Geometric and induced effects in binocular stereopsis and motion parallax

This paper examines and contrasts motion-parallax analogues of the induced-size and induced-shear effects with the equivalent induced effects from binocular disparity. During lateral head motion or with binocular stereopsis, vertical-shear and vertical-size transformations produced 'induced effects' of apparent inclination and slant that are not predicted geometrically. With vertical head motion, horizontal-shear and horizontal-size transformations produced similar analogues of the disparity induced effects. Typically, the induced effects were opposite in direction and slightly smaller in size than the geometric effects. Local induced-shear and induced-size effects could be elicited from motion parallax, but not from disparity, and were most pronounced when the stimulus contained discontinuities in velocity gradient. The implications of these results are discussed in the context of models of depth perception from disparity and structure from motion.     

3D after-effects are due to shape and not disparity adaptation

There are a variety of stereoscopic after-effects in which exposure to a stimulus with a particular slant or curvature affects the perceived slant or curvature of a subsequently presented stimulus. These after-effects have been explained as a consequence of fatigue (a decrease in responsiveness) among neural mechanisms that are tuned to particular disparities or patterns of disparity. In fact, a given disparity pattern is consistent with numerous slants or curvatures; to determine slant or curvature, the visual system must take the viewing distance into account. We took advantage of this property to examine whether the mechanisms underlying the stereoscopic curvature after-effect are tuned to particular disparity patterns or to some other property such as surface curvature. The results clearly support the second hypothesis. Thus, 3D after-effects appear to be caused by adaptation among mechanisms specifying surface shape rather than among mechanisms signaling the disparity pattern.     

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.     

Temporal dependencies in resolving monocular and binocular cue conflict in slant perception

Observers viewed large dichoptic patterns undergoing smooth temporal modulations or step changes in simulated slant or inclination under various conditions of disparity-perspective cue conflict and concordance. After presentation of each test surface, subjects adjusted a comparison surface to match the perceived slant or inclination of the test surface. Addition of conflicting perspective to disparity affected slant and inclination perception more for brief than for long presentations. Perspective had more influence for smooth temporal changes than for step changes in slant or inclination and for surfaces presented in isolation rather than with a zero disparity frame. These results indicate that conflicting perspective information plays a dominant role in determining the temporal properties of perceived slant and inclination.     

Spatio-temporal properties of Panum's fusional area

Horizontal and vertical disparity limits of binocular fusion were examined with temporal or with spatio-temporal modulation of binocular disparity for two lines at 0.5 deg retinal eccentricity. For both horizontal and vertical disparity, Panum's fusional areas became extended by low spatial frequency variation in disparity. The horizontal extent of Panum's area also varied with temporal frequency of disparity variation. Under low spatial frequency conditions, Panum's area increased horizontally by as much as a factor of 10 (but vertically by less than a factor of 2) when temporal frequency decreased from 5.0 Hz to 0.1 Hz. At high spatial frequencies the horizontal and vertical extents of Panum's area remained almost constant (within a factor of 1.5) over the entire range of temporal frequencies. These results illustrated that Panum's area is the combination of a constant minimum area with an extended area that responds to low frequency time-varying disparities.     

Slant perception, and its voluntary control, do not govern the slant aftereffect: multiple slant signals adapt independently

Although it is known that high-level spatial attention affects adaptation for a variety of stimulus features (including binocular disparity), the influence of voluntary attentional control-and the associated awareness-on adaptation has remained unexplored. We developed an ambiguous surface slant adaptation stimulus with conflicting monocular and binocular slant signals that instigated two mutually exclusive surface percepts with opposite slants. Using intermittent stimulus removal, subjects were able to voluntarily select one of the two rivaling slant percepts for extended adaptation periods, enabling us to dissociate slant adaptation due to awareness from stimulus-induced slant adaptation. We found that slant aftereffects (SAE) for monocular and binocular test patterns had opposite signs when measured simultaneously. There was no significant influence of voluntarily controlled perceptual state during adaptation on SAEs of monocular or binocular signals. In addition, the magnitude of the binocular SAE did not correlate with the magnitude of perceived slant. Using adaptation to one slant cue, and testing with the other cue, we demonstrated that multiple slant signals adapt independently. We conclude that slant adaptation occurs before the level of slant awareness. Our findings place the site of stereoscopic slant adaptation after disparity and eye posture are interpreted for slant [as demonstrated by Berends et al. (Berends, E. M., Liu, B., & Schor, C. M. (2005). Stereo-slant adaptation is high level and does not involve disparity coding. Journal of Vision 5 (1), 71-80), using that disparity scales with distance], but before other slant signals are integrated for the resulting awareness of the presented slant stimulus.     

The effects of interocular correlation and contrast on stereoscopic depth magnitude estimation.

PURPOSE: Decreasing the interocular correlation in random dot stereograms elevates disparity detection thresholds. Whether decorrelation also affects perceived depth from suprathreshold disparity magnitudes is unknown. The present study investigated the effects of interocular correlation and contrast on the magnitude of perceived depth in suprathreshold random dot stereograms. METHODS: Stereoscopic depth magnitude estimation as a function of percent interocular correlation of dynamic random dot stimuli was measured for five human subjects with clinically normal binocular vision. Each trial's stimulus was randomly assigned one of two magnitudes of either crossed or uncrossed relative disparity. Subjects verbally reported the direction and magnitude of perceived relative depth for each trial using a modulus-free scale. Normalized depth magnitude estimations as a function of the percent interocular correlation demonstrated the relationship between perceived depth, interocular correlation and contrast within subjects. Inter-subject variability was examined with comparisons of data across subjects. RESULTS: The depth magnitude perceived for a given magnitude of disparity declined as the percent of correlation of elements between the eyes decreased for both crossed and uncrossed directions. The effect generally was greater for uncrossed disparities and lower contrast. Some subjects demonstrated asymmetries in perceived depth for crossed vs. uncrossed disparities of the same magnitude. CONCLUSIONS: Magnitude estimation of suprathreshold stimuli provided a method of studying performance characteristics of stereoscopic depth perception across the range of functional disparities. Differences found in depth magnitude estimation as a function of the sign of disparity suggest that the neural mechanisms underlying depth perception from uncrossed disparity are more sensitive to image decorrelation, particularly at low contrast, than the mechanisms underlying depth estimation from crossed disparity. These results could occur from differences in near and far disparity-sensitive neurons, from the geometrical relationship between disparity and physical distance in normal viewing, or from the response measure independent of perception.     

Bi-stability in perceived slant when binocular disparity and monocular perspective specify different slants

We examined how much depth we perceive when viewing a depiction of a slanted plane in which binocular disparity and monocular perspective provide different slant information. We exposed observers to a grid stimulus in which the monocular--and binocular-specified grid orientations were varied independently across stimulus presentations. The grids were slanted about the vertical axis and observers estimated the slant relative to the frontal plane. We were particularly interested in the metrical aspects of perceived slant for a broad spectrum of possible combinations of disparity--and perspective-specified slants. We found that observers perceived only one grid orientation when the two specified orientations were similar. More interestingly, when the monocular--and binocular-specified orientations were rather different, observers experienced perceptual bi-stability (they were able to select either a perspective--or a disparity-dominated percept).     

The role of (micro)saccades and blinks in perceptual bi-stability from slant rivalry

We exposed the visual system to an ambiguous 3D slant rivalry stimulus consisting of a grid for which monocular (perspective) and binocular (disparity) cues independently specified a slant about a horizontal axis. When those cues specified similar slants, observers perceived a single slant. When the difference between the specified slants was large, observers alternatively perceived a perspective- or a disparity-dominated slant. Eye movement measurements revealed that there was no positive correlation between a perceptual flip and both saccades (microsaccades as well as larger saccades) and blinks that occurred prior to a perceptual flip. We also found that changes in horizontal vergence were not responsible for perceptual flips. Thus, eye movements were not essential to flip from one percept to the other. After the moment of a perceptual flip the occurrence probabilities of both saccades and blinks were reduced. The reduced probability of saccades mainly occurred for larger voluntary saccades, rather than for involuntary microsaccades. We suggest that the reduced probability of voluntary saccades reflects a reset of saccade planning.     

Stereo channels with different temporal frequency tunings

To investigate the spatial and temporal frequency tunings for stereopsis, we measured the contrast sensitivity for depth discrimination with variable spatiotemporal frequencies and disparities using drifting sinusoidal gratings. The results showed that the contrast sensitivity changed with the stimulus disparity and the disparity tuning function varied with the spatial frequency. The disparity in the peak sensitivity decreased proportionally with the spatial frequency (size-disparity correlation). Although the temporal frequency exhibited a limited influence on the peak disparity, the temporal frequency tuning varied with the spatial frequency. The shape of the temporal frequency tuning function was lowpass for higher spatial frequencies, whereas it was bandpass for low spatial frequencies. These results suggest that more than one channel with different temporal as well as spatial frequency tunings contribute to stereopsis.     

The experience of stereoblindness does not improve use of texture for slant perception

Stereopsis is an important depth cue for normal people, but a subset of people suffer from stereoblindness and cannot use binocular disparity as a cue to depth. Does this experience of stereoblindness modulate use of other depth cues? We investigated this question by comparing perception of 3D slant from texture for stereoblind people and stereo-normal people. Subjects performed slant discrimination and slant estimation tasks using both monocular and binocular stimuli. We found that two groups had comparable ability to discriminate slant from texture information and showed similar mappings between texture information and slant perception (biased perception toward frontal surface with texture information indicating low slants). The results suggest that the experience of stereoblindness did not change the use of texture information for slant perception. In addition, we found that stereoblind people benefitted from binocular viewing in the slant estimation task, despite their inability to use binocular disparity information. These findings are generally consistent with the optimal cue combination model of slant perception.     

Stereoscopic depth perception from oblique phase disparities

In order to understand the role of oblique retinal image disparities in the perception of stereoscopic depth, we measured the depth perceived from random dot stereograms in which phase disparities were introduced in a selected band of stimulus orientations. A band of orientation was defined by a center orientation that ranged from 7.5 (near vertical) to 82.5 o[rientation]deg and by a bandwidth that was defined as the difference between the highest and the lowest orientation in the band. The bandwidths tested were 15, 30 and 45 odeg. A constant phase disparity of 90 p[hase]deg was introduced in all of the oriented spatial frequency components within the orientation band and the perceived depth of each stimulus was matched using a small square binocular probe. For each bandwidth, perceived depth increased with an increase in the center orientation up to approximately 60 odeg. This suggests that the human stereovision system derives a large proportion of information about perceived stereoscopic depth from oblique phase disparities. Simulations using an energy model of stereoscopic depth perception indicate that oblique phase disparities are unlikely to be processed by neural mechanisms tuned to near-vertical orientations within the stimulus. Our results therefore suggest that oblique retinal disparities are initially detected as oblique phase disparities by binocular mechanisms tuned to oblique orientations. Because the perceived depth from oblique phase disparities is consistent with the trigonometrically determined equivalent horizontal disparities, we presume that the information from oblique phase disparities is included in the visual system's computation of the horizontal retinal disparity.     

Assessment of Cyclodisparity-Induced Slant Perception with a Synoptophore

Purpose To evaluate with a synoptophore slant perception induced by binocular cyclodisparities in normal subjects and to argue for the possibility of abnormal slant perception in patients with cyclo-vertical strabismus.Methods A vertical line with cyclodisparities that ranged from 0° to ± 10° was presented to 17 normal subjects (mean ± SD age, 28.4 ± 5.6 years; 11 men and 6 women) with a synoptophore, and the perceived slant of the line in the pitch plane was measured by a matching method. Cyclodisparity thresholds for top-away and top-forward slants were also evaluated by the method of limits in a separate experiment.Results An incyclodisparity induced top-forward, while an excyclodisparity induced top-away, slant perception. The maximum slant angle was 37° on average at a cyclodisparity of 10°, and the mean slant gain (perceived angle/geometrically calculated angle) was 64 ± 13%. The mean cyclodisparity thresholds for top-away and top-forward slants were –1.1° and 0.6°, respectively.Conclusion The slant perception induced by cyclodisparities was reasonably assessed with the synoptophore. The cyclodisparity thresholds obtained in this experiment were much lower than the cyclodeviation range of the patients, indicating that a considerable number of patients may have abnormal slant perception once they achieve sensory fusion. Jpn J Ophthalmol 2005;49:137–142 © Japanese Ophthalmological Society 2005     

The effect of spatial parameters on oscillatory movement displacement thresholds

Earlier work has established that oscillatory movement displacement thresholds (OMDT) are a form of hyperacuity. There is speculation that the mechanism determining OMDT, like motion perception in general, involves direct motion sensing at high temporal frequencies of oscillation and spatial localization processes (from which motion is inferred) at low temporal frequencies, which are both hyperacuities in their own right. OMDT were determined, for three experienced observers, over the temporal frequency range 1-15 Hz, for three stimulus lengths and three stimulus widths. Both decreasing stimulus length and decreasing stimulus width increased OMDT at all temporal frequencies. Furthermore, the resulting functions consistently exhibit a "kink" in the temporal frequency midrange. The results are interpreted as evidence that there are two subsystems involved in the analysis of visual motion with the kink indicating the transition where one system begins to predominate over the other.     

Perceived slant of binocularly viewed large-scale surfaces: a common model from explicit and implicit measures

It is known that the perceived slants of large distal surfaces, such as hills, are exaggerated and that the exaggeration increases with distance. In a series of two experiments, we parametrically investigated the effect of viewing distance and slant on perceived slant using a high-fidelity virtual environment. An explicit numerical estimation method and an implicit aspect-ratio approach were separately used to assess the perceived optical slant of simulated large-scale surfaces with different slants and viewing distances while gaze direction was fixed. The results showed that perceived optical slant increased logarithmically with viewing distance and the increase was proportionally greater for shallow slants. At each viewing distance, perceived optical slant could be approximately fit by linear functions of actual slant that were parallel across distances. These linear functions demonstrated a fairly constant gain of about 1.5 and an intercept that increased logarithmically with distance. A comprehensive three-parameter model based on the present data provides a good fit to a number of previous empirical observations measured in real environments.     

Slant cue are combined early in visual processing: Evidence from visual search

When looking for a target with a different slant than all the other objects, the time needed is independent of the number of other objects. Surface slant can be inferred from the two-dimensional images on the retinas using various cues. The information from different cues is subsequently combined to get a single estimate of slant. Is information from the individual cues or from the combined percept responsible for us so easily finding the target? To find out we compared combinations of two slant cues. The cues that we chose are retinal shape and binocular disparity. We compared search times for conditions with the same differences between the target and the other objects in each individual cue, but for each object the two cues either indicated the same slant or opposite slants. Search times were independent of the number of other items if the target clearly differed in perceived slant from the other items. Subjects systematically found the target faster when the cues indicated the same slant. We conclude that slant cues are combined locally throughout the visual field before the search process begins.     

Temporal dynamics of the Venetian blind effect

When square wave gratings are viewed binocularly with lower luminance or contrast in one eye, the individual bars of the grating appear to rotate around a vertical axis (Venetian blind effect). The effect has typically been thought to occur due to retinal disparities that result from irradiation and, therefore, are entirely entoptic. If so, the visual system should process disparities from a luminance or contrast disparity and a geometric disparity at the same rate. Studies of motion-in-depth using geometric disparities have shown that the visual system is unable to process depth cues when those cues are oscillated at frequencies greater than 5 Hz. By changing contrast (experiments one and two) and geometric (experiment three) disparity cues over time, the present study measured the frequency at which both the perception of motion-in-depth and the perception of depth diminish. The perception of motion-in-depth from contrast disparities decreased near 1.1 Hz (experiments one and four) and the perception of depth from contrast disparities decreased near 1.3 Hz (experiments one, two and four); both of which are lower than the frequency where depth from a geometric disparity diminished (near 4.8 Hz in experiment three). The differences between the dynamics of depth from contrast and geometric disparities suggest that the perception arises from separate neural mechanisms.     

Spatial characteristics of static and dynamic stereoacuity in strabismus

The spatial and temporal organization of stereoscopic depth perception were compared in normal and strabismic observers. The minimum and maximum disparities for stimulating static and dynamic stereopsis in strabismus were examined as a function of spatial separation of disparate stimuli. Disparities and their spacing were produced by spatial modulation of two vertical lines viewed haploscopically. Most strabismics had normal upper disparity limits but elevated static and dynamic stereothresholds. Moderate stereothreshold elevations (100 arc sec) were constant for spatial separations greater than 15 arc min. Two new types of spatial crowding effects upon stereopsis were observed. The first type resulted from the constant elevation of the disparity threshold. The second type consisted of a reduced maximum disparity limit for stereopsis. In both cases, the constriction of the range of perceivable depth produced a reduction in the spatial and temporal frequency limits for depth perception. Clinical tests of stereoacuity that crowd stimuli closer than 0.25 degree underestimated the strabismic patients' potential stereoacuities by a factor of 2 to 4. Similarly, tests of dynamic stereopsis that use temporal frequencies greater than 1 Hz will underestimate optimal dynamic stereoacuity.     

The stroboscopic Pulfrich effect is not evidence for the joint encoding of motion and depth

In the Pulfrich effect, an illusion of depth is produced by introducing differences in the times at which a moving object is presented to the two eyes. In the classic form of the illusion, there is a simple explanation for the depth percept: the interocular delay introduces a spatial disparity into the stimulus. However, when the moving object is viewed stroboscopically, this simple explanation no longer holds. In recent years, depth perception in the stroboscopic Pulfrich effect has been explained by invoking neurons that are sensitive both to stereo disparity and to direction of motion. With such joint motion/disparity encoders, interocular delay causes a perception of depth by causing a shift in each neuron's preferred disparity. This model has been implemented by N. Qian and R. A. Andersen (1997). Here we show that this model's predictions for perceived disparity are quantitatively at odds with psychophysical measures. The joint-encoding model predicts that the perceived disparity is the virtual disparity implied by the apparent motion; in fact, the perceived disparity is smaller. We show that the percept can be quantitatively explained on the basis of spatial disparities present in the stimulus, which could be extracted from pure disparity sensors. These results suggest that joint encoding of motion and depth is not the dominant neuronal basis of depth perception in this stimulus.     

Slant or occlusion: global factors resolve stereoscopic ambiguity in sets of horizontal lines

Perceived slant was measured for horizontal lines aligned on one side and of varying lengths whose length disparity was either a constant linear amount for all lines (consistent with uniocular occlusion) or proportional to line length (consistent with global slant). Although the disparity of any line was ambiguous with respect to these two possibilities, slant of individual lines did not occur in the former case, but a subjective contour in depth was reported along the alignment. For proportional disparity of the set, global slant was seen. Adding a constant length to each line on the invalid eye for occlusion resulted in multiple slants. Smooth uniocular variations in alignment shape elicited subjective contours slanting or curving in depth. Global context can disambiguate the depth status of individual disparate lines.