from August 2019 to March 2020. METHODS: Twentynine subjects with anisometropic amblyopia played binocular games through "VisuoPrime" software 30 min daily for 6 weeks. Bestcorrected visual acuity (BCVA) and binocularity was assessed at 1 and 3 months. Student's paired ttest, Wilcoxon signedrank sum test and MannWhitney tests were used. Statistical package of SPSS version 20.0 was used for analysis, considering P < 0.05 as statistically significant.
Results:
BCVA of the amblyopic eye improved from 0.60 ± 0.40 logMAR to 0.45 ± 0.29 logMAR and 0.38 ± 0.23 logMAR at 1 and 3 months, respectively (P = 0.0001). Near acuity improved from 0.21 ± 0.14 to 0.14 ± 0.08 logMAR and 0.1 ± 0.04 logMAR at 1 and 3 months respectively (P < 0.0001). Improvement in stereopsis was observed in 24% of subjects which maintained at 3 month followup.
Conclusion:
Dichopticbased active vision therapy using "VisuoPrime" software was effective as a primary modality in adults with anisometropic amblyopia.
... The experiments were conducted in a dark environment in which the only ambient light came from the monitor. The "Tumbling-E pattern (TeVA) test" paradigm ( Fig. 2) was used [29][30][31]. Figure 3 shows a subject participating in the experiment. The stimuli consisted of 5 X 5 E-patterns, which correspond to a subset of the LogMAR chart, with a baseline pattern size (TeVA = 0) corresponding to the baseline 6/6 visual acuity. ...
In this research we present a basis for a solution for Age Related Macular Degeneration (AMD) patients. The proposed solution is a binocular passive optical device composed of a contact lens and spectacles, both coated by light-reflecting material in order to generate a Fabry-Perot-like resonator. This bounces the light rays several times between the two surfaces, achieving optical simultaneous magnifications for near and far distances as needed for AMD patients in early stages of the disease. Our work has two parts: numerical simulation of the magnification achieved by the device and a clinical experiment, with non-AMD patients, in which we examined visual skills with simultaneous magnifications. The numerical simulations proved mathematically that the device can produce several different magnifications simultaneously, Zemax simulations confirmed this. In the clinical study, simultaneous vision was found to have little effect on visual acuity, but slightly increased reaction time to stimuli. Thus, the proposed device may improve visual capabilities of AMD patients, allow patients in stages where the peripheral retina still functions to use these areas to maximize their remaining visual potential and thus function better in everyday life.
... 1 It encompasses other functional issues like low accommodation, decreased contrast sensitivity, spatial distortion, abnormal spatial localization, abnormalities in pursuit and saccadic eye movements, crowding, suppression, and abnormal interaction of spatial and temporal function. 2,3 Clinically amblyopia is defined when bestcorrected visual acuity is <20/40 in one eye, or there is a two-line difference in VA between the two eyes, which represents the threshold for treatment intervention. Amblyopia is one of the main causes of poor unilateral vision in children which is amenable to correction. ...
54 children with anisometropic amblyopia of ages 4-15 years were included in the study - 24 children in the patching and 30 children in the MFBF group. The study ran for a period of 6 months with BCVA assessed at each visit of follow up.
In the patching group, visual acuity (VA) gain was 0.37 ± 0.26 log Mar over the course of 6 months with continuous improvement in vision across the three time points (0.20 log mar in the first month, 0.11 log Mar from month1 to 3 and 0.06 log Mar from months 3 to 6).In the Amb I Net group, visual acuity gain was 0.54 ± 0.38 log Mar over the same course of time. A continuous improvement in vision was also noted, with gains of 0.16 in the first month, 0.19 from month 1 to 3 and 0.19 from month 3 to 6. The difference in log Mar gains in VA was not statistically significant between the treatment arms and the final VA was similar (0.25 log Mar for patching and 0.18 log Mar for Amb I net). However, clinically the patching group showed 3 log MAR lines, whereas the MFBF group showed 5 log MAR lines of vision improvement at the end of therapy.
MFBF was not superior to patching as far as efficacy is concerned in anisometropic amblyopia. However, the combination of more game options to sustain interest, a shorter duration of therapy over the course of a week and software directed compliance monitoring system favors the Amb I Net system from both patient and parental perspective. Amb I Net also negated discomfort, social stigma, and parental monitoring needed with patching regimens. It can be considered as a good alternative to patching in reluctant/non - compliant children with anisometropic amblyopia. This is the first large cohort study based on this therapy.
... We tested the crowding effect using the "Tumbling-E patterns (TeVA) test" paradigm [51]. The stimulus were black E-patterns on a white background, corresponding to a subset of the LogMar chart. ...
Young children exhibit poorer visual performance than adults due to immaturity of the fovea and of the fundamental processing of visual functions such as masking and crowding. Recent studies suggest that masking and crowding are closely related to the size of the fundamental processing unit—the perceptive field (PF). However, while it is known that the retina and basic visual functions develop throughout childhood, it is not clear whether and how changes in the size of the PF affect masking and crowding. Furthermore, no retinal and perceptual development data have been collected from the same cohort and time. Here we explored the developmental process of the PF and the basic visual functions. Psychophysical and imaging methods were used to test visual functions and foveal changes in participants ranging from 3–17 years old. Lateral masking, crowding and contrast sensitivity were tested using computerized tasks. Foveal measurements were obtained from spectral-domain optical coherence tomography (OCT). The children patterns below 6 years exhibited high crowding, while the expected facilitation was found only at a larger target-flanker distance than required for children above 6 years, who exhibited the typical adult. Foveal thickness and macular volume for the children below 6 years were significantly lower than for the older group. Significant correlation was found for contrast sensitivity, foveal thickness and macular volume with age and between contrast sensitivity and foveal thickness. Our data suggest that the developmental processes at the retina and visual cortex occur in the same age range. Thus, in parallel to maturation of the PF, which enables reduction in crowding, foveal development contributes to increasing contrast sensitivity.
... Many types of training tasks are used in the perceptual learning scheme (e.g., Levi, 2005;Sagi, 2011): vernier acuity (Levi, Klein, & Wang, 1994;Levi & Polat, 1996;Levi, Polat, & Hu, 1997), position discrimination (Li, Klein, & Levi, 2008;Li & Levi, 2004;Li, Provost, & Levi, 2007), contrast detection (Li, Polat, Makous, & Bavelier, 2009;Polat, Ma-Naim, Belkin, & Sagi, 2004;Zhou et al., 2006), and letter identification (Chung, Li, & Levi, 2008;Levi, 2005) in hoping that a certain degree of transfer of the gained capacity to improvements in Snellen acuity globally. Other types of visual deficits among amblyopes such as shape perception, positional uncertainty, contour integration and spatial interaction of surrounding visual object (Bonneh, Sagi, & Polat, 2004;Hess, McLlhagga, & Field, 1997;Levi, 2005) are left not much addressed directly. ...
Ocular dominance plasticity (ODP) is a type of cortical plasticity operating in visual cortex of mammals that are endowed with binocular vision based on the competition‐driven disparity.
Earlier, a molecular mechanism was proposed that catecholamines play an important role in the maintenance of ODP in kittens. Having survived the initial test, the hypothesis was further advanced to identify noradrenaline (NA) as a key factor that regulates ODP in the immature cortex. Later, the ODP‐promoting effect of NA is extended to the adult with age‐related limitations. Following the enhanced NA availability, the chain events downstream lead to the β‐adrenoreceptor‐induced cAMP accumulation, which in turn activates the protein kinase A. Eventually, the protein kinase translocates to the cell nucleus to activate cAMP responsive element binding protein (CREB). CREB is a cellular transcription factor that controls the transcription of various genes, underpinning neuronal plasticity and long‐term memory.
In the advent of molecular genetics in that various types of new tools have become available with relative ease, ODP research has lightly adopted in the rodent model the original concepts and methodologies. Here, after briefly tracing the strategic maturation of our quest, the review moves to the later development of the field, with the emphasis placed around the following issues: 1) Are we testing ODP per s e? 2) What does monocular deprivation deprive of the immature cortex? 3) The critical importance of binocular competition, 4) What is the adult plasticity? 5) Excitation‐Inhibition balance in local circuits, and 6) Species differences in the animal models.
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... 14 Vision charts in which a target is surrounded by flankers are recommended for children's vision screening programs to detect amblyopia, 25,26 as crowding is exaggerated if strabismus is present. [27][28][29][30][31] In currently available tests, targets are flanked by a box, [32][33][34] bars, 35 or other letters, 36 but these flanking features can have different effects on visual acuity in adults 4 and children. 37 Compared to effects of simple contours such as a box or bars, other letters placed around a target letter can lead to stronger interactions. ...
Purpose:
Grouping of flankers from the target can modulate crowding in adults. Visual acuity in children is measured clinically using charts with targets and different flankers to enhance spatial interactions. We investigated grouping effects on interactions using visual acuity letters, flanked by contours and letters, in children.
Methods:
Visual acuity for isolated and flanked letters was measured in 155 three- to 11-year old children and 32 adults. Flankers were one stroke width from the target and were a box or four bars and black or red letters. Magnitudes of interaction were flanked minus isolated logMAR acuities. Psychometric function slopes were also examined.
Results:
Magnitudes of interaction by contours did not change significantly with age. They were 0.047 ± 0.014 logMAR more with bars than a box. Interaction from flanking letters reduced with age, adults being not different from 9- to 11-year-olds for black and red letter surrounds. It was weaker by 0.033 ± 0.013 logMAR when a black letter was surrounded by red rather than black letters. Psychometric function slopes for visual acuity were steepest for the youngest children (3-5 years).
Conclusions:
For contour and letter flankers, grouping effects on interaction magnitude are age independent. Grouping bars into a box forming a single object reduces magnitude of effect. Grouping letter flankers by color and ungrouping them from the target reduce interaction magnitude by ∼8%, suggesting that luminance-defined form dominates. Differently colored letter flankers of high-luminance contrast on acuity charts could draw attention to the target but retain significant interaction strength.
... In a study by Y.S. Bonneh et al., who studied local and nonlocal deficits in amblyopia: acuity and spatial interactions on 60 subjects, it was found that a strabismic patient, who had almost normal acuity for a single optotype but very poor acuity for multiple patterns. Also, the strabismic patient showed a twofold more crowding as compared to a patient with anisometropic amblyopia [16]. In another study by Levi & Klein, 1985, they concluded that the crowding effect in strabismics, unlike in anisometropes, is larger than their acuity deficit would predict [17]. ...
Introduction: Amblyopia is defined as unilateral or bilateral dimness of vision caused by form vision deprivation and/or abnormal binocular interaction, is the most common cause of preventable monocular blindness and nearly all-amblyopic visual loss is reversible with timely detection and appropriate intervention. The conditions leading to functional amblyopia are well known and include strabismus, anisometropia, astigmatism, hypermetropia, cataract, and other forms of stimulus deprivation. Material &Method: The present hospital based cross sectional study has been carried out in Department of Ophthalmology at Acharya Vinoba Bhave Rural Hospital attached to Jawaharlal Nehru Medical College, Sawangi (Meghe), Wardha, a constituent of Datta Meghe Institute of Medical Sciences (Deemed University), Nagpur during the period from September 2012 to August 2014. 100 eyes of 91 patients having Amblyopia or amblyogenic factor (strabismus or anisometropia) in outdoor and indoor patients were selected for the study.All patients were enrolled in the study after due consideration of various exclusion criteria for selection. Results: The mean age of the study population was 15.00 ± 5.74 years, range 6-30 year. The present study found amblyopia to be more common in males than females. All the patients included in this study had one of the diagnoses from strabismic amblyopia, anisometropic amblyopia, isometropic amblyopia and stimulus deprivation amblyopia. Out of these maximum number of eyes was found to have anisometropic amblyopia (37%) and strabismic amblyopia (36%). Maximum numbers of eyes were seen with0.9-1.0 visual acuity (in LogMAR units).It was found to be highly significant in almost all except between strabismic and stimulus deprivation amblyopia; and between anisometropic and isometropic amblyopia.The present study found colour vision to be normal in all the various types of amblyopia. Most eyes were found to have hyperopic astigmatism/hyperopia (56%) followed by myopic astigmatism/myopia (31%) while mixed astigmatism was least common (13%). Maximum numbers of eyes were seen with 0.00 – 0.15 contrast sensitivity. 43% had normal while 57% had abnormal stereopsis. Conclusion: Amblyopia and associated strabismus can have devastating psychosocial and economic fallouts. Knowledge about the sub-types of amblyopia is important because the clinical presentations, management and outcomes of these different types are different. The data in the present study could be used to enhance screening efforts in an organized manner in those health-care groups which come in regular contact with infants and young children.
... A tumbling E test was used to measure visual acuity (VA) threshold 44 at 9 different visual field locations (centre, 2° and 4°, 4 locations at each eccentricity). Separate staircase procedures were applied for each of the 9 locations; trials of all locations were mixed randomly in each session. ...
Since perceptual and neural face sensitivity is associated with a foveal bias, and neural place sensitivity is associated with a peripheral bias (integration over space), we hypothesized that face perception ability will decline more with eccentricity than place perception ability. We also hypothesized that face perception ability may show an upper visual field (UVF) bias due to the proximity of face-related regions to UVF retinotopic representations, and a left visual field (LeVF) bias due to earlier reports suggesting right hemisphere dominance for faces. Participants performed fovea and parafoveal face discrimination tasks (≤4°) while their eye movements were monitored. Additional within-category discrimination performance was measured for houses, inverted faces, shapes and low-level visual acuity. While, as expected, eccentricity-related accuracy reductions were evident for all categories, in contrast to our hypothesis, there was no significant difference between face and house-related accuracy. Furthermore, RTs for houses were significantly faster than for faces at all locations including the fovea. Significant LeVF bias was evident for upright and inverted faces, and face inversion effect was found at all parafoveal eccentricities. Our results suggest that low-level and possibly top-down factors, and not only the face-fovea place-peripheral associations found in high-level visual cortex, influence perceptual performance.
... Recently, a number of perceptual learning paradigms have been developed to improve visual performance of observers with amblyopia. Many have focused on monocular training in the amblyopic eye, including vernier offset discrimination (Levi & Polat, 1996;Levi, Polat, & Hu, 1997), contrast detection with flankers (Bonneh, Sagi, & Polat, 2004;Polat, 2008;Zhou, Huang, Xu, et al., 2006), contrast detection at the cutoff spatial frequency (Zhou et al., 2006), contrast discrimination (Zhang, Cong, Levi, Klein, & Yu, 2014;Zhou et al., 2006), video game (Vedamurthy, Nahum, Huang, et al., 2015), and de-suppression (Hess, Mansouri, & Thompson, 2010), and found that monocular training significantly improved visual acuity in the amblyopic eye. A few studies evaluated the relationship between the magnitudes of visual acuity improvements in the amblyopic eye and enhancement of binocular vision measured in terms of stereoacuity, interocular suppression, lateral interactions and Gabor grating resolution under dichoptic viewing (Hess & Thompson, 2015;Polat, 2008;Vedamurthy, Nahum, Bavelier, & Levi, 2015;Xi, Jia, Feng, Lu, & Huang, 2014), but found no significant correlation. ...
Intensive monocular perceptual learning can improve visual acuity, contrast sensitivity, and vernier acuity in the amblyopic eye in adults with amblyopia. It is however not clear how much monocular training can enhance binocular visual functions. In the current study, we aimed to evaluate effects of monocular training on a variety of binocular functions. Nineteen anisometropic amblyopes (18.5 ± 1.26 yrs, mean ± s.e.) were trained in a grating contrast detection task near each individual’s cutoff spatial frequency for 6–10 days (630 trials/day). Visual acuity, stereoacuity, monocular and binocular contrast sensitivity functions (CSF), binocular phase combination and binocular rivalry were tested before and after training. Although monocular training can improve visual acuity and contrast sensitivity and eye dominance of the amblyopic eye, the magnitudes of improvements did not correlate with each other; the impact of monocular training on binocular phase combination was not significant. The results strongly suggest that structured monocular and binocular training is needed to fully recover deficient visual functions in anisometropic amblyopia.
... A range of behavioral studies also support a later-stage locus for visual crowding (3,13,(51)(52)(53). Furthermore, although acuity and crowding are correlated in the "normal" periphery (as in our gap-resolution data) and within-group for cases of amblyopia (54,55), between-group comparisons reveal a clear acuitycrowding dissociation (56,57). It may be that low-level spatial precision sets the precedence for crowding in development, but that disruptions to the visual system through additional factors such as a loss of binocularity (54) can dissociate these two factors. ...
Significance
Our ability to see, localize, and interact with stimuli varies depending on their location in the visual field. Here we consider the source of these variations for several aspects of spatial vision: crowding (the disruption of object recognition in clutter), spatial localization, and saccadic eye movements. We observe a range of variations across both individuals and the visual field with strong correlations between all tasks. However, a number of dissociations exclude the possibility that these correlations arise from the same spatial representation of the visual field. Rather, we propose a “topology of spatial vision,” whereby idiosyncratic variations in spatial precision are established early in the visual system and inherited up to the highest levels of object recognition and motor planning.
... Spatial integration is known to be deficient in the amblyopic eye. Some evidence implicates a greater deficit for strabismic than anisometropic amblyopia 188 : when stimulus visibility is controlled for, path detection is still impaired in strabismic amblyopia 189 but not in anisometropic amblyopia. 190 Adults with amblyopia also show deficits on oval contour detection tasks, particularly when more orientation noise is present, 191 and on dotted line detection tasks when noise is present. ...
Unilateral amblyopia is a visual disorder that arises after selective disruption of visual input to one eye during critical periods of development. In the clinic, amblyopia is understood as poor visual acuity in an eye that was deprived of pattern vision early in life. By its nature, however, amblyopia has an adverse effect on the development of a binocular visual system and the interactions between signals from two eyes. Visual functions aside from visual acuity are impacted, and many studies have indicated compromised sensitivity in the fellow eye even though it demonstrates normal visual acuity. While these fellow eye deficits have been noted, no overarching theory has been proposed to describe why and under what conditions the fellow eye is impacted by amblyopia. Here, we consider four explanations that may account for decreased fellow eye sensitivity: the fellow eye is adversely impacted by treatment for amblyopia; the maturation of the fellow eye is delayed by amblyopia; fellow eye sensitivity is impacted for visual functions that rely on binocular cortex; and fellow eye deficits reflect an adaptive mechanism that works to equalize the sensitivity of the two eyes. To evaluate these ideas, we describe five visual functions that are commonly reported to be deficient in the amblyopic eye (hyperacuity, contrast sensitivity, spatial integration, global motion, and motion-defined form), and unify the current evidence for fellow eye deficits. Further research targeted at exploring fellow eye deficits in amblyopia will provide us with a broader understanding of normal visual development and how amblyopia impacts the developing visual system.
... LogMAR in both eyes. In addition we found that a significant part of the acuity deficit was due to crowding (~0.3 log units, which is larger than normal as measured with crowded and uncrowded displays of tumbling E patterns (Bonneh et al. 2004). A conspicuous difficulty with dot grouping suggested problems of visual integration that were further investigated. ...
... 6 In both instances, crowding might predict (unidentified) brain damage and, indirectly, visual perception problems. 7---9 Children without any neurological impairment who have developmental visual problems, like strabismic amblyopia, are also at risk of crowding 10,11 as well as of cerebral visual impairments, for example motion-defined form perception. 12 All these visual problems in both children with amblyopia 13 and children with cerebral visual impairment are associated with anatomical and functional abnormalities of the brain's visual system. 13,14 If, indeed, abnormal crowding is more common in children with brain damage than in other children, a small addition to routine visual acuity assessment might improve the detection of children at risk of cerebral visual impairment. ...
Purpose:
To determine whether the assessment of visual acuity ratios might improve the referral of children with (sub)normal visual acuity but at risk of cerebral visual impairment.
Methods:
In an exploratory study, we assessed visual acuity, crowding ratio and the ratios between grating acuity (Teller Acuity Cards-II) and optotype acuity (Cambridge Crowding Cards) in 60 typically developing school children (mean age 5y8m±1y1m), 21 children with ocular abnormalities only (5y7m±1y9m) and 26 children with (suspected) brain damage (5y7m±1y11m). Sensitivities and specificities were calculated for targets and controls from the perspective of different groups of diagnosticians: youth health care professionals (target: children with any visual abnormalities), ophthalmologists and low vision experts (target: children at risk of cerebral visual impairment).
Results:
For youth health care professionals subnormal visual acuity had the best sensitivity (76%) and specificity (70%). For ophthalmologists and low vision experts the crowding ratio had the best sensitivity (67%) and specificity (79 and 86%).
Conclusion:
Youth health care professionals best continue applying subnormal visual acuity for screening, whereas ophthalmologists and low vision experts best add the crowding ratio to their routine diagnostics, to distinguish children at risk of visual impairment in the context of brain damage from children with ocular pathology only.
... I also include in this group three functions (matching static faces based on identity or expression, reading heavily masked text, and recognizing complex Kanizsa figures) that LG was impaired in before training, and was not tested on after training, as I find that their perceptual demands resemble those of tasks that LG is still impaired in (face perception, and shape recognition from incomplete information), yet I cannot rule out a possible post-training improvement, had LG been retested on these functions post-training. Explicitly, this group includes contrast sensitivity (Polat et al., 2005;Gilaie-Dotan et al., 2009;Lev et al., 2015), near and far visual acuity (Gilaie-Dotan et al., 2009;Lev et al., 2015), crowding (Bonneh et al., 2004;Gilaie-Dotan et al., 2009;Lev et al., 2015), lateral interactions (Polat and Sagi, 1993;Gilaie-Dotan et al., 2009;Lev et al., 2014Lev et al., , 2015, identifying contour in noise (Kovacs et al., 2000;Gilaie-Dotan et al., 2009;Lev et al., 2015), Table 2). ...
... Using a VEP measure, they found evidence for mature inhibitory interaction patterns by 6 months, which is consistent with the early development of suppressive interactions reported by Hou et al. (2003) and our results (note that for spatial vision, age in weeks for monkeys is approximately equivalent to age in months for human infants (Teller & Boothe, 1979)). It is also interesting to consider that in amblyopia, which is a developmental disorder of vision, there have been several reports of abnormal lateral interactions measured using VEPs or psychophysics (Polat, Sagi & Norcia, 1997;Ellemberg, Hess & Arsenault, 2002;Bonneh, Sagi & Polat, 2004;Polat et al., 2005;Wong, Levi & McGraw, 2005;Levi & Carney, 2011). In many cases, amblyopic observers fail to show facilitation with stimulus configurations that evoke facilitation in visually-normal observers. ...
Lateral spatial interactions among elements of a scene, which either enhance or degrade visual performance, are ubiquitous in vision. The neural mechanisms underlying lateral spatial interactions are a matter of debate, and various hypotheses have been proposed. Suppressive effects may be due to local inhibitory interactions, whereas facilitatory effects are typically ascribed either to the function of long-range horizontal projections in V1 or to uncertainty reduction. We investigated the development of lateral spatial interactions, facilitation and suppression, and compared their developmental profiles to those of potential underlying mechanisms in the visual system of infant macaques. Animals ranging in age from 10 weeks to 3 years were tested with a lateral masking paradigm. We found that suppressive interactions are present from very early in postnatal life, showing no change over the age range tested. However, facilitation develops slowly over the first year after birth. Our data suggest that the early maturation of suppressive interactions is related to the relatively mature receptive field properties of neurons in early visual cortical areas near birth in infant macaques, whereas the later maturation of facilitation is unlikely to be explained by development of local or long-range connectivity in primary visual cortex. Instead our data favor a late developing feedback or top-down cognitive process to explain the origin of facilitation.
... A pioneering PL procedure designed to improve the abnormal spatial interactions in amblyopia was recently developed [13,14,17]. Since the level of amblyopic deficit is not identical among subjects [36][37][38], the treatment was tailored and specifically designed for each individual's deficiencies. After the treatment, the impaired spatial interactions were significantly improved to the normal level [13,17]. ...
Binocular vision disorders or dysfunctions have considerable impact on daily visual activities such as reading. Heterophoria (phoria) is a latent eye misalignment (with a prevalence of up to 35%) that appears in conditions that disrupt binocular vision and it may affect the quality of binocular fusion. Our recent study, which used lateral masking (LM), suggests that subjects with binocular fusion disorders (horizontal phoria) exhibit an asymmetry and an abnormal pattern of both binocular and monocular lateral interactions, but only for the horizontal meridian (HM). The perceptive field (PF) is the fundamental processing unit of human vision and both masking and crowding depend on its size. An increased PF size is found in amblyopic populations or in young children. We hypothesized that the PF’s size would be asymmetric only for the phoric group (larger along the HM). We estimated the PF’s size using two different methods (LM with equal-phase and opposite-phase flankers). Phoric subjects exhibited a larger binocular PF size, only for the HM, confirming our hypothesis of an asymmetric PF size. However, the monocular PF size of phoric and control subjects was similar. Phoria affects the PF’s size similarly to meridional amblyopia but without being attributed to abnormal refraction. We suggest that these findings could help explain the inter-observer variability found in the masking literature and the reading difficulties often encountered in subjects with high heterophoria. Since perceptual learning can reduce the PF’s size, further investigation of training may provide a novel therapy to reduce some symptoms related to heterophoria.
Clinical relevance
Crowding limits many daily life activities, such as reading and the visual search for objects in cluttered environments. Excessive sensitivity to crowding, especially in central vision, may amplify the difficulties of patients with ocular pathologies. It is thus important to investigate what limits visual activities and how to improve it.
Background
Numerous studies have reported reduced contrast sensitivity in central vision in patients with glaucoma. However, deficits have also been observed for letter recognition at high contrast, suggesting that contrast alone cannot completely account for impaired central perception.
Method
Seventeen patients and fifteen age-matched controls were randomly presented with letters in central or parafoveal vision at 5° eccentricity for 200 ms. They were asked to decide whether the central T was upright or inverted. The T was either presented in isolation (uncrowded) or flanked by two Hs (crowded) at various spacings. Contrast was manipulated: 60% and 5%.
Results
Compared to controls, patients exhibited a significant effect of crowding in central vision, with higher accuracy for the isolated T than for HTH only at low contrast. In parafoveal vision, an effect of crowding was also observed only in patients. The spacing to escape crowding varied as a function of contrast. Larger spacing was required at low contrast than at high contrast. Susceptibility to crowding was related to central visual field defect for central presentations and to contrast sensitivity for parafoveal presentations, only at low contrast. Controls were at ceiling level both for central and parafoveal presentations.
Conclusion
Crowding limits visual perception, impeding reading and object recognition in cluttered environments. Visual field defects and lower contrast sensitivity in glaucoma can increase susceptibility to central and parafoveal crowding, the deleterious effect of which can be improved by manipulating contrast and spacing between elements.
Since perceptual and neural face sensitivity is associated with a foveal bias, and neural place sensitivity is associated with a peripheral bias (integration over space), we hypothesized that face perception ability will decline more with eccentricity than place perception ability. We also wanted to examine whether face perception ability would show a left visual field (LeVF) bias due to earlier reports suggesting right hemisphere dominance for faces, or would show an upper or lower visual field bias. Participants performed foveal and parafoveal face and house discrimination tasks for upright or inverted stimuli (≤4°) while their eye movements were monitored. Low-level visual tasks were also measured. The eccentricity-related accuracy reductions were evident for all categories. Through detailed analyses we found (i) a robust face inversion effect across the parafovea, while for houses an opposite effect was found, (ii) higher eccentricity-related sensitivity for face performance than for house performance (via inverted vs. upright within-category eccentricity-driven reductions), (iii) withincategory but not across-category performance associations across eccentricities, and (iv) no hemifield biases. Our central to parafoveal investigations suggest that high-level vision processing may be reflected in behavioural performance.
Background:
Amblyopic observers present abnormal spatial interactions between a low-contrast sinusoidal target and high-contrast collinear flankers. It has been demonstrated that perceptual learning (PL) can modulate these low-level lateral interactions, resulting in improved visual acuity and contrast sensitivity.
Objective:
We measured the extent and duration of generalization effects to various spatial tasks (i.e., visual acuity, Vernier acuity, and foveal crowding) through PL on the target's contrast detection.
Methods:
Amblyopic observers were trained on a contrast-detection task for a central target (i.e., a Gabor patch) flanked above and below by two high-contrast Gabor patches. The pre- and post-learning tasks included lateral interactions at different target-to-flankers separations (i.e., 2, 3, 4, 8λ) and included a range of spatial frequencies and stimulus durations as well as visual acuity, Vernier acuity, contrast-sensitivity function, and foveal crowding.
Results:
The results showed that perceptual training reduced the target's contrast-detection thresholds more for the longest target-to-flanker separation (i.e., 8λ). We also found generalization of PL to different stimuli and tasks: contrast sensitivity for both trained and untrained spatial frequencies, visual acuity for Sloan letters, and foveal crowding, and partially for Vernier acuity. Follow-ups after 5-7 months showed not only complete maintenance of PL effects on visual acuity and contrast sensitivity function but also further improvement in these tasks.
Conclusion:
These results suggest that PL improves facilitatory lateral interactions in amblyopic observers, which usually extend over larger separations than in typical foveal vision. The improvement in these basic visual spatial operations leads to a more efficient capability of performing spatial tasks involving high levels of visual processing, possibly due to the refinement of bottom-up and top-down networks of visual areas.
A general property of nervous system development is that correlated activity is used to organize topographic projections. This correlated activity is typically produced by electrical coupling of adjacent neurons; however, electrical coupling is not possible for retinal ganglion cells in separate eyes that need to be precisely mapped to the same place in the brain. This forces the visual system to rely on environmental stimuli to produce the correlated activity that drives the development of binocularity, with amblyopia as necessary consequence when visual experience is abnormal. The characteristic visual defi cits in both the amblyopic and the sound eyes can be understood in the context of these normal developmental processes. The auditory system provides another example-where precise connections between paired sense organs must rely on environmental stimuli for normal development-in which the analogous condition of amblyaudia occurs.
Comparison of eye movement mechanisms in natural reading with two commonly used instruments in the evaluation and/or treatment of SLD (Specific Learning Disorders): the naming of word lists (and/or pseudowords) and the naming of single word tachystoscopic projections. This critical literary revision demonstrates limitations of single-word paradigms and highlights the specific necessity of eye movements for efficient reading. New perspectives can also be demonstrated by means of eye movement tracing analysis. We propose a new hypothesis regarding both the dynamic perceptual span and the crowding of WHERE (compared to the crowding of WHAT). Therefore there is a need for a more ecological approach to the analysis of reading, based on the mechanics of eye movement found in normal adults.
These three volumes provide the only detailed review of all aspects of perceiving the three-dimensional world. They deal with all the senses involved in depth perception, although the visual system receives the most extensive treatment. Volume 1 deals with basic mechanisms underlying depth perception. It starts with an outline of the history of visual science from the Greeks to the early 20th century. Psychophysical methods, analytic procedures, and sensory coding and the physiology of the primate visual system are reviewed. An account of the evolution of visual systems is followed by an account of the development of the neural mechanisms of the visual system, with emphasis on development of mechanisms of depth perception. A description of the normal development of sensory and motor functions in humans is followed by an account of how these functions, especially depth perception, are disrupted by visual deprivation in early infancy. The two final chapters provide accounts of visual optics, accommodation, and vergence eye movements.
Young children are characterized by poor visual performances. Visual crowding, lateral interactions, and contour detection are critical functions for visual perception, context effect, and recognition that develop over the years up to maturity. The age at which the maturation's onset of the functions can be observed and the functions' underlying neural basis remain unclear. Here we used a development approach to investigate the onset of the foveal visual functions in order to learn about their neuronal basis and their relationships. We measured lateral interactions, crowding, and contour integration in participants aged 3-15 years. The results show that very young children do not exhibit collinear facilitation; rather, their vision is dominated by suppression and a high degree of crowding. Our results show sequential changes in the visual functions in parallel with the development of facilitation-that is, a significant reduction in crowding and an improved contour detection threshold. Our data suggest that the correlation between the onset age of maturation of collinear facilitation with crowding reduction and improvement of contour integration has underlying mutual neuronal mechanisms.
There is growing evidence for deficits in motion perception in amblyopia, but these are rarely assessed clinically. In this prospective study we examined the effect of occlusion therapy on motion-defined form perception and multiple-object tracking. Participants included children (3-10 years old) with unilateral anisometropic and/or strabismic amblyopia who were currently undergoing occlusion therapy and age-matched control children with normal vision. At the start of the study, deficits in motion-defined form perception were present in at least one eye in 69% of the children with amblyopia. These deficits were still present at the end of the study in 55% of the amblyopia group. For multiple-object tracking, deficits were present initially in 64% and finally in 55% of the children with amblyopia, even after completion of occlusion therapy. Many of these deficits persisted in spite of an improvement in amblyopic eye visual acuity in response to occlusion therapy. The prevalence of motion perception deficits in amblyopia as well as their resistance to occlusion therapy, support the need for new approaches to amblyopia treatment.
Copyright © 2015. Published by Elsevier Ltd.
Spatial crowding impairs conscious visual perception and object recognition in clutter.Short presentation times produce crowding in the normal fovea, in young participants and in uncorrected presbyopes ("aging eye"), measured as reduced visual acuity (VA). On the other hand, perceptual learning improves near VA in healthy young adults, in presbyopia, and in amblyopia. Here we aimed at exploring the effects of perceptual training on crowded VA in uncorrected presbyopes with naturally decreased VA, with two specific objectives: (a) to objectively measure crowded VA, taking advantage of the natural deterioration of near visual acuity from being normal or better than normal (i.e., 20/20 or better) in young participants to naturally decreasing in uncorrected presbyopes; and (b) to explore whether perceptual training previously shown to improve visual functions as complex as reading will affect crowded VA. Visual acuity was measured under crowded and uncrowded conditions by having subjects identify letters presented for short durations ranging from 34 to 116 msec. Training consisted of detecting brief Gabor stimuli under spatial and temporal masking conditions, using the GlassesOff mobile application (GlassesOff, Inc., New York, NY)on iOS devices from a distance of 40 cm. Before training, a robust reduction in crowded VA was found in the fovea of presbyopes. Training resulted in significant improvement of letter identification under both crowded and uncrowded VA conditions for all stimulus durations. After training, the crowded condition threshold reached the level of the uncrowded threshold measured before training. Thus, training enabled the subjects to overcome the effect of reduced VA under the crowded condition. We suggest that more efficient spatial and temporal processing induced by perceptual learning allows one to improve crowded VA, including that found on naturally reduced near VA, and that this effect may transfer to improve complex visual functions, such as reading and object recognition.
There is growing evidence that abnormal binocular interactions play a key role in amblyopia. In particular, stronger suppression of the amblyopic eye has been associated with poorer amblyopic eye visual acuity and a new therapy has been described that directly targets binocular function and has been found to improve both monocular and binocular vision in adults and children with amblyopia. Furthermore, non-invasive brain stimulation techniques that alter excitation and inhibition within the visual cortex have been shown to improve vision in the amblyopic eye. The aim of this review is to summarize this previous work and interpret the therapeutic effects of binocular therapy and non-invasive brain stimulation in the context of three potential neural mechanisms; active inhibition of signals from the amblyopic eye, attenuation of information from the amblyopic eye and metaplasticity of synaptic long term potentiation and long term depression.
Copyright © 2015. Published by Elsevier Ltd.
Amblyopia is a deficit in vision that arises from abnormal visual experience early in life. It was long thought to develop into a permanent deficit, unless properly treated before the end of the sensitive period for visual recovery. However, a number of studies now suggest that adults with long-standing amblyopia may at least partially recover visual acuity and stereopsis following perceptual training. Eliminating or reducing interocular suppression has been hypothesized to be at the root of these changes. Here we show that playing a novel dichoptic video game indeed results in reduced suppression, improved visual acuity and, in some cases, improved stereopsis. Our relatively large cohort of adults with amblyopia, allowed us, for the first time, to assess the link between visual function recovery and reduction in suppression. Surprisingly, no significant correlation was found between decreased suppression and improved visual function. This finding challenges the prevailing view and suggests that while dichoptic training improves visual acuity and stereopsis in adult amblyopia, reduced suppression is unlikely to be at the root of visual recovery. These results are discussed in the context of their implication on recovery of amblyopia in adults.
Purpose:
Amblyopia is a developmental disorder that results in both monocular and binocular deficits. Although traditional treatment in clinical practice (i.e., refractive correction, or occlusion by patching and penalization of the fellow eye) is effective in restoring monocular visual acuity, there is little information on how binocular function, especially stereopsis, responds to traditional amblyopia treatment. We aim to evaluate the effects of perceptual learning on stereopsis in observers with amblyopia in the current study.
Methods:
Eleven observers (21.1 ± 5.1 years, six females) with anisometropic or ametropic amblyopia were trained to judge depth in 10 to 13 sessions. Red-green glasses were used to present three different texture anaglyphs with different disparities but a fixed exposure duration. Stereoacuity was assessed with the Fly Stereo Acuity Test and visual acuity was assessed with the Chinese Tumbling E Chart before and after training.
Results:
Averaged across observers, training significantly reduced disparity threshold from 776.7″ to 490.4″ (P < 0.01) and improved stereoacuity from 200.3″ to 81.6″ (P < 0.01). Interestingly, visual acuity also significantly improved from 0.44 to 0.35 logMAR (approximately 0.9 lines, P < 0.05) in the amblyopic eye after training. Moreover, the learning effects in two of the three retested observers were largely retained over a 5-month period.
Conclusions:
Perceptual learning is effective in improving stereo vision in observers with amblyopia. These results, together with previous evidence, suggest that structured monocular and binocular training might be necessary to fully recover degraded visual functions in amblyopia. Chinese Abstract.
Background/aims:
To compare interocular acuity differences, crowding ratios, and binocular summation ratios in 4- to 8-year-old children with albinism (n = 16), children with infantile nystagmus syndrome (n = 10), and children with normal vision (n = 72).
Methods:
Interocular acuity differences and binocular summation ratios were compared between groups. Crowding ratios were calculated by dividing the single Landolt C decimal acuity with the crowded Landolt C decimal acuity mono- and binocularly. A linear regression analysis was conducted to investigate the contribution of 5 predictors to the monocular and binocular crowding ratio: nystagmus amplitude, nystagmus frequency, strabismus, astigmatism, and anisometropia.
Results:
Crowding ratios were higher under mono- and binocular viewing conditions for children with infantile nystagmus syndrome than for children with normal vision. Children with albinism showed higher crowding ratios in their poorer eye and under binocular viewing conditions than children with normal vision. Children with albinism and children with infantile nystagmus syndrome showed larger interocular acuity differences than children with normal vision (0.1 logMAR in our clinical groups and 0.0 logMAR in children with normal vision). Binocular summation ratios did not differ between groups. Strabismus and nystagmus amplitude predicted the crowding ratio in the poorer eye (p = 0.015 and p = 0.005, respectively). The crowding ratio in the better eye showed a marginally significant relation with nystagmus frequency and depth of anisometropia (p = 0.082 and p = 0.070, respectively). The binocular crowding ratio was not predicted by any of the variables.
Conclusions:
Children with albinism and children with infantile nystagmus syndrome show larger interocular acuity differences than children with normal vision. Strabismus and nystagmus amplitude are significant predictors of the crowding ratio in the poorer eye.
Purpose:
The detection of amblyopia in children relies on an accurate assessment of visual acuity. Visual deficits in anisometropic and strabismic amblyopia are different, but the influence of chart design, in particular position, and type of crowding features on visual acuity in the two types of amblyopia, is not clear. Certain aspects of amblyopic spatial vision are mimicked in healthy individuals by imposing increasing levels of blur and retinal eccentricity. We measured the effects of these conditions on visual acuity in healthy adults, with crowded and uncrowded vision tests.
Methods:
Visual acuity was measured under conditions of blur (0-4 D) and eccentric viewing (0-5 degrees) using high-contrast optotypes derived from common children's acuity charts. Optotypes were presented in isolation, in commercially available crowded configurations, and in configurations with closer target-flanker separations than those currently available.
Results:
Dioptric blur had similar degradative effects on crowded and isolated visual acuity (P > 0.05), whereas eccentric viewing resulted in a larger deterioration of visual acuity for crowded tests (E2 of 0.86-1.06) than for isolated optotypes (E2 of 1.57-1.72) (P < 0.05). Maximum crowding effects occurred for closer target-flanker separations than those currently used commercially.
Conclusions:
In so far as blur and eccentric viewing mimic spatial acuity deficits in amblyopia, the results suggest that crowded tests might be of limited value in the detection of anisometropic amblyopia, but should be valuable in the detection of strabismic amblyopia. Crowding effects would be greater if flanking features were placed closer to the target than they currently are in commercially available charts.
This paper describes a series of visual search experiments for targets defined by their binocular characteristics. In searches
for targets defined by binocular rivalry among fused distractors, or Vice versa, the rivalrous items do not “pop out” (reaction
time [RT] increases with number of distractors). Binocular luster, a variety of rivalry, is an exception. Luster, an important
property of visible surfaces, behaves like a basic feature or “texton” (RT independent of the number of nonlustrous distractors).
Searches for targets defined exclusively by eye-of-origin information are virtually impossible. Subjects respond randomly,
suggesting that purely monocular information is not available for visual search. Searches for cyclopean (but nonstereoscopic)
features are easy with RTs independent of set size, suggesting that some purely binocular information is available for visual
search.
Aubert and Foerster (1857) are frequently cited for having shown that the lower visual acuity of peripheral vision can be
compensated for by increasing stimulus size. This result is seemingly consistent with the concept of cortical magnification,
and it has been confirmed by many subsequent authors. Yet it is rarely noted that Aubert and Foerster also observed a loss
of the “quality of form.” We have studied the recognition of numeric characters in foveal and eccentric vision by determining
the contrast required for 67% correct identification. At each eccentricity, the lowest contrast threshold is achieved with
a specific stimulus size. But the contrast thresholds for these optimal stimuli are not independent of retinal eccentricity
as cortical magnification scaling would predict. With high-contrast targets, however, threshold target sizes were consistent
with cortical magnification out to 6° eccentricity. Beyond 6°, threshold target sizes were larger than cortical magnification
predicted. We also investigated recognition performance in the presence of neighboring characters (crowding phenomenon). Target
character size, distance of flanking characters, and precision of focusing of attention all affect recognition. The influence
of these parameters is different in the fovea and in the periphery. Our findings confirm Aubert and Foerster’s original observation
of a qualitative difference between foveal and peripheral vision.
Crowding, the difficulty in recognizing a letter flanked by other letters, has been explained as a lateral masking effect. The purpose of this study was to examine the spatial-frequency and contrast dependencies of crowding, and to compare them with the properties of pattern masking. In experiment 1, we measured contrast thresholds for identifying the middle letters in strings of three randomly chosen lower-case letters (trigrams), for a range of letter spacings. Letters were digitally filtered using a set of bandpass filters, with peak object spatial frequencies ranging from 0.63 to 10 c/letter. Bandwidth of the filters was 1 octave. Frequencies of the target and flanking letters were the same, or differed by up to 2 octaves. Contrast of the flanking letters was fixed at the maximum value. Testing was conducted at the fovea and 5° eccentricity. We found that crowding exhibits spatial-tuning functions like masking, but with generally broader bandwidths than those for masking. The spatial extent of crowding was found to be about 0.5 deg at the fovea and 2 deg at 5° eccentricity, independent of target letter frequency. In experiment 2, we measured the contrast thresholds for identifying the middle target letters in trigrams for a range of flanking letter contrasts at 5° eccentricity. At low flanker contrast, crowding does not show a facilitatory region, unlike pattern masking. At high flanker contrast, threshold rises with contrast with an exponent of 0.13–0.3, lower than corresponding exponents for pattern masking. In experiment 3, we varied the contrast ratio between the flanking letters and the target letters, and found that the magnitude of crowding increases monotonically with contrast ratio. This finding contradicts a prediction based on a grouping explanation for crowding. Our results are consistent with the postulation that crowding and masking may share the same first stage linear filtering process, and perhaps a similar second-stage process, with the additional property that the second-stage process in crowding pools information over a spatial extent that varies with eccentricity.
Resolution thresholds for Landolt C's and for vernier targets remain the same whether the target is stationary or moving with horizontal or vertical velocities of up to 2.5 degrees for foveal presentations lasting 0.1 and 0.2 s. Oblique target motions are tolerated only up to 1 degree. Because visual pursuit is ruled out by randomization of direction of motion and by the short exposure, it is concluded that a stationary retinal image is not a prerequisite for good acuity.
Aubert and Foerster (1857) are frequently cited for having shown that the lower visual acuity of peripheral vision can be compensated for by increasing stimulus size. This result is seemingly consistent with the concept of cortical magnification, and it has been confirmed by many subsequent authors. Yet it is rarely noted that Aubert and Foerster also observed a loss of the "quality of form." We have studied the recognition of numeric characters in foveal and eccentric vision by determining the contrast required for 67% correct identification. At each eccentricity, the lowest contrast threshold is achieved with a specific stimulus size. But the contrast thresholds for these optimal stimuli are not independent of retinal eccentricity as cortical magnification scaling would predict. With high-contrast targets, however, threshold target sizes were consistent with cortical magnification out to 6 degrees eccentricity. Beyond 6 degrees, threshold target sizes were larger than cortical magnification predicted. We also investigated recognition performance in the presence of neighboring characters (crowding phenomenon). Target character size, distance of flanking characters, and precision of focusing of attention all affect recognition. The influence of these parameters is different in the fovea and in the periphery. Our findings confirm Aubert and Foester's original observation of a qualitative difference between foveal and peripheral vision.
The most frequent cause of visual loss in childhood is functional amblyopia, an abnormality of visual acuity usually associated with either anisometropia (unequal refractive errors) or strabismus (turned eye) during early development. The usual clinical investigation of the visual acuity of amblyopes involves discrimination of the high contrast letters of a Snellen chart; however, there are other aspects of acuity, for example, grating acuity (the high spatial frequency limit of vision) and Vernier acuity (the smaller perceptible misalignment). Because of the extreme precision of Vernier acuity compared with either grafting or Snellen acuity, it is considered to be a form of hyperacuity which requires very precise positional information. In an effort to understand the nature of the neural abnormalities which cause the reduced acuity of amblyopes, we have measured here the Vernier acuity of amblyopic observers using an extended Vernier grating stimulus, and compared these results with their Snellen acuity and grating acuity. The results showed that different acuity losses are associated wih anisometropic versus strabismic amblyopia. When scaled with respect to their grating acuity, anisometropic amblyopes, like normals, showed hyperacuity, even at high spatial frequencies, while strabismic amblyopes showed severe losses in Vernier acuity. Snellen letter acuity showed a similar deficit relative to grating acuity in strabismic but not in anisometropic amblyopes. Contrary to some previous theories which have considered that all forms of amblyopia share a common neural basis, these results strongly support the view that different neural losses are associated with amblyopias of different aetiologies.
Two gratings composed of lines were displayed one above the other, and the vernier threshold was measured as a function of spatial frequency and vertical separation between gratings for 12 strabismic and/or anisometropic amblyopes. The vernier acuity of the anisometropic amblyopes was similar to that of nonamblyopic eyes when scaled proportionally to their reduced grating acuity. Strabismic amblyopes, on the other hand, have poorer vernier acuity than might be predicted from their grating resolution, even at low spatial frequencies. In addition, the strabismic amblyopes showed "crowding effects" for vernier gratings well within their resolution limit, while anisometropic amblyopes, like normals, showed no such effects.
Visual spatial resolution is limited by factors ranging from optics to neuronal filters in the visual cortex, but it is not known to what extent it is also limited by the resolving power of attention. To investigate this, we studied adaptation to lines of specific orientation, a process that occurs in primary visual cortex. When a single grating is presented in the periphery of the visual field, human observers are aware of its orientation, but when it is flanked by other similar gratings ('crowding'), its orientation becomes impossible to discern. Nevertheless, we show that orientation-specific adaptation is not affected by crowding, implying that spatial resolution is limited by an attentional filter acting beyond the primary visual cortex. Consistent with this, we find that attentional resolution is greater in the lower than in the upper visual field, whereas there is no corresponding asymmetry in the primary visual cortex. We suggest that the attentional filter acts in one or more higher visual cortical areas to restrict the availability of visual information to conscious awareness.
There are many influences on our perception of local features. What we see is not strictly a reflection of the physical characteristics of a scene but instead is highly dependent on the processes by which our brain attempts to interpret the scene. As a result, our percepts are shaped by the context within which local features are presented, by our previous visual experiences, operating over a wide range of time scales, and by our expectation of what is before us. The substrate for these influences is likely to be found in the lateral interactions operating within individual areas of the cerebral cortex and in the feedback from higher to lower order cortical areas. Even at early stages in the visual pathway, cells are far more flexible in their functional properties than previously thought. It had long been assumed that cells in primary visual cortex had fixed properties, passing along the product of a stereotyped operation to the next stage in the visual pathway. Any plasticity dependent on visual experience was thought to be restricted to a period early in the life of the animal, the critical period. Furthermore, the assembly of contours and surfaces into unified percepts was assumed to take place at high levels in the visual pathway, whereas the receptive fields of cells in primary visual cortex represented very small windows on the visual scene. These concepts of spatial integration and plasticity have been radically modified in the past few years. The emerging view is that even at the earliest stages in the cortical processing of visual information, cells are highly mutable in their functional properties and are capable of integrating information over a much larger part of visual space than originally believed.
To quantify the crowding effect with eccentric stimuli and to determine the relative contribution of neural interaction and attention to crowding in central and eccentric vision.
Monocular visual acuity was measured with computer-generated tumbling E and Landolt C targets presented centrally or at 2 degrees in the right visual field. Crowding distractors were designed to cause increasing contour interaction or increasing need for focused attention. A four-alternative forced-choice method of constant stimuli was used. In experiment 2 the distance between the target and the distractors was varied.
Data are presented in terms of normalized visual acuity. Crowding in central vision was minimal, mainly caused by contour interaction, and did not occur with distractors more than four stroke widths distant. Crowding in eccentric vision was far greater in magnitude and extent (occurring for distractors as far as 16 stroke widths distant) and was caused by contour interaction and attentional factors.
The results indicate that eccentric vision differs quantitatively and qualitatively from central vision. The extent of contour interaction effects are consistent with the proposed size of cortical processing zones. The results are discussed with reference to current theories of preattentive and attentive tasks and with reference to subjects with low vision due to central scotoma who use an eccentric retinal locus for fixation.
Amblyopia ("lazy eye") is an impairment in visual acuity resulting from abnormal neural development in the visual cortex. We tested the responses of ten amblyopic and six normal observers to illusions of perceived orientation in textures of Gabor patches: the "Fraser illusion," the "phase illusion," and a "tilted chain" illusion. The illusory tilt of the stimulus rows was matched by actual tilt in the opposite direction by using the method of constant stimuli. Amblyopes showed a significant increase in the Fraser illusion, a decrease in the phase illusion, and a reversal of the tilted chain illusion. Amblyopic performance could be simulated in normal observers by reducing the length of the rows. These results can be modeled by a theory which places the neural abnormality in amblyopia at the level of second stage grouping processes. Additionally, the illusions might be useful in the early diagnosis of amblyopia without the need for prior refractive correction.
Previous retrospective studies have found that integration of orientation information along contours defined by Gabor patches is abnormal in strabismic, but not in anisometropic, amblyopia. This study was conducted to reexamine the question of whether anisometropic amblyopes have contour integration deficits prospectively in an untreated sample, to isolate the effects of the disease from the effects of prior treatment-factors that may have confounded the results in previous retrospective studies.
Contour detection thresholds, optotype acuity, and stereoacuity were measured in a group of 19 newly diagnosed anisometropic amblyopes before initiation of occlusion therapy. Contour detection thresholds were measured using a card-based procedure.
Significant interocular differences in contour detection thresholds were present in 14 of the 19 patients with anisometropic amblyopia.
Contour integration deficits are a common, but not universal, finding in untreated anisometropic amblyopia. Differences in the prevalence of contour integration deficits between the present study and that of another study may lie in differences in treatment history and/or in the sensitivity of the two different contour integration tasks.
When a target stimulus is embedded in a high contrast surround, the target appears reduced in contrast and is harder to detect, and neural responses in visual cortex are suppressed. We used functional magnetic resonance imaging ( fMRI) and psychophysics to quantitatively compare these physiological and perceptual effects. Observers performed a contrast discrimination task on a contrast-reversing sinusoidal target grating. The target was either presented in isolation or embedded in a high-contrast surround. While observers performed the task, we also measured fMRI responses as a function of target contrast, both with and without a surround. We found that the surround substantially increased the psychophysical thresholds while reducing fMRI responses. The two data sets were compared, on the basis of the assumption that a fixed response difference is required for correct discrimination, and we found that the psychophysics accounted for 96.5% of the variance in the measured V1 responses. The suppression in visual areas V2 and V3 was stronger, too strong to agree with psychophysics. The good quantitative agreement between psychophysical thresholds and V1 responses suggests V1 as a plausible candidate for mediating surround masking.
Previous studies have suggested that the integration of orientation information across space is impaired in amblyopia. We developed a method for quantifying orientation-domain processing using a test format that is suitable for clinical application. The test comprises a graded series of cards where each card includes a closed path (contour) of high contrast Gabor signals embedded in a random background of Gabor signals. Contour visibility in both normals and patients with histories of abnormal binocular vision depends jointly on the spacing of elements on the contour as well as background element density. Strabismic amblyopes show significant degradation of performance compared to normals. Small but significant losses in sensitivity were also observed in a group of non-amblyopic strabismus patients. Threshold measurements made with contrast reducing diffusers indicated that the amblyopic loss is not due to the reduced contrast sensitivity of the amblyopic eye. An abnormal pattern of long-range connectivity between spatial filters or a loss of such connectivity appears to be the primary source of contour integration deficits in amblyopia and strabismus. 2000 Elsevier Science Ltd. All rights reserved.
It has been known for some time that visual acuity in amblyopia is higher for single letters than for letters in a row (termed crowding). Early work showed that this could not be accounted for on the basis of the destructive interaction of adjacent contours (termed contour interaction), which was shown to be, in resolution units, normal in amblyopia. We have re-examined this issue using a letter stimulus that is modulated about a mean light level. This allows an examination of the effects of contrast polarity and spatial filtering within the contour interaction paradigm. We show that the majority of strabismic amblyopes that we investigated exhibit an anomalous contour interaction that, in some cases, was dependent on the contrast polarity of the flanking stimuli. Furthermore, we show that while amblyopes do select the optimum scale of analysis for unflanked stimuli, they do not select the optimum scale of analysis for flanked stimuli. For reasons that may have to do with their poorer shape discrimination, they select a non-optimal scale to process flanked stimuli.
Two tasks were used to evaluate the grain of visual attention, the minimum spacing at which attention can select individual items. First, observers performed a tracking task at many viewing distances. When the display subtended less than 1° in size, tracking was no longer possible even though observers could resolve the items and their motions: The items were visible but could not be individuated one from the other. The limiting size for selection was roughly the same whether tracking one or three targets, suggesting that the resolution limit acts independently of the capacity limit of attention. Second, the closest spacing that still allowed individuation of single items in dense, static displays was examined. This critical spacing was about 50% coarser in the radial direction compared to the tangential direction and was coarser in the upper as opposed to the lower visual field. The results suggest that no more than about 60 items can be arrayed in the central 30° of the visual field while still allowing attentional access to each individually. Our data show that selection has a coarse grain, much coarser than visual resolution. These measures of the resolution of attention are based solely on the selection of location and are not confounded with preattentive feature interactions that may contribute to measures from flanker and crowding tasks. The results suggest that the parietal area is the most likely locus of this selection mechanism and that it acts by pointing to the spatial coordinates (or cortical coordinates) of items of interest rather than by holding a representation of the items themselves.
Spatial vision, refers to how the visual system encodes the relationships between simple features in the external world for extraction of meaningful information as to object form and location in space. Amblyopia (from the Greek, meaning "blunt vision") is a developmental monocular deficit arising in an estimated 5% of children and is the most common cause of unilateral blindness in the adult population. The condition is primarily associated with a strabismus (misalignment of an eye) or anisometropia (unequal refractive error in the two eyes) or both (mixed amblyopia) being present at or shortly after birth. The strabismic amblyopic deficit includes a decrease in contrast sensitivity, with an accompanying loss in acuity. This population is unique in that unlike the anisometropic group, they experience spatial distortions and non-contrast dependent anomalous spatial localization. This has led to the proposition that tarachopia or "distorted" vision may be a better descriptive term to use as opposed to amblyopia or "blunt" vision. In an effort to characterize the distortions that strabismic amblyopes perceive, we have quantified the deficit in spatial localization across the dimensions of spatial scale, eccentricity and exposure duration. The results show that: (1) the deficits of spatial localization, spatial uncertainty (increased alignment threshold) and bias (perceived point of subjective alignment) are uncorrelated; (2) both deficits are scale invariant for well separated elements; (3) the form of the regional distribution depends on the spatial measure used and the scale at which it is measured; (4) there is a poor correlation between the deficit for either spatial measure and the contrast sensitivity loss; and (5) spatial uncertainty is invariant with exposure duration. In addition we have sought to characterize the neural substrate of the spatial deficits. We psychophysically probe the sampling properties of the neuronal population and analyze the dependence of simple pattern discrimination on stimulus bandwidth properties. Our results show that the detection deficit due to neural pooling is effective in limiting the spatial frequencies where sampling considerations are important. Within this region of visible high frequencies, an irregular arrangement of neurons rather than neural loss is the greater contributor to the strabismic amblyopes' deficit. In addition, the amblyopic eye either has broader orientation and spatial frequency detectors or is deficient only for first-order orientation processing.
Acuity and contour interactions were assessed across the visual field of representative strabismic amblyopes. Acuity profiles display large variations between amblyopes who exhibit the same degree of eccentric fixation; however, in no case was foveal acuity significantly better than that found eccentrically. The acuity abnormality for most amblyopes is visual field locus dependent. Contour interactions are abnormal in both form and magnitude for some amblyopes. This abnormality does not depend upon the acuity deficit, is field locus dependent and cannot be easily accounted for by optical, oculomotor or eccentric fixation factors.
Threshold contrast sensitivity functions for 10 representative strabismic amblyopes are presented. The results indicate that there are two classes of response. For one class, contrast sensitivity is depressed for only high spatial frequencies. For the other class, contrast sensitivity is depressed for all spatial frequencies, including low spatial frequencies; this response has not been previously reported. It is proposed that some results from the latter class are not simply extensions in severity of the former class and therefore two types of amblyopia need to be postulated.
Contrast sensitivity functions for sinusoidal gratings of different spatial frequencies and stimulus durations have been determined for both eyes of amblyopic subjects. For both long and short stimulus durations, the entire contrast sensitivity function of the amblyopic eyes is lower than that of the nonamblyopic eyes. When the gratings were flickered at 10 Hz, the sensitivity for pattern and flicker detection were similar for both eyes at low spatial frequencies. Possible neural mechanisms for the differences in contrast sensitivity are considered with respect to recent psychophysical and electrophysiological evidence for channels with different spatio-temporal properties in the mammalian visual system.
Three visual tests were administered to a group of 15 amblyopic children, 15 adult amblyopes and two age-matched control groups, each of 20 subjects. Test results comprised visual acuity for recognizing high contrast letters presented in line (i.e. Snellen) format, isolated-letter format and repeat-letter format. The classical Snellen format confounds the effects of gaze control defects with the effects of adjacent contours on a patient's ability to recognize a foveated letter. We designed a repeat-letter format intended to unconfound these effects. The repeat letter format is much less sensitive to gaze control defects, and somewhat more sensitive to adjacent contour interactions than is the Snellen format. We report that amblyopic eyes can be subdivided empirically into three repeat-letter categories: repeat-letter acuity significantly better than Snellen acuity; repeat letter acuity not significantly different from Snellen acuity; and repeat letter acuity significantly worse than Snellen acuity. We report that this subdivision cuts across the clinical subclassification of amblyopia and also across the crowding/no crowding subclassification. We suggest that, rather than abnormal lateral interactions, defective selection and/or control of gaze is an important factor in depressed visual acuity in amblyopic eyes of the first repeat-letter category but not for the third type, in which abnormal lateral interactions may be important. To test the hypothesis that the response to patching and refractive therapy may be less satisfactory in our first category of amblyopic eyes, we are carrying out a prospective study.(ABSTRACT TRUNCATED AT 250 WORDS)
The authors assessed relationships between visual acuity and the amplitude, frequency, intensity, and duration of foveation periods in a retrospective study of 32 patients. Twenty-four patients had congenital idiopathic nystagmus, and eight patients had nystagmus and albinism. Visual acuity was determined for Landolt ring optotypes and, as the extrapolated high-frequency cutoff of the contrast sensitivity function, for horizontal and vertical gratings. No significant correlation existed between acuity and any measured eye movement parameter; however, optotype acuity was related to the magnitude of astigmatic refractive correction, both in patients with idiopathic nystagmus and in albinos. In a subgroup of patients with idiopathic nystagmus whose astigmatic refractive error was -1.50 D or less, nystagmus intensity (amplitude x frequency) correlated significantly with acuity for optotypes (r = 0.71), but not for gratings. Although resolution for vertical gratings was correlated with astigmatic refractive correction, the ratio of resolution for gratings parallel and orthogonal to the meridian of nystagmus was not. Thus, the belief that poorer acuity in patients with substantial astigmatism is attributable to an optically induced meridional amblyopia is supported only partly by these results. The authors concluded that among patients with congenital nystagmus, the influence of eye motion on visual acuity is not readily predicted either from the parameters of nystagmus that they evaluated or from the comparison of resolution for horizontal and vertical gratings.
Three experiments were performed to examine positional acuity and the role of spatial sampling in central, peripheral and amblyopic vision. In the first experiment, 3-line bisection acuity was compared to grating acuity. In normal foveal vision bisection acuity represents a hyperacuity. In anisometropic amblyopes, bisection acuity is reduced in rough proportion to their grating acuity. In strabismic amblyopes, and in the normal periphery, bisection acuity is reduced to a greater extent than grating acuity. This result implies that reduced contrast sensitivity of the spatial filters is not sufficient to account for the increased positional uncertainty found in peripheral vision and in strabismic amblyopia. In order to test the hypothesis that the high degree of positional uncertainty evident in these visual systems is a consequence of sparse spatial sampling, bisection thresholds and width discrimination thresholds were measured with stimuli comprised of discrete samples. The results showed that normal foveal vision and the vision of anisometropic amblyopes show little benefit from adding discrete samples to the stimulus. In contrast, the normal periphery, and the central field of strabismic amblyopes demonstrate marked positional uncertainty which can be efficiently reduced in proportion to the square root of the number of samples (up to about 10) comprising the stimulus in the direction orthogonal to the discrimination cue. In aggregate the results suggest that anisometropic and strabismic amblyopia are fundamentally different. The positional uncertainty in anisometropic amblyopia is consistent with the reduced sensitivity of the spatial filters. The data of the normal periphery and of the central field of strabismic amblyopes suggest that the cortical sampling grain imposes a fundamental limit upon their positional acuity.
The human visual system is capable of making spatial discriminations with extraordinary accuracy. In normal foveal vision, relative position, width or size can be judged with an accuracy much finer than the size or spacing of even the smallest foveal cones. This remarkable accuracy of spatial vision has been termed 'hyperacuity'. Almost a century ago Ewald Hering proposed that the accuracy of Vernier acuity could be accounted for by averaging of discrete samples along the length of the lines comprising the targets; however, the discovery that Vernier acuity of a few arc seconds could be achieved with dots has rendered the nature and role of sampling in spatial discrimination unclear. We have been investigating the sampling of spatial information in central and peripheral vision (the perifovea) of normal human observers and in observers with strabismic amblyopia. Our results, presented here, show that peripheral vision and central vision of strabismic amblyopes differ qualitatively in their sampling characteristics from those of the normal fovea. Both the periphery and the central visual field of strabismic amblyopes demonstrate marked positional uncertainty which can be reduced by averaging of spatial information from discrete samples.
Poor vernier acuity, exhibited by amblyopes, may reflect anomalies related to eccentric fixation, deficient position sensitivity, or reduced contour visibility. We have examined these factors by measuring contrast and vernier sensitivities with stimuli consisting of extended sinusoidal gratings of several spatial frequencies. Vernier thresholds were measured using both a classical single step position change and also a grating that was position-modulated sinusoidally along its entire length. For both types of target amblyopes exhibited deficient displacement sensitivity although accurate fixation was not strictly required. The vernier deficits were not critically dependent upon the type of displacement used, and the magnitudes of the vernier and contrast sensitivity deficits were closely related. Both were largest at high spatial frequencies, and those amblyopes with larger contrast sensitivity deficits also had larger vernier acuity deficiencies. Typically, contrast sensitivity and vernier acuity were normal, or nearly so, at very low spatial frequencies. Also, vernier acuities for the amblyopic and non-amblyopic eyes were approximately equal if grating contrast was set at some fixed multiple of detection threshold. We did not find a close relationship between the magnitude of the vernier deficit and reported perceptual distortions.
When a vernier target is flanked by a pair of optimally positioned flanks, offset discrimination is strongly degraded. Spatial interference with vernier acuity was studied in each eye of observers with unilateral amblyopia associated with strabismus, anisometropia or both, and were compared to the functions obtained in the normal periphery (Levi et al., 1985). The results showed that: (1) For both strabismic and anisometropic amblyopes, as in normal central and peripheral vision, the extent of spatial interference was proportional to the unflanked vernier threshold. (2) For anisometropic amblyopes, grating and vernier acuity are affected similarly. (3) For strabismic amblyopes, like the normal periphery, vernier and grating acuity are decoupled, with vernier falling off faster than grating acuity. (4) The preferred eyes of strabismic but not anisometropic amblyopes have poorer vernier acuity than the normal controls. A conceptual framework for amblyopia based upon spatial filtering and spatial sampling is discussed.
When a vernier target is flanked by optimally positioned lines, foveal vernier discrimination is strongly degraded (Westheimer and Hauske, 1975). We confirmed this observation (Experiment I) and have mapped out a 2 dimensional "perceptive field" for crowding in the fovea using a 2 dot target (Experiment II). Crowding was also measured in peripheral vision, using either small flanking dots as masks (Experiment III), or using repetitive vernier gratings (Experiment IV). The results showed that when scaled in proportion to recent estimates of the cortical magnification factor, vernier acuity is as good in the periphery as it is centrally. Both centrally and peripherally, there appears to be a psychophysical processing module which we term a "perceptive hypercolumn". At all eccentricities vernier thresholds were found to be approximately 1/40 of the size of a perceptive hypercolumn and were elevated if interfering contours are present in the same (or adjacent) hypercolumns.
The mononuclear spatial judgments made by normal adults are quite accurate and precise. Although relatively little is known about how the spatial sense develops, it is likely that it can be compromised by abnormal visual experience as provided, for example, by strabismus. Previously, we reported that the spatial sense of strabismic amblyopic eyes is distorted, having quantified these distortions as marked inaccuracies in equating spatial extents in the nasal and temporal field and specifying when one target is vertically aligned with another. Distortions--localized "expansions" and "compressions" of spatial values and "bending" of direction lines--characterize the amblyopic eyes of strabismics but not non-squinting anisometropes. Thus, according to our previously published hypothesis, spatial distortions can account for the reduced acuity and abnormal oculomotor behavior of strabismic (but not anisometropic) amblyopic eyes. In the present paper, we provide quantification of another spatial measures--spatial precision--and show that extensive imprecision characterizes the space sense of the strabismic amblyopic eye. We now add spatial imprecision to our original hypothesis as a significant element in accounting for the abnormal sensory and motor behaviors of strabismic amblyopic eyes.
Functional amblyopia (a typically unilateral loss of visual acuity of unknown origin) is frequently accompanied by unsteady fixation. Measurements taken under conditions of retinal-image stabilization indicate that this fixation problem does not contribute to the currently measured losses in spatial contrast sensitivity of the amblyopic eye. Indeed, retinal image motions recorded from unsteadily fixating eyes do not produce spatial contrast sensitivity losses when superimposed on the central field of a normal subject, indicating that such losses are not an immediate consequence of unsteady fixation.
Contrast sensitivity functions were measured for sinusoidal gratings from a sample of 10 anisometropic amblyopes. A high spatial frequency deficit was found from tests of the amblyopic eyes of all subjects. This defect decreased with spatial frequency and was correlated with the magnitude of anisometropia. Controls were instituted to rule out psychophysical method and residual defocus as possible causes of these effects. At low spatial frequencies, there were small differences between the two eyes. For some subjects, sensitivities of the amblyopic eyes appeared actually higher than normal whereas the reverse was found for most of the others. Additional tests demonstrated that the low-frequency differences could be accounted for by magnification differences (aniseikonia) between the two eyes. These findings are consistent with the idea that monocular contrast deprivation is the causal agent in anisometropic amblyopia.
The resolution for vertical gratings was tested at different eccentricities in the visual field of 9 strabismic and 2 anisometric amblyopes.It has been found that, within the central 20° of the visual field of esotropic amblyopes, the acuity of the nasal retina was much more reduced than that of the temporal retina. Outside this region, acuity was practically identical to that of the non-amblyopic eye. This asymmetric loss of acuity was well correlated with the interocular suppression as determined by a method of interocular perimetry.For anisometropic amblyopes, the acuity loss was symmetric, diminishing gradually towards the periphery. The interocular suppression was also symmetric, uniformly affecting all tested positions in the visual field of the amblyopic eye.
We examined monocular spatial vision of strabismic amblyopes by measuring errors of relative directionalization (specifying whether or not two targets are in vertical alignment) and partitioning (equating left- and right-field spaces). Abnormally large errors were made when fixation occurred with the amblyopic eye; these errors did not attributable to reduced acuity, unsteady fixation, or eccentric fixation. From the results we infer that monocular space perception of strabismic amblyopic eyes is severely distorted and is characterized by "bending" of vertical lines of direction and by local "compressions" and "expansions" of horizontal spatial values. Such distortions can readily account for many of the oculomotor abnormalities of the amblyopic eye as well as for the strabismic subject's phenomenological description of the difficulties experienced in using this eye--difficulties that are typically much worse than the reduced acuity would predict.
We examine two competing explanations for the spatial localization deficit in human strabismic amblyopia, namely neural undersampling and uncalibrated neural disarray. An undersampling hypothesis would predict an associated deficit for contrast discrimination for which we find no evidence in strabismic amblyopia. A neural disarray hypothesis would predict an associated deficit in the degree to which stimuli appear spatially distorted. We find evidence for such a deficit in strabismic amblyopia. We propose that the spatial deficit in strabismic amblyopia is due to a filter-based distortion which is unable to be re-calibrated by higher visual centres.
Synchronous firing of spatially separate neurons was studied with multi-electrode recordings in area 17 of the visual cortex of strabismic cats which had developed behaviourally verified amblyopia of the deviated eye. Responses of neurons were evoked with moving light bars or gratings of different spatial frequency. Neurons driven by the normal eye displayed stronger synchronization of their responses than neurons dominated by the amblyopic eye. These interocular differences were highly significant and particularly pronounced for grating stimuli of high spatial frequency. No interocular differences were noted with respect to the amplitudes of responses to the light bars and gratings. These results suggest reduced synchronization of population responses as a neurophysiological correlate of strabismic amblyopia and underline the importance of correlated firing of spatially separate cortical neurons for normal processing of visual information.
Patients with intermittent exotropia (strabismus) can either fixate normally or allow one eye to deviate outward by as much as 60 degrees. Two such patients (D.N. and K.C.) were studied and it was found that during eye deviation, binocular correspondence is maintained by completely 'remapping' egocentric space for the deviating eye alone using extraretinal signals from that eye. Also, by using foveal afterimages we showed that binocular rivalry occurs at a site earlier than this egocentric remapping, probably in area 17 itself. And finally, consistent with the neural remapping hypothesis, patient K.C. also experienced monocular diplopia; objects appeared double when viewed with the deviating eye.
Amblyopic subjects were asked to memorize circles of different radii (2, 4 and 6 degrees), and then to reconstruct them monocularly with each eye, point by point (12 points per circle). The resulting two-dimensional maps of visual space showed considerable distortions, including expansion, shrinkage and torsion of specific regions of the visual field of the amblyopic eye, but not the normal eye. Based on the differences between the two eyes, we computed complex two-dimensional patterns (gratings, checkerboards, optotypes, written text, natural scenes) as "seen" monocularly with the amblyopic eye. These reconstructed patterns were then compared with drawings of the same patterns observed through the amblyopic eyes of the same subjects. The reconstructed patterns only partially reflected the actual perception for the amblyopic eyes. The compensation of complex, globally-extended scenes in comparison to the distortions obtained by local, punctate settings probably reflects cooperative interactions occurring at higher brain levels.
We measured contrast detection thresholds for a foveal Gabor signal flanked by two high contrast Gabor signals. The spatially localized target and masks enabled investigation of space dependent lateral interactions between foveal and neighboring spatial channels. Our data show a suppressive region extending to a radius of two wavelengths, in which the presence of the masking signals have the effect of increasing target threshold. Beyond this range a much larger facilitatory region (up to a distance of ten wavelengths) is indicated, in which contrast thresholds were found to decrease by up to a factor of two. The interactions between the foveal target and the flanking Gabor signals are spatial-frequency and orientation specific in both regions, but less specific in the suppression region.
Neural interactions between widely separated stimuli were explored with psychophysical and visual evoked potential (VEP) measures in normal and amblyopic observers. Contrast detection thresholds were measured psychophysically for small foveally viewed Gabor patches presented in isolation and in the presence of similar, but laterally displaced flanks. The amplitude and phase of VEPs elicited by similar targets were also measured. The presence of neural interaction between the target and flank responses was assessed by comparing the unflanked threshold to the flanked threshold in the psychophysical experiments and by comparing the response predicted by the algebraic sum of test and flank responses to that measured when test and flanks were presented simultaneously. In normal observers simultaneous presentation of test and flank targets produces a VEP response that is up to a factor of two larger than the linear prediction (facilitation). Psychophysical threshold is also facilitated by a comparable factor. Facilitation was found mainly for configurations in which local (carrier) and global (patch) orientations resulted in collinearity, independent of global orientation (meridian). Amblyopic observers showed several deviations from the normal pattern. The facilitation for the collinear configurations was either markedly lower than normal or was replaced by inhibition. The normal pattern of spatial interaction may facilitate the grouping of collinear line segments into smooth curves. In contrast, abnormal long-range spatial interactions may underlie the grouping disorders and perceptual distortions found in amblyopia.
Contour integration was measured in a group of strabismic amblyopes to determine if an explanation based solely on positional uncertainty was sufficient to explain performance. The task involved the detection of paths composed of micropatterns with correlated carrier orientations embedded in a field of similar micropatterns of random position and orientation (Field et al. Contour integration by the human visual system; Evidence for a local "association field". Vision Research, 33, 173-193, 1993). The intrinsic positional uncertainty for each amblyopic eye was measured with the same stimulus and it was found that in 10 out of our 11 amblyopic subjects, the reduced performance of the amblyopic eye could be modelled by the normal eye with an equivalent amount of positional uncertainty added to the stimulus. We conclude that the rules by which cellular outputs are combined, at least as reflected by this task, are normal in amblyopia.
Neurons in the primary visual cortex are selective for the size, orientation and direction of motion of patterns falling within a restricted region of visual space known as the receptive field. The response to stimuli presented within the receptive field can be facilitated or suppressed by other stimuli falling outside the receptive field which, when presented in isolation, fail to activate the cell. Whether this interaction is facilitative or suppressive depends on the relative orientation of pattern elements inside and outside the receptive field. Here we show that neuronal facilitation preferentially occurs when a near-threshold stimulus inside the receptive field is flanked by higher-contrast, collinear elements located in surrounding regions of visual space. Collinear flanks and orthogonally oriented flanks, however, both act to reduce the response to high-contrast stimuli presented within the receptive field. The observed pattern of facilitation and suppression may be the cellular basis for the observation in humans that the detectability of an oriented pattern is enhanced by collinear flanking elements. Modulation of neuronal responses by stimuli falling outside their receptive fields may thus represent an early neural mechanism for encoding objects and enhancing their perceptual saliency.
Contour integration was measured in a group of anisometropic amblyopes to test the idea recently put forward that positional uncertainty sets a fundamental limit to contour integration in amblyopia. Anisometropic amblyopia, unlike strabismic amblyopia, has little or no positional uncertainty once the initial filtering loss has been taken into account. Therefore, according to the explanation put forward to explain strabismic amblyopia, anisometropes should exhibit normal contour integration. We show that this expectation is realized for five of our six anisometropic amblyopes.
Amblyopia is a developmental disorder of pattern vision. After surgical creation of esotropic strabismus in the first weeks of life or after wearing -10 diopter contact lenses in one eye to simulate anisometropia during the first months of life, macaques often develop amblyopia. We studied the response properties of visual cortex neurons in six amblyopic macaques; three monkeys were anisometropic, and three were strabismic. In all monkeys, cortical binocularity was reduced. In anisometropes, the amblyopic eye influenced a relatively small proportion of cortical neurons; in strabismics, the influence of the two eyes was more nearly equal. The severity of amblyopia was related to the relative strength of the input of the amblyopic eye to the cortex only for the more seriously affected amblyopes. Measurements of the spatial frequency tuning and contrast sensitivity of cortical neurons showed few differences between the eyes for the three less severe amblyopes (two strabismic and one anisometropic). In the three more severely affected animals (one strabismic and two anisometropic), the optimal spatial frequency and spatial resolution of cortical neurons driven by the amblyopic eye were substantially and significantly lower than for neurons driven by the nonamblyopic eye. There were no reliable differences in neuronal contrast sensitivity between the eyes. A sample of neurons recorded from cortex representing the peripheral visual field showed no interocular differences, suggesting that the effects of amblyopia were more pronounced in portions of the cortex subserving foveal vision. Qualitatively, abnormalities in both the eye dominance and spatial properties of visual cortex neurons were related on a case-by-case basis to the depth of amblyopia. Quantitative analysis suggests, however, that these abnormalities alone do not explain the full range of visual deficits in amblyopia. Studies of extrastriate cortical areas may uncover further abnormalities that explain these deficits.
We studied spatial integration at low contrasts by testing the detection thresholds of multi-Gabor element displays, examining configuration parameters such as orientation uniformity, contour smoothness, continuity, spacing and relative phase. We find that detectability depends on stimulus geometry and is constrained by collinearity and proximity spatial relationships. For textures, thresholds decrease with local orientation uniformity. For a 'coherent' contour (e.g. smooth and continuous), thresholds decrease linearly with increased number of elements, on a log-log scale, with a slope of -1/4 (sensitivity S proportional to N1/4). However, for a 'non-coherent' contour (e.g. jagged or with spacing > 5 lambda) thresholds are only slightly affected by the number of patches. Similar behavior is observed for supra-threshold stimuli embedded in band-pass noise. These results suggest that contrast integration is primarily based on local mechanisms and constrained by contour properties. These local mechanisms are possibly mediated by lateral interactions in the primary visual cortex.
We investigate the influence of stimulus contrast upon contour interaction in normal and amblyopic subjects. Using a computer generated acuity task, flanked and unflanked acuities were measured psychometrically at both high contrast (80%) and low contrast (6%), in a group of 19 normal and 11 amblyopic subjects. The crowding ratio for high contrast letters was found to be significantly higher than that for low contrast letters. The extent of the crowding zone was measured at high and low contrast by varying the separation of the optotype and flanking bars. The crowding zone measurement was repeated for the high contrast optotypes using dioptric blur. The position of the flanking contours was found to have a significant effect on letter resolution at high contrast but no significant effect was demonstrable at low contrast. With the addition of dioptric blur the effect of contour interaction became negligible at high contrast. These findings support the hypothesis that the crowding effect is: (1) similar in normal and amblyopic eyes when tested at threshold; (2) is contrast dependent appearing only for high contrast optotypes.