Postural changes during eye-head movements.

We investigated postural sway in young subjects during eye-head movements. Subjects stood on the force plate wearing a helmet on which a laser spot was attached. Visual targets were presented stepwise at 60 degrees to right and left or 50 degrees to upward and downward of the straight-ahead position. Subjects were asked to look at the targets quickly by directing the laser spot to the target. The anterior-posterior component of centre of pressures (COPs) exhibited the largest changes and shortest latencies during downward movements. Most of the subjects showed COP changes preceding the onset of downward head movements, and these changes were associated with EMG activity of biceps femoris and gastrocnemius. When subjects were asked to look at the target only by eyes, no consistent change was detected in ground reaction forces. These results indicate that the eye-head movements elicited the COP changes that preceded the onset of head movements.