Perceptually bistable three-dimensional figures evoke high choice probabilities in cortical area

The role of the primate middle temporal area (MT) in depth perception was e xamined by considering the trial-to-trial correlations between neuronal act ivity and reported depth sensations. A set of moving random dots portrayed a cylinder rotating about its principal axis. In this structure-from-motion stimulus, the direction of rotation is ambiguous and the resulting percept undergoes spontaneous fluctuations. The stimulus can be rendered unambiguo us by the addition of binocular disparities. We trained monkeys to report t he direction of rotation in a set of these stimuli, one of which had zero d isparity. Many disparity-selective neurons in area MT are selective for the direction of rotation defined by disparity. Across repeated presentations of the ambiguous (zero-disparity) stimulus, there was a correlation between neuronal firing and the reported direction of rotation, as found by Bradle y et al. (1998). Quantification of this effect using choice probabilities ( Britten et al., 1996) allowed us to demonstrate that the correlation cannot be explained by eye movements, behavioral biases, or attention to spatial location. MT neurons therefore appear to be involved in the perceptual deci sion process. The mean choice probability (0.67) was substantially larger t han that reported for MT neurons in a direction discrimination task (Britte n et al., 1996). This implies that MT neurons make a different contribution to the two tasks. For the depth task, either the pool of neurons used is s maller or the correlation between neurons in the pool is larger.