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Is peripheral visual acuity subject to perceptual learning in the adult
Abstract
While it is generally accepted that foveal visual acuity in the adult has reached an optimal value, claims for improvement of peripheral acuity with training in the adult persist in the literature. Practice effects in peripheral hyperacuity have been amply documented. A carefully controlled test is here reported to examine the influence of training on the resolution thresholds for two lines and on Landolt C acuity measurements in the retinal periphery in eight normal adults. It involved 11-30 daily sessions of 300 responses with feedback. In some observers the first day's results were somewhat poorer, but otherwise the threshold curves were essential flat. Yet in the same location vernier acuity could be improved by 50% in six training sessions. Sustained and lasting neural modifications in peripheral vision can take place in stereoscopic, orientation, vernier, bisection and time discriminations, but not in resolution and Landolt C acuities.
... The adult human visual system is well known to be plastic from the photoreceptors to the extrastriate visual areas of the cortex; sustained learning effects on visual tasks have been shown to take place after 1-2 hours of training (Fine & Jacobs, 2002). However, performance increases from perceptual learning vary considerably between different visual tasks, with low-level tasks such as foveal resolution showing little or no improvement with training (Fine et al., 2002;Johnson & Leibowitz, 1979;Westheimer, 2001). Determining whether a task can be improved with training can provide insight into the neural stages upon which that task relies (Westheimer, 2001). ...
... However, performance increases from perceptual learning vary considerably between different visual tasks, with low-level tasks such as foveal resolution showing little or no improvement with training (Fine et al., 2002;Johnson & Leibowitz, 1979;Westheimer, 2001). Determining whether a task can be improved with training can provide insight into the neural stages upon which that task relies (Westheimer, 2001). Westheimer has stated that "It can be argued that where there is no learning the processing is of a more primitive kind, more robust and nearer to sensory organs" (Westheimer, 2001). ...
... Determining whether a task can be improved with training can provide insight into the neural stages upon which that task relies (Westheimer, 2001). Westheimer has stated that "It can be argued that where there is no learning the processing is of a more primitive kind, more robust and nearer to sensory organs" (Westheimer, 2001). It is generally accepted that foveal visual resolution in the adult has reached an optimal value, has a strong retinal basis, and depends on the spacing of retinal elements in an elementary way (Westheimer, 2001). ...
Visual resolution, the ability to see fine spatial detail, emerges from the capacities of both the eye and the brain. A great deal of insight into the anatomical and physiological basis of human visual resolution has been gained since Helmholtz first proposed his sampling theory of visual resolution. Anatomical, physiological and psychophysical investigations have revealed in great detail the properties of the biological structures underlying visual resolution and identified many of the optical, retinal, and cortical factors that govern the limits of visual resolution. However, technological limitations have long prevented researchers from examining both the structure and function of the visual system simultaneously in the living eye. The microscopic photoreceptors of the retina have been inaccessible to optical examination, preventing high quality measurements of both visual resolution and retinal anatomy from being obtained in the same eyes. The present studies investigated the relationship between the optical, retinal, and cortical factors that govern visual resolution in humans. These experiments employed adaptive optics scanning laser ophthalmoscopy (AOSLO) as a tool to study how these factors govern visual resolution in normal and diseased eyes. The AOSLO is an ideal tool for studying the limits of vision because of its ability to present complex stimuli to the retina that are of higher optical quality than the visual system has ever experienced, while simultaneously imaging the underlying cone photoreceptor mosaic on a microscopic scale.
Adaptive optics correction of ocular aberrations allowed observers to achieve immediate and significant improvements in visual resolution. Training was not required to achieve this benefit, which allowed the resolving capacity of the retinal and cortical visual system to be assessed unobstructed by the optics of the eye. Not all participants in these studies benefited to the same extent from AO correction. Visual resolution was found to be significantly poorer in low myopia as compared to emmetropia, despite the similar optical quality afforded by AO correction, showing that retinal and cortical changes in myopia caused the observed deficit. Simultaneous imaging and visual resolution testing determined the precise relationship between the spatial sampling limit of the cone mosaic and visual resolution across the human fovea. These studies revealed that the spatial sampling limit of the cone mosaic largely governs visual resolution at the center of the fovea for normal eyes, but that outside the foveal center visual resolution falls off at a greater rate than predicted by cone spacing and is governed by the spatial sampling limit of the mosaic of midget retinal ganglion cells.
Significant differences between otherwise normally appearing observers were revealed using AOSLO, showing the power of visual resolution testing after AO correction for detecting small changes in the visual system resulting from disease. Significant retinal changes were revealed in female carriers of a rare X-linked genetic mutation in the L and M opsin gene array that causes blue cone monochromacy (the loss of all L and M cone function) in affected males. Retinal findings from AO imaging provided insight into the development and function of the carrier retina. Although carriers had visual resolution within the normal range when tested clinically, visual resolution testing in AOSLO revealed significantly reduced visual resolution compared to normal eyes. Resolution testing across the fovea in normal and diseased eyes provided insight into the relationship between cones, ganglion cells and visual resolution across the visual field. Retinal imaging showed that carriers had fairly normal cone topography despite peak cone density that was ~50% lower than normal; showing that cones destined to express a non-functional photopigment degenerated early in development. Drastic reductions in visual resolution across the fovea in the carrier are best explained by ganglion cell loss that resulted from the loss of cones in the carrier. The relationship between resolution and the spatial sampling limit of the carrier cone mosaic was similar to what was predicted for normal eyes, suggesting an organizing principle in the visual system whereby cone density is the primary determinant of the retinal circuitry that governs visual resolution across the visual field. Together, these studies provide an important contribution to the understanding of the limits of human visual resolution.
... In other words, the tasks for which learning does not occur are considered to have a process which is of a more primitive kind, more robust and closer to sensory origins. 73 For example, resolution tasks such as grating acuity have a strong retinal basis and depend on the spacing of retinal elements, while other localisation tasks of the hyperacuity kind have an element of cortical processing. 43 Following this idea, if word acuity thresholds follow the cortical magnification factor, 48 such as vernier acuity, which can be trained, 73 then it is plausible to propose that practice will also improve word recognition thresholds. ...
... 73 For example, resolution tasks such as grating acuity have a strong retinal basis and depend on the spacing of retinal elements, while other localisation tasks of the hyperacuity kind have an element of cortical processing. 43 Following this idea, if word acuity thresholds follow the cortical magnification factor, 48 such as vernier acuity, which can be trained, 73 then it is plausible to propose that practice will also improve word recognition thresholds. ...
... While controversy surrounds the notion that simple peripheral resolution performance can be improved with training, 73 recent studies have demonstrated trainingrelated improvements in peripheral reading performance, 74, 75 Chung, Legge and Cheung 75 specifically examined whether visual span profiles (plot of letter recognition accuracy as a function of letter positions left or right of the midline) could be modified through repeated training on a letter recognition task in peripheral vision. The results from this study showed that training with a letter recognition task at an eccentric location led to changes in the visual span profiles. ...
Age-related macular degeneration (AMD) is the leading cause of blindness in developed countries. With an ageing population, the prevalence of such a condition has resulted in a large proportion of the population relying on peripheral vision to undertake activities of daily living. Peripheral vision is not a scaled-down version of the fovea, simply requiring larger print or increased contrast for detection of objects or reading text. Even when print size is scaled and eye movements are minimised, the peripheral retina cannot perform at the level of the foveal region. Understanding how and why reading performance is limited as a function of eccentricity has important implications for how we approach rehabilitation of patients with central visual loss. This brief review of the extensive literature on reading with peripheral vision and the research aimed at better reading rehabilitation for low vision patients focuses on why many of the problems associated with the reduced reading capability of peripheral vision cannot be completely solved with magnification, reducing eye movements or modifying print.
... Unlike sensitivity changes resulting from repetitive stimulation, such as desensitization or habituation, perceptual learning tends to persist over months, and in some cases, years (Fahle, 2005). Perceptual learning has been shown to improve performance of visual tasks, including: Vernier acuity (Saarinen & Levi, 1995; Fahle, 1997; Herzog & Fahle, 1997; Westheimer, 2001), curvature acuity (Fahle,1997), resolution acuity (Westheimer, 2001), bisection acuity (Westheimer, 2001), orientation discrimination (Karni & Sagi, 1993; Dosher & Lu,1999; *corresponding author. Tel: +1-713-743-1940; fax: +1-713-743-2053, Email address: E-mail: rharwerth@uh.edu ...
... Unlike sensitivity changes resulting from repetitive stimulation, such as desensitization or habituation, perceptual learning tends to persist over months, and in some cases, years (Fahle, 2005). Perceptual learning has been shown to improve performance of visual tasks, including: Vernier acuity (Saarinen & Levi, 1995; Fahle, 1997; Herzog & Fahle, 1997; Westheimer, 2001), curvature acuity (Fahle,1997), resolution acuity (Westheimer, 2001), bisection acuity (Westheimer, 2001), orientation discrimination (Karni & Sagi, 1993; Dosher & Lu,1999; *corresponding author. Tel: +1-713-743-1940; fax: +1-713-743-2053, Email address: E-mail: rharwerth@uh.edu ...
... Unlike sensitivity changes resulting from repetitive stimulation, such as desensitization or habituation, perceptual learning tends to persist over months, and in some cases, years (Fahle, 2005). Perceptual learning has been shown to improve performance of visual tasks, including: Vernier acuity (Saarinen & Levi, 1995; Fahle, 1997; Herzog & Fahle, 1997; Westheimer, 2001), curvature acuity (Fahle,1997), resolution acuity (Westheimer, 2001), bisection acuity (Westheimer, 2001), orientation discrimination (Karni & Sagi, 1993; Dosher & Lu,1999; *corresponding author. Tel: +1-713-743-1940; fax: +1-713-743-2053, Email address: E-mail: rharwerth@uh.edu ...
Perceptual learning is a training induced improvement in performance. Mechanisms underlying the perceptual learning of depth discrimination in dynamic random dot stereograms were examined by assessing stereothresholds as a function of decorrelation. The inflection point of the decorrelation function was defined as the level of decorrelation corresponding to 1.4 times the threshold when decorrelation is 0%. In general, stereothresholds increased with increasing decorrelation. Following training, stereothresholds and standard errors of measurement decreased systematically for all tested decorrelation values. Post training decorrelation functions were reduced by a multiplicative constant (approximately 5), exhibiting changes in stereothresholds without changes in the inflection points. Disparity energy model simulations indicate that a post-training reduction in neuronal noise can sufficiently account for the perceptual learning effects. In two subjects, learning effects were retained over a period of six months, which may have application for training stereo deficient subjects.
... This skill is constrained by high spatial resolution, and it is considered to be mediated by low-level cortical mechanisms (Ahissar & Hochstein, 1997). Landolt-C acuity in the periphery has been shown to not improve with practice, possibly due to resolution constraints (Westheimer, 2001). Vernier offset discrimination thresholds improve with practice, but only for the trained orientation, location, or eye (Fahle, 2004;Fahle & Edelman, 1993;Fahle & Morgan, 1996;Fiorentini & Berardi, 1980;Poggio, Fahle, & Edelman, 1992; for reviews, see Sagi, 2011;Watanabe & Sasaki, 2015). ...
... Our Experiment 1 results are inconsistent with earlier findings that Landolt C acuity does not improve with training (Westheimer, 2001). On the contrary, we found improvement at the trained location and at the untrained location, with both selective and distributed attention. ...
Visual perceptual learning (VPL) refers to the improvement in performance on a visual task due to practice. A hallmark of VPL is specificity, as improvements are often confined to the trained retinal locations or stimulus features. We have previously found that exogenous (involuntary, stimulus-driven) and endogenous (voluntary, goal-driven) spatial attention can facilitate the transfer of VPL across locations in orientation discrimination tasks mediated by contrast sensitivity. Here, we investigated whether exogenous spatial attention can facilitate such transfer in acuity tasks that have been associated with higher specificity. We trained observers for 3 days (days 2-4) in a Landolt acuity task (Experiment 1) or a Vernier hyperacuity task (Experiment 2), with either exogenous precues (attention group) or neutral precues (neutral group). Importantly, during pre-tests (day 1) and post-tests (day 5), all observers were tested with neutral precues; thus, groups differed only in their attentional allocation during training. For the Landolt acuity task, we found evidence of location transfer in both the neutral and attention groups, suggesting weak location specificity of VPL. For the Vernier hyperacuity task, we found evidence of location and feature specificity in the neutral group, and learning transfer in the attention group-similar improvement at trained and untrained locations and features. Our results reveal that, when there is specificity in a perceptual acuity task, exogenous spatial attention can overcome that specificity and facilitate learning transfer to both untrained locations and features simultaneously with the same training. Thus, in addition to improving performance, exogenous attention generalizes perceptual learning across locations and features.
... This task was developed by optometrists to examine visual acuity, testing resolution -the ability to detect contrast between the black target and the white gap (Westheimer, 2017). It is a lowlevel perceptual task, in which performance ceiling can be reached within a small number of trials (Westheimer, 2001). Due to its simplicity, the Landolt gap task became customary in research of visual attention. ...
... Moreover, the group expressed an advantage in learning. This finding was enlightening, as some theoretical assumptions imply that no further learning should take place in the Landolt gap task after the initial 200 trials (Westheimer, 2001); on the other hand, Bonder et al. (2018) demonstrated an improvement in performance afterward. We questioned whether the Confidence Rating group would show improved learning throughout the experiment, capitalizing on the processing strategies formulated during task formation. ...
The current study explored the influence of confidence rating on visual acuity. We used brief exposures of the Landolt gap discrimination task, probing the primary visual ability to detect contrast. During 200 practice trials, participants in the Confidence Rating group rated their response-confidence in each trial. A second (Time Delay) group received a short break at the end of each trial, equivalent to the average rating response time of the Confidence Rating group. The third (Standard Task) group performed the Landolt gap task in its original form. During practice, the Confidence Rating group developed an efficient monitoring ability indicated by a significant correlation between accuracy and confidence rating and a moderate calibration index score. Following practice, all groups performed 400 identical test trials of the standard Landolt gap task. In the test trials, the Confidence Rating group responded more accurately than the control groups, though it did not differ from them in response time for correct answers. Remarkably, the Confidence Rating group was significantly slower when making errors, compared the control groups. An interaction in learning efficiency occurred: the Confidence Rating group significantly improved its reaction times after the initial practice, as compared to both control groups. The findings demonstrate an effect of confidence rating on the formation of processing and response strategies, which granted participants significant benefits in later performance.
... Some individual participants in this report reached final threshold values of nearly 3 seconds of arc (which is about 1/10 th of the diameter of a photoreceptor). The basic finding that vernier acuity is subject to substantial perceptual learning has been repeatedly seen throughout the literature, including in foveal vision (Saarinen & Levi, 1995), parafoveal vision (Beard, Levi, & Reich, 1995), and peripheral vision (Westheimer, 2001). ...
... Perceptual learning in orientation discrimination in the fovea only occurs for stimuli embedded in high levels of external noise in oblique directions, but not in zero noise or along cardinal directions . Finally, while vernier acuity is subject to very consistent learning effects, Landolt C acuity does not appear to show the same level of improvement with practice (Westheimer, 2001). Auditory analogues to these examples are hard to find. ...
Perceptual learning refers to a long‐term change in the ability to extract perceptual information from the environment that arises via experience or practice. In this chapter, we first explore many examples of perceptual learning in relatively simple as well as in more complex tasks. Furthermore, we do so across sensory modalities (vision, audition, touch, taste, smell) with the goal of highlighting the commonalities seen in perceptual learning across systems that are typically considered separately. We then consider the most substantial issues in the domain, including the question of learning specificity (i.e., when do improvements on one task transfer to new unpracticed tasks?), the conditions under which learning occurs, the neural basis of perceptual learning, computational models of perceptual learning, and potential real‐world applications of perceptual learning. We conclude with future directions; although the field has clearly made tremendous progress over the past century or more, there remain critical gaps in our knowledge that preclude our ability to fully harness perceptual learning to make a real‐world impact.
... Importantly, the learning effects are long-lasting (Fig. 1b), and only a few re-training sessions are needed to compensate for a slight loss of previously gained improvement. One might argue that the improvement in performance might be in part attributed to high level cognitive task learning or instrument learning (Westheimer, 2001). For example, amblyopes might learn to fixate and/or accommodate more accurately with their amblyopic eye. ...
... But further practice resulted in an additional 52% improvement over 40 more hours. In contrast to cognitive task learning (Westheimer, 2001) that is rapid, this gradual improvement reflects genuine experience-dependent neural alternations. Similar prolonged learning effects have also been recently reported in animals with degraded auditory systems (Zhou and Merzenich, 2007). ...
Amblyopia is a developmental abnormality that results in physiological alterations in the visual cortex and impairs form vision. It is often successfully treated by patching the sound eye in infants and young children, but is generally considered to be untreatable in adults. However, a number of recent studies suggest that repetitive practice of a visual task using the amblyopic eye results in improved performance in both children and adults with amblyopia. These perceptual learning studies have used relatively brief periods of practice; however, clinical studies have shown that the time-constant for successful patching is long. The time-constant for perceptual learning in amblyopia is still unknown. Here we show that the time-constant for perceptual learning depends on the degree of amblyopia. Severe amblyopia requires >50 h (approximately equal to 35,000 trials) to reach plateau, yielding as much as a five-fold improvement in performance at a rate of approximately equal to 1.5%/h. There is significant transfer of learning from the amblyopic to the dominant eye, suggesting that the learning reflects alterations in higher decision stages of processing. Using a reverse correlation technique, we document, for the first time, a dynamic retuning of the amblyopic perceptual decision template and a substantial reduction in internal spatial distortion. These results show that the mature amblyopic brain is surprisingly malleable, and point to more intensive treatment methods for amblyopia.
... Perceptual learning research has indicated that practice can result in improvements on many (but not all e.g. Westheimer, 2001) visual tasks by human adults (see Karni & Bertini, 1997;Sagi & Tanne, 1994;Sathian, 1998 for reviews). For instance, improvement has been shown in visual detection or discrimination of orientation (e.g. ...
... De Valois & De Valois, 1988, pp. 204-205). A peripheral training location was selected because previous work typically reports greater perceptual learning in the periphery than the fovea (see most recently Westheimer, 2001). A translucent lens with a refractive power of 0 diopters was placed in front of the non-observing eye to occlude the stimulus. ...
Performance of a wide range of simple visual tasks improves with practice. Here we ask whether such learning occurs for the fundamental visual task of luminance contrast detection. In two experiments we find that contrast sensitivity increases following extensive practice at detecting briefly presented sinusoidal luminance gratings and that learning is maintained after six months. Learning is spatial frequency tuned, specific to retinal location and can be specific to one eye, but is not selective for orientation. The selectivity of learning implies that it is based on plasticity in early visual, as opposed to central cognitive, processing mechanisms.
... Perceptual learning in the visual periphery has been reported for vernier stimuli but not for conventional visual acuity (Westheimer, 2001). Crist et al. (1997) showed that perceptual learning appeared to be specific for visual field location of the training stimuli. ...
... These results confirm previous observations that perceptual learning may take place with peripheral vision where observers were requested to pay attention to peripheral stimuli and they had to detect vernier offsets (Westheimer, 2001) or the relative position of a line (Crist et al., 1997). Shiu and Pashler (1992) reported that there was no improvement for orientation detection if subjects attended to luminance changes of lines, but others reported that perceptual learning occurred without attention to the peripheral stimuli (Watanabe et al., 2001(Watanabe et al., , 2002Seitz and Watanabe, 2003). ...
We studied human perceptual learning in the peripheral visual field in 16 healthy adults. Horizontal or vertical vernier stimuli were presented simultaneously at 8 locations at an eccentricity of 4 degrees . One of the stimuli displayed an offset, and subjects were asked to detect the target offset. Training was performed with either vertical or horizontal stimuli by the repeated presentation of stimuli. Discrimination performance was also measured with the untrained stimuli. Before and after the psychophysical experiment, EEG was recorded from 30 electrodes over the occipital areas (between the inion and Cz) while targets were presented at all locations as vernier onset/offset stimuli. The EEG was averaged for each orientation separately. Improvement in discrimination performance was observed in about 70% of the subjects with the trained orientation only. The evoked potential maps displayed three components occurring between 80 and 160, 180 and 260, and 280 and 340 ms. The potential field topography of the first and third component showed significant differences before and after learning. In addition, field strength (global field power) of the second and third component increased with learning. No effects were seen with the untrained stimuli in the psychophysical and electrophysiological experiments. Our findings suggest that perceptual learning in the peripheral visual field is specifically related to neurophysiological changes induced by training, and it is not caused by unspecific changes of spatial attention. The changes of electrical brain activity reflect short-term plasticity related to human perceptual learning.
... As a matter of fact, crowding effects have been reported to be larger in children than in adults (Atkinson, Anker, Evans, Hall, & Pimm-Smith, 1988), which again suggest that learning may alter these effects in adults. However, few studies have directly investigated this issue (for a review of learning effects in peripheral vision, see Westheimer, 2001). To our knowledge, only Wolford, Marchak, and Hughes (1988) have tested how training influences crowding. ...
... The differences between performance for isolated letters and letters presented in nonwords are referred to as crowding effect; the differences between targets presented in words and in nonwords are referred to as word superiority. monotonic increase with training (see also Westheimer, 2001). This assertion has to be considered with caution though because ceiling effects cannot be entirely ruled out for isolated letters. ...
Processes underlying crowding in visual letter recognition were examined by investigating effects of training. Experiment 1 revealed that training reduces crowding mainly for trained strings. This was corroborated in Experiment 2, where no training effects were obvious after 3 days of training when strings changed from trial to trial. Experiment 3 specified that after a short amount of training, learning effects remained specific to trained strings and also to the trained retinal eccentricity and the interletter spacing used in training. Transfer to other than trained conditions was observed only after further training. Experiment 4 showed that transfer occurred earlier when words were used as stimuli. These results thus demonstrate that part of crowding results from the absence of higher level representations of the stimulus. Such representations can be acquired through learning visual properties of the stimulus.
... The literature on visual perception notes a physiological limit beyond which certain low-level discrimination tasks cannot be further improved. [52][53][54][55] Very difficult discrimination tasks require excessive practice time and may reach a perceptual limit. 29 Beyond this, continuing to train may produce small incremental improvements, but there are diminishing skill returns. ...
Background:
Visual assessment of equine lameness is an everyday veterinary task suffering from poor diagnostic accuracy. The aim of this study was to quantify the impact of the perceptual learning game 'LamenessTrainer' on skill development.
Methods:
Thirty-six undergraduate veterinary students engaged in four game modules teaching the assessment of fore- and hindlimb lameness. Computer animations of horses in this game displayed 0% (sound) to 70% (moderately lame) vertical movement asymmetry of head and pelvis. Performance, learning effects, diagnostic accuracy, detection thresholds and survey responses were analysed.
Results:
Following staircase learning, more than 80% of students reliably classified horses with ≥20% asymmetry for forelimb lameness, ≥40% asymmetry for simplified hindlimb lameness and ≥50% asymmetry for realistic hindlimb lameness. During random presentation, on average 82% of sound and 65% of lame horses were assessed correctly during forelimb lameness evaluation, dropping to 39% of sound and 56% of lame horses for hindlimb lameness.
Conclusion:
In less than two hours, systematic perceptual learning through deliberate practice can develop visual assessment skills to an accuracy level comparable to expert assessors scoring the same animations. Skills should be developed further to improve misclassifications of sound and mildly lame horses, especially for hindlimb lameness evaluation.
... We chose a Landolt gap task because it is considered to involve early visual areas. Contrary to more complicated visual tasks, in which significant improvement is produced after extensive perceptual learning, in this basic task learning asymptote is achieved quickly, and typically no improvement is found after the initial practice (Westheimer, 2001). According to Fiorentini and Berardi (1981), objective 2-Alternative-Forced-Choice tasks, such as the Landolt gap task, show a learning effect that develops over ∼200 trials. ...
The present study examined the mutual influence of cortical neuroenhancement and allocation of spatial attention on perception. Specifically, it explored the effects of transcranial Direct Current Stimulation (tDCS) on visual acuity measured with a Landolt gap task and attentional precues. The exogenous cues were used to draw attention either to the location of the target or away from it, generating significant performance benefits and costs. Anodal tDCS applied to posterior occipital area for 15 min improved performance during stimulation, reflecting heightened visual acuity. Reaction times were lower, and accuracy was higher in the tDCS group, compared to a sham control group. Additionally, in post-stimulation trials tDCS significantly interacted with the effect of precuing. Reaction times were lower in valid cued trials (benefit) and higher in invalid trials (cost) compared to neutrally cued trials, the effect which was pronounced stronger in tDCS group than in sham control group. The increase of cost and benefit effects in the tDCS group was of a similar magnitude, suggesting that anodal tDCS influenced the overall process of attention orienting. The observed interaction between the stimulation of the visual cortex and precueing indicates a magnification of attention modulation.
... Perceptual learning occurs for different kinds of tasks at different levels of visual analysis (figure 1). It improves detection or discrimination for single features such as orientation , Schoups et al. 1995, Vogels & Orban 1985, spatial frequency (Bennett & Westheimer 1991, Fiorentini & Berardi 1981, phase , Fiorentini & Berardi 1980, contrast (Adini et al. 2004, Dorais & Sagi 1997, Sowden et al. 2002, color (Casey & Sowden 2012, Özgen & Davies 2002, Thurston & Dobkins 2007, acuity (Bennett & Westheimer 1991, Westheimer 2001, and hyper-acuity (Crist et al. 1997, McKee & Westheimer 1978, Poggio et al. 1992. It improves pattern discrimination in tasks involving compound stimuli (Fiorentini & Berardi 1980, Fiorentini & Berardi 1981, textures (Ahissar & Hochstein 1993, Karni & Sagi 1991), depth (Fendick & Westheimer 1983, Ramachandran & Braddick 1973, and motion (Ball & Sekuler 1982, Matthews & Welch 1997, Watanabe et al. 2002. ...
Visual perceptual learning through practice or training can significantly improve performance on visual tasks. Originally seen as a manifestation of plasticity in the primary visual cortex, perceptual learning is more readily understood as improvements in the function of brain networks that integrate processes, including sensory representations, decision, attention, and reward, and balance plasticity with system stability. This review considers the primary phenomena of perceptual learning, theories of perceptual learning, and perceptual learning's effect on signal and noise in visual processing and decision. Models, especially computational models, play a key role in behavioral and physiological investigations of the mechanisms of perceptual learning and for understanding, predicting, and optimizing human perceptual processes, learning, and performance. Performance improvements resulting from reweighting or readout of sensory inputs to decision provide a strong theoretical framework for interpreting perceptual learning and transfer that may prove useful in optimizing learning in real-world applications. Expected final online publication date for the Annual Review of Vision Science Volume 3 is September 15, 2017. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
... Perceptual learning is likely to be very different in prosthesis users than in sighted subjects. In subjects with normal vision it can be assumed that the organization of the visual pathways is already close to optimal, especially in the fovea (Westheimer 2001). As a result, task performance for naturalistic tasks such as object recognition is generally at ceiling, making it necessary to titrate difficulty by reducing stimulus difference (e.g. ...
The "bionic eye" - so long a dream of the future - is finally becoming a reality with retinal prostheses available to patients in both the US and Europe. However, clinical experience with these implants has made it apparent that the vision provided by these devices differs substantially from normal sight. Consequently, the ability to learn to make use of this abnormal retinal input plays a critical role in whether or not some functional vision is successfully regained. The goal of the present review is to summarize the vast basic science literature on developmental and adult cortical plasticity with an emphasis on how this literature might relate to the field of prosthetic vision. We begin with describing the distortion and information loss likely to be experienced by visual prosthesis users. We then define cortical plasticity and perceptual learning, and describe what is known, and what is unknown, about visual plasticity across the hierarchy of brain regions involved in visual processing, and across different stages of life. We close by discussing what is known about brain plasticity in sight restoration patients and discuss biological mechanisms that might eventually be harnessed to improve visual learning in these patients.
... Sin embargo, y a pesar de que existe un acuerdo unánime en aceptar que visión es mucho más que agudeza visual, rebasada la frontera del nuevo milenio, la agudeza visual estática, sigue siendo la habilidad visual más frecuentemente evaluada y analizada a nivel clínico. Los optotipos más utilizados para medir la agudeza visual estática son las letras de Snellen y la C o anillo de Landolt, tests de más de 100 años (Westheimer, 2001). ...
... This effect is lower than that reported in previous studies (Durrie and McMinn, 2007; Tan and Fong, 2008), probably because we used only a vertically oriented training stimulus. The transfer of learning with vertically oriented Gabors to complex stimuli such as Sloan letters and Landolt-C could be explained by assuming that these visual acuity tasks can be performed by detecting intensity changes (Westheimer, 2001), a task similar to the (trained) contrast detection. ...
Purpose:
In this study we investigated in observers with low myopia: (i) the pattern of lateral interactions between stimuli activating early cortical analyzers and its modulation by perceptual learning (PL), and (ii) whether PL transferred to untrained stimuli and tasks and whether it exhibits interocular transfer.
Method:
Participants (seven adults with low myopia) performed 12 training sessions. Participants were trained on a contrast detection task of a central Gabor target flanked by two co-oriented and co-aligned high contrast Gabor patches. Target-to-flankers separation along the vertical axis was varied from 2 wavelengths (λ) to 8λ.
Results:
The results showed that before PL facilitatory lateral interactions in the myopic eye were reduced in strength, but PL increased contrast sensitivity and improved facilitatory lateral interactions. However, PL did not transfer to different local/global orientations and lower spatial frequencies. On the other hand, PL resulted in an enhancement of the contrast sensitivity function (CSF) and of the uncorrected visual acuity (UCVA) both in the trained and untrained eye.
Conclusions:
Such improvements seem to be associated to a modulation of lateral interactions between target and flankers and it is likely to take place at a level in which the inputs from the two eyes converge.
... Comparing baseline visual acuity in the chronology of sessions, we detected a small and continuous increase of performance, indicating a training effect. Previous findings [32] showed some evidence for fast perceptual learning, not consistent over subjects and tasks. This suggests that learning in peripheral visual acuity may depend on the task as well as on the subject per se. ...
Repetitive transcranial magnetic stimulation is usually applied to visual cortex to explore the effects on cortical excitability. Most researchers therefore concentrate on changes of phosphene threshold, rarely on consequences for visual performance. Thus, we investigated peripheral visual acuity in the four quadrants of the visual field using Landolt C optotypes before and after repetitive stimulation of the visual cortex. We applied continuous and intermittend theta burst stimulation with various stimulation intensities (60%, 80%, 100%, 120% of individual phosphene threshold) as well as monophasic and biphasic 1 Hz stimulation, respectively. As an important result, no serious adverse effects were observed. In particular, no seizure was induced, even with theta burst stimulation applied with 120% of individual phosphene threshold. In only one case stimulation was ceased because the subject reported intolerable pain. Baseline visual acuity decreased over sessions, indicating a continuous training effect. Unexpectedly, none of the applied transcranial magnetic stimulation protocols had an effect on performance: no change in visual acuity was found in any of the four quadrants of the visual field. Binocular viewing as well as the use of peripheral instead of foveal presentation of the stimuli might have contributed to this result. Furthermore, intraindividual variability could have masked the TMS- induced effects on visual acuity.
... Three times the critical number of trials provides an estimate the number of trials beyond which stereothresholds level off and no longer improve. These values are 3168 ± 384 for the sparse stereograms and 3771 ± 795 for the dense stereograms, which are consistent with other studies of perceptual learning of depth discrimination using RDSs (Gantz, Chung & Harwerth, 2007a,Westheimer, 2001. The high variability obtained in the group data for the critical number of training trials (see also the individual functions shown in Figure 2) is consistent with the large inter-individual variability of perceptual learning that is reported by others (Schmitt, Kromeier, Bach & Kommerell, 2002). ...
Several studies reported that stereothresholds assessed with local-contour stereograms and complex random-dot stereograms (RDSs) are different. Dissimilar thresholds may be due to differences in the properties of the stereograms (e.g., spatial frequency content, contrast, inter-element separation, area) or to different underlying processing mechanisms. This study examined the transfer of perceptual learning of depth discrimination for local RDSs to global RDSs with similar properties, and vice versa. If global and local stereograms are processed by separate neural mechanisms, then the magnitude and rate of training for the two types of stimuli are likely to differ, and the transfer of training from one stimulus type to the other should be minimal. Based on the results of a previous study, we chose 3.7-deg RDSs with element densities of 1.15% and 15% to serve as the local and global stereograms, respectively. Fourteen inexperienced subjects with normal binocular vision were randomly assigned to either a local- or global-RDS training group. Stereothresholds for both stimulus types were measured before and after 7700 training trials (10 sessions X 10 blocks X 77 trials). Each subject's stereothresholds were normalized to the pre-training measurement for the trained condition and the average data were fit with an exponential equation. Stereothresholds for the trained condition improve for approximately 3000 trials, by approximately 0.23 log units for local and 0.15 log units for global RDSs, and level off thereafter. Neither the rate nor the magnitude of improvement differ statistically between the local- and global-training groups. Further, no significant difference exists in the amount of improvement on the trained vs. the untrained targets for either training group. These results are consistent with the operation of a single mechanism to process both local and global stereograms.
... Importantly, any investigation of peripheral vision introduces a number of caveats that are normally not present in the investigations of central field visual function. These caveats concern such factors as the effects of practice (16), the potential task-dependent effects of practice (17), and the effect of attention (18) on peripheral visual performance. ...
The U.S. Army acquisition community uses models of human and systems performance to evaluate materiel. One such model, used to assess equipment for the dismounted Soldier, is the Individual Warrior Simulation (IWARS). When Soldier modeling target acquisition, IWARS applies the ACQUIRE model to some portion of the visual scene, determines the probability of detecting a target if one is present, then repeats the process for a different portion of the field. ACQUIRE predicts the probability of visually acquiring a target presented anywhere within its field of view (FOV) without regard for the location of the target in the scene. Human visual performance varies dramatically across the FOV, with acuity decreasing rapidly as objects move away from the center of gaze or away from the central vertical and horizontal axes of vision. The current research used Gabor patches to characterize visual detection thresholds at various locations around central vision. Results showed a marked increase in contrast threshold for targets of a y of spatial frequencies ranging from 0.5 to 18.2 cycles/degree appearing greater than plus or minus 3 degrees minus 4 degrees horizontal eccentricity. These results can be used to intelligently constrain the portion of the visual field over which IWARS applies the ACQUIRE detection model.
... Sports vision-training programmes have been suggested to athletes to improve performance. Although in certain cases specific vision training has not led to improvements in visual acuity and peripheral acuity (Westheimer, 2001;Abernethy & Wood, 2001), vision function and shooting skills were enhanced in Olympic shooters who participated in an unspecified vision training programme lasting approximately 20 hours over a 3-month period (Quevedo & Sole, 1995). Furthermore, West and Bressan (1996) showed improvement in visual skills of cricket players with a vision training programme involving seven skills. ...
Many sports require fine spatiotemporal resolution for optimal performance. Previous studies have compared anticipatory skills and the decision-making process in athletes; however, there is little information on visual skills of elite athletes, particularly hockey players. To assess visual skills of Olympic hockey players and analyze differences by playing position, and to analyze improvement of visual skills after training, 21 Olympic field hockey players were pre- and post-tested on 11 visual tasks following a 10-wk. visual training program consisting of computer-based visual exercises. There were no mean differences at pre-test between players of different positions, suggesting that performance on these visual skills was independent of playing position. However, after training, an improvement was seen in all players (when scores were averaged across all 11 visual tasks) with goalkeepers improving significantly more than any other position. This suggests the possibility of improving visual skills even in an elite population.
Read More: http://www.amsciepub.com/doi/abs/10.2466/05.22.24.PMS.114.1.204-216
... This experiment involves letter recognition in peripheral vision. Although some types of visual performance improve with practice in peripheral vision, such as vernier acuity (Beard, Levi, and Reich, 1995), recent evidence indicates that practice has little or no effect on acuity measured with Landolt C's (Westheimer, 2001). ...
Our goal is to link spatial and temporal properties of letter recognition to reading speed for text viewed centrally or in peripheral vision. We propose that the size of the visual span — the number of letters recognizable in a glance — imposes a fundamental limit on reading speed, and that shrinkage of the visual span in peripheral vision accounts for slower peripheral reading. In Experiment 1, we estimated the size of the visual span in the lower visual field by measuring RSVP (rapid serial visual presentation) reading times as a function of word length. The size of the visual span decreased from at least 10 letters in central vision to 1.7 letters at 15° eccentricity, in good agreement with the corresponding reduction of reading speed measured by Chung and coworkers (Chung, S. T. L., Mansfield, J. S., & Legge, G. E. (1998). Psychophysics of reading. XVIII. The effect of print size on reading speed in normal peripheral vision. Vision Research, 38, 2949–2962). In Exp. 2, we measured letter recognition for trigrams (random strings of three letters) as a function of their position on horizontal lines passing through fixation (central vision) or displaced downward into the lower visual field (5, 10 and 20°). We also varied trigram presentation time. We used these data to construct visual-span profiles of letter accuracy versus letter position. These profiles were used as input to a parameter-free model whose output was RSVP reading speed. A version of this model containing a simple lexical-matching rule accounted for RSVP reading speed in central vision. Failure of this version of the model in peripheral vision indicated that people rely more on lexical inference to support peripheral reading. We conclude that spatiotemporal characteristics of the visual span limit RSVP reading speed in central vision, and that shrinkage of the visual span results in slower reading in peripheral vision.
... Nevertheless, practice effects have received only little attention with respect to standard acuity testing. Westheimer [6] argued that " the widespread utilization of foveal Snellen acuity measurements in the clinic, relied on for over 100 years as an indicator of possible change in visual function, makes it unlikely that significant training factors in the adult have been overlooked " , and he focused on assessing practice effects in peripheral acuity, where he did not find an improvement with practice. However, there is some evidence for an improvement of standard (central) acuity over the course of several sessions. ...
Practice-related improvements of performance are common in many areas of visual processing. There is preliminary evidence that this is also the case for standard optotype acuity tasks. The present study was designed to confirm and quantify the effect of practice under different feedback conditions and to track the dynamics of practice over several sessions.
Subjects completed a total of 56 runs of a computer-based acuity test with randomly oriented Landolt C optotypes, split evenly over four sessions at intervals of 1 week. Half of the subjects received feedback indicating the correct response.
Over the course of the sessions, the test outcomes increased significantly by 0.11 logMAR with feedback and by 0.055 logMAR without feedback. In addition to an increase in acuity over the first few runs of the first session, a major part of the practice effect with feedback occurred not during a session, but in between the first and the second session. Without feedback, the increase in acuity occurred mainly within the first half of the first session.
Feedback has a drastic effect on the magnitude and dynamics of the practice effect, which is not explained by simple familiarization with the test procedure. If feedback is not given, practice effects can be neglected in most clinical routine applications even when many test repetitions are performed. However, they may become relevant on a group level in clinical studies without an appropriate control.
... Similarly, the ability to discriminate between two contrast levels of otherwise identical grating patches does not improve with practice (Adini, Sagi, & Tsodyks, 2002; Dorais & Sagi, 1997; Maehara & Goryo, 2007 ). Landolt C acuity and two-line resolution thresholds, unlike Vernier, do not improve much with practice, if at all, possibly because these discriminations can be reduced to a task involving detection of an intensity change (Westheimer, 2001). Westheimer argued that ''where there is no learning the processing is of a more primitive kind, more robust and nearer to sensory origins''. ...
Reports published in Vision Research during the late years of the 20th century described surprising effects of long-term sensitivity improvement with some basic visual tasks as a result of training. These improvements, found in adult human observers, were highly specific to simple visual features, such as location in the visual field, spatial-frequency, local and global orientation, and in some cases even the eye of origin. The results were interpreted as arising from the plasticity of sensory brain regions that display those features of specificity within their constituting neuronal subpopulations. A new view of the visual cortex has emerged, according to which a degree of plasticity is retained at adult age, allowing flexibility in acquiring new visual skills when the need arises. Although this "sensory plasticity" interpretation is often questioned, it is commonly believed that learning has access to detailed low-level visual representations residing within the visual cortex. More recent studies during the last decade revealed the conditions needed for learning and the conditions under which learning can be generalized across stimuli and tasks. The results are consistent with an account of perceptual learning according to which visual processing is remodeled by the brain, utilizing sensory information acquired during task performance. The stability of the visual system is viewed as an adaptation to a stable environment and instances of perceptual learning as a reaction of the brain to abrupt changes in the environment. Training on a restricted stimulus set may lead to perceptual overfitting and over-specificity. The systemic methodology developed for perceptual learning, and the accumulated knowledge, allows us to explore issues related to learning and memory in general, such as learning rules, reinforcement, memory consolidation, and neural rehabilitation. A persistent open question is the neuro-anatomical substrate underlying these learning effects.
... Three times the critical number of trials provides an estimate the number of trials beyond which stereothresholds level off and no longer improve. These values are 3168 ± 384 for the sparse stereograms and 3771 ± 795 for the dense stereograms, which are consistent with other studies of perceptual learning of depth discrimination using RDSs (Gantz, Chung & Harwerth, 2007a,Westheimer, 2001). The high variability obtained in the group data for the critical number of training trials (see also the individual functions shown inFigure 2) is consistent with the large inter-individual variability of perceptual learning that is reported by others (Schmitt, Kromeier, Bach & Kommerell, 2002). ...
Several previous studies reported differences when stereothresholds are assessed with local-contour stereograms vs. complex random-dot stereograms (RDSs). Dissimilar thresholds may be due to differences in the properties of the stereograms (e.g. spatial frequency content, contrast, inter-element separation, area) or to different underlying processing mechanisms. This study examined the transfer of perceptual learning of depth discrimination between local and global RDSs with similar properties, and vice versa. If global and local stereograms are processed by separate neural mechanisms, then the magnitude and rate of training for the two types of stimuli are likely to differ, and the transfer of training from one stimulus type to the other should be minimal. Based on previous results, we chose RDSs with element densities of 0.17% and 28.3% to serve as the local and global stereograms, respectively. Fourteen inexperienced subjects with normal binocular vision were randomly assigned to either a local- or global- RDS training group. Stereothresholds for both stimulus types were measured before and after 7700 training trials distributed over 10 sessions. Stereothresholds for the trained condition improve for approximately 3000 trials, by an average of 0.36+/-0.08 for local and 0.29+/-0.10 for global RDSs, and level off thereafter. Neither the rate nor the magnitude of improvement differ statistically between the local- and global-training groups. Further, no significant difference exists in the amount of improvement on the trained vs. the untrained targets for either training group. These results are consistent with the operation of a single mechanism to process both local and global stereograms.
... However, it is not surprising, as several experimental observations suggest that eccentric reading can be improved with training. Westheimer (2001) demonstrated that learning in peripheral vision is taskspecific. Improvements were observed for stereoscopic, orientation, vernier acuity, bisection, and time discrimination tasks, but not for resolution or Landolt C acuities. ...
Le but de ce projet est de déterminer les caractéristiques minimales pour qu'une prothèse rétinienne permette une vision utile. Pour aborder ce sujet, nous avons simulé une vision prosthétique chez des sujets normaux. Les résultats suggèrent que 400-500 phosphènes, arrangés rétinotopiquement sur une surface rétinienne de 3x2 mm², sont nécessaires pour coder l'information visuelle pour la majorité des tâches. Le champ visuel effectif doit cependant être optimisé (en utilisant par exemple un zoom) pour chaque tâche. Un champ visuel effectif de 2°x1.4° est nécessaire pour permettre une lecture efficace. Un champ visuel effectif d'environ 16°x12° permet une performance efficace pour les tâches de coordination visuomotrice. Enfin, les tâches de mobilité requièrent un champ visuel effectif d'environ 33°x23°. Si ces implants devaient être placés loin de la fovéa, une période d'apprentissage relativement longue sera nécessaire à l'obtention des performances optimales.
... For example, learning has been observed for vernier acuity and bisection, for stereoscopic orientation and time discrimination tasks, but not for resolution tasks or Landolt C acuities (e.g. Beard, Levi, & Reich, 1995;Crist, Kapadia, Westheimer, & Gilbert, 1997;Schoups, Vogels, & Orban, 1995;Westheimer, 2001). Taken together these findings imply that spatial visual functions, which rely on important processing in higher cortical areas, can be improved by training in the visual periphery. ...
Simulations of artificial vision were performed to assess “minimum requirements for useful artificial vision”. Retinal prostheses will be implanted at a fixed (and probably eccentric) location of the retina. To mimic this condition on normal observers, we projected stimuli of various sizes and content on a defined stabilised area of the visual field. In experiment 1, we asked subjects to read isolated 4-letter words presented at various degrees of pixelisation and at various eccentricities. Reading performance dropped abruptly when the number of pixels was reduced below a certain threshold. For central reading, a viewing area containing about 300 pixels was necessary for close to perfect reading (>90% correctly read words). At eccentricities beyond 10°, close to perfect reading was never achieved even if more than 300 pixels were used. A control experiment using isolated letter recognition in the same conditions suggested that lower reading performance at high eccentricity was in part due to the “crowding effect”. In experiment 2, we investigated whether the task of eccentric reading under such specific conditions could be improved by training. Two subjects, naive to this task, were trained to read pixelised 4-letter words presented at 15° eccentricity. Reading performance of both subjects increased impressively throughout the experiment. Low initial reading scores (range 6%-23% correct) improved impressively (range 64%-85% correct) after about one month of training (about 1 h/day). Control tests demonstrated that the learning process consisted essentially in an adaptation to use an eccentric area of the retina for reading. These results indicate that functional retinal implants consisting of more than 300 stimulation contacts will be needed. They might successfully restore some reading abilities in blind patients, even if they have to be placed outside the foveal area. Reaching optimal performance may, however, require a significant adaptation process.
... If an observer does not fully understand the task and the required response, then training could lead to a general adaptation to the overall experimental situation, the kind of stimulus presentation, how to respond and so forth. Although this factor has a strong impact especially in animal research where simple demonstrations and verbal explanations are not possible (Westheimer, 2001), it is less likely in the present case. Our human observers were carefully instructed, and the demanded responsepressing a key on the keyboardis surely an easy and over-learned motor response. ...
Gottschaldt's (1926) study on embedded figures serves as the foundation for the claim that Gestalt factors limit the impact of experience on perception. The present experiments studied the effects of practice and transfer on the detection of embedded figures. Experiment 1 showed the clear effects of practice that might have been due to recognition. Moreover, it demonstrated the effects of transfer, thus reducing the potential impact of recognition. Experiment 2 replicated practice effects and extended them to other types of part and whole figures. It was conducted to evaluate the specificity of the transfer in more detail. In Experiments 1 and 2, transfer between old and new items only occurred when either the figures and their basic elements were of the same kind or the same search strategies could be used. Thus, perceptual abilities improve by training even when Gestalt factors are at work. Possible sources of these improvements are discussed with special attention to a distinction between figural and procedural aspects.
... The ratios are similar across all categories (ANOVA: F (df=2,33) = 1.38, p = 0.26). Because these ratios are all very close to 1 (no change in critical print size), the results show that there is minimal, if any, reduction in the critical print size, consistent with the finding that resolution acuity in peripheral vision does not improve with training (Westheimer, 2001). Practically, our results imply that training on a letter-recognition task may help observers read faster, but it does not improve observers' ability to resolve fine details.Fig. ...
Visual-span profiles are plots of letter-recognition accuracy as a function of letter position left or right of the midline. Previously, we have shown that contraction of these profiles in peripheral vision can account for slow reading speed in peripheral vision. In this study, we asked two questions: (1) can we modify visual-span profiles through training on letter-recognition, and if so, (2) are these changes accompanied by changes in reading speed? Eighteen normally sighted observers were randomly assigned to one of three groups: training at 10 degrees in the upper visual field, training at 10 degrees in the lower visual field and a no-training control group. We compared observers' characteristics of reading (maximum reading speed and critical print size) and visual-span profiles (peak amplitude and bits of information transmitted) before and after training, and at trained and untrained retinal locations (10 degrees upper and lower visual fields). Reading speeds were measured for six print sizes at each retinal location, using the rapid serial visual presentation paradigm. Visual-span profiles were measured using a trigram letter-recognition task, for a letter size equivalent to 1.4x the critical print size for reading. Training consisted of the repeated measurement of 20 visual-span profiles (over four consecutive days) in either the upper or lower visual field. We also tracked the changes in performance in a sub-group of observers for up to three months following training. We found that the visual-span profiles can be expanded (bits of information transmitted increased by 6 bits) through training with a letter-recognition task, and that there is an accompanying increase (41%) in the maximum reading speed. These improvements transferred, to a large extent, from the trained to an untrained retinal location, and were retained, to a large extent, for at least three months following training. Our results are consistent with the view that the visual span is a bottleneck on reading speed, but a bottleneck that can be increased with practice.
... An alternative hypothesis is that the improvement in performance might have a more trivial explanation. For example, improvement might be in part attributed to highlevel cognitive task learning [what Westheimer (2001) refers to as " instrument learning " ]. Our 3AFC " odd man out " paradigm requires no memory and has little cognitive load (even a 4-year-old can do the task). ...
Adult amblyopes can improve positional acuity through practice; however, the neural mechanisms underlying this improvement are still not clear. In this study, seven adult amblyopes repeatedly practiced a position discrimination task in the presence of positional noise. We found that six of the seven showed systematic and significant improvements in position acuity that were both eye and orientation specific. Using a position-averaging model, we were able to parse the improvement in performance with practice into two factors: improvement in sampling efficiency and reduction of equivalent input noise. Three of the seven showed improved efficiency with no change in equivalent noise, two showed a significant reduction in equivalent noise with no change in efficiency, and one showed both improved efficiency and reduced equivalent noise. Interestingly, all observers showed substantial improvement in visual acuity, and one observer showed substantial improvement in stereoacuity. Three observers were also tested on a counting task, and all three improved after practicing positional discrimination. Our results reveal the mechanisms underlying perceptual learning in amblyopic vision, and may provide a basis for developing more effective and efficient strategies for the treatment of amblyopia.
... If this is the case, these patients might show more facilitation than normally-sighted observers using a peripheral target at the same eccentricity. Such increased facilitation could account for the earlier reports of improved spatial vision in the near-periphery with training (reviewed by Westheimer, 1998). Westheimer (1998) found no long-term learning effect for resolution threshold or Landolt-C acuity, while noting that other tasks that, presumably, required higher-level processing do demonstrate long-term learning. ...
Foveal detection of a Gabor patch (target) is facilitated by collinear, displaced high-contrast flankers. Polat and Sagi reported that the same phenomenon occurred in the periphery, but no data were presented [Proc. Natl. Acad. Sci. 91 (1994) 1206]. Others have found no facilitation in a limited number of conditions tested. To resolve this apparent conflict, we measured lateral facilitation in the near-periphery using a range of stimulus parameters. We found facilitation for a range of target-flanker distances for peripheral eccentricities up to 6 degrees , but the magnitude of the effect was less than found in central vision. Facilitation varied across subjects and with spatial frequency. Flanker contrast had no effect over the range evaluated (10-80%). Equal facilitation was found for two global arrangements of the stimulus pattern. Facilitation was found using a temporal, but not a spatial two-alternative forced-choice paradigm, accounting for the different results among previous studies. This finding supports previous indications of the role of attention in altering such facilitation. The value of facilitation from lateral interactions for persons with central vision impairment, who have to shift their attention to a peripheral locus constantly, needs to be examined.
... ent (a reduction in contrast threshold) in their letter identification performance at the trained eccentric retinal location (10° eccentricity in the inferior visual field). This finding is consistent with that of Chung et al. (2004) in which percentcorrect performance of letter identification at 10° eccentricity was shown to improve with training. Westheimer (2001) showed that peripheral Landolt C acuity does not benefit from training. While these results may seem to be at odds with ours, there are several reasons why they might be expected to differ. First, we measured contrast thresholds for identifying letters from amongst a large array (26); a more demanding task than identifying the orientati ...
Performance for a variety of visual tasks improves with practice. The purpose of this study was to determine the nature of the processes underlying perceptual learning of identifying letters in peripheral vision. To do so, we tracked changes in contrast thresholds for identifying single letters presented at 10 degrees in the inferior visual field, over a period of six consecutive days. The letters (26 lowercase Times-Roman letters, subtending 1.7 degrees) were embedded within static two-dimensional Gaussian luminance noise, with rms contrast ranging from 0% (no noise) to 20%. We also measured the observers' response consistency using a double-pass method on days 1, 3 and 6, by testing two additional blocks on each of these days at luminance noise of 3% and 20%. These additional blocks were the exact replicates of the corresponding block at the same noise contrast that was tested on the same day. We analyzed our results using both the linear amplifier model (LAM) and the perceptual template model (PTM). Our results showed that following six days of training, the overall reduction (improvement across all noise levels) in contrast threshold for our seven observers averaged 21.6% (range: 17.2-31%). Despite fundamental differences between LAM and PTM, both models show that learning leads to an improvement of the perceptual template (filter) such that the template is more capable of extracting the crucial information from the signal. Results from both the PTM analysis and the double-pass experiment imply that the stimulus-dependent component of the internal noise does not change with learning.
... In the two previous and the current studies, training in some basic psychophysical task also improved visual acuity. On the other hand, ''direct'' training in a letter acuity task did not produce considerable improvements in peripheral visual acuity (Westheimer, 2001). Perhaps training in basic psychophysical tasks improved processing/coding of basic visual features that in turn facilitated performance in the high-level visual acuity task, while training of visual acuity may not have allowed direct access to some of the basic visual features (Ahissar & Hochstein, 1996;Dosher & Lu, 1998). ...
To evaluate the effects of perceptual learning on contrast-sensitivity function and visual acuity in adult observers with amblyopia, 23 anisometropic amblyopes with a mean age of 19.3 years were recruited and divided into three groups. Subjects in Group I were trained in grating detection in the amblyopic eye near pre-training cut-off spatial frequency. Group II received a training regimen of repeated contrast-sensitivity function measurements in the amblyopic eye. Group III received no training. We found that training substantially improved visual acuity and contrast-sensitivity functions in the amblyopic eyes of all the observers in Groups I and II, although no significant performance improvement was observed in Group III. For observers in Group I, performance improvements in the amblyopic eyes were broadly tuned in spatial frequency and generalized to the fellow eyes. The latter result was not found in Group II. In a few cases tested, improvements in visual acuity following training showed about 90% retention for at least 1 year. We concluded that the visual system of adult amblyopes might still retain substantial plasticity. Perceptual learning shows potential as a clinical tool for treating child and adult amblyopia.
... Interestingly, the effects of deprivation or training in adult visual cortex seem to be fairly limited, suggesting that the correlation between visual acuity and cortical size found in our previous study may be primarily established in development. Visual hyperacuity, but not visual acuity, was shown to improve with practice (Westheimer 2001 ). Similarly, psychophysical evidence demonstrates an improvement in hyperacuity thresholds with training (Sathian and Zangaladze 1998; Grant et al. 2000). ...
Behavioral tactile discrimination thresholds were compared with functional magnetic resonance imaging measurements of cortical finger representations within primary somatosensory cortex (S1) for 10 human subjects to determine whether cortical magnification in S1 could account for the variation in tactile hyperacuity thresholds of the fingers. Across 10 subjects, the increase in tactile thresholds from the index finger to the little finger correlated with the decrease in cortical representation across fingers in S1. Additionally, representations of the fingers within S1, in Brodmann areas 3b and 1, were also correlated with the thresholds. These results suggest that tactile hyperacuity is largely determined by the cortical representation of the fingers in S1.
... Optometrists assumed that any improvements in vision achieved through training would transfer automatically to improved sports performance. However, it was shown that specifically optometric tests such as visual acuity do not improve with training in general [4]. In sports research, Abernethy and Wood [5] showed that visual acuity and stereopsis in a prepost training paradigm did not lead to improvements in optometric vision beyond those resulting from familiarity. ...
In tennis, as in many disciplines of sport, fine spatio-temporal resolution is required to reach optimal performance. While many studies on tennis have focused on anticipatory skills or decision making, fewer have investigated the underlying visual perception abilities. In this study, we used a battery of seven visual tests that allowed us to assess which kind of visual information processing is performed better by tennis players than other athletes (triathletes) and non-athletes. We found that certain time-related skills, such as speed discrimination, are superior in tennis players compared to non-athletes and triathletes. Such tasks might be used to improve tennis performance in the future.
Choroideremia (CHM) is an X-linked recessive form of hereditary retinal degeneration, which preserves only small islands of central retinal tissue. Previously, we demonstrated the relationship between central vision and structure and population receptive fields (pRF) using functional magnetic resonance imaging (fMRI) in untreated CHM subjects. Here, we replicate and extend this work, providing a more in-depth analysis of the visual responses in a cohort of CHM subjects who participated in a retinal gene therapy clinical trial. fMRI was conducted in six CHM subjects and six age-matched healthy controls (HC’s) while they viewed drifting contrast pattern stimuli monocularly. A single ∼3-minute fMRI run was collected for each eye. Participants also underwent ophthalmic evaluations of visual acuity and static automatic perimetry (SAP). Consistent with our previous report, a single ∼ 3 min fMRI run accurately characterized ophthalmic evaluations of visual function in most CHM subjects. In-depth analyses of the cortical distribution of pRF responses revealed that the motion-selective regions V5/MT and MST appear resistant to progressive retinal degenerations in CHM subjects. This effect was restricted to V5/MT and MST and was not present in either primary visual cortex (V1), motion-selective V3A or regions within the ventral visual pathway. Motion-selective areas V5/MT and MST appear to be resistant to the continuous detrimental impact of CHM. Such resilience appears selective to these areas and may be mediated by independent retina-V5/MT anatomical connections that bypass V1. We did not observe any significant impact of gene therapy.
The human retina is the most susceptible part of the human visual system to the radiation effects of space flight. Unfortunately, it is difficult to dissect the visual system in a non-invasive way using conventional methods in order to quantify such radiation effects and attempt to mitigate them. The present study employs the sandwich model to take advantage of a nonlinear stage in the visual system and measure separately the stages before, during and after the nonlinearity. If the results can be correlated to some part of retinal function then it would provide a new testing method for the effects of retinal trauma e.g. due to radiation in space. A five-channel Maxwellian-view system was used to generate contrast-modulated sinusoidally-flickering stimuli. The subjects adjusted the modulation of the signal in order to set the thresholds for the perception of flicker and distortion in hue. The temporal properties of the visual system prior to the nonlinearity were measured by varying the carrier frequency and those after the nonlinearity were measured by varying the contrast-modulation frequency. Subjects measured the input-output function of the nonlinearity by matching the distortion in the contrast-modulated stimulus with up to three sinusoidally-flickering monochromatic lights, which allowed the measurement of colour change, brightness enhancement and desaturation. The results were compared with existing studies on contrast-modulated gratings and contrast perception. The results show that the pre-nonlinearity stage can be described well by photoreceptor function. In addition, the results can be modelled by a single nonlinear stage underlying colour, brightness and saturation changes. This nonlinearity is concluded to exist early within the retina prior to the separation of the chromatic and luminance channels. The method thai: is used in this study is concluded to be effective in diagnosing significant changes in retinal function that can be caused by trauma such as space radiation. Based on the conclusions, this study provides recommendations on monitoring human vision during spaceflyyight using a non-invasive and efficient test.
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.
The two volumes in this book contain a survey of knowledge about the visual perception of the three-dimensional visual world. The primary interest is biological vision. Machine vision and computational models are mentioned only where they contribute to an understanding of the living system. This book grew out of Howard and Rogers' Binocular Vision and Stereopsis, which was published in 1995. The topics have been reorganized, many new sections and chapters have been added, and the literature review has been brought up to date. The present book covers all aspects of depth perception, including a review of monocular cues to depth. Volume I deals with the basic visual mechanisms used in depth perception. Volume II deals with the perception of three-dimensional space.
We investigated perceptual learning in 85 healthy adults with stereoscopic information contained in dynamic random dot stimuli or with vernier targets. Stimuli were flashed simultaneously at 8 locations at an eccentricity of 1.15 degrees or 2.3 degrees, and subjects had to detect a target in an "8 Alternative Forced Choice" task. For training at a given eccentricity stimuli at the other eccentricity served as a "no training" control. Viewing of visual targets for about 20 minutes resulted in a significant increase of discrimination performance only for the trained stimuli (significant interaction between training and time). Thus, learning is position specific: improved performance can be demonstrated only when test and training stimuli are presented to the same retinal areas. In combination with our earlier electrophysiological results, this study illustrates how perceptual training induces stimulus and visual field specific neural plasticity in adults.
We summarize the various strands of research on peripheral vision and relate them to theories of form perception. After a historical overview, we describe quantifications of the cortical magnification hypothesis, including an extension of Schwartz's cortical mapping function. The merits of this concept are considered across a wide range of psychophysical tasks, followed by a discussion of its limitations and the need for non-spatial scaling. We also review the eccentricity dependence of other low-level functions including reaction time, temporal resolution, and spatial summation, as well as perimetric methods. A central topic is then the recognition of characters in peripheral vision, both at low and high levels of contrast, and the impact of surrounding contours known as crowding. We demonstrate how Bouma's law, specifying the critical distance for the onset of crowding, can be stated in terms of the retinocortical mapping. The recognition of more complex stimuli, like textures, faces, and scenes, reveals a substantial impact of mid-level vision and cognitive factors. We further consider eccentricity-dependent limitations of learning, both at the level of perceptual learning and pattern category learning. Generic limitations of extrafoveal vision are observed for the latter in categorization tasks involving multiple stimulus classes. Finally, models of peripheral form vision are discussed. We report that peripheral vision is limited with regard to pattern categorization by a distinctly lower representational complexity and processing speed. Taken together, the limitations of cognitive processing in peripheral vision appear to be as significant as those imposed on low-level functions and by way of crowding.
In visual search one tries to find the currently relevant item among other, irrelevant items. In the present study, visual search performance for complex objects (characters, faces, computer icons and words) was investigated, and the contribution of different stimulus properties, such as luminance contrast between characters and background, set size, stimulus size, colour contrast, spatial frequency, and stimulus layout were investigated. Subjects were required to search for a target object among distracter objects in two-dimensional stimulus arrays. The outcome measure was threshold search time, that is, the presentation duration of the stimulus array required by the subject to find the target with a certain probability. It reflects the time used for visual processing separated from the time used for decision making and manual reactions. The duration of stimulus presentation was controlled by an adaptive staircase method. The number and duration of eye fixations, saccade amplitude, and perceptual span, i.e., the number of items that can be processed during a single fixation, were measured. It was found that search performance was correlated with the number of fixations needed to find the target. Search time and the number of fixations increased with increasing stimulus set size. On the other hand, several complex objects could be processed during a single fixation, i.e., within the perceptual span. Search time and the number of fixations depended on object type as well as luminance contrast. The size of the perceptual span was smaller for more complex objects, and decreased with decreasing luminance contrast within object type, especially for very low contrasts. In addition, the size and shape of perceptual span explained the changes in search performance for different stimulus layouts in word search. Perceptual span was scale invariant for a 16-fold range of stimulus sizes, i.e., the number of items processed during a single fixation was independent of retinal stimulus size or viewing distance. It is suggested that saccadic visual search consists of both serial (eye movements) and parallel (processing within perceptual span) components, and that the size of the perceptual span may explain the effectiveness of saccadic search in different stimulus conditions. Further, low-level visual factors, such as the anatomical structure of the retina, peripheral stimulus visibility and resolution requirements for the identification of different object types are proposed to constrain the size of the perceptual span, and thus, limit visual search performance. Similar methods were used in a clinical study to characterise the visual search performance and eye movements of neurological patients with chronic solvent-induced encephalopathy (CSE). In addition, the data about the effects of different stimulus properties on visual search in normal subjects were presented as simple practical guidelines, so that the limits of human visual perception could be taken into account in the design of user interfaces. Visuaalisella haulla tarkoitetaan jonkin kohteen näönvaraista etsimistä muiden ärsykkeiden joukosta. Visuaalista hakua on esimerkiksi se, kun etsitään tiettyä henkilöä ihmisjoukosta, lyijykynää työpöydältä tai vaikkapa kuvaketta tietokoneen näytöltä. Visuaalinen haku on keskeistä monissa työtehtävissä sekä liikenteessä. Erilaiset käyttöliittymät ja niiden sisältämät ärsykkeet, kuten tekstit ja symbolit, tulisi suunnitella ihmisen näköjärjestelmän kannalta mahdollisimman helposti havaittaviksi. Koska näöntarkkuus alenee ääreisnäössä voimakkaasti, kykenemme havaitsemaan yksityiskohtia vain pieneltä näkökentän alueelta. Luonnollisessa tilanteessa tarkimman näön aluetta siirretäänkin kiinnostuksen kohteesta toiseen silmiä liikuttamalla. Silmänliikkeitä mittaamalla saadaan tietoa visuaalisen haun tehokkuuteen vaikuttavista seikoista. Tässä tutkimuksessa selvitettiin monimutkaisten objektien, kuten kasvokuvien, kirjainten, kuvakkeiden sekä sanojen hakua. Haun aikana tapahtuvat silmänliikkeet rekisteröitiin. Tutkimuksessa pyrittiin selvittämään, miten erilaiset ärsykkeiden ominaisuudet, kuten vaaleus- ja värikontrasti, lukumäärä ja asettelu vaikuttavat visuaalisen haun tehokkuuteen ja silmänliikkeisiin. Erityisesti tutkittiin havaintokentän kokoa, eli sitä, kuinka paljon eri tyyppistä informaatiota yhdellä katseen kohdistuksella pystytään käsittelemään. Yhdessä osatutkimuksessa selvitettiin myös liuotinaineaivosairaudesta kärsivien potilaiden visuaalista hakua. Tutkimuksessa havaittiin, että yhdellä katseen kohdistuksella kyetään prosessoimaan useita ärsykkeitä. Sana tai kasvokuva löydettiin hakutilanteessa yhdellä katseen kohdistuksella 4 - 5 muun sanan tai kasvon joukosta, kun taas yksinkertaisempia kirjaimia ja kuvakkeita kyettiin prosessoimaan kerralla jopa 30 - 40. Ärsykkeiden ominaisuudet vaikuttivat havaintokentän kokoon: esimerkiksi vaaleuskontrastin alentuessa yhdellä katseen kohdistuksella havaittiin vähemmän ärsykkeitä ja vastaavasti hakuaika kasvoi. Liuotinaineaivosairaudesta kärsivien potilaiden havaintokenttä oli kapeutunut ja visuaalinen haku hidastunut. Tulokset osoittavat, että visuaalisen haun tehokkuus eri tilanteissa on yhteydessä havaintokentän kokoon, joka puolestaan vaikuttaa tarvittavien katseen kohdistusten lukumäärään. Erityisesti verkkokalvon rakenteesta johtuva näöntarkkuuden aleneminen ääreisnäössä näyttää määräävän havaintokentän kokoa ja siten selittävän visuaalisen haun tehokkuutta eri tilanteissa. Myöhemmän tason tiedonkäsittely, kuten oppiminen ja kyky suunnata tarkkaavaisuutta havaintokentässä, näyttävät vaikuttavan haun tehokkuuteen välillisemmin näiden varhaisemman tason rajoitusten sallimissa puitteissa. Tutkimuksen liitteeksi on koottu muutamia yksinkertaisia ohjeita siitä, kuinka käyttöliittymissä tapahtuvaa visuaalista hakua voidaan tehostaa suunnittelemalla niiden visuaaliset ärsykkeet riittävän suuriksi ja selkeästi erottuviksi, jotta havaintokentän koko olisi mahdollisimman suuri.
The visual benefit of correcting high-order aberrations may not be fully realized due to neural mechanisms that compensate for the aberrations of the eye. We examined the extent to which these neural mechanisms might be susceptible to perceptual learning in an adaptive optics (AO)-corrected test of visual resolution. Visual resolution was measured in an adaptive optics scanning laser ophthalmoscope (AOSLO) in 3 conditions: (1) low-order correction (defocus and astigmatism) without AO, (2) 3-mm pupil with AO correction, and (3) 5.81-mm pupil with AO correction. Measurements were made on both eyes in all three conditions before training. Subjects underwent 5 days of monocular training in both AO-corrected conditions and were retested in all three conditions in both eyes after training. The range of minimum angle of resolution (MAR) for each condition was: (1) without AO: 0.53-0.95 arcmin, (2) AO 3-mm pupil: 0.33-0.6 arcmin, and (3) AO 5.81-mm pupil: 0.36-0.56 arcmin. AO correction provided an immediate and significant improvement in visual resolution. There was no significant difference in resolution when correcting aberrations over a 5.81-mm pupil versus a 3-mm pupil. Training on this task provided a minimal improvement in performance. Adaptation to aberrations did not hinder AO correction from providing an immediate visual benefit.
Perceptual learning in the peripheral visual field was studied in 24 adults using vernier targets. The aim was to relate perceptual improvements to changes of electrical brain activity. Thresholds were measured before, during, and after training, and on the next day. During training, the subjects passively looked at suprathreshold targets, and EEG activity was recorded from 30 electrodes over the occipital brain areas. Mean evoked potentials were computed for the first and second block of 1200 stimulus presentations, and the scalp topography of visual evoked potential (VEP) activity was analysed. Only for the stimulated area, training resulted initially in increased thresholds that, however, decreased significantly after consolidation. Electrical brain activity displayed smaller field strength and altered topography after training. Some of the effects were caused by habituation or adaptation to the training stimuli resulting in less efficient neurophysiological processing. The topographical changes indicate that different neuronal elements were activated after perceptual learning.
The classical account of visual acuity incorporates optical, anatomical and physiological components. Knowledge of these helps to put into perspective recent scientific advances that have clinical implications. They include the development of active optical devices for neutralising aberrations in peripheral zones of the pupil, with the potential of improving the eye's imagery beyond its customary limit. In another application of modern optical and electronic instrumentation, the quality of retinal images in young eyes has been compared with that in the aged. This has led to a better understanding of the choice of contrast patterns to enhance visual acuity in older patients.
We compared perceptual learning in 16 psychophysical studies, ranging from low-level spatial frequency and orientation discrimination tasks to high-level object and face-recognition tasks. All studies examined learning over at least four sessions and were carried out foveally or using free fixation. Comparison of learning effects across this wide range of tasks demonstrates that the amount of learning varies widely between different tasks. A variety of factors seems to affect learning, including the number of perceptual dimensions relevant to the task, external noise, familiarity, and task complexity.
A patient can demonstrate a poor stereoscopic test or task performance for reasons inherent within the test/task itself for reasons dependent on normal physiology common to all human subjects, and also for reasons that are outside of normal physiology and are unique or idiosyncratic to a particular person's visual system. This article reviews the literature for the first two reasons, but emphasizes the pathophysiology involved in the idiosyncratic and abnormal reasons.
Using control systems analysis, it is shown that deficits in stereoscopic performance can be explained by reference to the quantitative aspect of stereoscopic threshold and qualitative aspects such as speed of response, reliability-robustness, and strength of percept. The relationship between fixation disparity and stereopsis is seen to be central to this explanation.
Proceeding from diagnosis to treatment, control systems analysis offers physiologically based explanations for the corrective procedures necessary to ameliorate abnormal conditions. Several topics for applied research in stereopsis are suggested.
Immediately after focal retinal lesions, receptive fields (RFs) in primary visual cortex expand considerably, even when the retinal damage is limited to the photoreceptor layer. The time course of these changes suggests that mere lack of stimulation in the vicinity of the RF accompanied by stimulation in the surrounding region causes the RF expansion. While recording from single cells in cat area 17, we simulated this pattern of stimulation with a pattern of moving lines in the visual field, masking out an area covering the RF of the recorded cell, thereby producing an "artificial scotoma." Over approximately 10 min this masking resulted in a 5-fold average expansion in RF area. Stimulating the RF center caused the field to collapse in size, returning to near its original extent; reconditioning with the masked stimulus led to RF reexpansion. Stimulation in the surrounding region was required for the RF expansion to occur--little expansion was seen during exposure to a blank screen. We propose that the expansion may account for visual illusions, such as perceptual fill-in of stabilized images and illusory contours and may constitute the prodrome of altered cortical topography after retinal lesions. These findings support the idea that even in adult animals RFs are dynamic, capable of being altered by the sensory context.
The human visual system is normally very good at determining the relative positions of objects in space, but under certain conditions contextual influences can cause significant errors in this process. We studied spatial localization around an artificial scotoma, a small mask that occludes part of the visual field while a dynamic pattern is shown over a surrounding region, and found that the ability to determine the position of short line segments was strongly biased toward the interior of the scotoma. We attribute this "shift" or misassignment of position to receptive field (RF) expansions within the artificial scotoma as seen in recent physiological studies. Furthermore, our findings show that this shift begins within 1 sec of stimulus presentation, suggesting that RFs are constantly altered by their local context and that these dynamics are a part of normal vision.
Discrimination of simple visual attributes can improve significantly with practice. We have trained human observers to perform peripherally presented tasks involving the localization of short line segments and examined the specificity of the learning for the visual location, orientation, and geometric arrangement of the trained stimulus. Several weeks of training resulted in dramatic threshold reductions. The learning was specific for the orientation and location of the trained stimulus, indicating the involvement of the earliest cortical stages in the visual pathway where the orientation and location of stimuli are mapped with highest resolution. Furthermore, improvement was also specific for both the configuration of the trained stimulus and the attribute of the stimulus that was under scrutiny during training. This degree of specificity suggests that the learning cannot be achieved by cortical recruitment alone, as proposed in current models, but is likely to involve a refinement of lateral interactions within the cortex and possibly a gating of lower level changes by attentional mechanisms.
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.
The effect of training on an observer's ability to detect the misalignment of three points, a hyperacuity, and to resolve a six-line grating was studied in a transfer-of-training design with observers (4 in each of two experiments) who were experienced in making psychophysical judgments of other visual stimuli. The transfer-of-training design enabled us to look for any training-based improvement. Long periods of training produced no statistically significant improvement in performance under any condition. There were small practice-based improvements, but the primary patterns indicated threshold fluctuation rather than improvement. We interpret the results to indicate that the neural mechanisms underlying three-point alignment and grating discrimination, like those for gap bisection (Klein & Levi, 1985), are not malleable to any significant extent.
Disparity thresholds were obtained for a single point target when surrounded by a hexagonal array of comparison point targets. The experiment was carried out in two observers at the fovea and at retinal eccentricities of 3, 6, and 9 degrees. In each case the array diameter for best stereoacuity was determined. Care was taken that the data represented the optimum performance of the observer for the level of training in peripheral stereo tasks and also for the distance from the horopter. The results show a steady rise with eccentricity both for the stereothreshold and for the minimum target separations needed for uncrowded stereo performance. Both increase by a factor of about 10 between the fovea and the 9 degrees periphery, where a clear zone of at least 2 degrees diameter between test and comparison targets is needed for best stereoacuity.
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 improvement in stereoacuity of two inexperienced, normal subjects was compared at foveal and at 2.5 degrees and 5 degrees peripheral target locations as a function of practice. Outlines of two squares differing only in binocular disparity were used as test stimuli and estimates of stereoacuity were obtained by application of the method of constant stimuli with feedback. The peripheral thresholds of both subjects improved 60-80% over the course of the first 3000-4000 responses at each stimulus location. Foveal improvement followed an identical time-course with a 73% improvement in one subject and only 23% in the other. This difference was reflected in the peripheral/foveal threshold ratios of the two subjects and underlines the necessity of ensuring the stability of thresholds. Stereoacuity measurements were also obtained using several different square separations at the fovea and at 2.5 degrees, 5 degrees and 10 degrees peripheral locations along the horizontal and vertical retinal meridians of two other normal subjects. Practice-stabilized disparity thresholds using optimal target separations revealed a steeper deterioration between the fovea and 2.5-5 degrees eccentricities than did measurements of the same subjects' minimum angles of resolution (MAR). The decrease of optimal stereoacuity at the more peripheral test locations was more gradual than has been previously reported but was not clearly related to that of the MAR.
Measurements for normal human photopic vision in the fovea and in 2.5, 5 and 10 degree positions in the periphery reveal that hyperacuity thresholds rise faster with eccentricity than visual resolution thresholds. On the other hand, the retinal distances over which hyperacuity detection remains optimal show a much slower rise. At 10 degrees eccentricity, hyperacuity thresholds are 10 times higher visual resolution thresholds 4-5 times higher, and optimal processing distances for hyperacuity only 2-3 times larger than at the fovea. Psychophysical procedures do not, therefore, provide a single unambiguous measure for the changes of spatial grain across the visual field.
The present study tests the effects of practice on parafoveal vernier and resolution acuity. By measuring task specificity, transfer of training to other retinal locations in the trained eye and transfer of training to the untrained eye, we directly address whether improvement on these tasks is the result of changes in the underlying physiological processes or simply the development of new cognitive strategies. We found that: (1) significant learning can occur for both vernier and resolution acuity in many (but not all) individuals; (2) there were significant individual differences in the degree and time-course of learning: (3) learning transfers to the untrained task; and (4) learning transfers to the other eye particularly when the visual pathway leads to the trained hemisphere. These results suggest that both physiological and cognitive processes contribute to the improvement seen after repetitive practice on these visual tasks.
1. Human perceptual learning in discrimination of the oblique orientation was studied using psychophysical methods. Subjects were trained daily to improve their ability to identify the orientation of a circular 2.5 deg diameter unidimensional noise field. Dramatic improvements in sensitivity to contour orientation occurred over a period of 15-20 days. The improved performance persisted for several months. Improvement was more evident between daily sessions than within sessions. This was partly due to fatigue interfering with the learning effect. Moreover, a consolidation period seemed to be required. 2. Improvement was restricted to the position of the stimulus being trained. This position dependency of the learning effect proved very precise. After training at a specific stimulus position, merely displacing the stimulus to an adjacent position caused a marked increase in thresholds. 3. No transfer of the training effect was observed between orientations. Following a shift of 90 deg away from the trained orientation, performance fell, even below the initial level. 4. We observed complete to almost complete transfer between the two eyes. 5. Our results suggest plastic changes at a level of the visual processing stream where input from both eyes has come together, but where generalization for spatial localization and orientation has not yet occurred.
We investigated fast improvement of visual performance in several hyperacuity tasks such as vernier acuity and stereoscopic depth perception in almost 100 observers. Results indicate that the fast phase of perceptual learning, occurring within less than 1 hr of training, is specific for the visual field position and for the particular hyperacuity task, but is only partly specific for the eye trained and for the offset tested. Learning occurs without feedback. We conjecture that the site of learning may be quite early in the visual pathway.
Ueber die Sehschaerfe (Form-sinn) an der Peripherie der Netzhaut. Pflueger's Archi6 f. Physi-ologie
- W Dobrowolsky
- A Gaine
Dobrowolsky, W., & Gaine, A. (1875). Ueber die Sehschaerfe (Form-sinn) an der Peripherie der Netzhaut. Pflueger's Archi6 f. Physi-ologie, 12, 411 – 432.
Sehschärfe (zentrale und periphere)
- Guillery
Guillery, H. (1931). Sehschä (zentrale und periphere). In A. Bethe, et al., Handbuch der normalen und pathologischen Physiologie, vol. XII/2 (pp. 745 – 811). Berlin: Springer.
Die Lehre vom Raumsinn
- Hoffmann
Hoffmann, F. P. (1920). Die Lehre vom Raumsinn. In Graefe/ Saemisch Handbuch der Augenheilkunde, vol. 3 (p. 70). Berlin: Springer.