Hand stabilization during body motion shares similar control processes with known vestibular-driven motor responses

Vestibular-driven motor responses have short latency and are largely controlled through automatic processes. Here we examined if the Compensatory Arm Movements (CAM) that allow subjects to keep the hand stationary in space during body motion have the same characteristics and therefore, if they belong to the same class of movements. If so, the CAM must fulfil specific criterions. The CAM must results from muscular activity. Indeed, as the arm’s inertia tends to stabilize the hand in space during body motion, the CAM could result from passive forces. We recorded EMG activity while subjects (Ss) had to keep the hand fixed in space during 35° body rotations in darkness. Marked EMG bursts were found in the Pectoralis major during clockwise (CW) rotations and in the posterior Deltoïdeus during counterclockwise (CCW) rotations. The swiftness of the CAM was assessed computing EMG reaction time (RT). This RT was compared with the RT measured when the arm movement responded to a visual stimulation. Here, the Ss tracked with the unseen finger a visual target that moved either CW or CCW. RT was shorter when the arm movement was a response to a body rotation than when it was a response to a visual stimulus (162 vs. 327 ms, P < 0.05). The automatic character of the CAM was assessed testing the effect of providing Ss with information about the required direction of the impending CAM. Before the trials, the Ss were informed of the direction of the arm movements to keep the hand stationary in space during rotations or to track the visual target with the finger (task that largely depends on cognitive processes). In 14 of the trials, false information about the required arm direction was given. In 83 of the catch trials where Ss tracked the visual target, EMG bursts first appeared in the muscles permitting arm motion in the direction provided by the false precueing, i.e. in the opposite direction to the actual target motion. For the CAM, only 10 of the catch trials affected the EMG response. We conclude that the CAM are essentially based on sensorimotor mechanisms, with negligible cognitive processes