"Binocular stereopsis and the ventral pathway"
The visual system has a remarkable capability of deriving 3-D surface structure from two retinal images. The computation for stereopsi starts in the striate cortex (V1), where signals from the two eyes converge onto single neurons and binocular disparity information is encoded. 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 routed exclusively to the dorsal visual pathway. Research in the past decade, however, revealed that binocular signals for stereopsis are processed both along the ventral and dorsal pathways. Analysis of single neuron activity recorded from awake monkeys suggests that neurons in the ventral pathway areas such as areas V4 solve the binocular correspondence problem (ref.1), compute relative disparity between visual features (ref.4), and exhibit activities correlated with behavioral judgment of fine disparity discrimination (ref.3). Microstimulation in area V4 biases judgment of fine disparity. Studies in other laboratories demonstrate that neurons in the dorsal pathway areas encode binocular correlation, signal local absolute-disparity, and are involved in judgment of coarse disparity. Psychophysics in human subjects suggests that perception of plane-in-depth depends on the ventral pathway mechanisms (ref.5). Thus, the two visual cortical pathways contribute to different aspects of stereopsis.
1. Tanabe, S., Umeda, K., Fujita,I. (2004) Rejection of false-matches for binocular correspondence in macaque visual cortical area V4. J. Neurosci. 24: 8170-8180.
2. Tanabe, S., Doi, T., Umeda, K., Fujita, I. (2005) Disparity-Tuning characteristics of neuronal responses to dynamic random-dot stereograms in macaque visual area V4. J. Neurophysiol. 94: 2683-2699.
3. Uka, T., Tanabe, S., Watanabe, M., Fujita, I. (2005) Neural correlates of fine depth discrimination in monkey inferior temporal cortex. J. Neurosci. 25: 10796-10802.
4. Umeda, K., Tanabe, S., Fujita, I. (2007) Representation of stereoscopic depth based on relative disparity in macaque area V4. J. Neurophysiol. 98: 241-252.
5. Tanabe, S., Yasuoka, S., Fujita, I. (2008) Disparity-energy signals in perceived stereoscopic depth. J. Vision 8(3):22, 1-10