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PubMed1988

Disparity acceleration effects in stereoscopy and their importance for theoretical neurology.

Stereoscopic perceptions have a special significance for brain modeling, because they require a special form of cortical integration for their appearance. We have newly observed, along ridges of very high stereoscopic disparity accelerations, narrow bands of exaggerated visual depth. These observations signal the importance of local excitation accelerations for future models of brain functions. Some key parameters of disparity acceleration bands are identified and their effects measured.

Cerebral Cortex↗
PubMed1995

Discriminability of random-dot stereograms in three-dimensional space.

The discriminability of crossed-disparity (near) and uncrossed-disparity as a function of their location in the upper-left, upper-right, lower-left, and lower-right quadrants of the visual field. Discriminability was assessed using choice reaction-time (RT) and accuracy measures. While near targets were recognized equally well in the upper and lower fields, far targets were perceived more easily in the upper visual field. The discriminability of far targets was particularly poor in the lower left quadrant. These results point to the existence of fundamental asymmetries in perceiving crossed and uncrossed disparities along the vertical and lateral axes.

Anisotropy↗
PubMed1990

Mechanisms underlying the anisotropy of stereoscopic tilt perception.

There is a marked anisotropy in the perception of stereoscopic tilt: vertical gradients of horizontal disparity are more easily perceived than horizontal gradients. This could be explained if orientation disparity (the orientation difference in the two eyes' views of the same line) were one of the cues used to determine tilt, since orientation disparities are in general larger for vertical gradients. We show here that a marked anisotropy in tilt perception is present even with stereograms which contain equally strong orientation disparity cues for horizontal and vertical gradients. This implies that there must be other mechanisms for stereoscopic tilt perception, or further processing steps in the use of orientation disparity, which are anisotropic in their mode of action.

Depth Perception↗
PubMed1996

Relative size disparities and the perception of surface slant.

Perceived slant produced by size disparities in random-dot displays was measured by tactile matching. For a 60 deg surface, slant produced by vertical-size disparity (the induced effect) was opposite to that produced by horizontal-size disparity. Overall-size disparity produced a little slant. With small displays, effects of horizontal and vertical disparities were reduced but not those of overall disparity. A zero-disparity surround increased effects of horizontal and overall disparities but reduced the induced effect. A mixture of horizontally disparate and zero-disparity dots produced two slanted surfaces. Vertically disparate and zero-disparity dots produced one slanted surface. Abutting opposite horizontal disparities produced surfaces with a sharp boundary. Abutting vertical disparities produced surfaces with a gradual boundary. Perceived slant depends on the difference between horizontal-size disparity detected locally and mean vertical-size disparity over a relatively large area.

Depth Perception↗
PubMed1999

The effects of contrast on perceived depth and depth discrimination.

The contrast dependence of perceived depth was quantified through a series of depth matching experiments. Perceived depth was found to be a power law function of contrast. In addition, subjects exhibited a large uncrossed depth bias indicating that low contrast test patterns appeared much farther away than high contrast patterns of equal disparity. For disparities in the range of +/- 4.0 arc min, matching disparities for low contrast patterns were shifted in the uncrossed direction by the same amount. In other words, while the magnitude of the uncrossed depth bias is a power law function of contrast, it is constant with respect to disparity. In a second series of experiments, the contrast dependence of stereo increment thresholds was measured. Like perceived depth and stereoacuity, stereo increment thresholds were found to be a power law function of contrast. These results suggest that contrast effects occur at or before the extraction of depth and have implications for the response properties of disparity-selective mechanisms.

Contrast Sensitivity↗
PubMed1993

Fixation disparity at different viewing distances of a visual display unit.

Fixation disparity was measured while subjects observed a realistic alphanumeric pattern on the cathode ray tube (CRT) of a visual display unit at viewing distances of 25-85 cm. The psychometric function of fixation disparity was determined by analysing the observers' responses to a series of short (100 ms) flashes of nonius targets that were superimposed on the CRT-image by means of a half-silvered mirror. Test-retest correlation of fixation disparity was high (r = 0.8) at viewing distances of 40 and 82 cm and lower (r = 0.55) at the shorter distance of 26.5 cm. Mean fixation disparity increased to a more exophoric condition as the display approached the eyes. However, this effect was observed only in some of the subjects, so the susceptibility of fixation disparity to viewing distance appears to be an individual characteristic. When subjects observed the VDU screen continuously for 30 min at viewing distances of 85, 47, 31 and 25 cm in different sessions, fixation disparity and accommodation remained unchanged over time.

Accommodation, Ocular↗
PubMed1993

Initial control component in disparity vergence eye movements.

Recent experimental evidence indicates that a portion of the oculomotor response to disparity stimulation is functionally open-loop; that is, the response occurs without the aid of visual feedback. To investigate the stimulus features that elicit or influence this dynamic movement, convergence responses to a step, a step followed by target disappearance, and a pulse followed by target disappearance were obtained from four subjects using infrared oculography. The target was a thin vertical line (0.25 degrees) either 2 or 10 degrees in height. Stimuli having different amplitudes (1, 2, 4 and 8 degrees) and disappearance times (50, 100 and 200 ms) were selected randomly along with occasional divergent stimuli to minimize prediction and voluntary vergence. Experiments showed that the dynamic characteristics of the initial portion of the response were essentially the same, even when the target disappeared before the movement took place. The magnitude of the initial response depended on the stimulus amplitude, but was not influenced by either stimulus duration or target height. For example, stimulus durations as short as 50 ms elicited responses similar to those caused by standard steps. The initial response was shown to be active over a well-defined time period of about 200 ms, after which the response appears to be mediated by a visually-guided control component. These results support the recently developed dual-mode theory of vergence control in which an initial preprogrammed (open-loop) control component is followed by a feedback (closed-loop) controlled component which reduces any remaining disparity.

Convergence, Ocular↗
PubMed1991

A freehand method for drawing stereoscopic pictures.

A method for the freehand drawing of anaglyphs is described. Usually textbooks cover the topic of stereopsis in an abstract way, making it difficult for students to grasp. The present method enables students to draw anaglyphs by hand and should make the concept of binocular disparity more easily learned.

Attention↗
PubMed1995

On the accuracy of surface reconstruction from disparity interpolation.

Observers viewed flashed random-dot stereograms depicting a pair of long, narrow, curved ribbons of textured surface defined by a Gabor function in disparity. Observers judged the location of the peak of the depth profile of one ribbon relative to that of the other. In one ribbon, disparity changed smoothly while in the other disparity was periodically sampled. Up to a limiting sampling period, disparity interpolation produced accurate surface reconstruction, but beyond that performance deteriorated rapidly. This interpolation limit depended on surface orientation (vertical vs horizontal) and disparity sign, but not Gabor spatial frequency.

Depth Perception↗
PubMed1992

Perspective, orientation disparity, and anisotropy in stereoscopic slant perception.

Stereoscopic depth estimates are not predictable from the geometry of point disparities. The configural properties of surfaces (surface contours) may play an important role in determining, for example, slant responses to a disparity gradient, and the marked anisotropy in favour of slant around a horizontal axis. It has been argued that variation in slant magnitude are attributable to the degree of perspective conflict present and that anisotropy is attributable to orientation disparity, which varies with the axis of slant. Three experiments were conducted in which configural properties were varied to try and tease apart the respective roles of orientation disparity and conflicting perspective in determining stereoscopic slant perception and slant axis anisotropy. The results could not be accounted for by the magnitude of the orientation disparities present. Conflicting perspective cues appeared to play a role but only for slant around a vertical axis. It was concluded that there are important configural effects in stereopsis attributable neither to orientation disparity nor to perspective.

Anisotropy↗
PubMed2003

Ocular dominance predicts neither strength nor class of disparity selectivity with random-dot stimuli in primate V1.

We address two unresolved issues concerning the coding of binocular disparity in primary visual cortex. Experimental studies and theoretical models have suggested a relationship between a cell's ocular dominance, assessed with monocular stimuli, and its tuning to binocular disparity. First, the disparity energy model of disparity selectivity suggests that there should be a correlation between ocular dominance and the strength of disparity tuning. Second, several studies have reported a relationship between ocular dominance and the shape of the disparity tuning curve, with cells dominated by one eye more likely to have disparity tuning of the tuned-inhibitory type. We investigated both of these relationships in single neurons recorded from the primary visual cortex of awake fixating macaques, using dynamic random-dot patterns as a stimulus. To classify disparity tuning curves quantitatively, we develop a new measure of symmetry, which can be applied to any function. We find no evidence for any correlation between ocular dominance and the nature of disparity tuning. This places constraints on the circuitry underlying disparity tuning.

Algorithms↗
PubMed2000

Vergence provides veridical depth perception from horizontal retinal image disparities.

One useful source of depth information available to the human nervous system is present in the horizontal disparities that exist between the two retinal images (stereoscopic depth). The relationship between horizontal disparity and depth varies with viewing distance so that an interpreting signal is required if disparities are to yield useful information. One potentially useful interpreting signal is available from ocular vergence. A number of studies have concluded, however, that a vergence signal does not provide veridical stereoscopic depth. All of these studies required observers to make a range of judgements under conditions of uncertainty (often using random dot stimuli) and we suggest that the lack of veridicality arose because of a contraction bias: a general tendency to bias judgements towards the centre of the range of possible responses. We re-examined the role of ocular vergence in the maintenance of stereoscopic depth constancy for real three-dimensional objects. Our results question the conclusions reached by previous studies and suggest that vergence can provide a veridical interpretation of stereoscopic depth. Our results indicate that horizontal retinal image disparities are not interpreted by a 'higher order' signal (i.e. the 'perceived distance' of the fixation point). The results of the experiment have significant implications for models of depth processing from disparity.

Adolescent↗
PubMed2005

Is depth perception of stereo plaids predicted by intersection of constraints, vector average or second-order feature?

Stereo plaid stimuli were created to investigate whether depth perception is determined by an intersection of constraints (IOC) or vector average (VA) operation on the Fourier components, or by the second-order (non-Fourier) feature in a pattern. We first created stereo plaid stimuli where IOC predicted vertical disparity, VA predicted positive diagonal disparity and the second-order feature predicted negative diagonal disparity. In a depth discrimination task, observers indicated whether they perceived the pattern as 'near' or 'far' relative to a zero-disparity aperture. Observers' perception was consistent with the disparity predicted by VA, indicating its dominance over IOC and the second-order feature in this condition. Additional stimuli in which VA predicted vertical disparity were created to investigate whether VA would dominate perception when it was a less reliable cue. In this case, observers' performance was consistent with disparity predicted by IOC or the second-order feature, not VA. Finally, in order to determine whether the second-order feature contributes to depth perception, stimuli were created where IOC and VA predicted positive horizontal disparity while the second-order feature predicted negative horizontal disparity. When the component gratings were oriented near horizontal (+/-83 degrees from vertical), depth perception corresponded to that predicted by the second-order feature. However, as the components moved away from horizontal (+/-75 degrees and +/-65 degrees from vertical), depth perception was increasingly likely to be predicted by an IOC or VA operation. These experiments suggest that the visual system does not rely exclusively on a single method for computing pattern disparity. Instead, it favours the most reliable method for a given condition.

Contrast Sensitivity↗
PubMed1993

Stereoscopic slant reversals: a new kind of 'induced' effect.

Data are presented from three experiments confirming an earlier finding that the stereoscopic slant perceived may be opposite to the geometrically predicted direction of slant (Gillam 1967). The stimulus for stereoscopic slant was created by imposing a disparity gradient on a frontal plane surface. Reversals are shown to occur readily for slants around a vertical axis but rarely for slant around a horizontal axis. Reversal frequency is greater for surfaces which have a regular pattern, providing good perspective information about slant. Cue conflict cannot explain reversals because adherence to perspective information predicts a perception of zero slant rather than reverse slant. A new explanation has been proposed attributing reversals to the ambiguity of horizontal disparity gradients and disambiguation of the disparity gradient by its relationship to the perspective gradient. It is shown that for any given disparity gradient there is a physical surface which would give rise to a slant reversed with respect to that normally predicted. Such a surface is eccentric in the field of view, with eccentricity given by the difference between the slants signalled by the disparity gradient and the perspective gradient. This explains why reversal responses to disparity gradients occur in the presence of perspective. It is proposed, on the basis of this analysis and the fact that reversals occur, that, like convergence and vertical disparity, perspective is a factor contributing to the correct scaling of disparity gradients in the horizontal meridian with respect to surface eccentricity.

Attention↗
PubMed1995

Depth from binocular rivalry without spatial disparity.

Some new stereoscopic effects are reported that arise from dichoptic stimuli containing no binocular disparity. In one effect, identical arrays of small black discs are presented to the two eyes and slightly smaller white discs are superimposed on one of each pair of black discs. This creates the impression of a surface with holes in it, through which is seen a surface with fluctuating black and white areas. This is referred to as the 'sieve effect'. The white discs must subtend less than about 1 deg of visual angle. With larger discs the black and white areas no longer exhibit alternating rivalry but combine to produce binocular lustre. This destroys the sieve effect. The sieve effect is weak or nonexistent when the black and white discs are the same size, showing that well-defined binocular rims are required for the effect. When the monocular white discs are reduced to dots, the impression of a surface seen through holes gives way to the impression of an array of dots behind or standing out from the background. In this case the monocular dots permanently dominate the homogeneous backgrounds in the other eye and the impression of depth can be explained in terms of apparent parallax or of disparity due to the instability of vergence.

Contrast Sensitivity↗
PubMed2001

Perceptual learning without feedback and the stability of stereoscopic slant estimation.

Subjects were examined for practice effects in a stereoscopic slant-estimation task involving surfaces that comprised a large portion of the visual field. In most subjects slant estimation was significantly affected by practice, but only when an isolated surface (an absolute disparity gradient) was present in the visual field. When a second, unslanted, surface was visible (providing a second disparity gradient and thereby also a relative disparity gradient) none of the subjects exhibited practice effects. Apparently, stereoscopic slant estimation is more robust or stable over time in the presence of a second surface than in its absence. In order to relate the practice effects, which occurred without feedback, to perceptual learning, results are interpreted within a cue-interaction framework. In this paradigm the contribution of a cue depends on its reliability. It is suggested that normally absolute disparity gradients contribute relatively little to perceived slant and that subjects learn to increase this contribution by utilizing proprioceptive information. It is argued that--given the limited computational power of the brain--a relatively small contribution of absolute disparity gradients in perceived slant enhances the stability of stereoscopic slant perception.

Cues↗
PubMed2002

Stereo transparency and the disparity gradient limit.

Several studies (Vision Research 15 (1975) 583; Perception 9 (1980) 671) have shown that binocular fusion is limited by the disparity gradient (disparity/distance) separating image points, rather than by their absolute disparity values. Points separated by a gradient >1 appear diplopic. These results are sometimes interpreted as a constraint on human stereo matching, rather than a constraint on fusion. Here we have used psychophysical measurements on stereo transparency to show that human stereo matching is not constrained by a gradient of 1. We created transparent surfaces composed of many pairs of dots, in which each member of a pair was assigned a disparity equal and opposite to the disparity of the other member. For example, each pair could be composed of one dot with a crossed disparity of 6' and the other with uncrossed disparity of 6', vertically separated by a parametrically varied distance. When the vertical separation between the paired dots was small, the disparity gradient for each pair was very steep. Nevertheless, these opponent-disparity dot pairs produced a striking appearance of two transparent surfaces for disparity gradients ranging between 0.5 and 3. The apparent depth separating the two transparent planes was correctly matched to an equivalent disparity defined by two opaque surfaces. A test target presented between the two transparent planes was easily detected, indicating robust segregation of the disparities associated with the paired dots into two transparent surfaces with few mismatches in the target plane. Our simulations using the Tsai-Victor model show that the response profiles produced by scaled disparity-energy mechanisms can account for many of our results on the transparency generated by steep gradients.

Female↗