Diminished spatial summation contributes to the age deficit in the discrimination of low-contrast vernier oscillation.

PURPOSE: This study sought to determine whether a decline in spatial summation contributes to age-related deficits on oscillatory displacement thresholds (ODT's). A secondary goal was to evaluate the extent of spatial summation on a dynamic version of vernier hyperacuity. METHODS: The ODT's and contrast sensitivity functions (CSF's) of optimally corrected young and old observers were compared as a function of vernier target length (4, 8, or 32 min), contrast level (5 or 30%), and oscillation rate (2 or 8 Hz). RESULTS: Age deficits on ODT's were related directly to rate of oscillation, but not target contrast. No age difference was seen in ODT's for short low-contrast targets; as target length increased thresholds improved more rapidly for young than old observers; this pattern was reversed at high contrast. ODT's were related strongly and consistently to contrast sensitivity for old but not young observers. CONCLUSIONS: Diminished spatial summation appears to contribute to the aging visual system's loss of temporal discrimination for low contrast oscillating targets. Spatial summation contributes more importantly to the displacement discrimination of oscillating than to static vernier targets.     

Detection of vernier and contrast-modulated stimuli with equal Fourier energy spectra by infants and adults

Infant and adult vernier acuity differed by a factor of only 4 to 6 when the stimuli were periodic and results were expressed in units of spatial phase. This ratio was much smaller than the factor of 50 to 100 obtained when we expressed our results and those of others in terms of the threshold spatial displacement in visual angle. We compared infant and adult vernier performance to performance on a "benchmark" contrast discrimination task, where the vernier and contrast discrimination stimuli contained identical Fourier contrast spectra. When we compared vernier performance directly to contrast discrimination performance, infant and adult data were remarkably similar, suggesting that similar parts of the visual system limit vernier and contrast performance of subjects of both ages. A control experiment on adults suggested that the superior performance of the contrast discrimination task is due to recruitment of visual pattern analyzers situated at a distance from the discontinuities in phase position and contrast.     

A comparison of the effects of ageing upon vernier and bisection acuity

While most positional acuity tasks exhibit an age-related decline in performance, the effect of ageing upon vernier acuity continues to be the subject of some debate. In the present study we employed a stimulus design that enabled the simultaneous determination of bisection and vernier acuities in 36 subjects, aged between 22 and 84 years. This approach provided a means for directly testing the hypothesis that ageing affects bisection acuity but not vernier acuity by ensuring that differences in stimulus configuration and in the subject's task were kept to an absolute minimum. Optimum thresholds increased as a function of age for both bisection and vernier tasks. Inter-subject threshold variability also increased with age. Issues surrounding the comparison of absolute vernier thresholds across different studies are discussed and two important methodological factors are identified: the precise statistical method used to estimate thresholds, and the magnitude, in angular terms, of the smallest spatial offset of the elements of the vernier stimulus which can be displayed. Comparison with previously published data indicates that the discrepancy between this study and most previous investigations with respect to the effect of age upon vernier performance can be at least partly accounted for by differences in the minimum displayable vernier offset. Vernier thresholds do increase with age. The increased variability of vernier thresholds in older subjects would appear to limit the diagnostic value of the test as a means of enabling normal ageing to be distinguished from visual loss due to pathology of the eye or visual system.     

Comparison of thresholds for high-speed drifting vernier and a matched temporal phase-discrimination task

For rapidly translating targets, vernier thresholds correspond to millisecond asynchronies between targets. The 'temporal hypothesis' is that these thresholds reflect the limiting sensitivity of asynchrony detectors. Previous studies showed that temporal thresholds are generally higher than vernier thresholds, but failed to reject the 'temporal hypothesis' because stimuli had differing spatiotemporal characteristics, and temporal thresholds depend strongly on stimulus and task. Here we use matched grating stimuli to test - and reject - the temporal hypothesis. Expressed as asynchrony, temporal phase discrimination was typically 10-fold poorer than vernier thresholds, and differed in dependence on spatial frequency, temporal frequency, contrast, and susceptibility to stroboscopic masks.     

Spatial summation determines the contrast response of displacement threshold hyperacuity

The effect of line length on displacement threshold hyperacuity at various levels of contrast was investigated. At high contrasts there was no significant effect of line length, but as contrast was reduced thresholds for shorter line lengths increased rapidly. Thresholds at longer line lengths demonstrated a form of contrast saturation which differed from vernier acuity whose thresholds increased consistently with a reduction in contrast. As line length in the displacement threshold task was reduced, the amount of contrast saturation became less and displacement thresholds therefore resembled the contrast response of vernier acuity. Results are explained in terms of spatial summation along the length of the lines, the effects of which depend upon the spatial configuration of the hyperacuity stimulus under consideration.     

Elevated vernier acuity thresholds in glaucoma

PURPOSE: In 1993, Piltz et al. observed that foveal vernier acuity thresholds for achromatic targets are elevated in patients with glaucoma. This study was undertaken to explore whether such elevated thresholds are present when subject groups are measured with targets of effectively equivalent contrast. Vernier acuity measures were also obtained with short-wavelength and frequency-doubled stimuli, to assess spatial hyperacuity performance in the short-wavelength-sensitive and magnocellular pathways, respectively. METHODS: Twenty patients with glaucoma and 19 subjects with normal vision participated. All subjects had visual acuity of 20/25 or better. Achromatic two-dot vernier thresholds were obtained for 90% contrast dots. In addition, individual contrast thresholds to the achromatic dots were measured for each subject, and vernier thresholds were measured at 4, 8, 12, and 16 times contrast threshold. Short-wavelength vernier acuity thresholds were measured for blue dots presented on a bright yellow background. The stimulus for the frequency-doubling grating vernier acuity task was a 90% contrast, 1-cyc/deg, 25-Hz sinusoidal grating. RESULTS: The glaucoma group demonstrated significantly higher foveal vernier acuity thresholds than control subjects for the blue-on-yellow stimulus (P = 0.002) and frequency-doubling grating stimulus (P < 0.001). No significant difference in vernier acuity between groups was found for the 90% contrast achromatic dots (P = 0.09), however a significant difference was found for the normalized contrast targets (P = 0.04). CONCLUSIONS: Vernier acuity tasks can be used to demonstrate abnormal foveal function in glaucoma. Testing with visual-function-specific stimuli may be effective in identifying such dysfunction. Vernier acuity, or other similar hyperacuity tasks that assess spatial sampling, may be useful in the detection of early glaucomatous damage, before it is detected with traditional perimetric tests.     

Electrophysiological correlates of vernier and relative motion mechanisms in human visual cortex

Vernier onset/offset thresholds were measured both psychophysically and with the steady-state VEP by introducing a series of horizontal breaks in a vertical square-wave luminance grating. Several diagnostic tests indicated that the first harmonic component of the evoked response generated by periodic modulation of offset gratings taps mechanisms that encode the relative position of spatial features. In the first test, a first harmonic component was only found with targets that contained transitions between collinear and noncollinear states. VEP vernier onset/offset thresholds obtained with foveal viewing were in the range of 15-22 arc sec. Control experiments with transitions between symmetrical, noncollinear patterns (relative motion) did not produce first harmonic components, nor did full-field motion of a collinear grating. A second series of experiments showed that VEP thresholds based on the first harmonic component of the vernier onset/offset response had an eccentricity dependence that was very similar to that found in a psychophysical discrimination task that required a left/right position judgment (vernier acuity). Other recordings showed that the first harmonic of the vernier onset/offset VEP was degraded by the introduction of a gap between stimulus elements, as is the displacement threshold. The vernier onset/offset target also produced a second harmonic component that was virtually identical to the one produced by a relative motion stimulus. Displacement thresholds based on these second harmonic components showed a more gradual decline with retinal eccentricity than did the first harmonic component elicited by vernier offsets. The second harmonic of the vernier onset/offset VEP was relatively unaffected by the introduction of gaps between the stimulus elements. The first and second harmonic components of the vernier onset/offset VEP thus tap different mechanisms, both of which support displacement thresholds that are finer than the resolution limits set by the spacing of the photoreceptors (hyperacuity).     

Is reduced vernier acuity in amblyopia due to position,contrast or fixation deficits?

Poor vernier acuity, exhibited by amblyopes, may reflect anomalies related to eccentric fixation, deficient position sensitivity, or reduced contour visibility. We have examined these factors by measuring contrast and vernier sensitivities with stimuli consisting of extended sinusoidal gratings of several spatial frequencies. Vernier thresholds were measured using both a classical single step position change and also a grating that was position-modulated sinusoidally along its entire length. For both types of target amblyopes exhibited deficient displacement sensitivity although accurate fixation was not strictly required. The vernier deficits were not critically dependent upon the type of displacement used, and the magnitudes of the vernier and contrast sensitivity deficits were closely related. Both were largest at high spatial frequencies, and those amblyopes with larger contrast sensitivity deficits also had larger vernier acuity deficiencies. Typically, contrast sensitivity and vernier acuity were normal, or nearly so, at very low spatial frequencies. Also, vernier acuities for the amblyopic and non-amblyopic eyes were approximately equal if grating contrast was set at some fixed multiple of detection threshold. We did not find a close relationship between the magnitude of the vernier deficit and reported perceptual distortions.     

Discrimination of position and contrast in amblyopic and peripheral vision

Many computational models of normal vernier acuity make predictions based on the just-noticeable contrast difference. Recently, Hu, Klein and Carney [(1993) Vision Research, 33, 1241–1258] compared vernier acuity and contrast discrimination (jnd) in normal foveal viewing using cosine gratings. In the jnd stimulus the test grating was added in-phase to the (sinusoidal) pedestal, whereas in the vernier stimulus the same test grating was added with an approx. 90 deg phase shift to the pedestal. In the present experiments, we measured thresholds for discriminating changes in relative position and changes in relative contrast for abutting, horizontal cosine gratings in a group of amblyopes using the Hu et al., test-pedestal approach. The approach here is to ask whether the reduced vernier acuity of amblyopes can be understood on the basis of reduced contrast sensitivity or contrast discrimination. Our results show that (i) abutting cosine vernier acuity is strongly dependent on stimulus contrast. (ii) In both anisometropic and strabismic amblyopes, abutting cosine vernier discrimination thresholds are elevated at all contrast levels, even after accounting for reduced target visibility, or contrast discrimination. (iii) For both strabismic and anisometropic amblyopes, the vernier Weber fraction is markedly degraded, while the contrast Weber fraction is normal or nearly so. (iv) In anisometropic amblyopes the elevated vernier thresholds are consistent with the observers' reduced cutoff spatial frequency, i.e. the loss can be accounted for on the basis of a shift in spatial scale. (v) In strabismic amblyopes and in the normal periphery, there appears to be an extra loss, which can not be accounted for by either reduced contrast sensitivity and contrast discrimination or by a shift in spatial scale. (vi) This extra loss cannot be quantitatively mimicked by “undersampling” the stimulus. (vii) Surprisingly, in some strabismics, and in the periphery, at relatively high spatial frequencies, vernier thresholds appear to lose their contrast dependence, suggesting the possibility that there may be qualitative differences between the normal fovea and these degraded visual systems. (viii) This contrast saturation can be mimicked by “undersampling” the target, or by introducing strips of mean luminance between the two vernier gratings, thus mimicking a “scotoma”. Taken together with the preceding paper, our results suggest that the extra loss in position acuity of strabismic amblyopes and the normal periphery may be a consequence of noise at a second stage of processing, which selectively degrades position but not contrast discrimination.     

Quantifying target conspicuity in contextual modulation by visual search

Contextual elements can strongly modulate visual performance. For example, performance deteriorates when a vernier is flanked by neighboring lines. On a neural level, such contextual modulation is often explained by local spatial interactions such as lateral inhibition or pooling. However, these mechanisms cannot account for a number of recent results which showed that global rather than local factors play a key role in contextual modulation. On a level of perceptual organization, we proposed that contextual modulation increases when the target groups with the flankers and decreases when the target stands out from the flankers. To quantify this "standing out" in foveal vision, here, we performed both a visual search and a vernier offset discrimination task on the same stimulus configurations. Stimulus configurations yielding short reaction times in visual search yielded good vernier discrimination performance. Stimulus configurations yielding long reaction times yielded weaker discrimination. Hence, vernier offset discrimination is superior for targets that are efficiently searched and vice versa.     

Senescent effects on binocular summation for contrast sensitivity and spatial interval acuity

PURPOSE: Age effects on binocular summation, a cortically mediated visual function, were compared for resolution acuity, contrast sensitivity (CS), and spatial interval (SI) hyperacuity. METHOD: The binocular and monocular thresholds of healthy, optimally-corrected healthy young (mean age 21.3 years) and old (mean age 69.9 years) observers were determined for acuity, SI discrimination, and CS at 1.0, 2.0, 4.0, 8.0 and 18.0 c/deg. RESULTS: No age effects were observed on monocular or binocular SI discrimination. The binocular summation ratios (BSRs) did not exceed the expected probability gain for resolution acuity or SI discrimination in either age group. Older observers showed a binocular inhibition effect on the SI task. On the CS task, the BSRs of the young significantly exceeded those of the old only at 18.0 c/deg. CONCLUSIONS: 1.) Binocular summation of high spatial frequency contrast information, which may be less robust in the senescent visual system, did not appear to be related to interocular CS differences, 2.) SI discrimination of high contrast, well-separated targets appears to be unaffected by aging, and 3.) Binocular neural summation on SI discrimination may be more likely to be seen with targets that are narrowly separated or low in contrast.     

Differences Between Fovea and Periphery in the Detection and Discrimination of Spatial Offsets

Previous work has shown that the ratio between the thresholds for detecting a spatial (vernier) offset and discriminating its direction is about two, if the targets are presented to the fovea, whether at a fixed, vertical, orientation, or a variable orientation. In the present study, vernier detection and discrimination thresholds were measured at the fovea and at two retinal eccentricities (3 and 10 deg), two presentation durations (300 and 1000 msec), and three target lengths (25, 50 or 100 min) with the targets either vertical or in a variable orientation. For vertical targets, thresholds rose at similar rates in the two tasks, so that their ratio was constant with eccentricity. For variable-orientation targets, thresholds rose with eccentricity at different rates in the two tasks, and more steeply for discrimination, so that at 10 deg, unlike in the fovea, detection performance was superior to discrimination performance. The implications for estimates of cortical magnification and possible differences in the specializations of foveal and peripheral vision are discussed. Copyright © 1996 Elsevier Science Ltd.     

OSCILLATORY MOVEMENT DISPLACEMENT THRESHOLDS: RESISTANCE TO OPTICAL IMAGE DEGRADATION

Vernier acuity, under optimal conditions, appears remarkably resistant to image degradation. This characteristic has been used to assess neural function in cataract patients who exhibit poor spatial resolution. Conventional resolution tests fail to differentiate between neural and optical causes of visual loss. The oscillatory movement displacement threshold--the smallest amplitude of oscillation which gives rise to the perception of movement--provides an alternative hyperacuity test. This may offer advantages over vernier acuity. The effect of image defocus upon displacement thresholds for a sinusoidal contrast grating of spatial frequency 2 c deg-1 was investigated for high (15 Hz) and low (2 Hz) oscillation frequencies. With visible stationary references, subjects were more sensitive to oscillation frequencies of 2 Hz than to 15 Hz (F 1,9 = 33.34, p less than 0.005) whilst blur of up to 2 dioptres had no significant effect (F3,27 = 1.73, p greater than 0.1). Simulating media opacities by spatial degradation affected both grating acuity and contrast sensitivity functions, whilst displacement thresholds remained largely unaffected. Results suggest that oscillatory movement displacement thresholds may be of value in assessing ocular neural dysfunction in the presence of media opacities.     

Effects of blur and eccentricity on differential spatial displacement discrimination

Differential spatial displacement discrimination thresholds were determined for stimuli consisting of blobs with Gaussian spatial and temporal contrast envelopes. The stimuli were presented at detection threshold luminance contrast. The tasks were similar to the two-point discrimination acuity task and the three-dot alignment hyperacuity task. Thresholds were determined as a function of eccentricity along the horizontal meridian of the visual field (from 45 degrees nasal to 65 degrees temporal). The spatial spread or blur parameter of the blobs was adopted as a scale parameter. The results show that the performance of the visual system in differential spatial displacement discrimination tasks becomes progressively more homogeneous for a progressive increase in the blur parameter of the stimuli. Scaling (i) the three-blob alignment results with estimates of the cortical magnification factor and (ii) the two-blob separation discrimination results with their corresponding neural blur parameter shows an impressive isotropy and blur scale-invariance for the mechanisms mediating differential spatial displacement discrimination across the visual field. These results are interpreted in terms of a scaled sampling lattice model of the visual system, in combination with an automatic scale-selection mechanism.     

A VEP measure of the binocular fusion of horizontal and vertical disparities

PURPOSE: Because of the lateral separation of the orbits, the retinal images differ in the two eyes. These differences are reconciled into a single image through sensory and motor fusional mechanisms. This study demonstrates electrophysiologically the effects that normal horizontal and vertical fusional processes have on the processing of monocular position signals. METHODS: VEPs were recorded in 16 healthy adults in response to a vernier onset-offset target presented to one eye. The vernier offsets appeared and disappeared at 2 Hz and were introduced into bar targets that were oriented either vertically (horizontal offsets) or horizontally (vertical offsets). The magnitude of the offsets was varied over the range of 0.5 to 10 arc min. VEP amplitude was measured as a function of the size of the dynamic offset under monocular viewing conditions and in the presence of two different static targets presented to the other eye. One of the static targets matched the dynamic test, except that it had no vernier offsets. The other static target, the static pedestal, matched the dynamic test, but contained a set of static vernier offsets in locations corresponding to the locations of the dynamic offsets presented to the other eye. RESULTS: VEP amplitude was a monotonically increasing function of vernier offset size under monocular viewing conditions. The addition of the static target without offsets in the other eye resulted in an increased amplitude VEP response. The addition of the static target with vernier offsets resulted in a decrease in VEP amplitude for both horizontal and vertical disparities. CONCLUSIONS: The normal process of fusion results in a single visual direction. To obtain a single visual direction, the visual system must synthesize a binocular visual direction that differs from the monocular components. One of the conditions (the static pedestal with offsets) produces binocular visual direction shifts that degrade the appearance of vernier onset-offset, and reduce VEP amplitude for both horizontal and vertical disparities. This characteristic evoked response marker is a promising tool for measuring binocular fusion objectively in patients with strabismus.     

Vernier in Motion: What Accounts for the Threshold Elevation?

Vernier acuity is susceptible to degradation by image motion. The purpose of this study was to determine to what extent vernier thresholds are elevated in the presence of image motion because of reduced stimulus visibility, due to contrast smearing, or to a shift in the spatial scale of analysis. To test the visibility hypothesis, we measured vernier thresholds as a function of stimulus velocity (0–6 deg/sec), for various levels of stimulus visibility, each normalized to the detection threshold at the respective velocity. Contrary to the prediction of the visibility hypothesis, vernier thresholds worsen as the velocity increases, even when the stimuli are equally visible. To test the shift in spatial scale hypothesis, we determined spatial frequency tuning functions for vernier discrimination and line detection tasks, using a masking paradigm. We measured vernier and line detection thresholds as a function of spatial frequency of a sine-wave mask (0.5-32 c/deg), and for stimulus and mask velocities ranging from 0 to 4 deg/sec. Peak masking for both vernier discrimination and line detection, which indicates the most sensitive band of spatial frequencies for each task, shifts systematically toward lower spatial frequencies as the velocity increases. The progressive increase in spatial scale largely accounts for the worsening of vernier thresholds for moving stimuli. Differences between peak masking for vernier discrimination and line detection were found at 0 and 1 deg/sec, suggesting that different mechanisms mediate the two tasks, at least at low velocities. The masking results are consistent with previous findings that directionally selective motion detectors mediate detection of moving stimuli, but suggest that these detectors do not analyze vernier offsets. We conclude that the elevation of vernier threshold for a moving stimulus is accounted for primarily by a shift of sensitivity to mechanisms of lower spatial frequency, and not by decreased stimulus visibility. Copyright © 1996 Elsevier Science Ltd.     

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.     

The development of vernier acuity in the cat

The development of vernier acuity was assessed in nine kittens using offsets in a series of square-wave gratings as stimulus targets. The kittens were tested on a jumping stand in a two-choice simultaneous discrimination paradigm, and daily thresholds were tracked with a modified staircase procedure. Vernier acuity improved gradually from initial values of 13-57' arc and reached asymptote at 1.5-5.4' arc by 70-90 days of age. The developmental function for vernier acuity is discussed in relation to physiologic development of the kitten visual system and is related to published data on the development of stereoacuity and spatial resolution in the same species.     

Contrast dependency of foveal spatial functions: orientation, vernier, separation, blur and displacement discrimination and the tilt and Poggendorff illusions.

To examine the effect of reducing luminance contrast in human foveal vision, discrimination thresholds were measured in four tasks and also a numerical measure of two visual illusions were obtained by a nulling technique. The patterns used for all tasks were made very similar to facilitate comparison between them--all featured luminance step edges whose contrast could be varied from near unity down to the detection threshold. Orientation, vernier and blur discrimination thresholds rise on average 5-6-fold when the contrast is reduced from near unity to a Michelson value of 0.03. Jump displacement thresholds are somewhat more robust to contrast reduction, and the curve of separation discrimination versus contrast is much shallower, rising by a factor of about 2. The magnitude of the Poggendorff and tilt illusions changes very little until the inducing contours are barely detectable.     

Variations in hyperacuity performance with age

The term hyperacuity has been applied to a group of stimuli which produce spatial thresholds smaller than those expected given the relatively large receptor spacing and the retinal image quality of the human eye. It is not yet firmly established whether hyperacuity performance declines with increasing age in the same way as most other measures of visual ability. This is perhaps due to the use of varying task configurations and criterion-dependent psychophysical techniques. The present study examines age-related performance in three different hyperacuity tasks using a criterion independent forced-choice method. Both displacement and bisection thresholds were found to increase with age, but there was no significant change in vernier acuity. This indicates that age has a differential effect on thresholds for various hyperacuities depending upon the task requirements. No significant age-related trend was observed in hyperacuity bias, which represents the difference between subjective and true physical alignment.