On Hearing by Electrical Stimulation

Abstract

The well-known fact that the ear generates an electric potential in response to stimulation by a sound wave has its counterpart in the fact that, when an alternating current is passed through the head, an auditory sensation results. The observer hears a tone whose pitch is determined by the frequency of the alternating current. Measurements were made of the power needed to elicit a sensation using currents of various frequencies. The power, rather than the simple voltage or current was measured, because the body presents a complex impedance to the current. The total impedance, when the electrodes are applied in a standard manner, decreases with increasing frequency. The power factor varies slightly with frequency. When the power is increased about 20 db above the threshold value, the threshold of electric shock—a stinging sensation—is reached. This effect severely limits the size of the auditory area under electrical stimulation. The amount of distortion present makes it difficult to understand speech when the observer is connected directly to the output circuit of a radio set, but music can be readily identified. The rectification producing this distortion appears to be largely electrical. It is probable that the alternating current sets up vibrations in the inner ear due, perhaps, to a residual charge on the basilar membrane which makes it behave as a condenser microphone.