Feature detectors and vernier acuity

Habitual higher order aberrations affect Landolt but not Vernier acuity

To assess whether the eye's optical imperfections are relevant for hyperacute vision, we measured ocular wave aberrations, visual hyperacuity, and acuity thresholds in 31 eyes of young adults. Although there was a significant positive correlation between the subjects' performance in Vernier- and Landolt-optotype acuity tasks, we found clear differences in how far both acuity measures correlate with the eyes' optics. Landolt acuity thresholds were significantly better in eyes with low higher order aberrations and high visual Strehl ratios (r² = 0.22, p = 0.009), and significantly positively correlated with axial length (r² = 0.15, p = 0.03). A retinal image quality metric, calculated as two-dimensional correlation between perfect and actual retinal image, was also correlated with Landolt acuity thresholds (r² = 0.27, p = 0.003). No such correlations were found with Vernier acuity performance (r² < 0.03, p > 0.3). Based on these results, hyperacuity thresholds are, contrary to resolution acuity, not affected by higher order aberrations of the eye.

The equivalent internal orientation and position noise for contour integration

Contour integration is the joining-up of local responses to parts of a contour into a continuous percept. In typical studies observers detect contours formed of discrete wavelets, presented against a background of random wavelets. This measures performance for detecting contours in the limiting external noise that background provides. Our novel task measures contour integration without requiring any background noise. This allowed us to perform noise-masking experiments using orientation and position noise. From these we measure the equivalent internal noise for contour integration. We found an orientation noise of 6° and position noise of 3 arcmin. Orientation noise was 2.6x higher in contour integration compared to an orientation discrimination control task. Comparing against a position discrimination task found position noise in contours to be 2.4x lower. This suggests contour integration involves intermediate processing that enhances the quality of element position representation at the expense of element orientation. Efficiency relative to the ideal observer was lower for the contour tasks (36% in orientation noise, 21% in position noise) compared to the controls (54% and 57%).

Cortical sources of Vernier acuity in the human visual system: An EEG-source imaging study

Vernier acuity determines the relative position of visual features with a precision better than the sampling resolution of cone receptors in the retina. Because Vernier displacement is thought to be mediated by orientation-tuned mechanisms, Vernier acuity is presumed to be processed in striate visual cortex (V1). However, there is considerable evidence suggesting that Vernier acuity is dependent not only on structures in V1 but also on processing in extrastriate cortical regions. Here we used functional magnetic resonance imaging–informed electroencephalogram source imaging to localize the cortical sources of Vernier acuity in observers with normal vision. We measured suprathreshold and near-threshold responses to Vernier onset/offset stimuli at different stages of the visual cortical hierarchy, including V1, hV4, lateral occipital cortex (LOC), and middle temporal cortex (hMT+). These responses were compared with responses to grating on/off stimuli, as well as to stimuli that control for lateral motion in the Vernier task. Our results show that all visual cortical regions of interest (ROIs) responded to both suprathreshold Vernier and grating stimuli. However, thresholds for Vernier displacement (Vernier acuity) were lowest in V1 and LOC compared with hV4 and hMT+, whereas all visual ROIs had identical thresholds for spatial frequency (grating acuity) and for relative motion. The cortical selectivity of sensitivity to Vernier displacement provides strong evidence that LOC, in addition to V1, is involved in Vernier acuity processing. The robust activation of LOC might be related to the sensitivity to the relative position of features, which is common to Vernier displacement and to some kinds of texture segmentation.

Detection of small orientation changes and the precision of visual working memory

We investigated the precision of orientation representations with two tasks, change detection and recall. Previously change detection has been measured only with relatively large orientation changes compared to psychophysical thresholds. In the first experiment, we measured the observers' ability (d') to detect small changes in orientation (5-30°) with 1-4 Gabor items. With one item even a 10° change was well detected (average d'=2.5). As the amount of change increased to 30°, the d' increased to 5.2. When the number of items was increased, the d's gradually decreased. In the second experiment, we used a recall task and the observers adjusted the orientation of a probe Gabor to match the orientation of a Gabor held in the memory. The standard deviation (s.d.) of errors was calculated from the Gaussian distribution fitted to the data. As the number of items increased from 1 to 6, the s.d. increased from 8.6° to 19.6°. Even with six items, the observers did not make any random adjustments. The results show a square root relation between the d'/s.d. and the number of items. The d' in change detection is directly proportional to the square root of (1/n) and the orientation change. The increase of the s.d. in recall task is inversely proportional to square root of (1/n). The results suggest that limited resources and precision of representations, without additional assumptions, determine the memory performance.

Relationship between threshold and suprathreshold perception of position and stereoscopic depth

We seek to determine the relationship between threshold and suprathreshold perception for position offset and stereoscopic depth perception under conditions that elevate their respective thresholds. Two threshold-elevating conditions were used: (1) increasing the interline gap and (2) dioptric blur. Although increasing the interline gap increases position (Vernier) offset and stereoscopic disparity thresholds substantially, the perception of suprathreshold position offset and stereoscopic depth remains unchanged. Perception of suprathreshold position offset also remains unchanged when the Vernier threshold is elevated by dioptric blur. We show that such normalization of suprathreshold position offset can be attributed to the topographical-map-based encoding of position. On the other hand, dioptric blur increases the stereoscopic disparity thresholds and reduces the perceived suprathreshold stereoscopic depth, which can be accounted for by a disparity-computation model in which the activities of absolute disparity encoders are multiplied by a Gaussian weighting function that is centered on the horopter. Overall, the statement "equal suprathreshold perception occurs in threshold-elevated and unelevated conditions when the stimuli are equally above their corresponding thresholds" describes the results better than the statement "suprathreshold stimuli are perceived as equal when they are equal multiples of their respective threshold values."

A psychophysical study of human binocular interactions in normal and amblyopic visual systems

During infancy and childhood, spatial contrast sensitivity and alignment sensitivity undergo maturation, and during this period the visual system has considerable plasticity. The purpose of this study was to compare the nature of interocular interactions of these spatial functions in normally sighted children and adults, and to study the extent to which interocular interactions are impaired in anisometropic amblyopia. Spatial functions were measured under three viewing conditions: monocular (fellow eye occluded), dichoptic (uniform stimulus presented to the fellow eye but with a peripheral fusion lock), and binocular. Measurements were made in each eye during monocular and dichoptic viewing. In the contrast sensitivity task, Gabor stimuli were presented in one of two temporal intervals. For the alignment task, a three-element Gabor stimulus was used. The task of the subject was to indicate the direction of displacement of the middle patch with respect to the outer patches. The findings indicate that in children, binocular contrast sensitivity was better than monocular (binocular summation) but so too was dichoptic sensitivity (dichoptic summation). The magnitude of binocular/dichoptic summation was significantly greater in children than in normally sighted adults for contrast sensitivity, but not for alignment sensitivity. In anisometropic amblyopes, however, we find that for the group as a whole the amblyopic eye does not benefit when the fellow eye views a dichoptic stimulus, compared to dark occlusion of that eye. In addition, we found considerable inter-individual variation within the amblyopic group. Implications of these findings for techniques used in vision therapy are discussed.

On the Benefits of Transient Attention across the Visual Field

There are well-known differences in resolution and performance across the visual field with performance generally better for the lower than the upper visual hemifield. Here we attempted to assess how transient attention summoned by a peripheral precue affects performance across the visual field. Four different attentional precueing tasks were used, varying in difficulty and attentional load. When a single discrimination target was presented (experiments 1 and 2), precues that summon transient attention had very little, if any, effect upon performance. However, when the target was presented among distractors (experiments 3 and 4), the precue had a substantial effect upon discrimination performance. The results showed that asymmetries in visual resolution between the upper and lower hemifields become more pronounced with increasing eccentricity. Furthermore, when the observers performed a precued acuity task with distractors, involving the judgment of the relative position of a small disk within a larger one, there was an asymmetry in the transient attentional effect on discrimination performance; the benefits of transient attention were larger in the upper than in the lower hemifield. Areas in the visual field where visual performance is generally worse thus appear to receive the largest attentional boost when needed. Possible ecological explanations for this are discussed.

Orientation repulsion and attraction in alignment perception

Orientation masking induces changes of discrimination thresholds and perceived orientation. Studies on alignment discrimination of Vernier stimuli concentrated on masking induced changes of discrimination thresholds, without considering possible changes of perceived orientation and/or alignment of the two-line segments. Measuring both parameters in an orientation discrimination task, we confirmed a standard repulsion effect between a single line target and a mask grating that co-varied with elevated orientation discrimination thresholds. Masking a Vernier stimulus in an alignment discrimination task, we observed a strong misperception of alignment that was accompanied with elevated alignment discrimination thresholds. Orientation masking on perceived orientation and alignment of a Vernier stimulus revealed orientation repulsion and attraction that depended on the spatio-orientation configuration of the superimposed stimuli. Control of task-dependent effects confirmed that our observed pattern of results was independent of attentional or cognitive demands.

Neural fine tuning during Vernier acuity training?

A superposition masking and summation to threshold paradigm was employed before and after unmasked Vernier acuity training to measure sensory changes of offset analysing mechanisms. Masking functions show a uniform downward translation after training and detection data reveal higher sensitivities to compound Gabor gratings in the post-test. These findings confirm the existence of learning related changes at early levels of information processing, but the results cannot be explained by neural fine tuning of offset analysing mechanisms. The data are consistent with the idea of task dependent broadening of orientation tuned mechanisms responsible for detecting small Vernier offsets.

Velocity dependence of Vernier and letter acuity for band-pass filtered moving stimuli

The ability to see fine detail diminishes when the target of interest moves at a speed greater than a few deg/s. The purpose of this study was to identify fundamental limitations on spatial acuity that result from image motion. Discrimination of Vernier offset was measured for a pair of vertical abutting lines and letter resolution was measured using a four-orientation letter 'T'. These stimuli were digitally filtered using one of five band-pass (bandwidth=1.5 octaves) filters with a center frequency between 0.83 and 13.2 c/deg, and presented at velocities that ranged from 0 to 12 deg/s. Filtered and unfiltered stimuli were presented for 150 ms at a constant multiple (4x or 2x) of the contrast-detection threshold at each velocity. For stimuli of low to middle spatial frequency (up to 3.3 c/deg), Vernier and letter acuity for equally detectable targets are essentially unaffected by velocity up to 12 deg/s, i.e., for temporal frequencies of motion (velocity x spatial frequency) up to approximately 50 Hz. For stimuli of higher spatial frequency, acuity remains essentially constant until the velocity corresponds to a temporal frequency of about 30 Hz, and increases thereafter. Both Vernier and letter acuities worsen by approximately a factor of two for each one-octave decrease in filter spatial frequency. Both types of acuities worsen also as the contrast of the stimulus is reduced, but Vernier discrimination exhibits a stronger contrast-dependence than letter resolution. Our results support previous suggestions that a shift in the spatial scale used by the visual system to analyze spatial stimuli is principally responsible for the degradation of acuity in the presence of image motion. The results are consistent with a spatio-temporal-frequency limitation on spatial thresholds for moving stimuli, and not with a temporal-frequency limitation per se.

Late maturation of visual hyperacuity

We used a visual evoked-potential measure to study the development of two components of pattern vision, vernier acuity and grating acuity, in humans from early infancy through adolescence. These two visual functions develop at similar rates and have nearly the same absolute values between 1 month and 6 years of age. After age 6, grating acuity is constant at the adult level, but vernier acuity continues to improve, becoming a hyperacuity. Vernier acuity reaches asymptotic levels around age 14 years. These results suggest that adultlike vernier hyperacuity is not limited by spatial resolution or sensitivity of small receptive fields, but rather that the limitation is imposed by higher-level processing. Sensitivity, connections in visual cortical areas, or both therefore retain plasticity throughout childhood and into adolescence.

Micropattern orientation and spatial localization

A current, popular, theory of spatial localization holds that the visual system represents the location of simple objects by a single positional tag, the accuracy of which is largely independent of the internal properties of the object. We have already presented evidence of the limitations of such a view (Keeble & Hess (1998). Vision Research, 38, 827-840) in that 3-micropattern alignment performance was found to be dependent on the orientation of the micropatterns. We tested whether this was caused by a local anisotropy in positional coding by conducting 3-micropattern bisection experiments with varying patch orientation. No corresponding effect of patch orientation was found, implying a difference in the mechanisms used for the two tasks. In a further experiment we show that alignment task performance is very similar to the otherwise identical 2-patch orientation discrimination task. We conclude that the 3-micropattern alignment task is mediated by orientational mechanisms. We therefore present a 2nd-order orientation model for 3-patch alignment.

Unmasking the mechanisms for Vernier acuity: evidence for a template model for Vernier acuity

The goal of this study was to evaluate the mechanisms underlying Vernier acuity, over a range of spatial scales using narrow-band Vernier stimuli and oblique masking. Specifically, the test stimuli consisted of a pair of vertical ribbons of horizontal cosine grating with a vertical Vernier offset between the ribbons. These stimuli have two important advantages for studying Vernier acuity: (1) they are relatively well localized in vertical spatial frequency, and (2) they are localized in their horizontal extent (width). We measured the orientation, spatial frequency and width tuning of Vernier acuity over a wide range of ribbon spatial frequencies, using a simultaneous oblique masking paradigm. Our masking results suggest that the mechanisms underlying Vernier acuity are tuned to the orientation, spatial frequency and width of the ribbon stimuli. The peak of the bimodal orientation tuning function varies systematically with the spatial frequency of the ribbon. The peak of the spatial frequency tuning function varies systematically with both the ribbon spatial frequency, and the ribbon width (i.e. the grating length). A 'template' model, in which the 'mechanism' is a windowed version of the stimulus is able to account for many features of the data, including results which cannot be easily accounted for by standard multi-scale filter models. Specifically, the template model can account for: (i) the bimodal orientation tuning function, (ii) the systematic variation in the peak of the orientation and spatial frequency tuning functions with spatial frequency, and (iii) the systematic effect of ribbon width on spatial frequency tuning.

Vernier and contrast discrimination in central and peripheral vision

The present paper asks whether Vernier offset discrimination is limited by the observer's sensitivity to local contrast change in both central and peripheral vision. To answer this question we compared Vernier discrimination and contrast discrimination thresholds (specified in the same units) for a pair of narrow ribbons of cosine gratings. Because the ribbons are narrow, both the offset information (for Vernier discrimination) and the contrast information (for contrast discrimination) are highly localized. We found that when the stimuli are narrow ribbons, the local contrast cue is the limiting factor in Vernier discrimination. However, our results also show that integration of information along the length of the gratings (the ribbon width) is: (i) different for Vernier and contrast discrimination, and (ii) for Vernier discrimination the integration of information along the length of the gratings differs qualitatively in central and peripheral vision. For narrow ribbons, the peripheral 'template' for ribbon Vernier acuity is not as well matched to the stimulus (in two-dimensional spatial frequency space) as the foveal 'template'.

Elevation of Vernier thresholds during image motion depends on target configuration

Previously we showed that thresholds for abutting Vernier targets are unaffected by motion, as long as the targets are processed by the same spatial-frequency channel at each velocity and remain equally detectable [Invest. Ophthalmol. Visual Sci. (Suppl.) 37, S734 (1996)]. In this study we compared Vernier thresholds for stationary and moving abutting and nonabutting targets (gaps = 0, 18, and 36 arc min) for velocities of 0-16 deg/s. The Vernier targets were spatially filtered vertical lines (peak spatial frequency = 3.3 or 6.6 c/deg), presented at contrast levels of two, four, and eight times the detection threshold of each component line. Unlike the results for abutting targets, Vernier thresholds for nonabutting targets worsen with velocity as well as gap size. The results for abutting Vernier targets are consistent with the hypothesis that thresholds are mediated by oriented spatial filters, whose responses increase proportionally with the stimulus contrast. The velocity-dependent thresholds found for nonabutting Vernier targets can be explained on the basis of local-sign comparisons if the comparison process is assumed to include a small amount of temporal noise.

Progress and paradigm shifts in spatial vision over the 20 years of ECVP

In the beginning there was light, and form, and visual mechanisms. This paper traces developments in research on spatial vision over the 20 years of ECVP, with particular emphasis on (1) hyperacuity, (2) peripheral vision, (3) amblyopia and development, and (4) learning and plasticity.

The combination of filters in early spatial vision: a retrospective analysis of the MIRAGE model

Since the discovery of spatial-frequency-tuned channels in the visual system, most theories attempting to account for pattern encoding have assumed that the filters can be independently accessed and flexibly combined. We review here an alternative model, 'MIRAGE', in which the filters are inflexibly combined before pattern analysis. In the MIRAGE model the half-wave rectified outputs of all spatial-frequency channels are combined before locating spatial zero-bounded regions in the neural image, which serve as the spatial primitives for pattern analysis. We describe the evidence that led to this model, and review recent evidence on the rules of filter combination.

How the human visual system encodes the orientation of a texture, and why it makes mistakes

Human observers are exquisitely sensitive to tilt in the orientation of a line. We can detect rotations away from the vertical of 0.5 degrees. It has been suggested [1,2] that this accuracy is a result of the orientation-selectivity of simple cells in the primary visual cortex (V1), many of which have receptive fields with an elliptical shape [3]. However, it is possible to sense the tilt of many stimuli that are unlikely to have their tilt directly encoded by such cells. For example, a garment such as a tie with diagonal stripes would predominantly stimulate cells in V1 tuned to an orientation of the stripes; yet we could tell whether or not the garment as a whole was tilted from the vertical. The perception of oriented textures is subject to systematic errors, however. A striking example is the Fraser 'twisted cord' illusion (Fig. 1) in which we see the global orientation of the horizontal texture-defined lines as being tilted in the direction of its locally tilted segments. If the component segments are at a larger angle (30 degrees) to the global orientation, on the other hand, the perceived shift is in the opposite direction. We have measured these effects psychophysically, and we propose a model in which second-order orientation units receive excitation from V1 units of similar orientation, but inhibition from V1 units of dissimilar orientation. Our model correctly predicts that making the textures different in average brightness from the background will reduce the illusions.

Vernier acuity with plaid masks: the role of oriented filters in vernier acuity

Superimposition of oriented grating masks on vernier targets results in bimodal patterns of vernier threshold elevation, with peaks occurring on either side of vernier target orientation. These bimodal masking effects suggest a contribution to vernier acuity from spatial filters tuned to orientations on either side of the target. We report similar bimodal threshold elevation with plaid masks composed of symmetrically oriented pairs of gratings. Since filters oriented to either side of the vernier stimulus will be affected similarly by plaid masks, it is unlikely that threshold elevation reflects disruption of relative filter activity that is used to code for change in target orientation. Instead, the results support the proposition that misalignments are detected on the basis of differential (i.e. absolute rather than relative) activity of spatial filters. Our plaid-mask data also rule out the possibility that: (i) "off-channel" looking; or (ii) detection of orientation shifts (e.g. tilt illusions), underlie bimodal masking effects. The finding that weak bimodal threshold elevation occurs with dot targets separated by 40 min arc further suggests that the mechanisms involved in detecting misalignments over large regions [possibly collator/collector-type mechanisms] also do so via analysis of their differential activity.

Resolution acuity is better than vernier acuity

Our impressive sensitivity to vernier offsets as compared to resolution acuity has long inspired vision researchers to study the phenomena in great detail. In this study we use the test-pedestal framework to compare resolution and vernier acuity. In these experiments the test stimulus is the same for both tasks, only the pedestals differ. When thresholds are expressed in common units of test strength, vernier acuity thresholds are higher (worse) than for resolution and contrast discrimination tasks over the range of pedestal strengths tested. This apparent reversal of sensitivity is actually consistent with expectations based on the presumed underlying visual mechanisms involved in the tasks.

Spatial properties of filters underlying vernier acuity revealed by masking: evidence for collator mechanisms

Models of vernier acuity based on the differential response of oriented filters receive support from the finding that vernier threshold elevation peaks for grating mask orientations which are slightly different from the orientation of the vernier bars. We replicate this effect using long, abutting vernier bars, and masks which possess gaps up to 22.5' wide (Experiments 1 and 3); a surprising result considering that vernier acuity improves little for bars longer than 10'. To account for this we suggest the involvement of elongated mechanisms (referred to as collators or collectors) that "integrate" responses of numerous smaller filters along the axis of their common orientation. The collator model explains patterns of threshold elevation obtained with a variety of mask-vernier configurations. In particular, the model predicts that masks located midway between separated vernier bars will interfere with integrative processes occurring over the entire region encompassing both bars (Experiment 2). In confirmation of this prediction we find that centrally placed masks produce significant orientation-specific threshold elevation. In suggesting a contribution to vernier acuity from integrative mechanisms, our results, along with others, emphasize the importance of global processes in vernier acuity.

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.

Position acuity with opposite-contrast polarity features: evidence for a nonlinear collector mechanism for position acuity?

Vernier acuity for opposite-contrast polarity stimuli clearly poses problems for local contrast models of relative position processing. In Expt 1 we show that vernier thresholds for abutting, or closely separated features of opposite-contrast polarity, are degraded across a wide range of stimulus strengths and configurations; but for widely separated stimuli they are more or less independent of contrast polarity (confirming and extending previous work). In Expts 2 and 3 we use a one-dimensional spatial noise masking paradigm to investigate to what extent the same mechanisms masked by this noise contribute to the relative position processing of same and opposite polarity stimuli. The orientation tuning functions determined using this paradigm are quite different for same and opposite polarity targets, for both line vernier acuity, and closely spaced two-dot alignment. However, for widely separated targets (24 min arc or more), they are similar. Over a range of separations from 3 to 30 min arc, for same and opposite polarity dots, masking is strongest at a spatial frequency of about 10 c/deg. Our results are consistent with the notion that signals from early (and relatively high spatial frequency) linear filters are collected in a second-stage nonlinear mechanism, which collates information along an orientation trajectory. We suggest that different properties of the mechanisms at each level of processing, can constrain positional acuity at small and large separations.

Vernier acuity during image rotation and translation: visual performance limits

Our capacity to detect spatial misalignments a fraction of the distance between retinal receptors in the presence of image motion challenges our understanding of spatial vision. We find that vernier acuity, while robust to image translation, rapidly degrades during image rotation. This indicates that orientation is a critical cue utilized by the visual system in vernier acuity tasks. Moreover, vernier acuity is robust to translational motion only at high target strengths. Vernier acuity for translating 3-dot targets over midrange velocities can be predicted from vernier acuity data derived from static targets of different presentation durations. However, the degradation observed at higher velocities is greater than predicted. The high velocity degradation reveals that performance is limited by a 1 msec asynchrony sensitivity. The moving vernier stimulus appears to constitute an optimal configuration for the visual system to achieve a 1 msec asynchrony sensitivity by making use of an orientation cue.

Binocular processes in vernier acuity

We investigate the role of binocular mechanisms in vernier acuity, using dichoptic variants of spatial-frequency masking and flank-line interference paradigms. The finding that grating masks and flanking lines presented to one eye elevate (worsen) thresholds for detecting vernier offsets presented to the other eye suggests that neural mechanisms mediating vernier acuity receive binocular inputs, thus placing the loci of these mechanisms at postreceptoral sites. The observation that these threshold elevation effects are orientation dependent is consistent with a contribution to vernier acuity from oriented cortical filters.

Perceptual learning in vernier acuity: what is learned?

It has been suggested that the improvement of vernier acuity in the course of practice reflects "fine tuning" of the visual mechanisms underlying vernier acuity. Masking studies suggest that an important source of information by which the visual system may accomplish fine vernier acuity is the activity in orientation tuned channels. Therefore, we investigated whether improvement in vernier acuity after training was accompanied by systematic changes in the orientation tuning characteristics of vernier acuity (as revealed by simultaneous spatial noise masking). The results show large interindividual variation in learning vernier acuity. However, they reveal a close correspondence between the improvement in vernier acuity and the narrowing of the orientation tuning function. Thus, the results provide some support for the notion of narrowing of the orientation characteristics of vernier acuity in the course of learning.

Spatial scale shifts in amblyopia

We used a masking paradigm to uncover the properties of the mechanisms engaged by the amblyopic visual system for vernier acuity and line detection. Line vernier and line detection thresholds were measured in the presence of one-dimensional noise masks varying in orientation, spatial frequency content or contrast. Our results reveal that in both normal and amblyopic eyes, there is a bimodal orientation tuning function for vernier acuity, i.e. vernier acuity is most strongly masked by mask orientations approx. +/- 10 deg on either side of the target lines. In contrast, in both normal and amblyopic eyes, line detection is most strongly masked when the mask and line target have the same orientation. In the normal fovea, the spatial frequency tuning is bandpass, with a peak spatial frequency of about 10 c/deg. In the amblyopic eyes, the spatial tuning is similar in specificity; however the peak is shifted to lower spatial frequencies, suggesting a shift in the scale of spatial processing of line stimuli. For all of the amblyopic eyes, the increased line detection thresholds are approximately proportional to the shift in spatial scale. In anisometropic amblyopes, the (unmasked) vernier threshold is elevated in proportion to the shift in spatial scale; however in some amblyopes with constant strabismus the shift in spatial scale is not sufficient to account for the degraded vernier acuity. The "extra" increase in vernier thresholds associated with strabismus may be a consequence of a high degree of positional uncertainty which adds noise at a stage following the combination of filter responses.

Spatial integration in position acuity

To determine whether and how spatial integration takes place in position acuity, bisection and Vernier thresholds were measured in the fovea of four normal observers with spatially "undersampled" dark lines (i.e. lines comprised of discrete samples). The size, contrast, and density of samples, and the separation of the lines were varied. For a given sampling density, sample size (0.17-2.72 min) has negligible effect on position threshold. For all sample sizes, position threshold decreases as sampling density increases, indicating that spatial integration takes place. The form of spatial integration depends on line separation. At the optimal line separation (2 min for bisection and 0 min for Vernier), position threshold decreases as sampling density increases with a slope of about -0.8 on log axes, steeper than a slope of -0.5 as would be expected from statistical position averaging. This effect of sampling density can be completely explained by spatial contrast summation for visibility. At the 16 min line separation, position threshold also decreases as sampling density increases but with a slope shallower than -0.5. However, this effect of sampling density can not be explained by contrast summation. Position thresholds decrease even after discounting the effect of contrast summation on visibility, suggesting a genuine position averaging. These findings are independent of line orientation (horizontal or vertical), and hold for both random and uniform dot distributions, and for both bisection and Vernier. Thus, two separate mechanisms of position acuity are suggested. A spatial filter mechanism operates at the optimal (or narrow) line separation where position threshold is critically dependent on stimulus visibility. A local sign mechanism operates at the relatively wider line separation where position acuity benefits from local sign position averaging. For both mechanisms, spatial integration is not perfect.

Spatial scale shifts in peripheral vernier acuity

Abutting line vernier acuity thresholds are markedly degraded in peripheral vision, while line detection thresholds are elevated to a much lesser extent. To study the spatial and orientation tuning properties of the mechanisms underlying peripheral line vernier acuity, abutting vernier thresholds were measured in the presence of one-dimensional band-limited spatial noise masks varying in orientation and spatial frequency. To examine the effects of these masks on target visibility, line detection thresholds were also measured. We find that in both the fovea and the periphery, noise masking produces marked elevations of vernier thresholds, which are tuned to both spatial frequency and orientation. (i) Spatial frequency tuning: in the fovea, the spatial frequency tuning is bandpass, with a bandwidth of approximately 2.5 octaves, and a peak spatial frequency of about 10 c/deg. In the periphery the spatial tuning is similar in bandwidth, however the peak shifts systematically to lower spatial frequencies with increasing eccentricity, implying that thresholds are mediated by spatial mechanisms tuned to progressively larger spatial scales with eccentricity. (ii) Orientation tuning: at all eccentricities there is a bimodal orientation tuning function for vernier acuity, consistent with the hypothesis that the responses of at least two filters, whose orientations straddle the target lines, are combined to extract vernier offset information. In contrast, at all eccentricities, line detection is most strongly masked when the mask and line target have the same orientation. For both the line detection and line vernier tasks, the scale of the most sensitive spatial mechanisms shifts systematically with eccentricity. The change in line detection threshold with eccentricity is approximately proportional to the variation in spatial scale; however this shift in spatial scale is not sufficient to account for the degraded peripheral vernier acuity. The extra increase in peripheral vernier thresholds may be a consequence of a high degree of positional uncertainty which adds noise at a stage following the combination of filter responses.

Binocular rivalry disrupts stereopsis

Does the shift from binocular rivalry to fusion or stereopsis take time? We measured stereoacuity after rivalry suppression of one half-image of a stereoacuity line target. After the observer signalled that the single stereo half-image had been suppressed, the other half-image was presented for a variable duration. Stereoacuity thresholds were elevated for 150-200 ms. A control experiment demonstrated that the threshold elevation was due to rivalry suppression per se, rather than masking effects associated with the rivalry-inducing target. Monocular Vernier thresholds, measured as the smallest identifiable abrupt shift in the upper line of an aligned Vernier target that had previously been suppressed by rivalry, were elevated for a much longer duration. This result shows that an appropriately matched stereo pair can break rivalry suppression more easily than can monocular changes in position. With the aid of a similar paradigm, we also measured the duration needed to detect a disparate feature in a random-dot stereogram after rivalry suppression of one half-image of the stereogram. Observers could correctly identify the location of the disparate feature (upper or lower visual field) when the other half-image was presented for a duration ranging from 150-650 ms. In the absence of the matching half-image, the first half-image was suppressed by the rival target for a far longer duration (a few seconds). These findings show that although stereopsis and fusion terminate rivalry, both are initially disrupted for a few hundred milliseconds by rivalry suppression.

Orientation, masking, and vernier acuity for line targets

In an attempt to uncover the properties of the psychophysical spatial mechanisms which optimally respond to the vernier offset between two abutting lines, we investigated the effects of one-dimensional band-limited spatial noise masks superimposed with the target, on vernier thresholds. Unidirectional vernier thresholds were measured in the presence of masks varying in orientation, spatial frequency content and luminance modulation. Because of the dependence of vernier thresholds on target visibility, the effects of these masks on target detection thresholds were also measured. In accordance with the results of Findlay [(1973) Nature, 241, 135-137] but contrary to an hypothesis that the direction of the vernier offset is mediated by the differential output of spatial filters of a single orientation, our results reveal a bimodal orientation tuning function for vernier acuity. We propose that, for offset line targets, the differential responses of at least two filters with orientations which straddle the target lines are combined to extract relative position information. The spatial frequency tuning characteristics of the optimal mechanisms for mediating vernier information are similar to those optimal for detecting the target lines themselves, except that they are tuned to a slightly higher spatial frequency and have a slightly narrower bandwidth. The spatial mechanisms most sensitive to the vernier offset and to target detection exhibit similar responses to increases in mask modulation. This finding suggests that these tasks are limited by the same source of noise, and explains why under a variety of experimental manipulations, equally visible vernier targets result in similar vernier thresholds.

Can sinusoidal vernier acuity be predicted by contrast discrimination?

A test-pedestal approach, with a test grating superimposed on a masking pedestal, was used to compare sinusoidal grating vernier acuity and contrast discrimination thresholds. The goal is to develop a simple model for vernier acuity without assumptions about underlying mechanisms. In the contrast discrimination task, subjects were asked to detect contrast increments in the presence of a base pedestal. In the vernier task, a test grating shifted by 90 deg relative to the pedestal grating was added to one-half of the pedestal grating to produce a vernier offset. When expressed in the same contrast units and compared under optimal conditions, vernier and contrast discrimination thresholds agree well at spatial frequencies between 2 and 20 c/deg and at pedestal contrasts above 10 times detection threshold. Thus, under these conditions, contrast discrimination predicts grating vernier acuity. To account for the discrepancies between vernier thresholds and contrast just noticeable difference (JND) when conditions deviate from optimal, one needs to make assumptions about the underlying mechanisms.

Visibility, timing and vernier acuity

To investigate the relationship between contrast detection and vernier acuity for abutting targets, the effects of varying target exposure duration (12-2000 msec) on vernier and contrast detection thresholds for long, thin lines and sinusoidal gratings (1 and 8 c/deg), were measured. Vernier thresholds decreased with both increasing exposure duration and increasing target contrast. Predictions made for equally visible targets show that the effect of exposure duration on vernier thresholds is almost completely accounted for by its effect on target visibility. Vernier thresholds and contrast detection thresholds for line targets were also measured in the presence of a spatiotemporal mask, for different exposure durations. Again, once the effect of this mask on target visibility was accounted for, there was virtually no remaining effect of exposure duration on vernier thresholds. The results of these experiments suggest that similar spatial mechanisms mediate both contrast detection thresholds and vernier thresholds for abutting targets; and that the processes involved in target detection and the extraction of relative position information are limited by the same factors.

The coding of spatial position by the human visual system: effects of spatial scale and contrast

In this study we investigate the nature of the computations that underlie the encoding of spatial position by the human visual system. Specifically, we explore the relationship between alignment accuracy and spatial scale on the one hand, and between alignment accuracy and contrast on the other. We do this for stimuli where local luminance, local contrast, and orientation cues do not underlie performance. The results suggest that spatial localisation is independent of spatial scale and weakly dependent on contrast. We present subsequent models based on the properties of some classes of visual cortical neurones, namely multiplicative noise and contrast energy detection of complex cells, which describe the form of these relationships.

Vernier acuity: effects of chromatic content, blur and contrast

Offset thresholds were measured for targets whose horizontal profiles were either Gaussian or odd-symmetric Gabor functions. The targets were defined either by variation along the constant B or the constant R & G axes of color space or by luminance variation. Blur was varied in the case of the Gaussian targets by varying the standard deviation of the distribution and in the case of the Gabor functions by varying the spatial frequency of the sinusoidal component. Detection thresholds for all the stimuli were measured. The contrast of the targets used in the measurement of offset thresholds was varied from just above detection threshold to the maximum that could be produced. The offset thresholds obtained with targets of different chromatic composition are nearly identical when blur and contrast relative to detection threshold are held constant. We attribute the slight advantage held by luminance targets over chromatic targets for narrow Gaussians to the detectability of low frequency components of the chromatic targets which are of little use in the assessment of offsets. This conjecture is supported by the complete absence of such an advantage in the case of Gabor targets.

Positional acuity without monocular cues

The accuracy with which human observers can determine the spatial location of a shape boundary was measured by vernier alignment. The vernier targets were presented as random-dot stereograms, with varying amounts of camouflage in the monocular image. Camouflage decreased vernier acuity, but when the camouflage was broken by stereoscopic disparity, acuity was improved. In the limiting case when the shape boundaries were defined by disparity information alone, vernier thresholds (75% correct, binary forced-choice) were in the region of 40 s visual angle. This is poor acuity in comparison to vernier thresholds with monocular contour, but if the limited resolution acuity for stereopsis is taken into account, cyclopean and monocular positional acuities can be considered quite similar in relation to their respective resolution limits.

The development of vernier acuity in infants

Vernier and grating acuity were measured in infants up to 6 months of age using a two-alternative forced-choice preferential looking technique. Vernier resolution was measured using a horizontal vernier grating. The grating was presented on half of the screen while the other half contained horizontal lines which were continuous with the vernier grating. Movement artifacts were eliminated by making both halves of the display move as the offsets were introduced in half of the display field. Vernier acuity, like grating acuity, improved about 3 octaves over the first 6 months of life. Moreover, the vernier resolution of infants, as of adults, was better than grating resolution.

Abnormal orientation selectivity in both eyes of strabismic amblyopes

In normal observers preadaptation to a parallel grating increases the contrast threshold for a line whereas a perpendicular grating has no effect. Such orientation selectivity was not found in the amblyopic eye of two out of five squinters. Only a weak after-effect produced with a grating parallel to the line was obtained in the good eye of four of the amblyopes while all of them show an abnormal threshold reduction following adaptation to a perpendicular grating. This suggests a relationship between abnormal binocular interaction during visual development and the organization of orientational mechanisms but does not explain the loss of visual acuity in amblyopia.

Orientation specificity and spatial selectivity in human vision

An adaptation method is used to determine the orientation specificity of channels sensitive to different spatial frequencies in the human visual system. Comparison between different frequencies is made possible by a data transformation in which orientational effects are expressed in terms of equivalent contrast (the contrast of a vertical grating producing the same adaptational effect as a high-contrast grating of a given orientation). It is shown that, despite great variances in the range of orientations affected by adaptation at different spatial frequencies (±10° to ±50°), the half-width at half-amplitude of the orientation channels does not vary systematically as a function of spatial frequency over the range tested (2·5 to 20 cycles deg−1). Two subjects were used and they showed significantly different orientation tuning across the range of spatial frequencies. The results are discussed with reference to previous determinations of orientation specificity, and to related psychophysical and neurophysiological phenomena.