VESTIBULAR-EVOKED ELECTROMYOGRAPHIC RESPONSES IN SOLEUS - A COMPARISON BETWEEN CLICK AND GALVANIC STIMULATION

The aim of this study was to demonstrate, if possible, vestibulospinal reflex responses in soleus using a stimulus known to be capable of ex citing vestibular afferents, namely 100-dB (NHL) clicks. We were able to show short-latency electromyographic (EMG) responses after clicks i n five of eight normal subjects, and then we compared these responses with those after transmastoid galvanic stimulation (12 normal subjects ). Stimulation of the side towards which the head was rotated (i.e. th e side facing backwards) with either clicks or the cathode (anode appl ied to the opposite side) gave an initial excitatory response in soleu s, while click or cathodal stimulation of the opposite side (i.e. the side facing forwards) gave an initial inhibitory response. Onset laten cies and modulation with changes in postural task were identical for b oth click- and galvanic-evoked responses. In addition, there was a sig nificant correlation between the amplitudes of the responses in soleus after click and galvanic stimulation (R-2=0.72). These similarities s uggest that the earliest reflex responses in soleus after clicks and g alvanic stimulation may be mediated by a common central pathway. In co ntrast, there was no correlation between the amplitudes of responses e voked by 100-dB clicks in soleus and those evoked by the same stimulus in the sternocleidomastoid. We conclude that vestibular activation by clicks can evoke reflex responses in lower-limb muscles and these res ponses have similar characteristics to the earliest responses evoked b y galvanic vestibular stimulation.