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PubMed1991

Systematic distortions of shape from stereopsis.

The effectiveness of disparity information in defining 3-D shape was investigated by means of judgements of the shape of cylindrical continuous curved surfaces presented as random dot stereograms. At a close viewing distance, truly circular cylinders appeared elongated; at an intermediate distance, perception was close to veridical; and, at a far distance, cylinders appeared flattened. Indirect measures of scaling distance were calculated from these data. The results strongly suggest that the observed shape distortions are a consequence of scaling horizontal disparities with an incorrect measure of egocentric distance.

Depth Perception↗
PubMed2000

The task-dependent use of binocular disparity and motion parallax information.

Binocular disparity and motion parallax are powerful cues to the relative depth between objects. However to recover absolute depth, either additional scaling parameters are required to calibrate the information provided by each cue, or it can be recovered through the combination of information from both cues (Richards, W. (1985). Structure from stereo and motion. Journal of the Optical Society of America, 2, 343-349). However, not all tasks necessarily require a full specification of the absolute depth structure of a scene and so psychophysical performance may vary depending on the amount of information available, and the degree to which absolute depth structure is required. The experiments reported here used three different tasks that varied in the type of geometric information required in order for them to be completed successfully. These included a depth nulling task, a depth-matching task, and an absolute depth judgement (shape) task. Real world stimuli were viewed (i) monocularly with head movements, (ii) binocularly and static, or (iii) binocularly with head movements. No effect of viewing condition was found whereas there was a large effect of task. Performance was accurate on the matching and nulling tasks and much less accurate on the shape task. The fact that the same perceptual distortions were not evident in all tasks suggests that the visual system can switch strategy according to the demands of the particular task. No evidence was found to suggest that the visual system could exploit the simultaneous presence of disparity and motion parallax.

Depth Perception↗
PubMed1993

A proximity-contingent stereoscopic depth aftereffect: evidence for adaptation to disparity gradients.

Prolonged inspection of a surface slanted in the third dimension of visual space typically results in a negative aftereffect such that, after adaptation, a surface in the fronto-parallel plane will appear slanted in the opposite direction. Binocular disparity is not necessary to generate such effects, since they can be obtained monocularly, presumably via adaptation to texture gradient. Six experiments demonstrated durable stereoscopic depth aftereffects in the absence of a texture gradient--by using discrete disparate objects rather than slanted surfaces--and demonstrated that adaptation was to the interobject disparity gradient rather than to the relative disparity of the objects per se. The disparity required to null the obtained aftereffects was inversely proportional to the horizontal separation of elements, for a constant disparity, and directly proportional to the separation of subsequently presented probes. When elements differed in depth (disparity), but were not laterally separated, nulling disparity was significant but invariant with changes in the horizontal separation of probe elements. In that case, adaptation was (i) either to the disparity gradient generated by the vertical separation of probe elements (of which the relative disparity component was tapped); or (ii) to relative disparity per se.

Adult↗
PubMed1992

Vergence errors: some hitherto unreported aspects of fixation disparity.

Measurement of the monocular components of fixation disparity indicated a higher prevalence of asymmetric contributions to the total deviation than previously reported. Furthermore, the exact proportion varied from moment to moment. Two of six subjects showed significant changes in fixation disparity over a period of 1 week. For all six subjects the changes in fixation deviation of one eye were virtually independent of those changes occurring in the other eye. In other words, these monocular variances were uncorrelated. Settings of the monocular components of binocular fixation disparity were accomplished at an accuracy close to that achieved by using a monocular vernier technique. The remaining differences appeared to be due to occasional instabilities during binocular viewing. The usual method of clinical measurement in which only one element is moved is not always equivalent to that determined by summing the two monocular components. The principal process measured by subjective fixation disparity appears to be either oculomotor or localized directional shifts of a monocular nature.

Convergence, Ocular↗
PubMed1998

Depth perception in random dot stereograms is not affected by changes in either vergence or accommodation.

PURPOSE: To test the hypothesis that extraretinal cues related to vergence angle and lens accommodation are used to scale horizontal disparities for fixation distance. METHODS: Depth perception of random dot stereograms was studied in 10 healthy adult subjects with normal visual acuity by modifying retinal disparity, fixation distance, vergence angle, and accommodation. Statistical analysis was used to compare the data. RESULTS: Depth perception increased with fixation distance. The increment of depth perception persisted even when horizontal retinal disparity was kept constant. The magnitude of depth perception was independent of vergence angle. Depth perception did not vary with changes in accommodation. CONCLUSIONS: Extraretinal cues related to vergence angle and accommodation seem to be not necessary to scale horizontal disparities for viewing distance.

Accommodation, Ocular↗
PubMed2004

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.

Adult↗
PubMed2005

Differences in perceived depth for temporally correlated and uncorrelated dynamic random-dot stereograms.

We investigated the influence of temporal frequency on binocular depth perception in dynamic random-dot stereograms (DRS). We used (i) temporally correlated DRS in which a single pair of images alternated between two disparity values, and (ii) temporally uncorrelated DRS consisting of the repeated alternation of two uncorrelated image pairs each having one of two disparity values. Our results show that disparity-defined depth is judged differently in temporally correlated and temporally uncorrelated DRS above a temporal frequency of about 3 Hz. The results and simulations indicate that (i) above about 20 Hz, the complete absence of stereomotion is caused by temporal integration of luminance, (ii) the difference in perceived depth in temporally correlated and temporally uncorrelated DRS for temporal frequencies between 20 and 3 Hz, is caused by temporal integration of disparity.

Convergence, Ocular↗
PubMed1998

A computational model of depth perception based on headcentric disparity.

It is now well established that depth is coded by local horizontal disparity and global vertical disparity. We present a computational model which explains how depth is extracted from these two types of disparities. The model uses the two (one for each eye) headcentric directions of binocular targets, derived from retinal signals and oculomotor signals. Headcentric disparity is defined as the difference between headcentric directions of corresponding features in the left and right eye's images. Using Helmholtz's coordinate systems we decompose headcentric disparity into azimuthal and elevational disparity. Elevational disparities of real objects are zero if the signals which contribute to headcentric disparity do not contain any errors. Azimuthal headcentric disparity is a 1D quantity from which an exact equation relating distance and disparity can be derived. The equation is valid for all headcentric directions and for all binocular fixation positions. Such an equation does not exist if disparity is expressed in retinal coordinates. Possible types of errors in oculomotor signals (six) produce global elevational disparity fields which are characterised by different gradients in the azimuthal and elevational directions. Computations show that the elevational disparity fields uniquely characterise both the type and size of the errors in oculomotor signals. Our model uses a measure of the global elevational disparity field together with local azimuthal disparity to accurately derive headcentric distance throughout the visual field. The model explains existing data on whole-field disparity transformations as well as hitherto unexplained aspects of stereoscopic depth perception.

Computational Biology↗
PubMed1999

Interaction between the perceived shape of two objects.

The difference between the way in which binocular disparity scales with viewing distance and the way in which motion parallax scales with viewing distance introduces a potential indirect cue for viewing distance: the viewing distance is the only distance at which disparity and motion specify the same depth. The present study examines whether this information is used. Two simulated ellipsoids were presented on a computer screen in complete darkness. The two ellipsoids were 6 degrees to the left and right of straight ahead. Subjects set the width and depth of each ellipsoid to match a tennis ball, and set the distance of the one on the right to half that of the one on the left. The distance of the left ellipsoid varied between trials. On half of the trials it was static. On the other half it was rotating up and down around its frontal horizontal axis. Rotating the left ellipsoid influenced its set depth: rotating ellipsoids were set to be much more spherical. There was no influence on the set depth of the other ellipsoid, or on the set width of either. The set distance of the right ellipsoid was also unaffected. We conclude that subjects do not combine binocular disparity and motion parallax to obtain more veridical information about viewing distance.

Distance Perception↗
PubMed2000

Motor execution is necessary to memorize disparity.

Binocular saccades in response to briefly flashed, memorized disparate targets (different for the two eyes) become disconjugate following repeated trials. After 15 min of such training, the disconjugacy persists, even when the target to memorize is no longer disparate. This study examines the hypothesis that disparity memorization has a motor basis. We report here three experiments in which subjects were trained for 15-min periods. In experiment 1, subjects made no saccade after target presentation (static training); in experiment 2 subjects intended to make a saccade, but they actually made a saccade in only 10% or 20% of the trials; in experiment 3 subjects made anti-saccades. For all three experiments, the flashed target was disparate and the memory delay for each trial was 1 s. To examine the effects of learning for all three experiments, before and after training, we recorded memory-guided saccades to non-disparate targets (monocular viewing). Experiments 1 and 2 produced inconsistent (before/after training) changes in the disconjugacy of saccades. Thus, the disparity of potential saccade targets had no lasting effect on the disconjugacy of saccades if a saccade was not made. In contrast, the anti-saccades in experiment 3 developed a disconjugacy opposite to the disparity of the remembered target. These findings indicate that the execution of the saccade is necessary to memorize disparity of the target.

Conditioning, Psychological↗
PubMed1996

FPL and VEP measures of fusion, stereopsis and stereoacuity in normal infants.

Dynamic random dot fusion, stereopsis and stereoacuity were evaluated in 149 healthy, fullterm infants, using both forced-choice preferential looking (FPL) and steady-state visual evoked potential (VEP) protocols. Few infants aged 2-3 months demonstrated fusion or stereopsis in either the FPL or VEP protocol; most infants aged 5 months and older demonstrated fusion and stereopsis in both protocols. Both FLP and VEP stereoacuity approached adult-level (< 60 sec) by 6-7 months of age. Both infants and adults exhibited non-monotonic VEP amplitude vs disparity functions with a step change in phase at an intermediate disparity, consistent with separate fine and coarse disparity mechanisms.

Aging↗
PubMed2006

Stereo dynamics are not scale-dependent.

The experiments reported here focus on the temporal dynamics of stereopsis in an effort to shed light on how low level mechanisms might contribute to the execution of coarse-to-fine processing in the human stereo system. Because previous studies have used a variety of stimuli and configurations, we assess the effect of exposure duration on stereo thresholds using band-limited Gabor patches for a range of stimulus configurations. In preliminary studies, we found that the best stereo sensitivity-spatial frequency relationship was obtained when using configurations in which the size and target-reference spacing were consistent with spatially scaled stimuli. Sub-optimal stereo sensitivity as a function of spatial frequency was observed when the size and separation were fixed. Further, we found that the temporal properties of stereopsis were consistently sustained in nature irrespective of the stimulus spatial frequency content. This latter finding suggests that if coarse-to-fine stereo processing does occur it does not follow as a consequence of the dynamics of low-level disparity transduction.

Contrast Sensitivity↗
PubMed2003

Adaptation to vertical disparity induced-depth: implications for disparity processing.

Depth aftereffects produced by prolonged inspection of an object in depth can be mediated by monocular and binocular depth cues. The adapting mechanisms responsible for such effects have not yet been fully determined. Theories of binocular depth aftereffects typically posit a role of an adaptive horizontal disparity sensitive mechanism, implying multiple cue-specific mechanisms for depth aftereffects. Here we examined whether binocular depth aftereffects can be attributed to such a cue-specific mechanism. In Experiment 1 we did so using a technique allowing us to maintain horizontal disparities and vergence constant for our adaptation stimuli, whilst manipulating simulated depth by virtue of a vertical disparity induced-depth effect. We found that depth aftereffects were almost identical to those produced by adaptation to stimuli of equivalent depth produced by conventional horizontal disparity modulations. In Experiment 2, we examined depth aftereffects following adaptation to apparently frontal surfaces produced by different combinations of horizontal and vertical disparity modulations. Aftereffects were close to zero. These results suggest that binocular depth aftereffects are not due to adaptation of a horizontal disparity sensitive mechanism, and we argue that adaptation occurs at the level of a 3D shape sensitive mechanism derived from multiple cues. Experiment 3 was a control to examine whether the two types of adaptation stimuli in Experiment 1 were indeed perceptually the same, since in theory they may differ if vertical disparities influenced metric depth scaling. We found no evidence of this, and concluded that the two classes of stimuli used in Experiment 1, though consisting of very different patterns of disparity, were perceptually equivalent.

Adaptation, Physiological↗
PubMed2006

Bayesian models of binocular 3-D motion perception.

Psychophysical studies on three-dimensional (3-D) motion perception have shown that perceived trajectory angles of a small target traveling in depth are systematically biased. Here, predictions from Bayesian models, which extend existing models of motion-first and stereo-first processing, are investigated. These statistical models are based on stochastic representations of monocular velocity and binocular disparity input in a binocular viewing geometry. The assumption of noise in these inputs together with a plausible prior for 3-D motion leads to testable predictions of perceived trajectory angle and velocity. Results from two experiments are reported, suggesting that disparity rather than motion processing introduces perceptual bias.

Bayes Theorem↗
PubMed2002

Changes in Listing's plane after sustained vertical fusion.

PURPOSE: To determine whether prolonged fusion of an imposed vertical disparity leads to a change in the orientation of Listing's plane, even when measured during monocular viewing. METHODS: Four normal subjects (age range, 24-37 years) wore Fresnel prisms of increasing power for 72 hours to produce a final left-over-right disparity (range, 7-11 prism diopters [approximately 3.9 - 6.2 degrees]) that was still fusible. Eye movements were measured binocularly, using three-axis search coils, as subjects fixed on an array of light-emitting diodes (LEDs) arranged on a flat screen, 124 cm away. A regression was used to fit the data points to a plane (Listing's plane) during monocular and binocular viewing. From each planar fit, the horizontal and vertical components of primary position (the direction of gaze that is perpendicular to Listing's plane) were calculated. Baseline data were collected in the unadapted state, either just before or at least 4 days after wearing the prisms. RESULTS: After the period of viewing through the prisms, there was a change in vertical phoria (prism adaptation) ranging from 1.6 to 3.3. There was a significant (P < 0.01) shift of the relative orientation of the vertical component of primary position between the two eyes of 6.3 +/- 1.7 degrees (right eye value minus left eye, up being positive, each measured during monocular viewing). There was no consistent pattern of change in the horizontal component of primary position. CONCLUSIONS: Prolonged fusion of a vertical disparity is associated with a change in the orientation of Listing's plane that persists under monocular viewing. Possible mechanisms include phoria adaptation, the prolonged fusional effort itself, and the residual disparity that must be overcome by sensory mechanisms.

Adult↗
PubMed1995

Fixation disparity and near visual acuity.

Twenty patients having associated heterophoria values of 2 delta and greater, as recorded with a Mallett unit, had their near monocular and binocular visual acuities measured. In the latter case, measurements were taken both with the associated heterophoria corrected by prisms and without prism correction. Bradford Near Vision charts, specially designed to measure visual acuity at near in seconds of arc, were used in the acuity measurement. The improvement of binocular acuity compared with monocular acuity was less than would occur in normal subjects without associated heterophorias. When the associated heterophorias were corrected with prisms the improvement in binocular over monocular acuity was similar to that found in normal subjects, with zero associated heterophoria readings.

Adolescent↗