Electrical microstimulation of the fastigial oculomotor region in the head-unrestrained monkey.

It has been shown that inactivation of the caudal fastigial nucleus (cFN) by local injection of muscimol leads to inaccurate gaze shifts in the head-unrestrained monkey and that the gaze dysmetria is primarily due to changes in the horizontal amplitude of eye saccades in the orbit. Moreover, changes in the relationship between amplitude and duration are observed for only the eye saccades and not for the head movements. These results suggest that the cFN output primarily influences a neural network involved in moving the eyes in the orbit. The present study further tested this hypothesis by examining whether head movements could be evoked by electrical microstimulation of the saccade-related region in the cFN. Long stimulation trains (200-300 ms) evoked staircase gaze shifts that were ipsi- or contralateral, depending on the stimulated site. These gaze shifts were small in amplitude and were essentially accomplished by saccadic movements of the eyes. Head movements were observed in some sites but their amplitudes were very small (mean=2.4 degrees). The occurrence of head movements and their amplitude were not enhanced by increasing stimulation frequency or intensity. In several cases, electrically evoked gaze shifts exhibited an eye-head coupling that was different from that observed in visually triggered gaze shifts. This study provides additional observations suggesting that the saccade-related region in the cFN modulates the generation of eye movements and that the deep cerebellar output region involved in influencing head movements is located elsewhere.