Head-unrestrained gaze adaptation in the rhesus macaque.

The ability to adjust the amplitude of gaze shifts in response to persistent visual errors ("gaze adaptation") has been investigated primarily by introducing visual errors at the end of saccades produced by head-restrained primates. Very little is known about the behavior and neural mechanisms underlying gaze adaptation when the head is free to move. We tested alternative hypotheses about the signals that are altered during gaze adaptation by increasing (25 degrees --> 50 degrees ; "forward adaptation") or decreasing (50 degrees --> 25 degrees ; "backward adaptation") the size of large, head-unrestrained gaze shifts. In our three rhesus monkey subjects, changes to primary gaze shift amplitude occurred regardless of the particular combinations of eye and head movements that made up the amplitude-altered gaze shifts. The relative changes to eye and head movements that occurred during adaptation could be predicted based on the magnitude of gaze adaptation and the positions of the eyes in the orbits at gaze onset. These results are consistent with the hypothesis that gaze adaptation occurs at the level of a gaze shift command and inconsistent with hypotheses based on the assumption that gaze adaptation results from alterations to eye- and/or head-specific signals.