Temporary Threshold Shifts in Chinchilla: Electrophysiological Correlates

Abstract

Carder and Miller [Trans. Amer. Acad. Ophthalmol. Otolaryngol. 75, 1346–1354 (1972)] exposed chinchillas for seven days to an octave band of noise centered at 500 Hz and at 95 dB SPL to produce temporary shifts of behavioral auditory thresholds (TTS) of about 48 dB which required four to seven days to recover to normal. In the present study physiological potentials were measured about 5, 24, and 48 h after exposures to the same noise for two or three days. Cochlear microphonic (CM) responses and dc endocochlear potentials were measured in each of the three cochlear turns. Input-output functions for whole-nerve action potential (AP) responses to clicks and visual detection levels (VDL) for early averaged evoked responses (AER) arising in the brain stem were also measured. Endocochlear potentials were unchanged. Shifts of sensitivity for CM in the second and third turns showed the closest numerical correspondence to behavioral TTS. Loss of sensitivity for AP was greater. Changes in VDL for the AER were consistent with behavioral TTS. This pattern of physiological changes was consistent with a disorder in the mechano-electric modulation by the hair cells plus failure to synchronize primary neural responses. Just-detectable loss of outer hair cells was also observed anatomically.