Electrophysiological Evidence for Auditory Sensitization

Abstract

This study was designed to investigate the properties of the first neural response (N1) to tone pips; the responses were recorded from the round window of the cat. Amplitude, latency, and adaptation (equilibration) curves were studied. The major part of the investigation was concerned with the effect of low-tone exposure on the amplitude of the N1 response to tone pips. After a relatively intense exposure, the amplitude of the N1 response, compared with the pre-exposure level, shows an initial subnormality (decreased response), a sensitization (increased response), and then a second subnormality. Sensitization was found when the exposure tone was the same, higher and lower in frequency than the basic frequency of the test tone pip. Exposure to low-frequency noise gives rise to a monotonic recovery process; i.e., a subnormality without subsequent sensitization. Contralateral effects were not detected. This sensitization of the N1 response is compared with related psychophysical data and may be regarded as an example of post-tetanic potentiation. Psychophysical experiments [J. R. Hughes, Ph.D. thesis, Harvard University (1954) and reference 19] have demonstrated the existence of “auditory sensitization” (a lowering of the absolute threshold for pure tones after low-tone exposure). After a relatively intense exposure, there is a brief rise in threshold, then a sensitization which lasts for approximately 2 min, followed by a long-lasting second rise in threshold. This psychophysical study also demonstrated that a relatively wide range of exposure frequencies can sensitize the auditory system to a given test tone pip. In order to obtain comparable evidence for sensitization in the electrical activity of the auditory nervous system, electrophysiological experiments were conducted on cats. The present study was designed to investigate the effect of a pure-tone stimulus on the properties of the first neural response (N1) recorded from the vicinity of the round window of the cat's cochlea. The major part of the investigation was concerned with whether, after low-tone exposure, the N1 response exceeds in amplitude the pre-exposure value: this phenomenon, when it occurs, will be labeled “sensitization of N1.” These electrophysiological studies on sensitization may be related to post-tetanic potentiation (PTP) (see references 11 and 21); this possible relationship will be discussed later. PTP refers to the increased responsiveness of the nervous system after application of a tetanizing stimulus.