Sign Of Disparity Research Articles

Depth cue integration: stereopsis and image blur

Depth-of-focus limitations introduce spatial blur in images of three-dimensional scenes. It is not clear how the visual system combines depth information derived from image blur with information from other depth cues. Stereoscopic disparity is the pre-eminent depth cue, so experiments were conducted to investigate interactions between image blur and stereoscopic disparity. Observers viewed two random dot stereograms (RDSs) in a 2AFC task, and were required to identify the RDS depicting the greatest depth. In control observations, all dots in both RDSs were sharply defined. In experimental observations, one RDS contained only sharply defined dots, but the other contained differential spatial blur to introduce an additional depth cue. Results showed that the addition of differential blur made only a marginal difference to apparent depth separation, and only when the blur difference was consistent with the sign of disparity. Cue combination between blur and disparity cues is thus weighted very heavily in favour of the latter. It is shown that blur and disparity cues co-vary according to geometric optics. Since the two cues are effective over different distances, the visual system is not normally called upon to integrate them, and is most likely to make use of blur cues over distances beyond the range of disparity mechanisms.

Disparités spatiales en Syrie : dynamiques citadines, politiques d'aménagement territorial, régions

Littoralization is a recent sign of spatial disparity in the southern Mediterranean. Syria differs from most of the countries of the region by virtue if its very light littoralization, but can it be considered a coherent entity without Beirut and Lebanon or without of the Gulf cities, which constitute an economic and banking platform essential to the whole region ? The spatial disparities are thus to be found outside of littoralization. Are they regional disparities ? In point of fact we can even wonder if «regions» exist in the Syria which was inherited from the Ottoman empire. It would seem to be the city/countryside disparity that is most deeply inscribed, and in any case it is against that phenomenon that the government has chosen to invest the most heavily, making rural improvement on of its most essential stands. But has that policy succeeded in erasing inherited disparities as well as rendering inoperative what is a tendancy to constitute hegemonic urban regions around the great historic centers ? The recent development of Damascus as a capital seems currently to be the principal catalyst for the institution of regional imbalance and spatial disparities.

The Minimum Contrast Requirements for Stereopsis

A number of researchers have compared the contrast requirements for stereopsis with those for detection of the stereoscopic stimulus, but they have generally failed to allow for the fact that stereopsis requires a detectable stimulus in both eyes at the same time. It is argued that the most appropriate detection threshold for this comparison is that for simultaneous monocular detection (SMD) of the stereoscopic half images. Experiments in which this comparison threshold has been used are summarised and the hypothesis generated that, on using stimuli that are localised in both space and spatial frequency (e.g. Gabor patches or differences of Gaussians), a range of disparities can always be found over which contrast thresholds for depth identification are less than or equal to this SMD threshold (the SMD hypothesis). It is argued that the success of this hypothesis in describing data obtained with these stimuli is consistent with the notions of labelled lines for disparity sign and the size--disparity correlation. Last, experiments are reported in which contrast thresholds for stereoscopic depth identification (front/back) were measured with interocular differences in contrast. The data obtained are consistent with the presence of both inhibitory and excitatory interactions between the eyes when unequal monocular contrasts are presented. The implications of these results and the SMD hypothesis for theories of stereopsis and binocular function are discussed.

Effects of Cue Context on the Perception of Depth from Combined Disparity and Perspective Cues

In normal vision, stereoscopic cues are combined with perspective cues to provide veridical depth perception. The relative strengths of these depth cues, however, may be dependent upon context effects. We investigated the role of stimulus context on the interactions of binocular disparity, contrast, and size. The subjects, four observers with normal stereoacuity and one stereo-amblyope, discriminated far vs. near perceived depth of Gabor patches; feedback was based on the sign of binocular disparity. Depth discrimination functions were measured under conditions in which depth cues were consonant or in conflict. Three stimulus contexts were used: (1) variable disparity with fixed spatial frequency and contrast; (2) variable contrast with fixed spatial frequency and disparity; and (3) variable spatial frequency with fixed contrast and disparity. The effects of stimulus context were derived from comparisons of discrimination rates for identical stimuli across the three sets of conditions. In subjects with normal stereopsis, for disparities less than 2 arcmin, depth perception was dominated by contrast in contrast-varying sessions, or by size in spatial frequency-varying sessions. With larger disparities, depth perception became dependent on disparity, regardless of the contrast or spatial frequency of the test stimulus. The results for the stereo-amblyope showed much greater dependence on perspective cues and, in most cases, the transition from perspective- to disparity-based depth perception did not occur. These investigations demonstrate strong stimulus context effects and have important implications for the combination rules of stereoscopic and perspective cues in depth perception of normal and stereo-deficient subjects.

Neural mechanisms underlying stereoscopic vision

The progressive frontalization of both eyes in mammals causes overlap of the left and right visual fields, having as a consequence a region of binocular field with single vision and stereopsis. The horizontal separation of the eyes makes the retinal images of the objects lying in this binocular field have slight horizontal and vertical differences, termed disparities. Horizontal disparities are the main cue for stereopsis. In the past decades numerous physiological studies made on monkeys, which have in many aspects a similar visual system to humans, showed that a population of visual cells are capable of encoding the amplitude and sign of horizontal disparity. Such disparity detectors were found in cortical visual areas V1, V2, V3, V3A, VP, MT (V5) and MST of monkeys and in the superior colliculus of the cat and opossum. According to their disparity tuning function, these cells were first grouped into tuned excitatory, tuned inhibitory, near and far sub-groups. Subsequent studies added two more categories, tuned near and tuned far cells. Asymmetries between left and right receptive field position, on and off regions, and intra-receptive field wiring are believed to be the neural mechanisms of disparity detection. Because horizontal disparity alone is insufficient to compute reliable stereopsis, additional information about fixation distance and angle of gaze is required. Thus, while there is unequivocal evidence of cells capable of detecting horizontal disparities, it is not known how horizontal disparity is calibrated. Sensitivity to vertical disparity and information about the vergence angle or eye position may be the source of this additional information.

Temporal Phase Affects Spatial Binding in Depth Perception

The temporal linking of spatial information is essential for coherent space perception. We investigated the effect of the temporal phase of flickering ‘inducers’ in depth perception. Stereo half-images were generated on the left and right halves of a large-screen video monitor and viewed through a mirror stereoscope. The half-images portrayed a solid-black vertically oriented bar seen against a white background. Two brackets immediately flanking this bar could be placed in crossed or uncrossed disparity relative to the bar. A pair of thin white ‘bridging lines’ could appear on the black bar, always at zero disparity. Brackets and bridging lines could be flickered either in-phase or out-of-phase. Observers judged whether the brackets appeared in front of or behind the black bar, with disparity varied over trials according to a method of constant stimuli. We found that when bridging lines were absent, the depth location of the brackets was predictably dependent on disparity sign, with a small bias for “behind” judgements owing to occlusion. When bridging lines and brackets flashed in temporal phase, depth judgements were markedly biased toward “in front”; when the bridging lines and brackets flashed out of phase, this bias was much reduced. This biasing effect also depended on the spatial offset of lines and brackets. We conclude that depth perception is strongly influenced by the temporal phase of the spatial information.

Fine-scale processing in human binocular stereopsis.

Many studies have demonstrated that the human visual system is sensitive to very small differences in relative binocular disparity. It is not known over what monocular regions information is spatially integrated to mediate performance in such tasks. In this study we present psychophysical observations that define the smallest spatial scale involved in disparity processing, and we indicate the nature of the computations performed by the units mediating that disparity discrimination. We show that human observers can identify the sign of disparity of a single target dot when it is embedded in a row of identical dots, with these noise dots presented either in the fixation plane or with a proportion binocularly uncorrelated. In conjunction with the psychophysical data, we explore how a class of simple correlator models of stereopsis must be constrained in order to account for human performance for the same fine-scale tasks. Such models can perform the task only when the correlation is carried out over a very small region of the image, for a very small range of disparities. Our results demonstrate that there is a fine-scale input to the stereo system, mediated by foveal mechanisms that spatially integrate visual signals over a region as small as 4-6 arcmin in diameter.

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.

Binocular displacement of unpaired region.

Binocular displacement of binocularly unpaired parts of the stimulus was examined by means of the Poggendorff figure. The Poggendorff figure can be used to investigate displacement since lateral displacement of the transversal may cause bias in judgments of its collinearity. In experiment 1, the transversal had a disparity, and thus binocularly unpaired parts, relative to the rectangle. The magnitude of the Poggendorff illusion should not have changed by addition of binocular disparity if displacement occurred. There was no or slight change when the transversal was seen behind the rectangle, but there was significant decrease when the transversal was seen in front of the rectangle, suggesting absence of displacement in this case. There were two possible explanations. One was that displacement depended on the positional relation between the unpaired stimuli and the binocularly presented rectangle, ie the occlusion constraint, which the case with the transversal in front did not satisfy. The alternative was that the decrease was due to the perceived front depth of the transversal, and not related to binocular displacement at all. In order to discriminate between these two possibilities, the transversal was reduced to only the unpaired parts, resulting in dichoptic stimulation in experiment 2. In this stimulus, the positional relation between the unpaired and the paired stimuli was the same as in the previous experiment, yet no front depth could be perceived. The results showed similar asymmetry as in experiment 1. Thus we conclude that binocular displacement depends on the positional relation between the unpaired and the paired stimuli, regardless of their perceived depth. This may imply that binocular displacement is not symmetric about the sign of disparity, hence that it is not just averaging but is a reconstruction of the spatial layout of objects in the outside world to keep the visual direction of the unsuppressed unpaired region veridical by using explicit cues to depth discontinuity.

Neurological Implications of Aniseikonic Tilts and Stereoscopic Completions Through Homogeneous Visual Space

In 1965, it was first reported that BROKEN visual contours complete themselves in textured stereovision across empty or homogeneous space and that they do so strongly following directly the optics of crossed versus uncrossed disparities. The importance was then also noted of this completion effect for any neurophysiological model of stereovision. We now extend these measurements with several additional targets and to an analogous aniseikonic target, and confirm that the completion range has a maximum of about 5 degrees. There is much dependence upon target eccentricity and disparity sign, and some on size. That aniseikonic tilts may also be generated over this same range confirms the fact that global neurointegrative processes are of critical importance in all facets of stereovision, and in texture vision in general.

Complementary spatial locations, width, and disparity

An alternative analysis is offered for human depth perception in addition to the depth cue of disparity. The new analysis considers locations both proximal and distal to the fixation point and offers an explanation as to why a stimulus presented at one disparity sign may be mistakenly considered to possess the opposite sign. Three descriptions of applications, the Pulfrich phenomenon, an interpretation of the Hornbostel effect (the three-dimensional Necker cube), and the determination of the limits of stereoscopic vision, are discussed. In addition the new analysis discloses a particular advantage of binocular over monocular vision which had not been appreciated formerly. The new analysis offers a powerful analytical tool of simple mathematical form. The means of conversion from the new analysis to and from disparity is included. In addition the similarity between the new approach and the classical lens equation is examined.

Detection of vertical disparities

Psychometric curves determined for the detection of differences between vertical disparities as a function of the reference disparity for three presentation times (20msec, 160msec and 2 sec) and for two eccentricities of the stimulus (0.5° and 4°) are presented. The results show that the detectability of disparity differences first increases and then decreases when the reference disparity is increased from zero. It further appears that the only effect of the stimulus presentation time and eccentricity on the psychometric curves and their dependency on the reference disparity is that the effective disparities observed are multiplied by a constant factor. The experimental data can be described by a signal detection model with two special features around zero disparity where binocular single vision occurs, viz. loss of information about the sign of disparity and presence of intrinsic noise at a level up to 25% higher than the minimum level found at intermediate disparities.