Central and near peripheral retinal contributions to the depth-of-focus using naturalistic stimulation

Although the depth-of-focus (DOF) has been investigated separately in the central retina and in the near retinal periphery, knowledge about their combined relative contribution to overall blur perception has remained unknown. In the present study, the DOF was measured psychophysically with a naturalistic pictorial stimulus as a function of spatial extent across the near retinal periphery under monocular Badal viewing conditions with accommodation paralyzed. The group mean total DOF progressively increased linearly with target size. Based on the individual DOF responses, the group was categorized into two subgroups: a predominantly centrally-driven and a centrally plus peripherally-driven subgroup. The results implicated partial cone pooling of blur information, as well as influence from perceptual, attentional, and optical aspects. However, the subgroup response profiles suggested individual differences in the weighting of the near peripheral blur information at the retinal level, and perhaps at higher-level areas of the visual system, involving spatial integration and global attentional processing.     

Equiblur zones at the fovea and near retinal periphery

Knowledge regarding successive blur discrimination thresholds (i.e., equiblur zones) in depth and across the near retinal periphery, and their relation to blur detection (i.e., depth-of-focus), remains unknown. The blur detection threshold and four successive blur discrimination thresholds were measured psychophysically at the fovea, as well as at retinal eccentricities of 0.25 degrees , 2 degrees , 4 degrees , and 8 degrees . A Badal optometer system was used to assess blur sensitivity monocularly in five visually normal young adults with cycloplegia. The foveal test stimulus consisted of a small irregularly shaped black form, and the peripheral test stimulus consisted of high contrast circular apertures of different radii. Both the group mean blur detection and successive blur discrimination thresholds progressively increased with retinal eccentricity. At retinal eccentricities of 0 degrees , 0.25 degrees , 2 degrees , 4 degrees , and 8 degrees , the group mean blur detection thresholds were 0.53+/-0.06 D, 0.59+/-0.10 D, 0.93+/-0.11 D, 0.98+/-0.16D, and 1.25+/-0.25 D, while the average values of the group mean blur discrimination thresholds across the steps were 0.29+/-0.01 D, 0.37+/-0.01 D, 0.48+/-0.00 D, 0.51+/-0.01 D, and 0.72+/-0.02 D, respectively. At each retinal eccentricity, the blur discrimination thresholds were similar to each other, and they were approximately 60% of the blur detection threshold magnitude. These findings provide a conceptual representation of blur perception throughout the central visual field. Possible mechanisms are proposed for the decreased blur sensitivity in the near retinal periphery, as well as for the difference between the blur detection and blur discrimination thresholds.     

Effect of nearwork-induced transient myopia on distance retinal defocus patterns

The purpose of the current study was to assess the effect of nearwork-induced transient myopia (NITM) on retinal defocus patterns during distance viewing. An empirically derived, conceptual model of human blur perception and related retinal defocus patterns has been extended to determine the effect of NITM on the relative contributions of myopic and hyperopic retinal defocus during distance viewing. Under the normal hyperfocal refractive condition during distance viewing with isolated stimulus conditions, there is very small myopic defocus (～0.25 diopters), and no hyperopic defocus, present. After sustained nearwork generating NITM, a moderate increase in this myopic defocus contribution takes place. In the normal isolated distance viewing situation with only very small myopic defocus present, as would occur with many far outdoor activities, the paucity of overall retinal defocus may provide a “protected” condition against myopia development. In contrast, with the addition of NITM producing increased myopic retinal defocus only, there is an imbalance of retinal defocus that may be myopigenic, especially in the context of foveal and peripheral retinal interactions.     

近周边视网膜区模糊认知感的研究

目的 :介绍作者所在的实验中心近年来对于黄斑区和近周边视网膜区 (≤ 8°)模糊认知感的研究结果。方法和结果 :先后介绍三个实验。实验一测量了近周边视网膜区的焦深 (即对视网膜离焦的主觉耐受程度 ) ,并将其与黄斑区的反应进行了比较。实验二测量了近周边视网膜区的模糊敏感阈值和模糊辨别阈值。上述两实验结果表明 ,人眼焦深在近周边视网膜区随其与黄斑的距离增大而逐渐升高 ,其值从黄斑区的 0 .89D增加到距黄斑 8度视角离心区的 3.51D。模糊敏感阈值 ,即导致初始模糊认知感的视网膜离焦度 ,也随其与黄斑的距离增大而逐渐升高 ;其值从黄斑区的 0 .85D增加到距黄斑 8度视角离心区的 1 .89D。模糊辨别阈值 ,即在初始模糊认知感的条件下觉察初始模糊程度改变的视网膜离焦度 ,同样亦随其与黄斑的距离增大而逐渐升高 ,其值从黄斑区的 0 .4 5D增加到距黄斑 8度视角离心区的 0 .93D。基于实验一 ,二的结果 ,作者设计了实验三 :通过测量视标大小对焦深的影响以研究近周边视网膜区和黄斑区对模糊认知感的综合效应。结果表明 ,人眼焦深在近周边视网膜区随着视标尺寸的增大而逐渐升高 ,且据其数据分析结果及回归方程斜率的个体差异性 ,可将总体实验对象区分为两个亚组 :即“黄斑主导型”和“黄斑 周边视网膜协     

An objective technique to measure the depth-of-focus in free space

Background Determining the depth-of-focus (DOF) objectively in free space is important, as this provides information regarding the range of clear vision under more natural viewing conditions in which target blur, size, and proximal information, as well as other monocular depth cues, are present.Methods The DOF was assessed objectively in free space in young adults [n=20, ages 24.9±3.1years: myopes (n=14), emmetropes (n=5), hyperope (n=1)] by monitoring accommodation continuously using the commercially available Power Refractor II (PR II) under monocular viewing conditions. A high-contrast target (∼83%) subtending a mean visual angle of 2.3 deg was placed on an optical bench at 25 cm (4 D) along the line-of-sight of the viewing right eye and was displaced slowly (∼0.1–0.15 D/s) proximally and distally. This target provided blur, size, and proximal information, and other depth cues, as normally found in free space. The first consistent change in steady-state accommodative baseline level reflected one edge of the DOF. The mean dioptric difference between these distal and proximal endpoints was averaged, and this represented the total objective DOF.Results The group mean total objective DOF (n=20) was ±0.61±0.09 D, with a range from ±0.46 D to ±0.81 D. Repeatability was excellent.Conclusion A technique was developed and tested to measure the DOF in free space objectively. Further development of this technique will allow the assessment of blur perception under more complex natural viewing conditions simulating the everyday environment.     

模糊适应对近视成人模糊阈值的影响

目的 研究近视成人对不同图片的模糊阈值在模糊适应前后的变化.方法 前瞻性自身对照研究.38例近视成人为实验组,屈光度-0.50～-6.00 D,散光度均＜-0.75 D,试镜架上放置等效球镜度数插片矫正至视力5.0以上.模糊阈值的测量由自主设计的电脑程序完成,依据double-staircase理论,随机呈现0～+2 D离焦量的3种图片(视力表E字母、Lena头像、街景图片).受检者在模糊适应前后均进行测量,判断图片“模糊”或“不模糊”.38例对象中有18例作为对照组隔天再次测量,测量方法一致但不进行模糊适应.数据采用重复测量方差分析处理.结果 ①对照组前后两次测量的模糊阈值差异无统计学意义.②实验组经模糊适应后模糊阈值降低,即模糊敏感性提高(E视标:F=5.883,P＜0.05;Lena:F=6.234,P＜0.05;街景图片:F=3.987,P＞0.05).③同一受检者在判断不同图片时的模糊阈值存在差异,Lena图像的阈值高于另2种图片(F=10.761,P＜0.01).结论 模糊适应能提高成人近视者对不同图片的模糊敏感性;模糊适应效应不只限于近视成人黄斑中心凹区;人眼对于不同图片的阈值存在显著差异.     

Peripheral optical anisotropy in refractive error groups

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.     

Depth-of-focus of the human eye in the near retinal periphery

Although the depth-of-focus in the foveal region has been well investigated, knowledge regarding the effect of retinal eccentricity on blur detection and sensitivity is limited. In the present study, the depth-of-focus at the fovea and in the near retinal periphery (0 degrees -8 degrees ) was assessed psychophysically in 7 human subjects using a 5 mm artificial pupil with accommodation paralyzed. The group mean total depth-of-focus progressively increased linearly from 0.89 D at the fovea to 3.51 D at a retinal eccentricity of 8 degrees at the rate of 0.29 D/degree, with response variability (S.E.M.) remaining relatively constant (+/-0.17 D). We speculate that the reduced detection and sensitivity to blur in the near periphery may be attributed to retinal topography, sharpness overconstancy, optical aberrations, and visual attention in peripheral vision.     

低视力优选视网膜区的定位假说

黄斑疾病造成中心视力损害后,患者常使用旁中心注视来克服视觉功能的障碍,而在此过程中起替代作用的周边视网膜区的选择和形成便成为一个关键环节.目前已经有大量研究证明,患者利用稳定有效的优选视网膜区(preferred retinal locus,PRL)可以显著提高其在阅读以及视觉寻找等多个方面的视觉表现,从而明确了进行周围非黄斑区视网膜感知训练的重要性和必要性.然而,很多关于优选视网膜区的重要理论尤其是定位的相关理论有待进一步建立和统一.本文就优选视网膜区的定位假说的功能-动力学说、执行-动力学说、视网膜投射-动力学说等进行文献综述.     

The blur horopter: Retinal conjugate surface in binocular viewing

From measurements of wavefront aberrations in 16 emmetropic eyes, we calculated where objects in the world create best-focused images across the central 27∘ (diameter) of the retina. This is the retinal conjugate surface. We calculated how the surface changes as the eye accommodates from near to far and found that it mostly maintains its shape. The conjugate surface is pitched top-back, meaning that the upper visual field is relatively hyperopic compared to the lower field. We extended the measurements of best image quality into the binocular domain by considering how the retinal conjugate surfaces for the two eyes overlap in binocular viewing. We call this binocular extension the blur horopter. We show that in combining the two images with possibly different sharpness, the visual system creates a larger depth of field of apparently sharp images than occurs with monocular viewing. We examined similarities between the blur horopter and its analog in binocular vision: the binocular horopter. We compared these horopters to the statistics of the natural visual environment. The binocular horopter and scene statistics are strikingly similar. The blur horopter and natural statistics are qualitatively, but not quantitatively, similar. Finally, we used the measurements to refine what is commonly referred to as the zone of clear single binocular vision.     

Depth-dependent blur adaptation

Variations in blur are present in retinal images of scenes containing objects at multiple depth planes. Here we examine whether neural representations of image blur can be recalibrated as a function of depth. Participants were exposed to textured images whose blur changed with depth in a novel manner. For one group of participants, image blur increased as the images moved closer; for the other group, blur increased as the images moved away. A comparison of post-test versus pre-test performances on a blur-matching task at near and far test positions revealed that both groups of participants showed significant experience-dependent recalibration of the relationship between depth and blur. These results demonstrate that blur adaptation is conditioned by 3D viewing contexts.     

Blur limits for defocus, astigmatism and trefoil

We investigated the limits at which blur due to defocus, crossed-cylinder astigmatism, and trefoil became noticeable, troublesome or objectionable. Black letter targets (0.1, 0.35 and 0.6 logMAR) were presented on white backgrounds. Subjects were cyclopleged and had effectively 5 mm pupils. Blur was induced with a deformable, adaptive-optics mirror operating under open-loop conditions. Mean defocus blur limits of six subjects with uncorrected intrinsic higher-order ocular aberrations ranged from 0.18 ± 0.08 D (noticeable blur criterion, 0.1 logMAR) to 1.01 ± 0.27 D (objectionable blur criterion, 0.6 logMAR). Crossed-cylinder astigmatic blur limits were approximately 90% of those for defocus, but with considerable meridional influences. In two of the subjects, the intrinsic aberrations of the eye were subsequently corrected before the defocus and astigmatic blur were added. This resulted in only minor reductions in their blur limits. When assessed with trefoil blur and corrected intrinsic ocular aberrations, the ratio of objectionable to noticeable blur limits in these two subjects was much higher for trefoil (3.5) than for defocus (2.5) and astigmatism (2.2).     

Persistent biases in subjective image focus following cataract surgery

We explored the perception of image focus in patients with cataracts, and how this perception changed following cataract removal and implantation of an intraocular lens. Thirty-three patients with immature senile cataract and with normal retinal function were tested before surgery and 2 days after surgery, with 18 of the patients retested again at 2 months following surgery. The subjective focus of natural images was quantified in each session by varying the slope of the image amplitude spectra. At each time, short-term adaptation to the spectral slope was also determined by repeating the measurements after exposure to images with blurred or sharpened spectra. Despite pronounced acuity deficits, before surgery images appeared “best-focused” when they were only slightly blurred, consistent with a strong compensation for the acuity losses. Post-operatively, the image slopes that were judged “in focus” before surgery appeared too sharp. This bias remained strong at 2 months, and was independent of the rapid blur aftereffects induced by viewing filtered images. The focus settings tended to renormalize more rapidly in patients with higher post-operative acuity, while acuity differences were unrelated to the magnitude of the short-term blur aftereffects. Our results suggest that subjective judgments of image focus are largely compensated as cataracts develop, but potentially through a very long-term form of adaptation that results in persistent biases after the cataract is removed.     

Simulating refractive errors: source and observer methods

There are two principal methods of simulating refractive errors. Either the retinal image can be defocused by an optical system, usually a positive lens, placed in front of an observer's eye (observer method), or the source of the retinal image can be defocused as it is projected onto a screen or photograph (source method). There are significant differences between the two methods, differences that make it difficult to compare results. However, the source method, which is the more artificial, seems to be superior for a number of reasons. The results of these two methods can be compared using a common or interchangeable parameter for specifying the level of defocus. A convenient parameter is the size of the defocused image of a point, measured either in image space (linear or angular diameter on the retina) or in object space (angular diameter of the blur disc projected back into object space), with the angular diameter measured from the respective nodal point of the eye. Methods of measuring the angular blur-disc diameter for both methods are discussed and the validity of the formula omega = D delta L, is investigated, where omega is the angular diameter of the blur disc, D is the observer's pupil diameter, and delta L is the dioptric defocus.     

Sensitivity to retinal defocus with aspheric soft lenses--predictions and clinical validation

The purpose of this study was to obtain a numerically accurate discrete model of the effect of front aspheric soft lenses of various asphericities on the retinal illuminance profiles within the blur circles of a model eye. A computer program was developed for a model eye based on geometrical optics. The ray density was assumed to be proportional to the flux density or illumination. Retinal illuminance profiles are calculated for several values of defocus and asphericity. These results are validated by calculations based on modulation transfer functions (MTFs), observational data from contrast sensitivity and clinical results with Cals aspheric soft lenses. The results suggest that aspheric soft lenses maximize the central illuminance of blur circles and extend the range of desensitivity to retinal defocus in the eye to 4.00 D at the largest asphericity constant investigated.     

Retinal blur and the perception of egocentric distance

A central function of vision is determining the layout and size of objects in the visual field, both of which require knowledge of egocentric distance (the distance of an object from the observer). A wide range of visual cues can reliably signal relative depth relations among objects, but retinal signals directly specifying distance to an object are limited. A potential source of distance information is the pattern of blurring on the retina, since nearer fixation generally produces larger gradients of blur on the extra-foveal retina. While prior studies implicated blur as only a qualitative cue for relative depth ordering, we find that retinal blur gradients can act as a quantitative cue to distance. Surfaces depicted with blur gradients were judged as significantly closer than those without, with the size of the effect modulated by the degree of blur, as well as the availability of other extra-retinal cues to distance. Blur gradients produced substantial changes in perceived distance regardless of relative depth relations of the surfaces indicated by other cues, suggesting that it operates as a robust cue to distance, consistent with the empirical relationship between blur and fixation distance.     

Blur discrimination of the human eye in the near retinal periphery.

BACKGROUND: Although blur discrimination in the foveal region has been investigated, knowledge regarding the effect of retinal eccentricity is limited. METHODS: The initial blur discrimination threshold and the blur detection threshold were assessed psychophysically and compared at the fovea and in the near retinal periphery (up to 8 degrees) with accommodation paralyzed. RESULTS: The blur discrimination and blur detection thresholds increased progressively with retinal eccentricity, with the latter being about twice as large and increasing twice as fast as the former. However, the group mean blur ratio between these two parameters remained relatively constant (0.56) with retinal eccentricity. CONCLUSIONS: The more sensitive blur discrimination vs. blur detection findings may be attributed to variation in the optical modulation transfer function with retinal defocus and to a perceptually based blur-buffering mechanism. The reduced blur sensitivity found with increased retinal eccentricity might be attributed to cone photoreceptor/ganglion cell sampling limitations, sharpness overconstancy, reduced visual attention, and slightly degraded visual optics.     

Retinal Responses to Simulated Optical Blur Using a Novel Dead Leaves ERG Stimulus

PurposeThe purpose of this study was to evaluate retinal responses to different types and magnitudes of simulated optical blur presented at specific retinal eccentricities using naturalistic images.MethodsElectroretinograms (ERGs) were recorded from 27 adults using 30-degree dead leaves naturalistic images, digitally blurred with one of three types of optical blur (defocus, astigmatism, and spherical aberrations), and one of three magnitudes (0.1, 0.3, or 0.5 µm) of blur. Digitally computed blur was applied to the entire image, or on an area outside the central 6 degrees or 12 degrees of retinal eccentricity.ResultsERGs were significantly affected by blur type, magnitude, and retinal eccentricity. ERGs were differentially affected by defocus and spherical aberrations; however, astigmatism had no effect on the ERGs. When blur was applied only beyond the central 12 degrees eccentricity, the ERGs were unaffected. However, when blur was applied outside the central 6 degrees, the ERG responses were significantly reduced and were no different from the ERGs recorded with entirely blurred images.ConclusionsBlur type, magnitude, and location all affect the retinal responses. Our data indicate that the retinal area between 6 and 12 degrees eccentricity has the largest effect on the retinal responses to blur. In addition, certain optical blur types appear to have a more detrimental effect on the ERGs than others. These results cannot be solely explained by changes to image contrast and spatial frequency content, suggesting that retinal neurons might be sensitive to spatial cues in order to differentiate between different blur types.     

Dynamic accommodation with simulated targets blurred with high order aberrations

High order aberrations have been suggested to play a role in determining the direction of accommodation. We have explored the effect of retinal blur induced by high order aberrations on dynamic accommodation by measuring the accommodative response to sinusoidal variations in accommodative demand (1-3D). The targets were blurred with 0.3 and 1 μm (for a 3-mm pupil) of defocus, coma, trefoil and spherical aberration. Accommodative gain decreased significantly when 1-μm of aberration was induced. We found a strong correlation between the relative accommodative gain (and phase lag) and the contrast degradation imposed on the target at relevant spatial frequencies.     

Variability of the accommodation response in early onset myopia.

PURPOSE: Hyperopic retinal defocus (blur) is thought to be a cause of myopia. If the retinal image of an object is not clearly focused, the resulting blur is thought to cause the continuing lengthening of the eyeball during development causing a permanent refractive error. Both lag of accommodation, especially for near targets, and greater variability in the accommodative response, have been suggested as causes of increased hyperopic retinal blur. Previous studies of lag of accommodation show variable findings. In comparison, greater variability in the accommodative response has been demonstrated in adults with late onset myopia but has not been tested in children. This study looked at the lag and variability of accommodation in children with early onset myopia. METHODS: Twenty-one myopic and 18 emmetropic children were tested. Dynamic measures of accommodation and pupil size were made using eccentric photorefraction (PowerRefractor) while children viewed targets set at three different accommodative demands (0.25, 2, and 4 D). RESULTS: We found no difference in accommodative lag between groups. However, the accommodative response was more variable in the myopes than emmetropes when viewing both the near (4 D) and far (0.25 D) targets. Since pupil size and variability also varied, we analyzed the data to determine whether this could account for the inter-group differences in accommodation variability. Variation in these factors was not found to be sufficient to explain these differences. Changes in the accommodative response variability with target distance were similar to patterns reported previously in adult emmetropes and late onset myopes. CONCLUSIONS: Children with early onset myopia demonstrate greater accommodative variability than emmetropic children, and have similar patterns of response to adult late onset myopes. This increased variability could result in an increase in retinal blur for both near and far targets. The role of accommodative variability in the etiology of myopia is discussed.