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Movement of an observer through the environment generates motion on the retina. This optic flow contains information about the direction of self-motion. To accurately signal the direction of self-motion however, the optic flow has to carry some depth information: there has to be differential motion of elements at different depths. One depth cue that is available to an organism with frontal eyes is binocular disparity. Cells in the dorsal subdivision of the Medial Superior Temporal area (area MSTd) have been proposed to play a role in the analysis of optic flow. We have examined the disparity sensitivity of neurons from MSTd in awake behaving monkeys in an attempt to understand the possible contribution of disparity to the computation of the direction of self-motion. Cells with a response to fronto-parallel motion were examined. While the monkey looked at a fixation spot on a screen in front of it, random dot stimuli moved in the preferred direction of the cell under study, and the disparity of the dots made the stimuli appear to move in a fronto-parellel plane in front of, on, or behind the screen. Over 90% of the neurons studied were sensitive to the disparity of the visual stimulus. Of those disparity sensitive cells, 95 % responded best either to near stimuli (stimuli with crossed disparities appearing to move in front of the screen) or to far stimuli (stimuli with uncrossed disparities appearing to move behind the screen). In 40% of the disparity sensitive cells, we found cells whose preferred disparity reversed as the direction of stimulus motion was reversed. For example, a cell that responded best to crossed disparities (foreground) for rightward motion, responded best to uncrossed disparities (background) for leftward motion. Such an opposite motion of foreground and background occurs when an organism tracks a stationary object while translating in a direction different from the line of gaze. We propose that the reversal of disparity selectivity with a reversal in direction selectivity indicates one way in which these neurons could signal the direction of self-motion of the organism in its environment.
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