Use of psychometric-function slopes for forward-masked tones to investigate cochlear nonlinearity

Auditory enhancement in younger and older listeners with normal and impaired hearinga)

Auditory enhancement is a spectral contrast aftereffect that can facilitate the detection of novel events in an ongoing background. A single-interval paradigm combined with roved frequency content between trials can yield as much as 20 dB enhancement in young normal-hearing listeners. This study compared such enhancement in 15 listeners with sensorineural hearing loss with that in 15 age-matched adults and 15 young adults with normal audiograms. All groups were presented with stimulus levels of 70 dB sound pressure level (SPL) per component. The two groups with normal hearing were also tested at 45 dB SPL per component. The hearing-impaired listeners showed very little enhancement overall. However, when tested at the same high (70-dB) level, both young and age-matched normal-hearing listeners also showed substantially reduced enhancement, relative to that found at 45 dB SPL. Some differences in enhancement emerged between young and older normal-hearing listeners at the lower sound level. The results suggest that enhancement is highly level-dependent and may also decrease somewhat with age or slight hearing loss. Implications for hearing-impaired listeners may include a poorer ability to adapt to real-world acoustic variability, due in part to the higher levels at which sound must be presented to be audible.

The influence of hearing-aid compression on forward-masked thresholds for adults with hearing loss

This paper describes forward-masked thresholds for adults with hearing loss. Previous research has demonstrated that the loss of cochlear compression contributes to deficits in this measure of temporal resolution. Cochlear compression can be mimicked with fast-acting compression where the normal dynamic range is mapped to the impaired dynamic range. To test the hypothesis that fast-acting compression will most-closely approximate the normal ability to perceive forward-masked pure-tones, forward-masked thresholds were measured for two groups of adults (normal hearing, hearing loss). Adults with normal hearing were tested without amplification. Adults with hearing loss were tested with three different compression speeds and two different prescriptive procedures using a hearing-aid simulator. The two prescriptive procedures differed in the extent to which the normal dynamic range was mapped onto the impaired dynamic range. When using a faster compression speed with the prescriptive procedure that best restored the lost dynamic range, forward-masked thresholds for the listeners with hearing loss approximated those observed for the listeners with normal hearing.

Comparison of distortion-product otoacoustic emission growth rates and slopes of forward-masked psychometric functions

Slopes of forward-masked psychometric functions (FM PFs) were compared with distortion-product otoacoustic emission (DPOAE) input/output (I/O) parameters at 1 and 6 kHz to test the hypothesis that these measures provide similar estimates of cochlear compression. Implicit in this hypothesis is the assumption that both DPOAE I/O and FM PF slopes are functionally related to basilar-membrane (BM) response growth. FM PF-slope decreased with signal level, but this effect was reduced or reversed with increasing hearing loss; there was a trend of decreasing psychometric function (PF) slope with increasing frequency, consistent with greater compression at higher frequencies. DPOAE I/O functions at 6 kHz exhibited an increase in the breakpoint of a two-segment slope as a function of hearing loss with a concomitant decrease in the level of the distortion product (L(d)). Results of the comparison between FM PF and DPOAE I/O parameters revealed only a weak correlation, suggesting that one or both of these measures may provide unreliable information about BM compression.

A simple single-interval adaptive procedure for estimating thresholds in normal and impaired listeners

This report presents a single-interval adaptive procedure for measuring thresholds in untrained normal and impaired listeners. The accuracy of the procedure is evaluated using Monte Carlo methods and human data allowing a method to be proposed for deciding in advance the number of trials required to achieve a specified level of accuracy. The number of trials depends on the slope of the psychometric function. The slope of the psychometric function is evaluated in normal and impaired listeners, and is found to give a useful guide to the required number of trials. The single-interval up/down procedure is subsequently compared with two other popular traditional methods (two-interval forced-choice, two-down/one-up and maximum-likelihood procedures) and is shown to yield similar thresholds and be more efficient.

Low-frequency and high-frequency cochlear nonlinearity in humans

Low- and high-frequency cochlear nonlinearity was studied by measuring distortion product otoacoustic emission input/output (DPOAE I/O) functions at 0.5 and 4 kHz in 103 normal-hearing subjects. Behavioral thresholds at both f2's were used to set L2 in dB SL for each subject. Primary levels were optimized by determining the L1 resulting in the largest L(dp) for each L2 for each subject and both f2's. DPOAE I/O functions were measured using L2 inputs from -10 dB SL (0.5 kHz) or -20 dB SL (4 kHz) to 65 dB SL (both frequencies). Mean DPOAE I/O functions, averaged across subjects, differed between the two frequencies, even when threshold was taken into account. The slopes of the I/O functions were similar at 0.5 and 4 kHz for high-level inputs, with maximum compression ratios of about 4:1. At both frequencies, the maximum slope near DPOAE threshold was approximately 1, which occurred at lower levels at 4 kHz, compared to 0.5 kHz. These results suggest that there is a wider dynamic range and perhaps greater cochlear-amplifier gain at 4 kHz, compared to 0.5 kHz. Caution is indicated, however, because of uncertainties in the interpretation of slope and because the confounding influence of differences in noise level could not be completely controlled.