Human gaze shifts in which head and eyes are not initially aligned

Most studies of rapid orienting gaze shifts generated by combined eye and head movements have focused on an experimental condition in which gaze displacements are started with the subject's eyes in the normal straight-ahead position in the orbit. Such an experimental approach does not permit a clear identification of the input signal to the head motor system, because target offset angle is the same for both the eye and head. We have studied gaze shifts in human subjects which began with the visual axis straight ahead relative to the body (i.e., gaze or line of sight aligned with body sagittal plane) and with head offset from straight ahead at various angular positions. In our experimental conditions, the amplitude of head movement during a gaze shift was nearly equal to the angular distance between the target position and the starting head position (target-re-head), even though subjects were not specifically instructed to move their heads. This observation contrasts with other published reports in the literature showing considerable varibility amongst subjects in the amplitude of head rotation within a given task and between tasks. The difference may be related to the initial conditions which required subjects to align the eye and head on specific starting targets, since others have shown that requiring head alignment enhances head displacement. The amplitude of the saccadic eye movement was not determined by either the target's position relative to the starting eye or head positions. The value that best described the eye movement amplitude was the eye position in the orbit at the end of the saccade. This was nearly equal to target-rehead until a saturation eye position in the orbit was attained.

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