Electrophysiological investigation of the human cochlea. Recruitment, masking and adaptation.

Abstract

Action potentials from the human cochlea in response to short-tone bursts of different frequencies were recorded by means of a transtympanic needle electrode placed on the promontory. The input-output relations for amplitude (AN1) and peak latency (τN1) of the N1 deflecetion have been studied in a large number of human cochleas and are compared with similar results obtained by round window recording in guinea pigs. The amplitude-intensity relation is qualitatively the same for normal human and guinea pig cochleas. At high stimulus intensities, the peak latencies are the same for human and guinea pig cochleas. At high stimulus intensities, the peak latencies are the same for human and guinea pig cochlear action potentials, but at threshold, the human τN1 is about 3 msec longer than for the guinea pig. In recruiting ears, the input-output relation have been measured for both AN1 and τN1 and compared to those obtained from guinea pig cochleas suffering a high-frequency sensorineural hearing loss.On the basis of the AN1–ττN1 relation for 15 normal cochleas at three different frequencies, a statistical criterion is proposed to assist in clinical detection and diagnosis of abnormal cochleas. The dependency of the AN1 on the interval between successive tone bursts, usually called adaptation time-constant for human cochlear is about 4 times longer than for guinea pig cochleas. Also the masking curves for human cochlear action potentials are different from those found in guinea pig.The differences in latency, adaptation and masking properties in normal human ears, as compared with the guinea pig, indicate the existence of two distinct populations of neural elements in the human cochlea. There is a longer modal latency-differences in man than in the two corresponding populations found in guinea pig.