SACCADE-RELATED ACTIVITY IN THE FASTIGIAL OCULOMOTOR REGION OF THE MACAQUE MONKEY DURING SPONTANEOUS EYE-MOVEMENTS IN LIGHT AND DARKNESS

Saccade-related burst neurons were recorded in the caudal part of the fastigial nucleus (fastigial oculomotor region) during spontaneous eye movements and fast phases of optokinetic and vestibular nystagmus in light and darkness from three macaque monkeys. All neurons (n=47) were spontaneously active and exhibited a burst of activity with each sacc ade and fast phase of nystagmus. Most neurons (n=31) only exhibited a burst of activity, whereas those remaining also exhibited a pause in f iring rate before or after the burst. Burst parameters varied consider ably for similar saccades. For horizontal saccades all neurons, except for three, had a preferred direction with an earlier onset of burst a ctivity to the contralateral side. For contralateral saccades the burs t started on average 17.5 ms before saccade onset, whereas the average lead-time for ipsilateral saccades was only 6.5 ms. Three neurons wer e classified as isotropic with similar latencies and peak burst activi ty in all directions. None of the neurons had a preferred direction wi th an earlier onset of burst activity to the ipsilateral side. Burst d uration increased with saccade amplitude, whereas peak burst activity was not correlated with amplitude. There was no relationship between p eak burst activity and peak eye velocity. In the dark, neurons general ly continued to burst with each saccade and fast phase of nystagmus. B urst for saccades in the dark was compared with burst for saccades of similar amplitude and direction in the light. Saccades in the dark had a longer duration and peak burst activity was reduced on average to 6 2% (range 36-105%). In three neurons a burst in the dark was no longer clearly distinguishable above the ongoing spontaneous activity. These data suggest that the saccade-related burst neurons in the FOR modify saccadic profiles by directly influencing acceleration and decelerati on, respectively, of individual eye movements. This could be achieved by an input to the inhibitory and excitatory burst neurons of the sacc adic burst generator in the brainstem. From neuroanatomical studies it is known that FOR neurons project directly to the brainstem regions c ontaining the immediate premotor structures for saccade generation.