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The mean accommodation response obtained with the even-order aberrations being corrected in the subjects' eyes Each data point represents the mean±standard deviation (SD) of each accommodation demand. The dashed line displays the theoretical accommodation response.
Source publication
AIM: To investigate the potential effect that odd and even-order monochromatic aberrations may have on the accommodation response of the human eye.
METHODS: Eight healthy subjects with astigmatism below 1 D, best corrected visual acuity 20/20 or better and normal findings in an ophthalmic examination were enrolled. An adaptive optics system was use...
Context in source publication
Context 1
... both figures, the obtained accommodation responses are similar indicating a similar accommodative lag for both conditions with reference to the theoretical line. Nevertheless, in Figure 4 the lag of accommodation is greater for the accommodation demands of 1.5, 3, 3.5 and 4.0 D, in comparison to that obtained in Figure 3 for the same accommodation demands. As already mentioned, a statistical analysis was conducted to analyze whether the measurements obtained for the three different conditions were statistically different or not. ...
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Citations
... To speed and bolster the process, the eye makes use of some cues that stimulate the accommodation mechanism to obtain a sharp retinal image [Bharadwaj, 2017]. These cues can come from the environment outside or from the eye itself [Moulakaki et al., 2017]. In the remainder of this subsection, we will list the cues related to image blur. ...
(Title in Spanish: Estudio de la dinámica de la acomodación y las fluctuaciones del desenfoque en el ojo humano)
The quality of the retinal image is the first, physical limiting factor of visual quality. Defocus is the most common source of blur leading to retinal image quality loss. It depends on the interrelationship between the eye's axial length, optical power, and distance to the object. Until it is lost with age, the eye has the ability to modify its optical power (i.e., to accommodate) to produce focused retinal images. Although this process is not instantaneous, accommodation is a fast and fairly accurate mechanism in most young subjects, that results in a clear vision. However, it has been suggested that myopia onset and/or progression may be related to alterations in the accommodative process that could upset the emmetropization process. On the other hand, even when steadily looking at an object at a fixed distance, the optical power of the eye fluctuates more or less randomly. It is unclear if this fluctuation is an undesired inability of the eye to keep a constant focus or may serve a purpose in the accommodative process. In any case, from an optical point of view, fast fluctuations of defocus would be expected to produce some kind of blurring in the retinal images. In those circumstances, a short integration time may allow the visual system to select the best focused position in the sequence to maximize visual quality. In this context, this thesis studies the effects of changes in focus, both discrete and progressive, aiming to discern how the visual system copes with them. Two separate experiments were carried out with an open-view Hartmann-Shack sensor measuring refraction and high-order aberrations in real time. First, the dynamics of the accommodative response was analyzed in realistic binocular viewing conditions, both for emmetropic subjects and myopes, when the fixation abruptly changed from far to near. In a second experiment, we studied the effect on contrast sensitivity of fast oscillations of defocus with different magnitudes and temporal frequencies, generated with a tunable lens attached to the system. During the accommodation mechanism, convergence of the eyeballs and miosis of the pupils accompany the change in optical power of the crystalline lens. There is extensive literature on these processes but relatively few studies simultaneously measuring all three of them in binocular vision. To the best of our knowledge, this is the first study of their combined dynamics in real time under realistic viewing conditions. Furthermore, it was performed in both myopic and emmetropic young individuals. Eighteen young subjects participated in the first experiment, with an average refractive error of -2.3 D and a range from -7.5 D to 0 D. Cylinder was below 2 D in all cases. Excluding refractive errors, no subject had a history of visual problems and all of them reached 20/20 VA or better in both eyes. They were corrected during the measurements. The near stimulus, located at 2.8 D, and far target, at 0.36 D, were both black-on-white Maltese crosses with 1.3° width. Each subject underwent 3 cycles of 6 target switching (far-near-far-near-far-near). The data was analyzed with a threshold method consisting of calculating the initial and final states for each studied variable and considering the central 80% of the variation. Several far-to-near response parameters were calculated, including accommodation speed and amplitude, convergence speed and amplitude, pupil miosis speed, and amplitude, high-order aberration RMS, spherical aberration, lag of accommodation, and duration of accommodation, convergence, and pupil miosis. Correlation analysis between refractive error and accommodation speed and of these two variables with various far-to-near response parameters was performed. The correlation analysis of refraction (spherical equivalent, SE) with accommodation dynamics parameters suggests that myopia mildly affects or is affected by accommodation. The lag of accommodation was found to be linked to refractive error (R = -0.57, p = 0.01). Moreover, the correlation between miosis speed and refractive error also had a p-value below 0.05 (R = -0.49, p = 0.04). In other words, myopes may tend to have less precise accommodation and slower pupil constriction. The correlation coefficients between SE and the rest of accommodation-related parameters were small, with p-values well above 0.05. A substantial, low-p-value correlation was found between accommodation speed and convergence speed (R = 0.48, p = 0.04). To the best of our knowledge, this finding has not been previously reported. Furthermore, the correlation was stronger between accommodation speed and convergence duration (R = 0.57, p = 0.01), which may reflect the differences in the dynamics of these two processes. In addition, there may be a correlation between accommodation speed and miosis amplitude since the p-value was below 0.05 (R = 0.47, p = 0.049). These analyses showed that slower accommodation might be a function of slow convergence and more evident pupil miosis. For the second part of the thesis, a faster HS sensor with a refresh rate of 60 Hz and higher sensitivity to 1050 nm IR light was developed. This sensor was employed to characterize an optically tunable lens both in the typical static mode and, for the first time to our knowledge, in dynamic mode. After calibration, the tunable lens was used to apply defocus oscillations during contrast sensitivity measurements. Different amplitudes and frequencies were induced in 5 young emmetropes with 20/20 or better VA and no previous history of visual troubles. The visual stimulus was a 12 c/deg Gabor patch of 1º angular diameter located at 3 m. It was tilted 10 degrees left or right and a two-choice forced-choice protocol was used to determine the contrast threshold for each oscillation condition. The measurements were carried out in monocular mode, and the subjects viewed the stimulus through the tunable lens with their right eye. The sinusoidal waves induced included combinations of 3 temporal frequencies, 5, 15, and 25 Hz, and 8 peak-to-valley defocus values, ranging from 0.15 to 3 D, presented in fully random order. To the best of our knowledge, the effect of this kind of fast fluctuations of defocus on visual quality has not been previously studied. Visual performance, in the form of contrast threshold, was found resilient to induced defocus oscillations. The data showed that only for fast, large variations (25 Hz, ± 1.5 D), there was a noticeable reduction in contrast sensitivity. This indicates that for the eye to clearly perceive visual stimuli, the retinal image only needs to be in focus for a short time. A quantitative model was developed for predicting the deterioration in retinal image quality due to periodic defocus fluctuations. For the amplitudes and frequencies of oscillation used in the experiment, the average PSF was calculated for several integration times and the loss in the ensuing MTF was computed. Comparison between experimental results and simulated data suggests that the eye may be integrating defocus blur at 10 to 20 ms intervals.
El proceso de la visión que finaliza con la sensación de ver algo, es decir, con la percepción de un estímulo visual, comienza con la formación de la imagen del mundo sobre la retina. Esto hace que, aunque pueden surgir múltiples complicaciones a distintos niveles que empeoren la calidad visual, la calidad de la imagen retiniana impone un primer límite físico a nuestra capacidad de distinguir detalles y extraer información de lo que vemos. El desenfoque es la causa más importante y a la vez más común de emborronamiento de la imagen en la retina. Esta borrosidad reduce la calidad de imagen, disminuyendo su resolución y contraste y haciendo que se pierdan detalles. La nitidez de la imagen depende de la interrelación entre la longitud axial del ojo, su potencia óptica y la distancia al objeto. Hasta que se vuelve rígido con la edad, el ojo es capaz de modificar la forma del cristalino para ajustar su potencia óptica, un proceso que se denomina acomodación. De esta forma el sistema visual puede enfocar sobre la retina las imágenes de objetos situados a distintas distancias. Aunque este proceso no es instantáneo, la acomodación es un mecanismo rápido y bastante preciso en la mayoría de los sujetos jóvenes, que da como resultado una visión clara del mundo tridimensional. Sin embargo, se ha sugerido que la aparición y/o progresión de la miopía podría estar relacionada con alteraciones en el proceso acomodativo que podrían alterar el proceso de emetropización. Por otro lado, incluso cuando se mira fijamente un objeto a una distancia determinada, la potencia óptica del ojo fluctúa de forma más o menos aleatoria. No está claro si esta inestabilidad es un resultado no deseado de la incapacidad del ojo para mantener un enfoque constante o puede ser parte integrante del proceso de acomodación con un propósito concreto, por ejemplo, aumentando la velocidad de respuesta. Sea como fuere, considerando el fenómeno desde un punto de vista óptico, sería de esperar que la imagen retiniana sufriera algún tipo de degradación al verse sometida a una fluctuación de foco, perdiendo nitidez. En esas circunstancias, un tiempo de integración corto podría permitir que el sistema visual percibiera una secuencia instantáneas con distintos niveles de emborronamiento, y que empleara la más nítida para extraer información sobre el objeto, maximizando la calidad visual. En este contexto, esta tesis estudia los efectos de distintos tipos de variaciones de enfoque, tanto discretas como progresivas, con el objetivo de analizar cómo son manejados por el sistema visual. Para ello se llevaron a cabo dos experimentos separados empleando un sensor Hartmann-Shack (HS) de campo abierto, que mide tanto la refracción como las aberraciones de alto orden de ambos ojos en tiempo real. En primer lugar, se analizó la dinámica de la respuesta acomodativa en condiciones realistas de visión binocular, tanto en sujetos emétropes como miopes, cuando cambiaban su fijación abruptamente de un objeto lejano a otro cercano. Posteriormente, en un segundo experimento estudiamos el efecto sobre la sensibilidad al contraste de oscilaciones rápidas de desenfoque con diferentes amplitudes y frecuencias temporales, generadas con una lente sintonizable acoplada al sistema para este propósito. En el primer experimento, se midió la dinámica en tiempo real de las tres componentes de la respuesta acomodativa binocular (acomodación, convergencia y miosis pupilar) en emétropes y sujetos con distintos grados de miopía. El mecanismo de acomodación no solo conlleva el cambio de potencia óptica del cristalino sino que además incluye una rotación coordinada de los globos oculares para hacer que las líneas de mirada converjan sobre el objeto observado y una reducción del tamaño (miosis) de las pupilas. Existe una extensa literatura sobre estos procesos y sus combinaciones, pero relativamente pocos estudios los miden simultáneamente en visión binocular. Hasta donde sabemos, este es el primer estudio de la dinámica combinada de las tres componentes de la respuesta acomodativa se mide de forma precisa, en tiempo real y en condiciones realistas de observación. Además, se realizaron medidas tanto en miopes como emétropes, con el objeto de analizar las posibles diferencias de comportamiento entre ellos. En el estudio participaron 18 sujetos jóvenes, con un error refractivo promedio de -2.3 D en un rango de 0 D a -7.5 D, todos ellos con valores de cilindro por debajo de 2 D. Con su mejor corrección, que portaron durante las medidas en caso necesario, todos los sujetos alcanzaron una agudeza visual decimal por encima de la unidad y ninguno presentó antecedentes de enfermedades oculares o problemas visuales. Tanto el estímulo cercano, ubicado a 2.8 D, como el lejano, a 0.36 D, consistieron en cruces de Malta negras sobre un fondo blanco, con una anchura angular de 1.3° en ambos casos. Cada sujeto se sometió a 3 ciclos de 6 cambios de distancia de fijación (lejos-cerca-lejos-cerca-lejos-cerca). Para el análisis de datos se empleó un método de umbralización consistente determinar los instantes en los que se alcanza el 10% y se supera el 90% del rango de variación entre los estados inicial y final para cada variable estudiada. A partir de los datos experimentales se calcularon varios parámetros relacionados con el cambio de fijación de lejos a cerca, incluyendo las amplitudes de acomodación, convergencia y miosis pupilar, las duraciones de estos tres procesos y sus velocidades medias, el retraso acomodativo, el RMS total de las aberraciones de alto orden y la magnitud de la aberración esférica, y se analizaron los coeficientes de correlación de todas estas variables con el error refractivo y con la velocidad de acomodación. El análisis de correlación de la refracción (en forma de equivalente esférico) con los parámetros dinámicos de la respuesta acomodativa sugiere que la miopía afecta levemente o se ve afectada por la acomodación. Se encontró una relación de proporcionalidad entre el retraso acomodativo y el error refractivo (R = -0.57, p = 0.01), así como una correlación apreciable entre la velocidad de constricción pupilar y la refracción del sujeto (R = -0.49, p = 0.04), en ambos casos con valores de p por debajo de 0.05. En otras palabras, la acomodación en los miopes parece tender a ser menos precisa y la contracción de su pupila a ser más lenta. Los coeficientes de correlación entre el equivalente esférico y el resto de parámetros relacionados con la respuesta acomodativa fueron en general pequeños, con valores de p muy por encima de 0.05. Por otro lado, se encontró una correlación sustancial, con valor de p bajo, entre la velocidad de acomodación y la velocidad de convergencia (R = 0.48, p = 0.04), una asociación que no hemos encontrado mencionada en la literatura previa. Y todavía más fuerte fue la correlación la velocidad de acomodación y la duración de la convergencia (R = 0.57, p = 0.01), lo que puede reflejar las diferencias entre las dinámicas de estos dos procesos. Además, parece haber una correlación entre la velocidad de acomodación y la amplitud de la miosis pupilar, ya que el valor de p fue inferior a 0.05 (R = 0.47, p = 0.049). En resumen, el análisis de correlación de la velocidad de acomodación sugiere que una acomodación lenta puede estar asociada a una convergencia lente y a una constricción pupilar más evidente. Para la segunda parte de esta tesis se construyó un sensor HS más rápido, con una frecuencia de actualización de 60 Hz, y con mayor sensibilidad a la luz infrarroja de 1050 nm de longitud de onda. Este sensor se empleó para caracterizar una lente sintonizable, no solo para la producción en modo estático de valores fijos de desenfoque sino también, por primera vez que sepamos, en modo dinámico para la generación de variaciones sinusoidales de desenfoque. Después de la calibración, la lente sintonizable se utilizó para provocar oscilaciones periódicas de desenfoque, de distintas amplitudes y frecuencias, a 5 jóvenes emétropes mientras realizaban una tarea de sensibilidad al contraste. Todos los sujetos alcanzaron agudeza visual unidad o mejor y carecían de antecedentes de problemas visuales. El estímulo visual fue un test de Gabor de 1º de diámetro angular y una frecuencia espacial de 12 c/grado, ubicado a 3 m de distancia del observador. Las franjas se inclinaron aleatoriamente 10º hacia la izquierda o la derecha de la vertical y se utilizó un protocolo de elección forzada de dos opciones para determinar el umbral de contraste para cada condición de oscilación. Las medidas se tomaron monocularmente con el ojo derecho. En total se estudiaron 24 casos de fluctuación sinusoidal de desenfoque presentados en orden aleatorio, correspondientes a las combinaciones de 3 frecuencias temporales (5, 15 y 25 Hz) y 8 valores de pico-valle de desenfoque entre un mínimo de 0,15 D y 3 D de máximo. No se ha encontrado literatura previa analizando el efecto de este tipo de fluctuaciones rápidas de desenfoque en la calidad visual. Los resultados de este estudio indican que la calidad visual, cuantificada mediante el umbral de contraste, es resistente a las oscilaciones de desenfoque inducidas. Solo se encontró una reducción notable de la sensibilidad al contraste para las variaciones más grandes y rápidas (25 Hz, ± 1,5 D) consideradas. Este hecho se puede tomar como una indicación de que el ojo humano solo necesita que la imagen retiniana esté enfocada durante un periodo corto de tiempo para poder percibir claramente los estímulos visuales. Como complemento a las medidas experimentales, en la última sección de esta tesis se desarrolló un modelo cuantitativo para predecir el deterioro en la calidad de imagen retiniana que puede producir una fluctuación periódica de desenfoque. Para las amplitudes y frecuencias de oscilación utilizadas en el experimento, se calculó la PSF promedio para varios tiempos de integración. A partir de ellas se evaluó la MTF compuesta para 12 c/grado y se comparó con la MTF limitada por difracción a dicha frecuencia, con el objeto de determinar la pérdida de modulación inducida por las variaciones de enfoque. La comparación entre los resultados experimentales y los datos simulados sugiere que el ojo puede estar integrando el emborronamiento causado por desenfoque en intervalos de 10 a 20 ms, y que una instantánea más o menos enfocada en una secuencia muy emborronada resulta suficiente para que el sistema visual extraiga información relevante para percibir el estímulo.
... Combinations of even and odd-order aberrations result in shape changes of the PSF in response to defocus. The effects of even and odd-order aberrations as possible cues to accommodation have been reported by others (Wilson et al., 2002;Moulakaki et al., 2017). The human eye has typically positive spherical aberration when relaxed but decreasing spherical aberration when accommodated with a corresponding change in the PSF (Thibos et al., 2013;López-Gil and Fernández-Sánchez, 2010). ...
Accommodation of the human eye relies on multiple factors and visual cues that include object size, monochromatic and chromatic aberrations, and vergence. Yet, even in monocular conditions, accommodation corrects for defocus. Studies of eye growth in chicks have addressed whether the retina can decode the sign of defocus as this may play a role for emmetropization and possibly also accommodation. However, findings have not been unambiguous and questions remain. Here, we report on monocular accommodation studies of emmetropic and myopic human subjects to clarify whether foveal vision drives accommodation in the correct direction by removing out-of-focus blur potentially before relying on other cues. Subjects viewed monocularly a green target at 1-meter distance while being presented with a random sequence of negative defocus step changes induced by a pupil-conjugated current-driven tuneable lens. The natural pupil was constricted by a pupil-conjugated motorized iris using three different diameters and target brightness was set with a liquid crystal variable attenuator. A Hartmann-Shack wavefront sensor with an infrared beacon captured real-time changes of defocus and Zernike polynomial coefficients up to 4th radial order. We find that the young adult eye accommodates reliably in the correct direction but with a latency of 300-700 ms. The findings are discussed in relation to an absorption model of light in outer segments that breaks the defocus symmetry and thus may serve as a plausible guide for accommodation and emmetropization.
... Figure 1 shows a detailed description of the experimental setup used. The system is composed of a Hartmann-Shack wavefront sensor (Haso 32, Imagine Eyes, France) and a 52-actuator deformable mirror (Mirao 52, Imagine Eyes, France) that was used to compensate for the internal aberrations of the optical system [9]. The wavefront sensor employs a square array of 1024 microlenses and a near-infrared light source with a wavelength of 850 nm. ...
Purpose
To evaluate the static measurement of the accommodative stimulus-response curve with emoji symbols.
Methods
The accommodative stimulus-response curve was measured in 18 subjects using a Hartmann-Shack sensor to obtain the objective accommodative response from the Zernike defocus term. Measurements were acquired at different accommodative demands, from 0 to 3 D with a step of 0.5 D. Detailed and nondetailed emoji targets were used with two different sizes, corresponding to the two most common visual angles used in smartphones.
Results
A regression analysis was performed to fit the mean results obtained for each target. The determination coefficient was R² ≥ 0.988 for all targets. For the detailed targets, the slopes for the averaged stimulus-response curve were 0.65 and 0.66 for the bigger and smaller sizes, respectively. For the nondetailed targets, the slopes were 0.60 and 0.58 for the bigger and smaller sizes, respectively. p values for these slopes were statistically significant for the two types of targets (p < 0.01).
Conclusions
Our results reveal that the replacement of a word or several words by detailed or nondetailed emoji symbols seems not to provoke a different accommodative response in normal subjects and under standard viewing conditions in the use of smartphones.
Introduction:
This study investigated differences in peripheral image quality with refractive error. Peripheral blur orientation is determined by the interaction of optical aberrations (such as oblique astigmatism) and retinal shape. By providing the eye with an optical signal for determining the sign of defocus, peripheral blur anisotropy may play a role in mechanisms of accommodation, emmetropisation and optical myopia control interventions. This study investigated peripheral through-focus optical anisotropy and image quality and how it varies with the eye's refractive error.
Methods:
Previously published Zernike coefficients across retinal eccentricity (0, 10, 20 and 30° horizontal nasal visual field) were used to compute the through-focus modulation transfer function (MTF) for a 4 mm pupil. Image quality was defined as the volume under the MTF, and blur anisotropy was defined as the ratio of the horizontal to vertical meridians of the MTF (HVRatio).
Results:
Across the horizontal nasal visual field (at 10, 20 and 30°), the peak image quality for emmetropes was within 0.3 D of the retina, as opposed to myopes whose best focus was behind the retina (-0.1, 0.4 and 1.5 D, respectively), while for hyperopes it lay in front of the retina (-0.5, -0.6 and -0.6 D). At 0.0 D (i.e., on the retina), emmetropes and hyperopes both exhibited horizontally elongated blur, whereas myopes had vertically elongated blur (HVRatio = 0.3, 0.7 and 2.8, respectively, at 30° eccentricity).
Conclusions:
Blur in the peripheral retina is dominated by the so-called "odd-error" blur signals, primarily due to oblique astigmatism. The orientation of peripheral blur (horizontal or vertical) provides the eye with an optical cue for the sign of defocus. All subject groups had anisotropic blur in the nasal visual field; myopes exhibited vertically elongated blur, perpendicular to the blur orientation of emmetropes and hyperopes.
Accommodative responses of humans operate seamlessly to ensure clear vision of targets at different viewing distances, up until the onset of presbyopia. To achieve this, the visual system must correctly decipher the polarity and magnitude of retinal defocus in real-time, and often under very challenging viewing conditions. The past seven decades of research in this area has identified several retinotopic cues that may potentially provide the desired odd- and even-error information to the visual system for solving these challenges. These studies have used a variety of technology, experimental paradigms and outcome measures to determine the putative contribution of a given cue, or set of cues, in solving this problem. A variety of results, some offering consensus and others conflicting, have been observed in these studies. The present review distils this large volume of literature into specific, take-away points for the early reader of this topic, acknowledging that the problem is non-trivial and far from being solved. The review also reveals that many of these studies may not have used appropriate/sensitive methodology or outcome measures to tease apart the relative contribution of a cue in solving the direction and magnitude challenge. The review concludes with the proposal that, since a multitude of cues may be used by the visual system for solving these problems, future studies could employ a Bayesian statistical cue-combination approach to address this problem. Such approaches have yielded very meaningful insights in other areas of human decision-making involving multiple inter- and intra-modal combination of cues.
