Three Mechanisms of Hearing by Electrical Stimulation

Abstract

It was previously shown that when an alternating electric current is passed through the head via a salt solution in the external ear canal, a normal observer hears a tone which is related to the stimulus by a square law, that is to say, if the sound heard is interpreted in terms of motion of the tympanic membrane, then the displacement of the membrane is proportional to the square of the instantaneous voltage. The writers suggested the hypothesis that the cavity formed by the middle ear acts as an electrostatic transducer. This hypothesis accounted for the square-law response and for certain quantitative aspects of the results. This hypothesis had to meet the objection, however, that persons without tympanic membranes are able to hear by electrical stimulation. In order to determine the relation between the hearing of normal and operated ears, twenty ears lacking tympanic membranes were stimulated electrically. Eleven ears heard pure tones corresponding in pitch to the frequency of the applied voltage; seven heard a buzzing noise whose character was roughly independent of the stimulating frequency. Examination showed that the pure-tone response in the operated ears was purely linear, in contradistinction to the quadratic response of normal ears. Hence, under electrical stimulation normal and operated ears hear by means of two distinctly different mechanisms. The square-law response in normal ears is apparently mediated by an electrostatic action in the middle ear. The linear response in operated ears may or may not be the inverse of the cochlear microphonic. Evidence is presented that direct stimulation of the auditory nerve occurs in those ears which bear a buzzing noise. This result is consistent with a “place theory” of hearing, and it suggests that the “frequency theory” of hearing is untenable even for low frequencies.