Thresholds of Auditory Sensitivity and Auditory Fatigue: Relation with Cochlear Mechanics

Abstract

The shape of the curve of the auditory sensitivity thresholds as a function of frequency is determined by the transfer function of the outer and middle ear. For a constant acoustic signal at the input to the cochlea the auditory sensitivity threshold is nearly constant. From theoretical results, it seems that the threshold of the neural responses appears always for the same velocity of the cochlear partition all along the cochlea.

Concerning the auditory fatigue, we applied to the guinea pig’s cochlea a constant acoustic energy (pure tones of frequency from 2 kHz to 11,3 kHz) during 20 minutes and we measured the TTS by electrocochleography twenty minutes after the end of the exposure (from 2 to 32 kHz by half-octave steps). For each animal, the level of the stimulus at the input to the cochlea was adjusted by means of cochlear microphonic measurements.

• a) In order to obtain a constant TTS for each frequency of stimulation, the acoustic level at the input to the cochlea must decrease by about 4 dB/octave.

• b) A constant TTS at 16 kHz and at 22.6 kHz, whatever the stimulation frequency is, is obtained for an acoustic level at the input of the cochlea decreasing by about 10 dB/octave.

Given the facts that, for a constant acoustic input to the cochlea:

• the amplitude of the displacements of the cochlear partition near the CF doubles each time that the frequency is halved and consequently the velocity of the cochlear partition is constant,

• the amplitude of the displacements of the cochlear partition at the base of the cochlea is constant for frequencies lower than CF and consequently the velocity of the cochlear partition at the base doubles with the frequency, the difference of slope between case a) and b), i.e. 6 dB/octave seems to indicate that the velocity of the cochlear partition is an important parameter which plays a significant role in the coming out of the auditory fatigue.