The effects of eccentric head positions on leftward and rightward turns of a handle-bar

We explored the structural constraints on concurrent movements and/or positions of the head and a steering device by means of a reaction-time task. In the first experiment, subjects had to respond rapidly to an imperative stimulus by way of rotating a handle-bar to the left or to the right and back to the central position while they maintained a left or right eccentric position of the head. Latency of the handle-bar responses did not depend on whether their initial directions were toward the eccentric head position or in the opposite direction, but kinematic characteristics did: iso-directional movements were of larger amplitude and longer duration until peak excursion. In the second experiment, the imperative stimuli for handle-bar rotations were presented at variable intervals after the head had been moved from the central to one of the eccentric positions and before its predictable return movement. Kinematic characteristics of the handle-bar rotations depended on the left and right eccentric head positions in the same way as in Experiment 1, but now iso-directional movements had a longer reaction time than movements in the direction of the forthcoming return movement of the head. These findings suggest that specifications of head-movement directions facilitate concurrent specifications of handle-bar rotations in the same direction and inhibit specifications of handle-bar rotations in the opposite direction, consistent with the notion of cross-talk during motor programming.