RETICULAR CONTROL OF VERTICAL SACCADIC EYE-MOVEMENTS BY MESENCEPHALIC BURST NEURONS

Extracellular recordings were obtained from units in the rostral mesencephalic reticular formation (MRF) of alert monkeys trained to perform a visual tracking task that permitted a quantitative analysis of discharge patterns in relation to saccadic eye movements. Units reported here discharged a high-frequency burst of spikes before and during saccades, but were otherwise silent. Based on their discharge patterns, these units were all classified as medium lead burst neurons (28). Three quantitative relationships were found consistently between parameters of unit discharge and of saccadic eye movements. First, the duration of any component of the saccade, regardless of its direction, was strongly correlated with the duration of unit discharge (burst duration). Second average intraburst frequency was correlated with the peak velocity of either the upward or downward component of the saccade (the on-direction component), but not with the velocities of other components. Third, the number of spikes in the burst was correlated with the amplitude of the on-direction component, but not with the amplitudes of other components. These results suggest that medium lead burst neurons in the MRF are output cells of pulse generators responsible for upward or downward components of saccades. Destruction of these cells or their axons may contribute to the vertical gaze paralysis often associated with pretectal or mesencephalic lesions. The results suggest further that medium lead burst neurons receive separate excitatory and inhibitory inputs that determine intraburst frequency and burst duration, respectively.