Cochlear nonlinearity between 500 and 8000 Hz in listeners with impaired hearing

Abstract

Cochlear nonlinearity was estimated in listeners with impaired hearing using a forward-masking method. For a fixed low-level probe, the masker level required to mask the probe was measured as a function of the masker-probe interval, to produce a temporal masking curve (TMC). TMCs were measured for probe frequencies from 500 to 8000 Hz, and for masker frequencies from 0.5 to 1.6 times the probe frequency. Unlike what happens for normal-hearing (NH) listeners [Lopez-Poveda et al., J. Acoust. Soc. Am. 113, 951–960 (2003)], TMCs for on-frequency maskers sometimes show a single slope, suggesting linear responses for tones at CF. Sometimes, however, two distinct slopes are visible, suggesting remaining compression. Both patterns are uncorrelated with the absolute threshold and are consistent with selective damage to outer or inner hair cells (IHC), respectively. Remarkably, the slope of the TMCs for very low-frequency maskers is shallower for the impaired ears than for normal ones. This result implies that for NH listeners, the slope of the TMCs for very low-frequency maskers reflects some kind of compression, even at high CFs. It is discussed that this compression is likely to occur at the IHC rather than at the basilar membrane. [Work supported by FIS PI020343, G03/203, and Oticon Spain.]