POSTURAL ELECTROMYOGRAPHIC RESPONSES IN THE ARM AND LEG FOLLOWING GALVANIC VESTIBULAR STIMULATION IN MAN

Application of a small (around 1 mA), constant electric current betwee n the mastoid processes (galvanic stimulation) of a standing subject p roduces enhanced body sway in the approximate direction of the ear beh ind which the anode is placed. We examined the electromyographic (EMG) responses evoked by such stimulation in the soleus and in the triceps brachii muscles. For soleus, subjects stood erect, with their eves cl osed, leaning slightly forward. The head was turned approximately 90 t o the right or left relative to the feet. In averaged records (n=40), current pulses of 25 ms or longer modulated the EMG in a biphasic mann er: a small early component (latency 62+/-2.4 ms, mean+/-SEM) was foll owed by a larger late component (latency 115+/-5.2 ms) of opposite sig n. which was appropriate to produce the observed body sway. The early component produced no measurable body movement. Lengthening the durati on of the stimulus pulse from 25 to 400 ms prolonged the late componen t of the response but had little effect on the early component. Short- and long-latency EMG responses were also evoked in the triceps brachi i muscle if subjects stood on a transversely pivoted platform and had to use the muscle to maintain their balance in the anteroposterior pla ne by holding a fixed handle placed by the side of their hip. The late ncy of the early component was 41+/-2.6 ms; the latency of the late co mponent was 138+/-4.3 ms and was again of appropriate sign for produci ng the observed body sway. Galvanic stimulation evoked no comparable r esponses in either triceps brachii or soleus muscles if these muscles were not being used posturally. The responses were most prominent if v estibular input provided the dominant source of information about post ural stability, and were much smaller if subjects lightly touched a fi xed support or opened their eyes. The difference in latency between th e onset of the early component of the response in arm and leg muscles suggests that this part of the response uses a descending pathway whic h conducts impulses down the spinal cord with a velocity comparable wi th that of the fast conducting component of the corticospinal tract. T he late component of the EMG response occurs earlier in the leg than t he arm. We suggest that it forms part of a patterned, functional respo nse which is computed independently of the early component.