The coordination of eye, head, and arm movements during rapid gaze orienting and arm pointing.

This study aimed to investigate the coordination of multiple control actions involved in human horizontal gaze orienting or arm pointing to a common visual target. The subjects performed a visually triggered reaction time task in three conditions: (1) gaze orienting with a combined eye saccade and head rotation (EH), (2) arm pointing with gaze orienting by an eye saccade without head rotation (EA), and (3) arm pointing with gaze orienting by a combined eye saccade and head rotation (EHA). The subjects initiated eye movement first with nearly constant latencies across all tasks, followed by head movement in the EH task, by arm movement in the EA task, and by head and then arm movements in the EHA task. The differences of onset times between eye and head movements in the EH task, and between eye and arm movements in the EA task, were both preserved in the EHA task, leading to an eye-to-head-to-arm sequence. The onset latencies of eye and head in the EH task, eye and arm in the EA task, and eye, head and arm in the EHA task, were all positively correlated on a trial-by-trial basis. In the EHA task, however, the correlation coefficients of eye-head coupling and of eye-arm coupling were reduced and increased, respectively, compared to those estimated in the two-effector conditions (EH, EA). These results suggest that motor commands for different motor effectors are linked differently to achieve coordination in a task-dependent manner.