Amplitude changes in response to target displacements during human eye–head movements

Sensorimotor adaptation, the ability to adjust motor output in response to persistent changes in sensory input, is a key function of the central nervous system. Although a great deal is known about vestibulo-ocular reflex and saccadic adaptation, relatively little is known about the behavior and neural mechanisms underlying gaze adaptation when the head is free to move. In an attempt to understand the mechanisms of gaze adaptation, and constrain hypotheses concerning the locus at which changes in gaze control may be implemented, we altered the size of large, head-unrestrained gaze shifts made to visual targets by surreptitiously moving the visual target forward (30° → 60°) or backwards (60° → 30°) during gaze shifts. In our 10 human subjects, after a few hundred back-step trials, gaze amplitudes were reduced by between 6° and 27°. Similarly, after a few hundred forward adaptation trials, our subjects increased gaze amplitude by between 0° and 26°. Changes in the amplitude of primary gaze shifts occurred regardless of the particular combinations of eye and head movements that made up the amplitude-altered gaze shifts. When gaze shifts were initiated with the eyes in systematically different positions relative to the head, the resulting changes in gaze, eye and head movement amplitudes were consistent with the hypothesis that gaze adaptation occurs at the level of a gaze shift command and not by altering separately the signals that produce eye and head movements.