Abstract
Psychophysical depth judgments depend primarily on relative disparity, the difference in the absolute disparities of two visible features in a scene. This suggests that there exists, within the visual cortex, processes capable of computing relative disparity values. Previous studies indicate that the coding process of relative disparity begins in V2, while neurons in V1 and MT signal absolute disparity. Here, we studied the responses of V4 neurons to relative disparities in two alert macaque monkeys. A circular patch of a dynamic random-dot stereogram was presented to animals over the receptive field of each tested neuron. The patch was bipartite, containing a center disk and surrounding annulus. We examined the effect of shifting the surround annulus at different disparities on the tuning curves for disparity of the center disk. If a neuron codes relative disparity between the center and the surround, the disparity-tuning curve should shift sideways along the disparity axis by the amount and direction of the shift in the surround disparity. If a neuron codes for the absolute disparity of the center, no shift will occur. We quantified such shifts by calculating a shift ratio (the amount of the shift in the tuning curve / the amount of the shift in the surround disparity). The distribution of the shift ratio was unimodal and biased towards the direction expected for relative-disparity coding (median=0.44, n=98). A comparison with V2 neurons demonstrates that V4 neurons as a population exhibit substantially higher sensitivity for relative disparity. 3D structural information in V4 is compressed from absolute-disparity based to relative-disparity based. Disparity signals in V4 may be utilized in fine stereo depth judgments requiring detection of a disparity difference between two objects or surfaces.
supported by grants from the MEXT (13308046, 15016067)