Binocular disparity provides a powerful cue for depth perception in a stereoscopic environment. Despite increasing knowledge of the cortical areas that process disparity from neuroimaging studies, the neural mechanism underlying disparity sign processing [crossed disparity (CD)/uncrossed disparity (UD)] is still poorly understood. In the present study, functional magnetic resonance imaging (fMRI) was used to explore different neural features that are relevant to disparity-sign processing. We performed an fMRI experiment on 27 right-handed healthy human volunteers by using both general linear model (GLM) and multi-voxel pattern analysis (MVPA) methods. First, GLM was used to determine the cortical areas that displayed different responses to different disparity signs. Second, MVPA was used to determine how the cortical areas discriminate different disparity signs. The GLM analysis results indicated that shapes with UD induced significantly stronger activity in the sub-region (LO) of the lateral occipital cortex (LOC) than those with CD. The results of MVPA based on region of interest indicated that areas V3d and V3A displayed higher accuracy in the discrimination of crossed and uncrossed disparities than LOC. The results of searchlight-based MVPA indicated that the dorsal visual cortex showed significantly higher prediction accuracy than the ventral visual cortex and the sub-region LO of LOC showed high accuracy in the discrimination of crossed and uncrossed disparities. The results may suggest the dorsal visual areas are more discriminative to the disparity signs than the ventral visual areas although they are not sensitive to the disparity sign processing. Moreover, the LO in the ventral visual cortex is relevant to the recognition of shapes with different disparity signs and discriminative to the disparity sign.

With visuospatial dysfunction emerging as a potential marker that can detect Alzheimer’s disease (AD) even in its earliest stages and with disturbance in stereopsis suspected to be the prime contributor to visuospatial deficits in AD, we assessed stereoscopic abilities of patients with AD and mild cognitive impairment (MCI). Whereas previous research assessing patients’ stereoacuity has yielded mixed results, we assessed patients’ capacity to process coarse disparities that can convey adequate depth information about objects in the environment. We produced two virtual cubes at two different distances from the observer by manipulating disparity type (absolute vs. relative), disparity direction (crossed vs. uncrossed) and disparity magnitude, then had participants judge the object that appeared closer to them. Two patient groups performed as well as, or even better than elderly controls, suggesting that AD patients’ coarse disparity processing capacity is capable of supporting common tasks involving reaching, grasping, driving, and navigation. Results may help researchers narrow down the exact cause(s) of visuospatial deficits in AD and develop and validate measures to assess visuospatial dysfunction in clinical trials and disease diagnosis.

When a real or subjective contour is superimposed on a repetitive texture or ‘wallpaper pattern’, the apparent depth of the elements of the wallpaper pattern may be influenced by the contour. When the contour has crossed disparity, the elements inside the contour are seen floating in the plane of the contour. This is called ‘stereoscopic capture’. On the other hand, when the contour has uncrossed disparity a different, somewhat unstable, percept is seen. The wallpaper elements are either seen to form a transparent surface floating in front of the contour, or all of the elements inside and outside of the contour are seen to lie in the uncrossed plane of the inducing contour. We suggest that the asymmetry between crossed and uncrossed stereo capture is a by-product of the different roles played by crossed and uncrossed disparity in stereoscopic surface construction: specifically, crossed and, perhaps, zero disparity spreads one-directionally into a figure from a contour whereas uncrossed disparity spreads out from a figure in all directions. These points are illustrated with a series of demonstrations.

<h4>Purpose</h4>To study the pattern of facilitatory and suppressive binocular interactions in stereodeficient patients with strabismus and in healthy controls.<h4>Methods</h4>Visual evoked potentials were recorded in response to a Vernier onset/offset pattern presented to one eye, either monocularly or paired dichoptically with a straight vertical square-wave grating, which, when fused with the target in the other eye, gave rise to a percept of a series of bands appearing in depth from an otherwise uniform plane or with a grating that contained offsets that produced a standing disparity and the appearance of a constantly segmented image, portions of which moved in depth.<h4>Results</h4>Participants with normal stereopsis showed facilitative and suppressive binocular interactions that depended on which dichoptic target was presented. Patients with longstanding, constant strabismus lacked normal facilitative binocular interactions. The response to a normally facilitative stimulus was reduced below the monocular level when it was presented to the dominant eye of patients without anisometropia, consistent with classical strabismic suppression of the nondominant eye. The dominant eye of strabismic patients without anisometropia retained suppressive input from crossed but not uncrossed disparity stimuli presented to the nondominant eye.<h4>Conclusions</h4>Abnormal disparity processing can be detected with the dichoptic VEP method we describe. Our results suggest that suppression in stereoblind, nonamblyopic observers is determined by a binocular mechanism responsive to disparity. In some cases, the sign of the disparity is important, and this suggests a mechanism that can explain diplopia in patients made exotropic after surgery for esotropia.

The encoding of three-dimensional visual information is of important in everyday life. Eye-movements challenge this spatial encoding: they shift the image of the outside world across the retina. In the macaque ventral intraparietal area (VIP), many neurons encode visual information irrespective of horizontal and vertical eye position. Does this gaze invariance of spatial encoding extend to egocentric distances? Such invariance would correspond to a shift of disparity-tuning curves by vergence angle. Here, monkeys fixated one of three distances (vergence), while a visual stimulus was shown at one of seven distances (disparity). Most neurons' activity was modulated independently by both disparity and eye vergence, and we did not observe shifts of disparity-tuning curves as expected from encoding egocentric distances at a single-cell level. By using population activity, however, we were able to decode egocentric distance. Our results provide further strong evidence for a role of area VIP in 3D space encoding.

A substantial proportion of neurons in the striate and prestriate cortex of monkeys have stereoscopic properties; that is, they respond differentially to binocular stimuli that are known in humans to provide cues for stereoscopic depth perception. Stereoscopic neurons, as these cells may be called, are selective for horizontal positional disparity (i.e., display disparity selectivity) and for the textural correlation between images over their receptive fields (i.e., they show correlation selectivity). Many neurons have tuned disparity response profiles that collectively cover the entire range of physiological disparities. Neurons with peak responses at or about the zero disparity ("tuned zero neurons," excitatory or inhibitory) have narrow and symmetrical profiles. Neurons that are tuned to larger disparities, either crossed ("tuned near neurons") or uncrossed ("tuned far neurons"), have broader excitatory profiles that are asymmetrically wider toward the smaller disparities, and commonly include an inhibitory component about the zero disparity. Other stereoscopic neurons have reciprocal profiles ("near" or "far" neurons, respectively) in the sense that they respond with excitation to crossed or uncrossed disparities, and with suppression to disparities of opposite sign. Stereoscopic neurons can also signal the textural correlation between paired retinal images by giving different responses to random-dot patterns that have, and to those that do not have, the same dot distribution over the neuron's left and right receptive fields. Tuned-zero excitatory neurons characteristically respond to uncorrelation with suppression; tuned-zero inhibitory neurons, with excitation; and both types give the opposite responses to correlated stereopatterns. Neurons selective for nonzero disparities, both tuned and reciprocal, also give excitatory responses to uncorrelated stimuli, but these responses are smaller and more variable than those evoked by correlated patterns at the effective disparities. These findings suggest that stereoscopic neurons in the visual cortex of the macaque comprise three operational systems: (1) a zero-disparity system that is involved in fine depth discrimination with the obligatory singleness of vision, and the maintenance of vergence; and (2) a near-, and (3) a far-disparity system that together signal qualitative estimates of depth with double vision, and vergence responses to large disparities.

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