Neuronal activity related to eye-hand coordination in the primate premotor cortex.
To test the functional implications of gaze signals that we previously reported in the dorsal premotor cortex (PMd), we trained two rhesus monkeys to point to visual targets presented on a touch screen while controlling their gaze orientation. Each monkey had to perform four different tasks. To initiate a trial, the monkey had to put his hand on a starting position at the center of the touch screen and fixate a fixation point. In one task, the animal had to make a reaching movement to a peripheral target randomly presented at one of eight possible locations on a circle while maintaining fixation at the center of this virtual circle (central fixation + reaching). In the second task, the monkey maintained fixation at the location of the upcoming peripheral target and, later, reached to that location. After a delay, the target was turned on and the monkey made a reaching arm movement (target fixation + reaching). In the third task, the monkey made a saccade to the target without any arm movement (saccade). Finally, in the fourth task, the monkey first made a saccade to the target, then reached to it after a delay (saccade + reaching). This design allowed us to examine the contribution of the oculomotor context to arm-related neuronal activity in PMd. We analyzed the effects of the task type on neuronal activity and found that many cells showed a task effect during the signal (26/60; 43%), set (16/49; 33%) and/or movement (15/54; 28%) epochs, depending on the oculomotor history. These findings, together with previously published data, suggest that PMd codes limb-movement direction in a gaze-dependent manner and may, thus, play an important role in the brain mechanisms of eye-hand coordination during visually guided reaching.