"Stereopsis: 3-D perception with two eyes and two cortical pathways"
The human visual system can derive 3-D surface structure from two retinal images. The computation for binocular depth perception starts in the striate cortex(V1). Properties of V1 cells, however, do not account for a number of aspects of stereo perception, suggesting that subsequent processing in cortical areas beyond V1 is responsible for conscious perception of stereoscopic depth. It has long been believed that binocular disparity information is processed along the dorsal visual pathway. In fact, binocular disparity signals are processed both along the ventral and dorsal pathways. Cells in the ventral pathway areas solve the binocular correspondence problem, compute relative disparity between visual features, and exhibit activities correlated with behavioral judgment of fine disparity descrimination. Those in the dorsal pathway areas encode binocular correlation, signal local absolute-disparity, and are involved in judgment of coarse disparity and in control of vergence angle. Psychophysics in human subjects suggests that perception of plane-in-depth depends on the ventral pathway mechanisms. Thus, the two cortical pathways contribute to stereopsis in a complementary manner.