Abstract: Close one eye and look around you. Then open that eye. You immediately notice that the visual world has vivid depth when viewed with both eyes. The human brain can derive 3-dimensional depth structure of objects and surfaces from two flat images on the two retinae. This magnificent, mysterious, yet daily experienced ability is dubbed stereopsis, and has long attracted both scientists and non-scientists. Stereopsis allows a highly quantitative and sensitive detection of depth compared to depth perception based on monocular cues (e.g., occlusion, shading, texture gradients, etc.). Due to the horizontal displacement from each other, our two eyes view the world from slightly different vantage points. This causes a tiny positional difference in the visual images in the two eyes. The brain exploits this small difference, or binocular disparity, to compute depth.
The neural processing for binocular depth perception starts in the primary visual cortex (V1) where signals from two eyes converge onto single neurons for the first time in the visual pathway. V1 neurons detect binocular disparity through a computation similar to cross-correlation between the left and right retinal images. However, this does not necessarily mean that V1 neurons directly underlie stereoscopic depth perception. In fact, properties of V1 cells do not account for a number of aspects of stereopsis, 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 visual pathway projecting from V1 to the parietal cortex (dorsal pathway). Studies from our and other laboratories in the past 15 years, however, have revealed that binocular disparity signals are processed both along the dorsal pathway and the pathway projecting from V1 to the temporal cortex (ventral pathway). We thus use the two major cortical pathways for stereopsis. Why?
Recent studies are now answering this question; the two cortical pathways contribute to stereopsis in a different manner. Neurons 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 discrimination. 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.
Biography: Ichiro Fujita received his Bachelor degree (1979) from University of Tokyo, and his M.Sc. degree (1981) and Ph.D. (1984) from University of Tokyo Graduate School. He then received his postdoctoral training at National Institute for Physiological Sciences (Okazaki, Japan), California Institute of Technology (Pasadena, U.S.A.) and RIKEN (Wako, Japan). In 1994, he became a professor at Osaka University Medical School and started his Laboratory of Cognitive Neuroscience. He is currently a professor at Osaka University Graduate School of Frontier Biosciences, joint-appointed at Center for the Study of Communication Design, Research Center for Behavioral Economics, and Center for Information and Neural Networks. He has been studying the neural mechanism of vision, particularly object recognition and binocular depth perception (stereopsis). |