Functional properties of neurons in middle temporal visual area of the macaque monkey. II. Binocular interactions and sensitivity to binocular disparity.

1. Electrophysiological recordings were made in the middle temporal visual area (MT) of five macaque monkeys. Binocularity and selectivity for disparity were examined using a computer-driven stimulator to activate each eye independently. Results were obtained from 91 single units in MT. 2. Most units in MT receive approximately balanced inputs from the two eyes, and very few could be driven through only one eye. 3. In one type of test for disparity selectivity, units were examined with stimuli that had different but fixed horizontal disparities, thereby simulating frontoparallel movements at different distances from the animal. About two-thirds of ;the units tested for fixed disparity selectivity (52/76) showed pronounced sensitivity to horizontal disparity. Most of these units could be grouped into the same four classes of disparity-tuned units that have previously been described in V1 and V2 of the macaque: near, far, tuned excitatory, and tuned inhibitory. 4. Twenty units were tested for sensitivity to vertical stimulus disparity, which does not normally contribute to stereopsis. Most were as sensitive to vertical disparities as to horizontal. 5. Units were also tested for selectivity for stimuli that moved with changing disparity, simulating trajectories with components of motion toward or away from the animal (motion in depth). No units were found to be truly selective for motion in depth. Units tuned for fixed disparity could appear to prefer motion in depth if tested only with trajectories whose common center point was far from the unit's optimal fixed disparity. However, we do not consider this to represent genuine selectivity for motion in depth, since 1) the responses ara adequately and more easily explained in terms of selectivity for fixed disparity and 2) the best overall response of these units is to frontoparallel motion at the optimal fixed disparity. This observation bears importantly on the interpretation of motion in depth selectivity in previous investigations. 6. The presence of a substantial degree of selectivity for fixed disparity in MT, together with previously demonstrated selectivities for direction and speed, indicates that MT is well suited for the analysis of motion in three-dimensional space.