COMPARISON BETWEEN AUDITORY BRAIN-STEM RESPONSES AND DISTORTION PRODUCTS OTOACOUSTIC EMISSIONS AFTER TEMPORARY THRESHOLD SHIFT IN GUINEA-PIG

The aim of this study was to determine the best indicator of the site of generation of distortion product otoacoustic emissions (DPOAEs) by studying changes in DPOAEs and auditory brainstem responses (ABRs) fol lowing a discrete and reversible damage in the guinea pig cochlea. Whe n a continuous pure sound at 6 kHz was presented during 15 minutes at 95 dB SPL, the largest auditory brainstem responses (ABRs) threshold s hift (25 dB) was recorded at 8.4 kHz (half an octave above the exposur e frequency) and completely recovers after one day. Although this soun d exposure induced little changes for the highest levels of DPOAEs (> 56 dB SPL), DPOAEs for low and moderate levels of stimulations (< 56 d B SPL) display the same amount of amplitude reduction as ABRs threshol d shift (1dB for 1dB). In our experimental conditions (L(1) = L(2), f( 2)/f(1) = 1.17), the best frequency fitting between the maximum ABRs t hreshold shift and the maximum DPOAEs reduction was seen when f(2) was chosen as the frequency reference of the DPOAEs (rather than f(1), ro ot f(1) x f(2) or (2)f(1) - f(2)). Altogether these results suggest th at auditory fatigue is due to a reversible dysfunction of the cochlear mechanical non-linearity and support the idea that DPOAEs could be a very precise tool to estimate the functional integrity of the cochlea.