Intracochlear Electrical Stimulation: Results of Psychoacoustic Studies in Man and Physiological Studies in Cats

Abstract

A series of simple psychoacoustic experiments has been conducted on subjects previously implanted with an intracochlear bipolar electrode. With use of simple ac electrical stimulation, subjects heard sounds described as “tones” over a frequency range from about 100 to at least several thousand Hz. The apparent pitch rose rapidly up to 400–600 Hz; at higher frequencies, it changed slowly if at all over a broad range of frequency. The dF was about 1–3 Hz at 100 Hz, 5–10 Hz at 500 Hz, and 50–100 Hz at 1 kHz. Physiological experiments were conducted to determine how sensation heard by these subjects is generated and encoded in the auditory nervous system. Study of single units of the inferior colliculus in cats using standard microelectrode techniques has revealed: (1) Tuning curves of colliculus neurons excited by electrical stimuli were flat, and all were roughly parallel. The normal “place” representation of stimulus frequency is obscured. (2) The period of the stimulus is represented in the time order of discharge of colliculus neurons at frequencies up to 500–600 Hz. (3) With the stimulus electrode employed, the threshold of the acoustic nerve at the cochlear base was 30–40 dB lower than at the cochlear apex. Psychoacoustic and physiological evidence strongly suggests that hearing in these subjects is akin to the described sensation of “periodicity pitch.”