Abstract
Slopes of psychometric functions (PF) for forward-masked tones in normal-hearing (NH) listeners decrease as a function of signal threshold in both on- and off-frequency masker conditions. This result is attributed to nonlinear cochlear response growth, and has been used to demonstrate comparable compression at 4000 and 250 Hz. The current study further tested the hypothesis that the slope effect is due to nonlinearity. In hearing-impaired (HI) listeners who presumably lack cochlear nonlinearity, PF slopes should remain steep across signal threshold levels. Four NH and six HI listeners with a range of hearing thresholds were tested in on- and off-frequency masker conditions, with signals of 4000 and 250 Hz. Otoacoustic emissions were measured to verify absent or decreased cochlear nonlinearity in the HI listeners. In general, PF slopes were steeper for the HI than NH listeners. One subject had normal hearing at 250 Hz, and hearing loss at 4000 Hz. Her PF slopes were comparable to the mean slopes across NH subjects in the 250-Hz signal conditions, but were elevated in comparison to the NH subjects in the 4000-Hz conditions. These results are consistent with predictions from a model of forward masking that incorporates cochlear nonlinearity as one of its stages.
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