Sinusoidal Electrical Stimulation of the Human Vestibular Apparatus

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Abstract
Sinusoidal electrical stimulation with eight frequencies ranging from 0.030 to 4.0 cps was applied between the mastoid processes of five male undergraduate Ss. Only ascending series in the method of limits were used, and the amplitude was increased continuously rather than discretely. Subjective reports of the presence of a sensation, and also of its quality were recorded and used to determine RLs at all the frequencies. An objective technique, using the recorded sideways movements of Ss, was developed and also used to determine RLs. These curves of RL versus frequency are termed electrical vestibulograms. The reported qualities were classed into three main categories, (a) swaying sideways, (b) swaying forward and backward, and (c) sideways oscillation of head and body. Subjective and objective electrical vestibulograms were similar in shape, being U-shaped between 0.50 and 4.0 cps, and approximately linear between 0.030 and 0.50 cps, but with the subjective having a slope of about one, and the objective, of zero. The latter vestibulogram was lower than the former. Sway frequencies were also estimated, using a new technique, and were found to be proportional to the stimulus, but the proportionality varied with the frequency. Fluctuation in amplitude of swaying to a constant amplitude stimulus was observed separately in two Ss at each of two frequencies. The results were interpreted as supporting a central, rather than peripheral, adaptation hypothesis.