Microstimulation of the lateral wall of the intraparietal sulcus compared with the frontal eye field during oculomotor tasks

We compared the effects of intracortical microstimulation (ICMS) of the lat eral wall of the intraparietal sulcus (LIP) with those of ICMS of the front al eye held (FEF) on monkeys performing oculomotor tasks. When ICMS was app lied during a task that involved fixation, contraversive saccades evoked in the LIP and FEF appeared similar. When ICMS was applied to the FEF at the onset of voluntary saccades, the evoked saccades collided with the ongoing voluntary saccade so that the trajectory of voluntary saccade was compensat ed by the stimulus. Thus the resultant saccade was redirected and came clos e to the endpoint of saccades evoked from the fixation point before the sta rt of voluntary saccade. In contrast, when ICMS was applied to the LIP at t he onset of voluntary saccades, the resultant saccade followed a trajectory that was different from that evoked from the FEF. In that case, the collid ing saccades were redirected toward an endpoint that was close to the endpo int of saccades evoked when animals were already fixating at the target of the voluntary saccade. This finding suggests that the colliding saccade was directed toward an endpoint calculated with reference to the target of the voluntary saccade. We hypothesize that, shortly before initiation of volun tary saccades, a dynamic process occurs in the LIP so that the reference po int for calculating the saccade target shifts from the fixation point to th e target of a voluntary saccade. Such predictive updating of reference poin ts seems useful for immediate reprogramming of upcoming saccades that can o ccur in rapid succession.