Effect of fast-acting compression on modulation detection interference for normal hearing and hearing impaired listeners

A review of auditory processing and cognitive change during normal ageing, and the implications for setting hearing aids for older adults

Throughout our adult lives there is a decline in peripheral hearing, auditory processing and elements of cognition that support listening ability. Audiometry provides no information about the status of auditory processing and cognition, and older adults often struggle with complex listening situations, such as speech in noise perception, even if their peripheral hearing appears normal. Hearing aids can address some aspects of peripheral hearing impairment and improve signal-to-noise ratios. However, they cannot directly enhance central processes and may introduce distortion to sound that might act to undermine listening ability. This review paper highlights the need to consider the distortion introduced by hearing aids, specifically when considering normally-ageing older adults. We focus on patients with age-related hearing loss because they represent the vast majority of the population attending audiology clinics. We believe that it is important to recognize that the combination of peripheral and central, auditory and cognitive decline make older adults some of the most complex patients seen in audiology services, so they should not be treated as "standard" despite the high prevalence of age-related hearing loss. We argue that a primary concern should be to avoid hearing aid settings that introduce distortion to speech envelope cues, which is not a new concept. The primary cause of distortion is the speed and range of change to hearing aid amplification (i.e., compression). We argue that slow-acting compression should be considered as a default for some users and that other advanced features should be reconsidered as they may also introduce distortion that some users may not be able to tolerate. We discuss how this can be incorporated into a pragmatic approach to hearing aid fitting that does not require increased loading on audiology services.

Suprathreshold auditory processes in listeners with normal audiograms but extended high-frequency hearing lossa)

Hearing loss in the extended high-frequency (EHF) range (>8 kHz) is widespread among young normal-hearing adults and could have perceptual consequences such as difficulty understanding speech in noise. However, it is unclear how EHF hearing loss might affect basic psychoacoustic processes. The hypothesis that EHF hearing loss is associated with poorer auditory resolution in the standard frequencies was tested. Temporal resolution was characterized by amplitude modulation detection thresholds (AMDTs), and spectral resolution was characterized by frequency change detection thresholds (FCDTs). AMDTs and FCDTs were measured in adults with or without EHF loss but with normal clinical audiograms. AMDTs were measured with 0.5- and 4-kHz carrier frequencies; similarly, FCDTs were measured for 0.5- and 4-kHz base frequencies. AMDTs were significantly higher with the 4 kHz than the 0.5 kHz carrier, but there was no significant effect of EHF loss. There was no significant effect of EHF loss on FCDTs at 0.5 kHz; however, FCDTs were significantly higher at 4 kHz for listeners with than without EHF loss. This suggests that some aspects of auditory resolution in the standard audiometric frequency range may be compromised in listeners with EHF hearing loss despite having a normal audiogram.

The effect of frequency cueing on the perceptual segregation of simultaneous tones: Bottom-up and top-down contributions

Listeners were presented with two simultaneous tones of different frequencies (more than one octave apart) and asked to identify the tone that was amplitude-modulated while a tonal precursor was presented to cue the frequency of the lower frequency tone. Performance thresholds were estimated based on the duration of the tone-pair. In Exp. I the duration of the precursor varied from 100 to 400 ms and the inter-stimulus interval (ISI) between the precursor and the tone-pair varied from 0 to 1 s. The presence of the precursor facilitated segregation. As the ISI increased, the facilitation effect of the precursor increased for the precursor durations of 100 and 200 ms, but not for the 400-ms precursor duration. When the precursor was presented to the contralateral ear relative to the tone-pair in Exp. II, no significant change to the precursor effect was observed. These observations contradict the predictions of the model based solely on bottom-up processing, suggesting the likely involvement of top-down processes.

Cochlear hearing loss and the detection of sinusoidal versus random amplitude modulation

This study assessed the effect of cochlear hearing loss on detection of random and sinusoidal amplitude modulation. Listeners with hearing loss and normal-hearing listeners (eight per group) generated temporal modulation transfer functions (TMTFs) for envelope fluctuations carried by a 2000-Hz pure tone. TMTFs for the two groups were similar at low modulation rates but diverged at higher rates presumably because of differences in frequency selectivity. For both groups, detection of random modulation was poorer than for sinusoidal modulation at lower rates but the reverse occurred at higher rates. No evidence was found that cochlear hearing loss, per se, affects modulation detection.

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.

Across-frequency processing of modulation phase differences in hearing-impaired listeners

Two experiments tested the influence of hearing impairment (HI) on representing across-frequency temporal coherence. In one experiment, HI listeners demonstrated similar abilities to normal-hearing listeners in detecting across-frequency differences in modulation phase. In another, spectral-shape discrimination was detrimentally affected by modulation phase disparities imposed on spectral components. Spectral-shape discrimination by HI listeners was less influenced by the disparities, suggesting that hearing loss alters the representation of envelope phase. Results suggest that multiple approaches may be necessary to determine alterations associated with hearing loss—detection tasks may not be sufficient to elucidate distortions to temporal envelope associated with hearing loss.

Influence of hearing loss on children's identification of spondee words in a speech-shaped noise or a two-talker masker

OBJECTIVE

This study evaluated the influence of hearing loss on children's speech-perception abilities in a speech-shaped noise or a two-talker masker. For both masker conditions, it was predicted that children with hearing loss would require a more advantageous signal to noise ratio (SNR) than children with normal hearing to achieve the same criterion level of performance. However, it was hypothesized that the performance gap between children with hearing loss and children with normal hearing would be larger in the two-talker than in the speech-shaped noise masker.

DESIGN

A repeated-measures design compared the spondee identification performance of two age groups of children with hearing loss (9-11 and 13-17 years of age) and a group of children with normal hearing (9-11 years of age) in continuous speech-shaped noise or a two-talker masker. Estimates of the SNR required for 70.7% correct spondee identification were obtained using an adaptive, four-alternative, forced-choice procedure. Children were tested in the sound field. Children with hearing loss wore their personal hearing aids at their regular settings during testing.

RESULTS

Both groups of children with hearing loss performed more poorly than children with normal hearing in the speech-shaped noise masker. Younger children required an additional 2.7 dB SNR and older children required an additional 4.7 dB SNR to achieve the same level of performance as children with normal hearing. This disadvantage decreased to 8.1 dB for both age groups of children with hearing loss in the two-talker masker. For children with hearing loss, degree of hearing loss was significantly correlated with performance in the speech-shaped noise masker, but not in the two-talker masker.

CONCLUSIONS

A larger performance gap was observed between children with hearing loss and children with normal hearing in competing speech than in steady state noise. These results are consistent with the hypothesis that hearing loss influenced children's perceptual processing abilities.

Temporal resolution with a prescriptive fitting formula

PURPOSE

Authors of previous work using laboratory-based paradigms documented that wide dynamic range compression (WDRC) may improve gap detection compared to linear amplification. The purpose of this study was to measure temporal resolution using WDRC fit with compression ratios set for each listener’s hearing loss.

METHOD

Nineteen adults with mild-to-moderate hearing loss fitted with WDRC or linear amplification set to a prescriptive fitting method participated in this study. Subjects detected amplitude modulations and gaps. Two types of noise carrier were used: narrowband (1995–2005 Hz) and broadband (100–8000 Hz).

RESULTS

Small differences between WDRC and linear amplification were observed in the measures of temporal resolution. Modulation detection thresholds worsened by a mean of 0.7 dB with WDRC compared to linear amplification. This reduction was observed for both carrier types. Gap detection thresholds did not differ between the 2 amplification conditions.

CONCLUSIONS

WDRC set using a prescriptive fitting method with individualized compression ratios had a small but statistically significant effect on measures of modulation thresholds. Differences were not observed between the two amplification conditions for the measures of gap detection. These findings contrast with previous work using fixed compression ratios, suggesting that the effect of the fitting method on the compression ratio should be considered when attempting to generalize the effect of WDRC on temporal resolution to the clinical setting.