Cochlear Responses to Acoustic Transients: An Interpretation of Whole-Nerve Action Potentials

Abstract

Intracochlear electrodes in the guinea pig are used to measure the relations among cochlear potentials in response to slow acoustic transients. The traveling wave of Békésy is described in terms of cochlear‐microphonic (CM) voltage as functions of time and place along the cochlear partition. The results are consistent with previous observations in the ear and on models of the basilar membrane. Interpolations of wavevelocity and wave amplitudes between places used for the measurements allow continuous representations of the traveling‐wave pattern of CM in either space or time. From these representations, it is clear that the duration of the stimulating phase of CM along the cochlear partition significantly exceeds the apparent duration of the whole‐nerve action‐potential (AP) response to these transients. Selective changes in the waveforms of the AP responses, as opposed to simple reductions in amplitude, are observed when the transients are accompanied by bands of noise and after local chemical or mechanical injury to the organ of Corti. The selective changes in waveform allow consideration of the waveform removed from the normal AP response by the noise as well as the response remaining during noise. The responses removed by each of successive increases the bandwidth of the noise reveal the presence of AP responses at times not apparent in the normal whole‐nerve AP waveform. These observations are most easily explained by assuming that the basic neural response is diphasic as conventionally recorded. When neurons become active in an orderly sequence, the positive phases of the earlier individual responses coincide with and may conceal the negative phases of later responses. The whole‐nerve AP waveform is thus considered as the convolution (complex product) of two functions in time, the diphasic unit of response and the numerical sequence of newly active neurons. An empirical model for the diphasic unit of response “divided” into the AP waveform reveals patterns of neural activity that are compatible with the traveling wave of CM. The same model satisfactorily explains several details of the whole‐nerve AP waveform recorded during stimulation with a burst of high‐frequency tone.