Psychophysical experiments have demonstrated that it is possible to simultaneously perceive binocular depth and rivalry in plaids (A. Buckthought & H. R. Wilson, 2007). Here, we used fMRI at 3T to image activity in the visual cortex while human subjects perceived depth and rivalry from plaids. Six subjects performed either a rivalry or depth task. The spatial frequencies of the near-vertical and diagonal components were, respectively: 2.5, 6.4 cpd; 6.4, 2.5 cpd; or 6.4, 6.4 cpd. The network of activated cortical areas was very similar for the depth compared to the rivalry task. Nevertheless, regions of superior and inferior parietal cortices (including intraparietal sulcus) were activated more during the depth than the rivalry task, independent of spatial frequency, whereas a bias toward rivalry was seen in a lateral occipital region, superior temporal sulcus, and retrosplenial and ventral temporal cortices. Several retinotopic areas in the visual cortex showed a preference for the task with the higher (V1, V2, V3) or lower spatial frequency component (MT+), regardless of the depth or rivalry condition. Our results indicate that depth and rivalry are processed in a similar network of cortical areas and are perceived simultaneously by coexisting in different spatial channels. These results place constraints on binocular vision models.

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Neural hysteresis plays a fundamental role in stereopsis and reveals the existence of positive feedback at the cortical level [Wilson, H. R., & Cowan, J. D. (1973). A mathematical theory of the functional dynamics of cortical and thalamic nervous tissue. Kybernetik 13(2), 55-80]. We measured hysteresis as a function of orientation disparity in tilted gratings in which a transition is perceived between stereopsis and binocular rivalry. The patterns consisted of sinusoidal gratings with orientation disparities (0 degrees, 1 degrees, 2 degrees,..., 40 degrees) resulting in various degrees of tilt. A movie of these 41 pattern pairs was shown at a rate of 0.5, 1 or 2 pattern pairs per second, in forward or reverse order. Two transition points were measured: the point at which the single tilted grating appeared to split into two rivalrous gratings (T1), and the point at which two rivalrous gratings appeared to merge into a single tilted grating (T2). The transitions occurred at different orientation disparities (T1=25.4 degrees, T2=17.0 degrees) which was consistent with hysteresis and far exceeded the difference which could be attributed to reaction time. The results are consistent with a cortical model which includes positive feedback and recurrent inhibition between neural units representing different eyes and orientations.

The neural correlates of binocular rivalry have been actively debated in recent years, and are of considerable interest as they may shed light on mechanisms of conscious awareness. In a related phenomenon, monocular rivalry, a composite image is shown to both eyes. The subject experiences perceptual alternations in which the two stimulus components alternate in clarity or salience. The experience is similar to perceptual alternations in binocular rivalry, although the reduction in visibility of the suppressed component is greater for binocular rivalry, especially at higher stimulus contrasts. We used fMRI at 3T to image activity in visual cortex while subjects perceived either monocular or binocular rivalry, or a matched non-rivalrous control condition. The stimulus patterns were left/right oblique gratings with the luminance contrast set at 9%, 18% or 36%. Compared to a blank screen, both binocular and monocular rivalry showed a U-shaped function of activation as a function of stimulus contrast, i.e. higher activity for most areas at 9% and 36%. The sites of cortical activation for monocular rivalry included occipital pole (V1, V2, V3), ventral temporal, and superior parietal cortex. The additional areas for binocular rivalry included lateral occipital regions, as well as inferior parietal cortex close to the temporoparietal junction (TPJ). In particular, higher-tier areas MT+ and V3A were more active for binocular than monocular rivalry for all contrasts. In comparison, activation in V2 and V3 was reduced for binocular compared to monocular rivalry at the higher contrasts that evoked stronger binocular perceptual suppression, indicating that the effects of suppression are not limited to interocular suppression in V1.

Binocular rivalry was studied using plaids which were the sum of orthogonal diagonal gratings plus identical vertical gratings in the two eyes. The rivalry alternations sped up as the spatial frequency difference between the vertical and diagonal gratings was increased above about one octave, but slowed down for smaller differences. The interaction between depth and rivalry was studied using similar plaids but with depth introduced in the vertical components. Depth and rivalry coexisted when the spatial frequency difference between the vertical and diagonal gratings was greater than about one octave, but rivalry slowed down and depth perception was reduced for smaller differences. Plaids consisting of square wave gratings were used to compare: (1) added gratings; (2) vertical gratings superimposed on (i.e. occluding) diagonal gratings; (3) diagonal gratings superimposed on vertical gratings. Rivalry alternations were fastest in condition (3), indicating that grouping effects played a role. The final experiment indicated that depth and rivalry coexisted within a spatial frequency band if the orientation difference between the vertical and diagonal components was 60-70 degrees . These results place constraints on models of stereopsis and rivalry, indicating that depth and rivalry can coexist in different spatial frequency and orientation bands but that each interferes with the other in the same band.