Haircell non-functionality and dead regions in the cochlea: an exploring study

Dead regions (DRs) refer originally to regions in the cochlea without evoked electrical potentials due to non-functional inner hair cells andor fibers of the auditory nerve. The focus of this study is to characterize hair cell non-functionality in the cochlea by means of psychophysical tests. A battery of tests was administered to a group of 13 subjects with a steep sloping tone threshold (>50 dBoctave) between 1 and 2 kHz and severe loss (>60 dB HL) at high frequencies. Psychophysical Tuning Curves (PTC) and Threshold Equalizing Noise (TEN) are the classical tests to diagnose dead regions. Both use as criteria the phenomenon of off-frequency listening. Based on the results of complementary tests like Notched Noise measurements (NN) and Otoacoustic Emissions (OAE), it is argued that off-frequency listening is not necessarily connected to loss of inner hair cells andor nerve fibers. Furthermore, combination tones produced by well functioning outer hair cells at places of severe hearing loss (>60 dB HL) are found. This may be explained by the presence of a dead region, which is verified with PTC andor TEN measurements in 3 out of 4 of the cases