HUMAN BODY-SEGMENT TILTS INDUCED BY GALVANIC STIMULATION - A VESTIBULARLY DRIVEN BALANCE PROTECTION MECHANISM

1. We have studied the effects of changes in posture on the motor resp onse to galvanic vestibular stimulation (GVS). The purpose of the expe riments was to investigate whether the function of the GVS-evoked resp onse is to stabilize the body or the head in space. Subjects faced for wards with eyes closed standing with various stance widths and sitting . In all cases tile GVS-evoked response consisted of a sway of the bod y towards the anodal ear. 2. In the first set of experiments the respo nse was measured from changes in (i) electromyographic activity of hip and ankle muscles, (ii) the lateral ground reaction force, and (iii) lateral motion of the body at the level of the neck (C7). For all meas urements the response became smaller as the feet were placed further a part. 3. In the second set of experiments we measured the GVS-evoked t ilts of the head, torso and pelvis. The basic response consisted of a tilt in space (anodal ear down) of all three segments. The head tilted more than the trunk and the trunk tilted more than the pelvis produci ng a leaning and bending of the body towards the anodal ear. This chan ge in posture was sustained for the duration of the stimulus. 4. The t ilt of all three segments was reduced by increasing the stance width. This was due to a reduction in evoked tilt of the pelvis, the bending of the upper body remaining relatively unchanged. Changing from a stan ding to a sitting posture produced additional reductions in tilt by re ducing the degree of upper body bending. 5. The results indicate that the response is organized to stabilize the body rather than the head i n space. We suggest that GVS produces a vestibular input akin to that experienced on an inclined support surface and that tile function of t he response is to counter any threat to balance by keeping the centre of mass of the body within safe limits.