| A large number of people complain of speech recognition problems in noise despite having normal conventional audiograms. Peripheral auditory processing linked to outer hair cell mechanisms appears to play an important role in improving signal detection in noise. However, it is not known whether outer hair cell dysfunction causes or correlates with the reduced speech recognition in noise reported in this population. The aim of this study was to investigate if deficits in the auditory periphery, especially the outer hair cells, contribute to speech recognition impairment in the presence of noise. Fourteen listeners who self-reported speech recognition problems in noise and performed poorly on a sentences-in-noise perception test despite having normal hearing thresholds at all standard audiometric frequencies as evaluated by conventional pure tone audiometry were selected for the study. Three indirect measures believed to be reasonably sensitive to outer hair cell alterations, namely extended high-frequency pure tone audiometry (ref. eardrum SPL) with frequencies of 10 k, 12.5 k and 14 kHz, distortion product otoacoustic emissions at 2k, 4k and 8kHz and psychophysical estimates of auditory filters at 2 kHz were evaluated in these impaired listeners and compared to ten normal-hearing listeners. We report that a subset of listeners (8 out of 14) with reduced speech recognition in noise and normal conventional audiograms reveal peripheral auditory processing deficits consistent with outer hair cell dysfunction as indicated by significantly reduced hearing sensitivity at higher frequencies (10 k, 12 k and 14 kHz), significantly reduced DPOAEs at 2, 4 and 8 kHz and markedly wider auditory filters at 2 kHz compared to the normal hearing listeners. It is possible that in these 8 listeners the altered peripheral auditory abilities associated with outer hair cell functioning, to a certain extent, contribute to the reduced speech processing capabilities in noise. Four other listeners with reduced speech recognition in noise deviated from the normal-hearing listeners only on the behaviorally derived auditory filter measure. In these listeners, it is more likely that the speech recognition problems in noise result from a central auditory processing deficit. |
