Temporal integration differences between crossed and uncrossed stereoscopic mechanisms

Effects of binocular disparity on binocular luminance combination

This study aimed to examine the effects of binocular disparity on binocular combination of brightness information coming from luminance increments and decrements. The point of subjective equality was determined by asking the observers to judge which stimulus appeared brighter-a bar stimulus with variable disparity or another stimulus with zero disparity. For the bar stimulus, the interocular luminance ratio was varied to trace an equal brightness curve. Binocular disparity had no effect on luminance increments presented on a gray or black background. In contrast, when luminance decrements were presented on a gray background, non-zero disparities elevated points of subjective equality for stimuli with interocular luminance differences. This means that the binocular brightness combination of the two monocular signals shifted from winner-take-all summation toward linear averaging. It has been argued that this effect may be caused by non-zero binocular disparities attenuating interocular suppression, which is deemed to operate normally with zero disparity.

Differences in stereoacuity between crossed and uncrossed disparities reduce with practice

INTRODUCTION

Stereoacuity, like many forms of hyperacuity, improves with practice. We investigated the effects of repeated measurements over multiple visits on stereoacuity using two commonly utilised clinical stereotests, for both crossed and uncrossed disparity stimuli.

METHODS

Participants were adults with normal binocular vision (n = 17) aged between 18 and 50 years. Stereoacuity was measured using the Randot and TNO stereotests on five separate occasions over a six week period. We utilised both crossed and uncrossed stimuli to separately evaluate stereoacuity in both disparity directions. A subset of the subject group also completed a further five visits over an additional six week period. Threshold stereoacuity was determined by the lowest disparity level at which the subjects could correctly identify both the position and disparity direction (crossed or uncrossed) of the stimulus. Data were analysed by repeated measures analysis of variance.

RESULTS

Stereoacuity for crossed and uncrossed stimuli improved significantly across the first five visits (F_1,21  = 4.24, p = 0.05). The main effect of disparity direction on stereoacuity was not significant (F₁  = 0.02, p = 0.91). However, a significant interaction between disparity direction and stereotest was identified (F₁  = 7.92, p = 0.01).

CONCLUSIONS

Stereoacuity measured with both the TNO and Randot stereotests improved significantly over the course of five repetitions. Although differences between crossed and uncrossed stereoacuity were evident, they depended on the stereotest used and reduced or disappeared after repeated measurements. A single measure of stereoacuity is inadequate for properly evaluating adult stereopsis clinically.

Factors limiting sensitivity to binocular disparity in human vision: Evidence from a noise-masking approach

Our visual system uses the disparity between the images received by the two eyes to judge three-dimensional distance to surfaces. We can measure this ability by having subjects discriminate the disparity of rendered surfaces. We wanted to know the basis of the individual differences in this ability. We tested 53 adults with normal vision using a relative disparity detection task. Targets were wedge-shaped surfaces formed from random dots. These were presented in either crossed or uncrossed disparity relative to a random dot background. The threshold disparity ranged from 24 arc seconds in the most-able subject to 275 arc seconds in the least-able subject. There was a small advantage for detecting crossed-disparity targets. We used the noise-masking paradigm to partition subject performance into two factors. These were the subject's equivalent internal noise and their processing efficiency. The parameters were estimated by fitting the linear amplifier model. We found both factors contributed to the individual differences in stereoacuity. Within subjects, those showing an advantage for one disparity direction had enhanced efficiency for that direction. Some subjects had a higher equivalent internal noise for one direction that was balanced out by an increased efficiency. Our approach provides a more thorough account of the stereo-ability of our subjects compared with measuring thresholds alone. We present a normative set of results that can be compared with clinical populations.

Infant perception of von Szily contours

This habituation-dishabituation study examined infants' perception of subjective von Szily contours, the illusory effect of which is generated by horizontal disparity and half-occlusions. In these contours, a foreground surface appears to partially occlude a background surface. In Experiment 1, participants aged 4 and 5 months were habituated to a von Szily figure and were then tested for their ability to perceive the difference between the habituation figure and the same figure with reversed depth relations. The infants displayed significant novelty preferences during the posthabituation period. This observation indicates that 4- and 5-month-olds respond to the stereoscopically specified depth difference between the two surfaces of von Szily figures. In Experiment 2, participants aged 4 and 5 months were tested for the ability to conduct modal completion, that is, to perceive the surface that is stereoscopically shifted into the foreground as a whole. The infants were habituated to a von Szily figure and then examined for their ability to distinguish between complete and incomplete versions of the foreground surface. Longer looking at the incomplete posthabituation pattern indicates modal completion; the infants recognize the complete pattern as familiar and regard the incomplete pattern as novel. Similarly, Experiment 3 investigated whether infants aged 5 and 7 months amodally complete the background surface, that is, the surface that is partially covered by the foreground surface. Experiment 2 found modal completion in 5-month-olds. Experiment 3 established that 5- and 7-month-olds have developed some ability to conduct amodal completion. In sum, infants perceive the depth information in von Szily contours and conduct modal and amodal completion.

The prevalence and diagnosis of 'stereoblindness' in adults less than 60 years of age: a best evidence synthesis

PURPOSE

Stereoscopic vision (or stereopsis) is the ability to perceive depth from binocular disparity - the difference of viewpoints between the two eyes. Interestingly, there are large individual differences as to how well one can appreciate depth from such a cue. The total absence of stereoscopic vision, called 'stereoblindness', has been associated with negative behavioural outcomes such as poor distance estimation. Surprisingly, the prevalence of stereoblindness remains unclear, as it appears highly dependent on the way in which stereopsis is measured.

RECENT FINDINGS

This review highlights the fact that stereopsis is not a unitary construct, but rather implies different systems. The optimal conditions for measuring these varieties of stereoscopic information processing are discussed given the goal of detecting stereoblindness, using either psychophysical or clinical stereotests. In that light, we then discuss the estimates of stereoblindness prevalence of past studies.

SUMMARY

We identify four different approaches that all converge toward a prevalence of stereoblindness of 7% (median approach: 7%; unambiguous-stereoblindness-criteria approach: 7%; visual-defect-included approach: 7%; multiple-criteria approach: 7%). We note that these estimates were derived considering adults of age <60 years old. Older adults may have a higher prevalence. Finally, we make recommendations for a new ecological definition of stereoblindness and for efficient clinical methods for determining stereoblindness by adapting existing tools.

Fixating at far distance shortens reaction time to peripheral visual stimuli at specific locations

The purpose of the present study was to examine whether the fixation distance in real three-dimensional space affects manual reaction time to peripheral visual stimuli. Light-emitting diodes were used for presenting a fixation point and four peripheral visual stimuli. The visual stimuli were located at a distance of 45cm and at 25° in the left, right, upper, and lower directions from the sagittal axis including the fixation point. Near (30cm), Middle (45cm), Far (90cm), and Very Far (300cm) fixation distance conditions were used. When one of the four visual stimuli was randomly illuminated, the participants released a button as quickly as possible. Results showed that overall peripheral reaction time decreased as the fixation distance increased. The significant interaction between fixation distance and stimulus location indicated that the effect of fixation distance on reaction time was observed at the left, right, and upper locations but not at the lower location. These results suggest that fixating at far distance would contribute to faster reaction and that the effect is specific to locations in the peripheral visual field. The present findings are discussed in terms of viewer-centered representation, the focus of attention in depth, and visual field asymmetry related to neurological and psychological aspects.

Simple reaction times to cyclopean stimuli reveal that the binocular system is tuned to react faster to near than to far objects

Binocular depth perception is an important mechanism to segregate the visual scene for mapping relevant objects in our environment. Convergent evidence from psychophysical and neurophysiological studies have revealed asymmetries between the processing of near (crossed) and far (uncrossed) binocular disparities. The aim of the present study was to test if near or far objects are processed faster and with higher contrast sensitivity in the visual system. We therefore measured the relationship between binocular disparity and simple reaction time (RT) as well as contrast gain based on the contrast-RT function in young healthy adults. RTs were measured to suddenly appearing cyclopean target stimuli, which were checkerboard patterns encoded by depth in dynamic random dot stereograms (DRDS). The DRDS technique allowed us to selectively study the stereoscopic processing system by eliminating all monocular cues. The results showed that disparity and contrast had significant effects on RTs. RTs as a function of disparity followed a U-shaped tuning curve indicating an optimum at around 15 arc min, where RTs were minimal. Surprisingly, the disparity tuning of RT was much less pronounced for far disparities. At the optimal disparity, we measured advantages of about 80 ms and 30 ms for near disparities at low (10%) and high (90%) contrasts, respectively. High contrast always reduced RTs as well as the disparity dependent differences. Furthermore, RT-based contrast gains were higher for near disparities in the range of disparities where RTs were the shortest. These results show that the sensitivity of the human visual system is biased for near versus far disparities and near stimuli can result in faster motor responses, probably because they bear higher biological relevance.

50 Years of Stereoblindness: Reconciliation of a Continuum of Disparity Detectors With Blindness for Disparity in Near or Far Depth

Whitman Richards (1932–2016) discovered some 50 years ago that about 30% of observers from the normal population exhibit stereoblindness: the disability to process binocular disparities in either far or near depth. We review the literature on stereoblindness entailing two insights. First, contemporary scholars in stereopsis undervalue the comprehension that disparity processing studies require precise assessments of observers’ stereoblindness. We argue that this frequently leads to suboptimal interpretations. Second, there is still an open conundrum: How can the established finding that disparity is processed by a continuum of detectors be reconciled with the disability of many observers to process a whole class of far or near disparities? We propose, based upon integration of literature, that an asymmetry between far and near disparity detection at birth—being present for a variety of reasons—can suppress the typical formation of binocular correlation during the critical period for the development of stereopsis early in life, thereby disabling a whole class of far or near disparities.

Influence of Test Distance on Stereoacuity in Intermittent Exotropia

AIMS

To investigate influence of test distance on stereoacuity in intermittent exotropia (X[T]) using the same test conditions for both near and far distances.

METHODS

Subjects were 38 consecutive patients with X(T). All the patients were between ages 6 and 15 years and had decimal visual acuity of 1.0 or better. Another inclusion criterion was presence of phoric condition at near and far distances. Stereoacuity was measured at a near distance of 40 cm and at a far distance of 5 m. The following test conditions were used for both test distances: separation of the two eyes using polarized glasses, and a target with a random dot pattern. All the stereograms had the same subtended angle of 2.5º, and binocular disparity of 480, 240, 120, and 60 arcsec. We used two stereogram types with crossed and uncrossed disparities.

RESULTS

Far stereoacuity of 38 subjects measured with the crossed disparity was significantly worse than near stereoacuity (P<0.05, Wilcoxon signed-ranks test), although 30 (78.9%) of the 38 subjects showed no differences in stereopsis between the near and far distances. Far stereoacuity of 38 cases measured with the uncrossed disparity was significantly worse than at near (P<0.05, Wilcoxon signed-ranks test), although 20 (52.6%) of the 38 subjects showed no differences between stereoacuity at near and far. In comparison of stereoacuity with crossed disparity and uncrossed disparity, stereoacuity with crossed disparity was significantly better than that with uncrossed disparity both at near and far (P<0.05, Wilcoxon signed-ranks test).

CONCLUSIONS

Stereoacuity in X(T) was different according to test distance when measured controlling subtended angle of stereogram at both distances. Far stereoacuity was significantly worse than near stereoacuity when measured using test targets with both crossed and uncrossed disparities. Additionally, stereoacuity measured with crossed disparity was better than that with uncrossed disparity at both distances.

The effects of the binocular disparity differences between targets and maskers on visual search

A visual search for targets is facilitated when the target objects are on a different depth plane than other masking objects cluttering the scene. The ability of observers to determine whether one of four letters presented stereoscopically at four symmetrically located positions on the fixation plane differed from the other three was assessed when the target letters were masked by other randomly positioned and oriented letters appearing on the same depth plane as the target letters, or in front, or behind it. Three additional control maskers, derived from the letter maskers, were also presented on the same three depth planes: (1) random-phase maskers (same spectral amplitude composition as the letter masker but with the phase spectrum randomized); (2) random-pixel maskers (the locations of the letter maskers' pixel amplitudes were randomized); (3) letter-fragment maskers (the same letters as in the letter masker but broken up into fragments). Performance improved with target duration when the target-letter plane was in front of the letter-masker plane, but not when the target letters were on the same plane as the masker, or behind it. A comparison of the results for the four different kinds of maskers indicated that maskers consisting of recognizable objects (letters or letter fragments) interfere more with search and comparison judgments than do visual noise maskers having the same spatial frequency profile and contrast. In addition, performance was poorer for letter maskers than for letter-masker fragments, suggesting that the letter maskers interfered more with performance than the letter-fragment maskers because of the lexical activity they elicit.

The absolute disparity anomaly and the mechanism of relative disparities

There has been a long-standing debate about the mechanisms underlying the perception of stereoscopic depth and the computation of the relative disparities that it relies on. Relative disparities between visual objects could be computed in two ways: (a) using the difference in the object's absolute disparities (Hypothesis 1) or (b) using relative disparities based on the differences in the monocular separations between objects (Hypothesis 2). To differentiate between these hypotheses, we measured stereoscopic discrimination thresholds for lines with different absolute and relative disparities. Participants were asked to judge the depth of two lines presented at the same distance from the fixation plane (absolute disparity) or the depth between two lines presented at different distances (relative disparity). We used a single stimulus method involving a unique memory component for both conditions, and no extraneous references were available. We also measured vergence noise using Nonius lines. Stereo thresholds were substantially worse for absolute disparities than for relative disparities, and the difference could not be explained by vergence noise. We attribute this difference to an absence of conscious readout of absolute disparities, termed the absolute disparity anomaly. We further show that the pattern of correlations between vergence noise and absolute and relative disparity acuities can be explained jointly by the existence of the absolute disparity anomaly and by the assumption that relative disparity information is computed from absolute disparities (Hypothesis 1).

Functional architecture for disparity in macaque inferior temporal cortex and its relationship to the architecture for faces, color, scenes, and visual field

Binocular disparity is a powerful depth cue for object perception. The computations for object vision culminate in inferior temporal cortex (IT), but the functional organization for disparity in IT is unknown. Here we addressed this question by measuring fMRI responses in alert monkeys to stimuli that appeared in front of (near), behind (far), or at the fixation plane. We discovered three regions that showed preferential responses for near and far stimuli, relative to zero-disparity stimuli at the fixation plane. These "near/far" disparity-biased regions were located within dorsal IT, as predicted by microelectrode studies, and on the posterior inferotemporal gyrus. In a second analysis, we instead compared responses to near stimuli with responses to far stimuli and discovered a separate network of "near" disparity-biased regions that extended along the crest of the superior temporal sulcus. We also measured in the same animals fMRI responses to faces, scenes, color, and checkerboard annuli at different visual field eccentricities. Disparity-biased regions defined in either analysis did not show a color bias, suggesting that disparity and color contribute to different computations within IT. Scene-biased regions responded preferentially to near and far stimuli (compared with stimuli without disparity) and had a peripheral visual field bias, whereas face patches had a marked near bias and a central visual field bias. These results support the idea that IT is organized by a coarse eccentricity map, and show that disparity likely contributes to computations associated with both central (face processing) and peripheral (scene processing) visual field biases, but likely does not contribute much to computations within IT that are implicated in processing color.

Stereo and Shading Contribute Independently to Shape Convexity-Concavity Discrimination

The present study examined the joint contribution of shading and stereopsis to the perception of shape convexity–concavity. The stimuli were the images of a synthetic convex 3-D shape seen from viewpoints leading to ambiguity as to its convexity. Illumination came from either above or below, and from either the right or the left, and stimuli were presented dichoptically with normal binocular disparity, reversed disparity, or no disparity. Participants responded “convex” more often when the lighting came from above than from below. Also, participants responded that the shape was convex more often with normal than with zero disparity, and more often with zero disparity than with reversed stereopsis. The effects of lighting direction and display mode were additive—that is, they did not interact. This indicates that shading and stereopsis contribute independently to shape perception.

The onset of sensitivity to horizontal disparity in infancy: a short-term longitudinal study

In this short-term longitudinal study, infants were examined for their natural preference of a square defined by crossed horizontal disparity (either 1° or 0.5°) over a square defined by a vertical disparity (either 1° or 0.5°). The square targets were embedded in a dynamic random dot stereogram. The stimuli were presented on an autostereoscopic monitor equipped with a face-tracking device. The infants were tested weekly between 6 and 16 weeks of age. Four experiments were conducted. In two experiments, the infants were examined with the forced-choice preferential looking (FPL) method for their ability to perceive either 1° or 0.5° horizontal disparity. In the remaining two experiments, the classical natural preference (CNP) method (measurement of looking times) was applied. According to the results of the FPL experiments, mean relative preference for the horizontal disparity square became significant at 8 weeks of age. The CNP data indicated an onset of stereopsis at 12-15 weeks. The mean relative preferences for horizontal disparity indicated by the CNP method were smaller than those found in the FPL experiments. Thus, the FPL method was more sensitive than the CNP method in the measurement of infant responsiveness to crossed horizontal disparity.

Integrating multiple cues to depth order at object boundaries

We examined the interaction between motion and stereo cues to depth order along object boundaries. Relative depth was conveyed by a change in the speed of image motion across a boundary (motion parallax), the disappearance of features on a surface moving behind an occluding object (motion occlusion), or a difference in the stereo disparity of adjacent surfaces. We compared the perceived depth orders for different combinations of cues, incorporating conditions with conflicting depth orders and conditions with varying reliability of the individual cues. We observed large differences in performance between subjects, ranging from those whose depth order judgments were driven largely by the stereo disparity cues to those whose judgments were dominated by motion occlusion. The relative strength of these cues influenced individual subjects' behavior in conditions of cue conflict and reduced reliability.

Differences in temporal frequency tuning between the two binocular mechanisms for seeing motion in depth

There are two types of binocular cues available for perception of motion in depth. One is the binocular disparity change in time and the other is the velocity difference between the left and the right retinal images (inter-ocular velocity differences). We measured the luminance contrast threshold for seeing motion in depth while isolating either of the cues at various temporal modulations of velocity in the stimulus. To isolate disparity cues, dynamic random-dot stereograms were used (the disparity condition) while binocularly uncorrelated random-dot kinematograms were used to isolate velocity cues (the velocity condition). Results showed that sensitivity peaked at a temporal frequency (approximately 1 cps) in the velocity condition while the peak in the disparity condition was at the lowest frequency (0.35 cps) or at least at a frequency lower than that in the velocity condition. This suggests that the visual system has different temporal frequency properties for the velocity and disparity cues for motion in depth.

How to use individual differences to isolate functional organization, biology, and utility of visual functions; with illustrative proposals for stereopsis

This paper is a call for greater use of individual differences in the basic science of visual perception. Individual differences yield insights into visual perception's functional organization, underlying biological/environmental mechanisms, and utility. I first explain the general approach advocated and where it comes from. Second, I describe five principles central to learning about the nature of visual perception through individual differences. Third, I elaborate on the use of individual differences to gain insights into the three areas mentioned above (function, biology/environment, utility), in each case describing the approach advocated, presenting model examples from the literature, and laying out illustrative research proposals for the case of stereopsis.

Disparity-Tuning Characteristics of Neuronal Responses to Dynamic Random-Dot Stereograms in Macaque Visual Area V4

Stereo processing begins in the striate cortex and involves several extrastriate visual areas. We quantitatively analyzed the disparity-tuning characteristics of neurons in area V4 of awake, fixating monkeys. Approximately half of the analyzed V4 cells were tuned for horizontal binocular disparities embedded in dynamic random-dot stereograms (RDSs). Their response preferences were strongly biased for crossed disparities. To characterize the disparity-tuning profile, we fitted a Gabor function to the disparity-tuning data. The distribution of V4 cells showed a single dense cluster in a joint parameter space of the center and the phase parameters of the fitted Gabor function; most V4 neurons were maximally sensitive to fine stereoscopic depth increments near zero disparity. Comparing single-cell responses with background multiunit responses at the same sites showed that disparity-sensitive cells were clustered within V4 and that nearby cells possessed similar preferred disparities. Consistent with a recent report by Hegdé and Van Essen, the disparity tuning for an RDS drastically differed from that for a solid-figure stereogram (SFS). Disparity-tuning curves were generally broader for SFSs than for RDSs, and there was no correlation between the fitted Gabor functions' amplitudes, widths, or peaks for the two types of stereograms. The differences were partially attributable to shifts in the monocular images of an SFS. Our results suggest that the representation of stereoscopic depth in V4 is suited for detecting fine structural features protruding from a background. The representation is not generic and differs when the stimulus is broad-band noise or a solid figure.

Stereoscopic depth magnitude estimation: effects of stimulus spatial frequency and eccentricity

To determine the effects of stimulus spatial frequency and retinal eccentricity on the perception of depth magnitude derived from disparity cues alone, subjects were asked to estimate the magnitude of depth of a stereoscopically viewed Gabor patch presented to the central or peripheral field with either crossed or uncrossed absolute disparity. Disparity vergence responses to the same Gabor stimuli were separately estimated subjectively by determining the offset required for dichoptic nonius alignment following presentation of the stimulus. The normalized stereoscopic magnitude estimation data generally showed that crossed disparities were perceived with greater depth than uncrossed disparities of the same magnitude, whether presented to the central or peripheral field. Asymmetries in magnitude of depth perception ranged from mild differences between depth directions to complete lack of depth perception for one direction. Disparity vergence response functions varied from (1) appropriate initiation of vergence to both directions of disparity, (2) initiation of vergence to only one direction of disparity, or (3) an attenuated initiation of vergence response to either direction of disparity. Within subjects, their asymmetries in magnitude of depth perception did not correlate with their asymmetries in vergence initiation. The similarity of the asymmetric depth magnitude estimation for a given individual at both stimulus locations tested suggests that common neural mechanisms are responsible for central and peripheral depth magnitude estimation. The lack of correlation between the perceptual and motor responses to the same stimuli suggests that the neural pathways for these responses diverge shortly after the detection of disparity in primary visual cortex.

Correlation between stereoanomaly and perceived depth when disparity and motion interact in binocular matching

The aim of this study was to find out to what extent binocular matching is facilitated by motion when stereoanomalous and normal subjects estimate the perceived depth of a 3-D stimulus containing excessive matching candidates. Thirty subjects viewed stimuli that consisted of bars uniformly distributed inside a volume. They judged the perceived depth-to-width ratio of the volume by adjusting the aspect ratio of an outline rectangle (a metrical 3-D task). Although there were large inter-subject differences in the depth perceived, the experimental results yielded a good correlation with stereoanomaly (the inability to distinguish disparities of different magnitudes and/or signs in part of the disparity spectrum). The results cannot be explained solely by depth-cue combination. Since up to 30% of the population is stereoanomalous, stereoscopic experiments would yield more informative results if subjects were first characterized with regard to their stereo capacities. Intriguingly, it was found that motion does not help to define disparities in subjects who are able to perceive depth-from-disparity in half of the disparity spectrum. These stereoanomalous subjects were found to rely completely on the motion signals. This suggests that the perception of volumetric depth in subjects with normal stereoscopic vision requires the joint processing of crossed and uncrossed disparities.

Stereoillusory motion concomitant with lateral head movements

Stationary objects in a stereogram can appear to move when viewed with lateral head movements. This illusory motion can be explained by the motion-distance invariance hypothesis, which states that illusory motion covaries with perceived depth in accordance with the geometric relationship between the position of the stereo stimuli and the head. We examined two predictions based on the hypothesis. In Experiment 1, illusory motion was studied while varying the magnitude of binocular disparity and the magnitude of lateral head movement, holding viewing distance constant. In Experiment 2, illusory motion was studied while varying binocular disparity and viewing distance, holding magnitude of head movement constant. Ancillary measures of perceived depth, perceived viewing distance, and perceived magnitude of lateral head movement were also obtained. The results from the two experiments show that the extent of illusory motion varies as a function of perceived depth, supporting the motion-distance invariance hypothesis. The results also show that the extent of illusory motion is close to that predicted from the geometry in crossed disparity conditions, whereas it is greater than the predicted motion in uncrossed disparity conditions. Furthermore, predictions based on perceptual variables were no more accurate than predictions based on geometry.

A planar and a volumetric test for stereoanomaly

Stereoanomaly is the failure to see differences in depth when the viewer is presented with stimuli having different magnitudes of stereoscopic disparity. In the absence of eye movements, everyone suffers from stereoanomaly for extremely large disparities. Typically, such disparities are seen at the same depth as monocular stimuli. However, about 30%, of the population exhibit some form of stereoanomaly even for very small disparities, provided eye movements are avoided. In some cases, the sign of the disparity will be confused, and the perceived depth will be incorrectly seen as 'behind' rather than 'in front of' the fixation point, for example. Because anomalies provide useful information about perceptual mechanisms, tests that measure and quantify the extent of a blindness are important investigative tools for research. Here we offer two easy-to-administer tests for stereoanomaly. The first test is based on depth judgments of two bars relative to a fixation point. The second test involves judgments of volumetric stimuli, seen stereoscopically. In each case, subjects indicate depth by setting a rectangle (with fixed base) to match the perceived depth. Although both tests are correlated, some differences in stereo processing are seen, depending upon whether or not the stimuli are presented near the point of fixation.

Crossed and uncrossed stereoacuity at distance and the effect from heterophoria

BACKGROUND

Previous studies have found that crossed disparities give better stereoacuity than uncrossed. When near phoria is considered, exophores had a better crossed stereoacuity and esophores had a better uncrossed stereoacuity. The current study investigated the effect of heterophoria on distant crossed and uncrossed stereoacuity.

METHODS

Seventy-two subjects were recruited and their distant heterophoria measured. Heterophoria within two prism dioptres was considered as orthophoric. Esophoric group had esophoria greater than two prism dioptres. Exophoric group had exophoria greater than two prism dioptres. There were 40 orthophores, 23 exophores and nine esophores. Their stereoacuity was measured with a three-rod apparatus at 6 m.

RESULTS

The mean crossed stereoacuity was 4.8" and uncrossed stereoacuity was 7.2" (t = -3.03, p < 0.01). The mean stereoacuity for orthophoric subjects was 5.31", 6.02" for exophores and 8.91" for esophores. The distant crossed stereoacuity is better than uncrossed stereoacuity in all three groups but the difference is only significant for the exophores. Exophoric subjects demonstrated a significant difference between crossed (4.10") and uncrossed (7.95") stereoacuity.

CONCLUSIONS

Orthophores have the best stereoacuity, followed by exophores and esophores. Exophoric subjects have a better crossed than uncrossed stereoacuity. More esophoric subjects should be recruited to confirm the difference between crossed and uncrossed stereoacuity.

Surface textures improve the robustness of stereoscopic depth cues

This research develops design recommendations for surface textures (patterns of color on object surfaces) rendered with stereoscopic displays. In 3 method-of-adjustment procedure experiments, 8 participants matched the disparity of a circular probe and a planar stimulus rendered using a single visible edge. The experiments varied stimulus orientation and surface texture. Participants more accurately matched the depth of vertical stimuli than that of horizontal stimuli, consistent with previous studies and existing theory. Participants matched the depth of surfaces with large pixel-to-pixel luminance variations more accurately than they did surfaces with a small pixel-to-pixel luminance variation. Finally, they matched the depth of surfaces with vertical line patterns more accurately than they did surfaces with horizontal-striped texture patterns. These results suggest that designers can enhance depth perception in stereoscopic displays, and also reduce undesirable sensitivity to orientation, by rendering objects with surface textures using large pixel-to-pixel luminance variations.

Occlusion contributes to temporal processing differences between crossed and uncrossed stereopsis in random-dot displays

Stereoscopic depth discrimination was investigated in crossed and uncrossed directions using stimuli defined by binocular disparity differences embedded in dynamic random-dot stereograms. Across three experiments, fixation was directed to a point on the display screen (which placed crossed stimuli in front of and uncrossed stimuli behind, the background dots of the stereogram), to a point in front of the display screen (which placed both crossed and uncrossed stimuli in front of the background dots), and to a point behind the display screen (which placed both crossed and uncrossed stimuli behind the background dots). Results showed that depth discrimination was always good when the stimuli appeared in front of the background dots of the stereogram, whereas discrimination was always poor when the stimuli appeared behind the background dots. These results suggest that differences between crossed and uncrossed stereopsis as reported in past research arose, in part, from effects related to occlusion.

Asymmetries and errors in perception of depth from disparity suggest a multicomponent model of disparity processing

In three experiments, asymmetries between the processing of crossed and uncrossed disparities were investigated. The target was a luminance-defined circle concentric to a fixation mark, viewed stereoscopically on a computer monitor for 105 msec. Fifteen disparities were presented according to the method of constant stimuli. Observers indicated the apparent direction of target depth relative to fixation. All experiments measured both the accuracy and latency of this response. Experiment 1 showed fewer errors and shorter reaction times for identifying crossed disparities. Experiments 2 and 3 replicated Experiment 1 and also showed that observers may often perceive a target in the direction opposite that prescribed by the disparity information. We propose that the asymmetries and reversals result from differences in computation of sign, not of magnitude. This notion is consistent with a scheme of continuous disparity tuning and accounts for such asymmetries and errors without positing disparity pooling mechanisms.

Disparity tuning of the stereoscopic (cyclopean) motion aftereffect

Across five experiments this study investigated the disparity tuning of the stereoscopic motion aftereffect (adaptation from moving retinal disparity). Adapting and test stimuli were moving and stationary stereoscopic grating patterns, respectively, created from dynamic random-dot stereograms. Observers adapted to moving stereoscopic grating patterns presented with a given disparity and viewed stationary test patterns presented with the same or differing disparity to examine whether the motion aftereffect is disparity contingent. Across experiments aftereffect duration was greatest when adapting motion and test pattern both were presented with zero disparity and in the plane of fixation. Aftereffect declined as disparity of adapting motion and/or test pattern increased away from fixation, even under conditions in which depth position of adapt and test was equal. This argues against a relative depth separation explanation of the decline, and instead suggests that the amount of adaptable substrate decreases away from fixation.