Influence of head position and proprioceptive cues on short latency postural reflexes evoked by galvanic stimulation of the human labyrinth

Abstract

This study described the effects of low level (75–250 μA) galvanic stimulation of the labyrinths of the human posture control system. Experiments demonstrated a transient EMG response in gastrocnemius-soleus (GS) muscle with a latency of 100 msec after initiation of galvanic current pulses of 75–250 μA and 50–150 msec duration. These transient EMG responses were modulated by position of the head. With the head turned 90° to one side so that the positive electrode faced posterior resulted in excitation of GS muscles. Turning the head 90° to the other side so that the positive electrode faced anterior resulted in inhibition of GS muscles. The transient EMG response was absent if the head were in its normal forward facing position. A 2 df platform enabled the ankle joints of subjects to be maintained at a fixed angle during sway; thus making vestibular cues critical during control of postural sway. With vestibular cues critical, the transient EMG response caused marked swaying and compensatory responses correlated with galvanic stimulus. Dynamics of these postural responses were tested using both currents of 150 msec duration (to gain an approximate measure of the impulse response of the system) and of 5 sec duration (to measure the step response of the system). A simplified third order model of the results showed that dynamic characteristics of body sway to galvanic stimulation resembled those induced by motion disturbances. The initial postural response to galvanic stimulation was uneffected by the availability of cues from the feet and lower legs, although later components were strongly effected.