Interactions between ocular motor and manual responses during two-dimensional tracking.

Tracking of a moving target usually involves coordinated movements of the eye and the hand. To study the extent to which one behavior influences the other, eye and hand movements were recorded during three conditions (eye alone, hand alone, and eye and hand together) where subjects tracked a target that initially moved in a straight line and then made an abrupt and unpredictable change in direction. The response latencies of the eye and hand were influenced by the presence of the other tracking modality. More specifically, the latency for the hand was decreased during concomitant ocular tracking, whereas the latency for the eye was increased during combined hand-eye tracking. Moreover, the velocity profile of the smooth pursuit component of ocular tracking was different when the hand also tracked the target. Taken together, these observations support the hypothesis that at least part of the neural substrate underlying tracking is shared by the two modalities.