The relation between otoacoustic emissions and the broadening of the auditory filter for higher levels

The active behaviour of outer hair cells (OHCs) is often used to explain two phenomena, namely otoacoustic emissions (OAEs) and the level dependence of auditory filters. Correlations between these two phenomena may contribute to the evidence of these hypotheses. In this study auditory filters were calculated from probe thresholds in notched-noise maskers over a range of at least 25 dB. Further. transient evoked otoacoustic emissions (TEOAEs) were measured at several stimulation levels. Ten normal-hearing and nine hearing-impaired subjects were tested. A linear increase of the width of the auditory filter with 2.2 dB/Hz was found up to a specific saturation level. The group of selected hearing-impaired subjects with mild hearing loss showed no wider than normal auditory filters. As expected, the increase of the width of the auditory filter correlated positively with the level of TEOAEs for click intensities of about 80 dB peak SPL. However, for subjects with TEOAEs wider auditory filters at a masker level of 65 dB/Hz were found for subjects with larger TEOAEs. This result cannot be explained by a model by which the cochlea shows an active behaviour for lower stimulation levels, influencing both the TEOAE levels and the filter skirts, and a passive behaviour for higher stimulation levels