Contribution of the superior colliculus and the mesencephalic reticular formation to gaze control.

Converging lines of evidence support a role for the intermediate and deep layers of the superior colliculus (SC) and the mesencephalic reticular formation (MRF) in the control of combined head and eye movements (i.e., gaze). Recent microstimulation, single-cell recording, and lesion experiments are reviewed in which monkeys are free to move their heads. Cells in the SC discharge in advance of combined head and eye movements and most likely provide a gaze error signal to downstream structures. In contrast, the neurons in the MRF are of at least two types. Eye cells have features that are similar to neurons in the rostral portion of the SC, but fire before the onset of horizontal eye movments. A second group of MRF neurons begin to fire after the onset of the gaze shift and are most closely associated with movements of the head. The peak discharge of these late-onset MRF neurons occurs near the peak head velocity. Stimulation in the rostral SC generates eye movements with fixed amplitude and direction. A similar response is noted after stimulation of the more dorsal portion of the caudal MRF. Stimulation in the caudal portion of the SC produces combined head and eye movements of fixed amplitude. Electrical activation of the more ventral portions of the caudal MRF generates goal-directed and centering eye movements. Temporary inactivation of the SC with the GABA agonist muscimol generated hypometria and curved trajectories of contralateral eye movements. Inactivation of the caudal MRF produced contralateral hypermetria and ipsilateral hypometria of saccades. Release of the monkey's head demonstrated a profound contralateral head tilt. Taken together, these data suggest that the gaze signal generated in the SC is filtered by neurons in the MRF to generate a feedback signal of eye motor error. The head signal found in the MRF could cancel a portion of the gaze signal coming from the SC in the form of head velocity feedback.