Seeing the world with two eyes: neural processes for stereopsis
Ichiro Fujita

Osaka University 21st century COE Program International Symposium "Dynamics of Biological Systems"
Osaka University Graduate School of Frontier Biosciences, Osaka, Japan

Our two eyes are horizontally displaced from each other and view slightly different scenes. Images of objects at a distance other than on the plane of fixation are projected to different positions in the two retinas. This positional difference of retinal images between the two eyes is called "horizontal binocular disparity and provides an important visual cue for the perception of depth and 3-dimensional scene, the perceptual capability dubbed stereopsis.

Horizontal binocular disparity (hereafter simply binocular disparity) is first computed in the primary kvisual cortex of the primate brain. Previous psychophysical, clinical and brain-imaging studies suggest that binocular disparity is processed mainly in the dorsal visual pathway, which projects from the primary visual cortex through areas MT and MST to the posterior parietal cortex. Recent studies from our and other laboratories, however, show that a large population of neurons in cortical areas in the ventral visual pathway, such as area V4 and the inferior temporal cortex (IT), respond to a particular range of binocular disparity (Uka et al., 2000; Watanabe et al., 2001; Tanabe et al., 2005). We now know that disparity signals are widely spread across visual cortices, but their relative role is poorly understood.

We addressed how binocular disparity signals are transformed along the ventral visual pathway, and whether neurons in V4 and IT differ from neurons in dorsal pathway areas such as MT and MST in terms of the processing of binocular disparity. We found that (1) when a random-dot stereogram is contrast-reversed between the left and right eye images, the majority of disparity-selective V4 neurons attenuate their disparity selectivity, a decrease in sensitivity also reflected in depth perception (Tanabe et al., 2004); (2) a large portion of V4 neurons encode relative disparity between two surfaces in a visual stimulus (Umeda et al., in prep); (3) trial-to-trial fluctuations of IT-neuron responses to a given stimulus correlate with the animal s behavioral report of fine-grade depth judgment (Uka et al., 2005). These results indicate that neural activity in V4 and IT surpasses the local filter-like processing of the primary visual cortex, and correlates more with binocular depth perception than that in areas MT and MST.

Base on the above results and those in the current literature, we suggest that disparity selective neurons in the ventral visual pathway areas are involved in discriminating fine-scale depth as well as in representation of 3-dimensional structure of objects, while disparity selective neurons in the dorsal visual pathway areas are involved in coarse stereopsis and in the control of vergence eye movement (Fujita et al., 2005).

Fujita, L, Yasuoka, S., Tanabe, S. (2005) Dissociation of stereoscopic depth judgment from perception of a plane-in-depth: implication for neural mechanism of stereopsis. Soc. Neurosci. Abstr.

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.

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.

Uka, T., Tanaka, H., Yoshiyama, K., Kato, M and Fujita, I. (2000) Disparity selectivity of neurons in inferior temporal cortex. J. Neurophysiol., 84:120-132.

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.

Watanabe, M., Tanaka, H., Uka, T. and Fujita, I. (2002) Disparity-selective neurons in area V4 of macaque monkeys. J. Neurophysiol., 87:1960-1973.