Normal Foveal Vision Research Articles

Editorial: Improving visual deficits with perceptual learning.

Editorial: Improving visual deficits with perceptual learning.

Foveal visual acuity is worse and shows stronger contour interaction effects for contrast-modulated than luminance-modulated Cs

Contrast-modulated (CM) stimuli are processed by spatial mechanisms that operate at larger spatial scales than those processing luminance-modulated (LM) stimuli and may be more prone to deficits in developing, amblyopic, and aging visual systems. Understanding neural mechanisms of contour interaction or crowding will help in detecting disorders of spatial vision. In this study, contour interaction effects on visual acuity for LM and CM C and bar stimuli are assessed in normal foveal vision. In Experiment 1, visual acuity is measured for all-LM and all-CM stimuli, at ~3.5× above their respective modulation thresholds. In Experiment 2, visual acuity is measured for Cs and bars of different type (LM C with CM bars and vice versa). Visual acuity is degraded for CM compared with LM Cs (0.46 ± 0.04 logMAR vs. 0.18 ± 0.04 logMAR). With nearby bars, CM acuity is degraded further (0.23 ± 0.01 logMAR or ~2 lines on an acuity chart), significantly more than LM acuity (0.11 ± 0.01 logMAR, ~1 line). Contour interaction for CM stimuli extends over greater distances (arcmin) than it does for LM stimuli, but extents are similar with respect to acuities (~3.5× the C gap width). Contour interaction is evident when the Cs and bars are defined differently: it is stronger when an LM C is flanked by CM bars (0.17 ± 0.03 logMAR) than when a CM C is flanked by LM bars (0.08 ± 0.02 logMAR). Our results suggest that contour interaction for foveally viewed acuity stimuli involves feature integration, such that the outputs of receptive fields representing Cs and bars are combined. Contour interaction operates at LM and CM representational stages, it can occur across stage, and it is enhanced at the CM stage. Greater contour interaction for CM Cs and bars could hold value for visual acuity testing and earlier diagnosis of conditions for which crowding is important, such as in amblyopia.

The effect of flankers on three tasks in central, peripheral, and amblyopic vision

Using identical stimuli and methods, we assessed the effects of flankers on three different tasks, orientation discrimination, contrast discrimination, and detection, in central, peripheral, and amblyopic vision. The goal was to understand the factors that limit performance of a task in the presence of flankers in each of these visual systems. The results demonstrate that: (1) For unflanked targets, the losses in peripheral and amblyopic vision (relative to the normal fovea) are ordered, with the loss of unflanked contrast discrimination thresholds considerably smaller than those for either detection or orientation discrimination. (2) For flanked targets, in normal foveal vision and anisometropic amblyopia, the critical distance is more or less proportional to the target size, whereas in peripheral and strabismic amblyopic vision, the critical distance shows much less (or no) dependence on target size. (3) For the normal fovea, and anisometropic amblyopia, when the target is large (>≈0.2 deg) the amount of threshold elevation induced by flankers is low, increasing when the target is very small. On the other hand, for the periphery and the amblyopic eyes of most strabismic amblyopes, the elevation is large over the range of sizes tested. (4) In peripheral and strabismic amblyopic vision, remote flankers elevate orientation discrimination and contrast discrimination thresholds but not detection thresholds. Our results show clearly that the effects of flanks depend on both the task and the type of visual system. We conclude that in normal foveal vision and anisometropic amblyopia, the effects of flankers largely reflects a reduction in visibility and may be explained by masking. On the other hand, in peripheral vision and strabismic amblyopia, the effects of flankers on orientation discrimination and to a lesser extent contrast discrimination cannot be explained by simple masking and are due to crowding.

Visual acuity and contour interaction for luminance-modulated and contrast-modulated Cs in normal foveal vision

Visual acuity and contour interaction for luminance-modulated and contrast-modulated Cs in normal foveal vision

Amblyopia masks the scale invariance of normal central vision

In normal vision, detecting a kink (a change in orientation) in a line is scale invariant: it depends solely on the length/width ratio of the line (D. Whitaker, D. M. Levi, & G. J. Kennedy, 2008). Here we measure detection of a change in the orientation of lines of different length and blur and show that strabismic amblyopia is qualitatively different from normal foveal vision, in that: 1) stimulus blur has little effect on performance in the amblyopic eye, and 2) integration of orientation information follows a different rule. In normal foveal vision, performance improves in proportion to the square root of the ratio of line length to blur (L:B). In strabismic amblyopia improvement is proportional to line length. Our results are consistent with a substantial degree of internal neural blur in first-order cortical filters. This internal blur results in a loss of scale invariance in the amblyopic visual system. Peripheral vision also shows much less effect of stimulus blur and a failure of scale invariance, similar to the central vision of strabismic amblyopes. Our results suggest that both peripheral vision and strabismic amblyopia share a common bottleneck in having a truncated range of spatial mechanisms--a range that becomes more restricted with increasing eccentricity and depth of amblyopia.

Suppressive and facilitatory spatial interactions in amblyopic vision

Amblyopic vision is characterized by reduced spatial resolution, and inhibitory spatial interactions (“crowding”) that extend over long distances. The present paper had three goals: (1) To ask whether the extensive crowding in amblyopic vision is a consequence of a shift in the spatial scale of analysis. To test this we measured the extent of crowding for targets that were limited in their spatial frequency content, over a large range of target sizes and spatial frequencies. (2) To ask whether crowding in amblyopic vision can be explained on the basis of contrast masking by remote flanks. To test this hypothesis we measured and compared crowding in a direction-identification experiment with masking by remote flanks in a detection experiment. In each of the experiments our targets and flanks were comprised of Gabor features, thus allowing us to control the feature contrast, spatial frequency and orientation. (3) To examine the relationship between the suppressive and facilitatory interactions in amblyopic contrast detection and “crowding”. Our results show that unlike the normal fovea [Levi, Klein, & Hariharan, Journal of Vision 2 (2002a) 140] crowding in amblyopia is neither scale invariant, nor is it attributable to simple contrast masking. Rather, our results suggest that suppressive spatial interactions in amblyopic vision extend over larger distances than in normal foveal vision, similar to peripheral vision of non-amblyopic observers [Levi, Hariharan, & Klein, Journal of Vision 2 (2002b) 167], for targets of the same size. Observers can easily detect the features that comprise our targets (Gabor patches) under conditions where crowding is strong. Thus, our speculation is that crowding occurs because the target and flanks are combined or pooled at a second stage that is coarse in the amblyopic visual system, following the stage of feature extraction. In amblyopic vision, this pooling takes place over a large spatial distance.

Detecting disorder in spatial vision

In normal foveal vision, visual space is accurately mapped from retina to cortex. However, the normal periphery, and the central field of strabismic amblyopes have elevated position discrimination thresholds, which have often been ascribed to increased ‘intrinsic’ spatial disorder. In the present study we evaluated the sensitivity of the human visual system (both normal and amblyopic) to spatial disorder, and asked whether there is increased ‘intrinsic’ topographical disorder in the amblyopic visual system. Specifically, we measured thresholds for detecting disorder (two-dimensional Gaussian position perturbations) either in a horizontal string of N equally spaced samples (Gabor patches), or in a ring of equally spaced samples over a wide range of feature separations. We also estimated both the ‘equivalent intrinsic spatial disorder’ and sampling efficiency using an equivalent noise approach. Our results suggest that both thresholds for detecting disorder, and equivalent intrinsic disorder depend strongly on separation, and are modestly increased in strabismic amblyopes. Strabismic amblyopes also show markedly reduced sampling efficiency. However, neither amblyopic nor peripheral vision performs like ideal or human observers with added separation-independent positional noise. Rather, the strong separation dependence suggests that the ‘equivalent intrinsic disorder’ may not reflect topographic disorder at all, but rather may reflect an abnormality in the amblyopes’ Weber relationship.

Equivalent intrinsic blur in amblyopia

We used Gaussian blurred stimuli to explore the effect of blur on three tasks: (i) 2-line resolution; (ii) line detection; and (iii) spatial interval discrimination, in observers with amblyopia due to anisometropia, strabismus, or both. The results of our experiments can be summarized as follows. (i) 2-Line resolution: in normal foveal vision, thresholds for unblurred stimuli are approx. 0.5 min arc in the fovea. When the standard deviation (σ) of the stimulus blur is less than 0.5 min, it has little effect upon 2-line resolution; however, thresholds are degraded when the stimulus blur, σ, exceeds 0.5 min. We operationally define this transition point, as the equivalent intrinsic blur, or B i . When the stimulus blur, σ, is greater than B i , then the resolution threshold is approximately equal to σ. In all of the amblyopic eyes, 2-line resolution thresholds for unblurred stimuli were elevated, and the equivalent intrinsic blur was much larger. When the stimulus blur exceeds the equivalent intrinsic blur, resolution thresholds were similar in amblyopic and nonamblyopic eyes. (ii) Line detection: in both normal and amblyopic eyes, when the stimulus blur, σ, is less than B i , then the line detection threshold is approximately inversely proportional to σ; i.e. (it obeys Ricco's law). When σ is greater than B i , the equivalent intrinsic blur, then the detection threshold is approximately a fixed contrast. All of the amblyopic eyes showed markedly elevated thresholds for detecting thin lines, but normal or near normal thresholds for detecting very blurred lines. Consquently, Ricco's diameter is larger in amblyopic than in normal eyes. (iii) Spatial interval discrimination: thresholds are proportional to the separation of the lines (i.e. Weber's law). At the optimal separation, spatial interval discrimination thresholds represent a “hyperacuity” (i.e. they are smaller than the resolution threshold). For unblurred lines, the optimal separation is approx. 2–3 times B i . In the normal fovea, and in the amblyopic eyes of anisometropic amblyopes the optimal spatial interval discrimination threshold is about one-fifth of the resolution threshold (i.e. a hyperacuity); and over a wide range of separations, spatial interval discrimination thresholds begin to rise when the stimulus blur exceeds about one-third of the separation between the lines as long as the contrast is sufficiently high. In contrast, in strabismic amblyopes, like the normal periphery, the optimal spatial interval discrimination thresholds are worse (higher) than would be expected based upon the resolution limit of the strabismic amblyopic eye. In anisometropic amblyopes the elevated resolution and spatial interval discrimination thresholds are consistent with a raised level of equivalent intrinsic blur, and a reduced contrast response function. In strabismic amblyopia, there appears to be an additional source of loss, which affects spatial localization to a greater degree than resolution. This extra loss may be modeled in terms of abnormal positional uncertainty due to a sparse cortical spatial sampling grain.

Positional uncertainty in peripheral and amblyopic vision

Three experiments were performed to examine positional acuity and the role of spatial sampling in central, peripheral and amblyopic vision. In the first experiment, 3-line bisection acuity was compared to grating acuity. In normal foveal vision bisection acuity represents a hyperacuity. In anisometropic amblyopes, bisection acuity is reduced in rough proportion to their grating acuity. In strabismic amblyopes, and in the normal periphery, bisection acuity is reduced to a greater extent than grating acuity. This result implies that reduced contrast sensitivity of the spatial filters is not sufficient to account for the increased positional uncertainty found in peripheral vision and in strabismic amblyopia. In order to test the hypothesis that the high degree of positional uncertainty evident in these visual systems is a consequence of sparse spatial sampling, bisection thresholds and width discrimination thresholds were measured with stimuli comprised of discrete samples. The results showed that normal foveal vision and the vision of anisometropic amblyopes show little benefit from adding discrete samples to the stimulus. In contrast, the normal periphery, and the central field of strabismic amblyopes demonstrate marked positional uncertainty which can be efficiently reduced in proportion to the square root of the number of samples (up to about 10) comprising the stimulus in the direction orthogonal to the discrimination cue. In aggregate the results suggest that anisometropic and strabismic amblyopia are fundamentally different. The positional uncertainty in anisometropic amblyopia is consistent with the reduced sensitivity of the spatial filters. The data of the normal periphery and of the central field of strabismic amblyopes suggest that the cortical sampling grain imposes a fundamental limit upon their positional acuity.

Sampling in spatial vision.

The human visual system is capable of making spatial discriminations with extraordinary accuracy. In normal foveal vision, relative position, width or size can be judged with an accuracy much finer than the size or spacing of even the smallest foveal cones. This remarkable accuracy of spatial vision has been termed 'hyperacuity'. Almost a century ago Ewald Hering proposed that the accuracy of Vernier acuity could be accounted for by averaging of discrete samples along the length of the lines comprising the targets; however, the discovery that Vernier acuity of a few arc seconds could be achieved with dots has rendered the nature and role of sampling in spatial discrimination unclear. We have been investigating the sampling of spatial information in central and peripheral vision (the perifovea) of normal human observers and in observers with strabismic amblyopia. Our results, presented here, show that peripheral vision and central vision of strabismic amblyopes differ qualitatively in their sampling characteristics from those of the normal fovea. Both the periphery and the central visual field of strabismic amblyopes demonstrate marked positional uncertainty which can be reduced by averaging of spatial information from discrete samples.

Chromaticity diagrams and retinal color processes.

The magnitudes of three color processes R, G and B at the fovea were measured by means of Motokawa's method, which consisted of measuring the electrical excitability of the eye after a brief illumination with monochromatic light of various wave-lengths. Color diagrams were constructed from the measured magnitudes of R, G and B. They were found very similar to the usual chromaticity diagram constructed from color mixing data. So they can be used for specification of colors. The fact that such a color diagram can be constructed from three physiological quantities suggests that the normal foveal vision is essentially trichromatic. The limitation of the validity of the trichromatic principle was further discussed on the basis of physiological data. We wish to thank Prof. K. Motokawa for his good advice and cordial guidance.