Pooled, but not single-neuron, responses in macaque V4 represent a solution to the stereo correspondence problem

Abstract

Binocular disparity is an important cue for depth perception. To correctly represent disparity, neurons must find corresponding visual features between the left- and right-eye images. The visual pathway ascending from V1 to inferior temporal cortex solves the correspondence problem. An intermediate area, V4, has been proposed to be a critical stage in the correspondence process. However, the distinction between V1 and V4 is unclear, because accumulating evidence suggests that the process begins within V1. In this article, we report that the pooled responses in macaque V4, but not responses of individual neurons, represent a solution to the correspondence problem. We recorded single-unit responses of V4 neurons to random-dot stereograms of varying degrees of anticorrelation. To achieve gradual anticorrelation, we reversed the contrast of an increasing proportion of dots as in our previous psychophysical studies, which predicted that the neural correlates of the solution to correspondence problem should gradually eliminate their disparity modulation as the level of anticorrelation increases. Inconsistent with this prediction, the tuning amplitudes of individual V4 neurons quickly decreased to a nonzero baseline with small anticorrelation. By contrast, the shapes of individual tuning curves changed more gradually so that the amplitude of population-pooled responses gradually decreased toward zero over the entire range of graded anticorrelation. We explain these results by combining multiple energy-model subunits. From a comparison with the population-pooled responses in V1, we suggest that disparity representation in V4 is distinctly advanced from that in V1. Population readout of V4 responses provides disparity information consistent with the correspondence solution.

Weighted parallel contributions of binocular correlation and match signals to conscious perception of depth

“…Remember, however, that the mean amplitude ratio of monkey V1 neurons is 0.52 [13], indicating that cRDS modulation is twice stronger than aRDS modulation. The reduced sensitivity to aRDSs in human V4 reported by Bridge & Parker [19] is also consistent with monkey neurophysiology [18,86,91], but not with the other fMRI study [20]. Furthermore, the better decoding accuracy for cRDSs rstb.royalsocietypublishing.org Phil.…”

“…The results in human psychophysics (figure 3b, leftmost) predict that neurons in the match-based representation should gradually decrease their disparity modulation over a large range of graded anti-correlation. In contrast with this prediction, the tuning amplitude of individual V4 neurons quickly decreased to a positive baseline level with small anticorrelation and stayed constant with further anti-correlation [86] (figure 6a,b). The individual tuning curves of many neurons also shifted their peak with graded anti-correlation.…”

“…The results in V4 and V5/MTþ in human by Preston et al [20] are not consistent with the reduced sensitivity of monkey V4 to aRDSs [18,86,91] or the retained sensitivity of monkey V5/MT and MST [17,87]. The discrepancy for area V4 may be partially explained by the controversial existence and location of the dorsal part of V4 in human cortex.…”

Specialized contributions of mid-tier stages of dorsal and ventral pathways to stereoscopic processing in macaque

“…The raw responses are linearly interpolated in both dimensions. The V4 example was previously shown in Abdolrahmani et al, 2016 .…”

“…C and F show cumulative distributions of the area ratio and amplitude ratio, respectively. The amplitude ratio histogram for V4 ( E ) is a replot of the result reported in Abdolrahmani et al, 2016 .…”