Specialization of mid-tier stages of dorsal and ventral pathways in stereoscopic processing

The division of labor between the dorsal and ventral visual pathways is an influential model of parallel information processing in the cerebral cortex. However, direct comparison of the two pathways at the single-neuron resolution has been scarce. Here we compare how MT and V4, mid-tier areas of the two pathways in the monkey, process binocular disparity, a powerful cue for depth perception and visually guided actions. We report a novel tradeoff where MT neurons transmit disparity signals quickly and robustly, whereas V4 neurons markedly transform the nature of the signals with extra time to solve the stereo correspondence problem. Therefore, signaling speed and robustness are traded for computational complexity. The key factor in this tradeoff was the shape of disparity tuning: V4 neurons had more even-symmetric tuning than MT neurons. Moreover, this correlation between tuning shape and signal transformation was present across individual neurons within both MT and V4. Overall, our results reveal both distinct signaling advantages and common tuning-curve features of the dorsal and ventral pathways in stereoscopic processing.