Head-unrestrained primate gaze adaptation

Changing the direction of the line of sight (gaze) allows primates to gather visual information from their surrounding environment. Gaze shifts can result from movements of the eyes, head, and/or body. In order to efficiently obtain information from the environment, the central nervous system must maintain the accuracy of gaze shifts by adjusting the metrics of these effectors in response to persistent changes in sensory input (sensorimotor adaptation) that result from aging, disease, or changes to the reliability of sensory cues. Studies of primate gaze adaptation have emphasized changes in eye movement amplitude in response to small (<5°) visual errors in head-restrained subjects. The work presented in the current thesis investigated the mechanism by which the central nervous system adjusts gaze amplitude in response to large (>20°) residual visual errors in head-unrestrained subjects. I tested hypotheses describing the nature of this mechanism in two primate species using behavioral (in humans and monkeys) and neurophysiological (in monkeys) methods that allowed gaze, eye, and head motor commands to be dissociated.;Three results from my behavioral experiments support a hypothesis that a gaze command signal is adjusted to maintain the accuracy of gaze shifts towards visual targets: (1) changes in gaze amplitude in response to large visual errors were the result of alterations to both eye and head movement amplitudes; (2) modifications to eye and head movements were dependent upon the position of the eyes at gaze onset, and; (3) changes to gaze amplitude at one initial eye position transferred to gaze shifts initiated from other eye positions. A neurophysiology experiment was designed to test the hypothesis that a modification to a gaze command generated by the superior colliculus (SC) could account for these behavioral observations. Our results suggest that gaze shifts evoked by microstimulation of the SC can be modified in response to large visual errors. The combined results of the experiments described in the current thesis are consistent with the hypothesis that a gaze command is altered prior to being decomposed into separate eye and head movement commands during gaze adaptation.