Coding of horizontal disparity and velocity by MT neurons in the alert macaque.

We performed the first large-scale (n = 501), quantitative study of horizontal disparity tuning in the middle temporal (MT) visual area of alert, fixating macaque monkeys. Using random-dot stereograms, we quantified the direction tuning, speed tuning, horizontal disparity tuning, and size tuning of each neuron. The vast majority (93%) of MT neurons were significantly tuned for horizontal disparity. Although disparity tuning was generally quite robust, the average disparity sensitivity of MT neurons was significantly weaker than their direction or speed sensitivity as quantified using both an index of response modulation and an index of signal-to-noise ratio. Disparity tuning was not correlated with direction or size tuning but tended to be broader and weaker for neurons that preferred faster speeds of motion. By comparison with recent studies, we find that disparity selectivity in MT is substantially stronger than that seen in either primary visual cortex (V1) or area V4. In addition, MT neurons are more broadly tuned for disparity than V1 neurons at comparable eccentricities. Disparity tuning curves are very well described by Gabor functions for >80% of MT neurons. The distribution of Gabor phases shows clear bimodality, indicating that MT neurons tend to have odd-symmetric disparity tuning (unlike neurons in V1). The preferred disparities were more strongly correlated with the phase parameter of the Gabor function than with the positional offset parameter. In fact, for neurons with preferred disparities close to zero, the positional offset tended to oppose the phase shift in specifying the disparity preference. We suggest that this result reflects a strategy used to finely distribute the disparity preferences of MT neurons, given the predominance of odd-symmetry and broad tuning.