Cochlear nonlinearity between 500 and 8000 Hz in listeners with moderate cochlear hearing loss

Abstract

For two listeners with flat hearing loss, temporal masking curves (TMCs) were measured for probe frequencies (Fp) of 0.5, 1.0, 2.0, 4.0, and 8.0 kHz and masker frequencies of 0.5, 0.6, 0.7, 0.9, 1.0, 1.05, 1.1, and 1.2 Fp. From these, basilar membrane (BM) input/output (IO) functions and psychophysical tuning curves (PTCs) were derived and compared with corresponding data for normal-hearing listeners. Linear IO functions were observed for one ear only and for probe frequencies of 4 kHz and above. This result is consistent with the consequences of severe outer hair cell (OHC) damage on the BM response. Elsewhere, however, IO functions showed residual compression, with slopes in the compression region close to normal. This suggests that either the impairment relates mostly to inner hair cell (IHC) dysfunction, or that OHC dysfunction reduces the gain of the BM at low levels while maintaining the degree of maximum compression. The data suggest that the active and the passive BM mechanisms have different relative positions (in frequency) in the apical and the basal regions of the cochlea. Across probe frequencies, the slopes of the TMCs for off-frequency maskers are shallower for the impaired ears. It is considered that this possibly reflects IHC-related compression.