Society for Neuroscience 2003

I.Fujita1*; T.Uka3; S.Tanabe2; M.Watanabe2
1. Lab. Cognit Neurosci, Grad. Sch. Frontier Biosci., Osaka Univ, Toyonaka, Japan
2. Grad Sch Eng Sci, Osaka Univ, Toyonaka, Japan
3. Grad Sch Med, Osaka Univ, Suita, Japan

Recent studies have shown that the inferior temporal cortex (IT), an area that belongs to the "what" pathway , contains a large population of neurons selective for horizontal binocular disparity. The specific role that disparity-selective IT neurons play in stereoscopic depth perception, however, is still unclear. To draw a link between neuronal responses of disparity-selective IT neurons and the perception of stereoscopic depth, we examined responses of IT neurons while monkeys were discriminating fine binocular disparity. Two monkeys were trained to discriminate whether a solid shape figure presented over the fixation point was in front of or behind the fixation point by making an eye movement to one of two choice targets. Monkeys' performance was pushed down to psychophysical threshold, by varying the horizontal disparity of the shape finely across trials. We recorded from 95 neurons from the lateral surface of the IT (areas TEO and TE) and analyzed 57 that had a preference for "near" or "far" disparities. Trial-to-trial fluctuations of IT responses co-varied with the choice the monkeys made. To quantify this co-variation, we calculated "choice probability" (Britten et al. 1996), which is the probability that an ideal observer can predict the monkeys' choice based on the responses of each neuron. Thirty-five out of 57 neurons had choice probabilities significantly different from 0.5 (permutation test, p<0.05), with the majority (28/35) being larger than 0.5. The mean choice probability across the 57 neurons was 0.56, which is significantly larger than 0.5 (sign-test, p<0.001). This means that one could predict the monkeys' choice by listening to the responses of a population of disparity-selective IT neurons. The present results suggest that the IT is functionally involved in fine localization of stereoscopic depth.