The relationship of monkey frontal eye field activity to saccade dynamics

1. In this study, we compared the temporal waveforms of the activity of monkey frontal eye field movement neurons with the dynamics of saccadic eye movements. 2. Movement neurons in the frontal eye field were selected according to previously published criteria. They had little or no response to visual stimuli in a fixation task, and equivalent activity before visually guided and memory-guided saccades. We studied corticotectal neurons and corticopontine neurons identified by antidromic excitation, as well as neurons whose projections were not identified. 3. These neurons had a peak activation at a mean of 13 ms before the saccade began. However, rather than falling off rapidly as the saccade ended, most neurons continued to fire after the saccade, returning to baseline at a mean of 93 ms after the end of the saccade. 4. We measured the decrement in activity for these neurons during the saccade. Although a few neurons showed decrements of > 60% of their peak activity level, the average activity dropped only 16.9%, with some neurons actually showing a rise in activity during the saccade. If we ignored the latency between peak in activity and saccade start and measured the fall in activity for a period equal to one saccade duration after the peak, the average drop in activity was still only 34.9%. Thus the activity of these neurons did not appear to be closely related to dynamic motor error, which falls from its maximum value to zero over the time course of a saccade. 5. These results suggest that a focus of movement activity within the topographic map in the frontal eye field specifies the amplitude and direction for an impending saccade, whereas the peak of movement activity signals the time to initiate a saccade. 6. Unlike the superior colliculus, the activity of frontal eye field movement neurons does not appear to be related to dynamic events that occur during the saccade, such as motor error.