Benefit and predictors of outcome from frequency compression hearing aid use

Non-linear frequency compression (NLFC) hearing aids are frequency lowering devices that compress a signal into a reduced bandwidth in order to maximise use of residual hearing. Only a few published studies have investigated benefit from NLFC hearing aids. Outcomes vary considerably between studies although most show large differences across listeners. This highlights the need for identification of reliable predictors of benefit. Furthermore, little is known about the time course and magnitude of perceptual learning associated with the use of NLFC amplification. A better understanding of these issues could potentially lead to significant clinical benefit and was therefore the focus of the present study.Two preliminary experiments were conducted on a total of 27 listeners with normal hearing in order to investigate the effect of NLFC on categorical perception and the role of cognition on NLFC outcome. The findings were used to inform the development (particularly with regards to the selection of NLFC fitting parameters) of a longitudinal study of 12 experienced adult hearing aid users with moderate-to-severe high frequency hearing loss. Participants wore the hearing aids, with and without NLFC enabled, in an A-B-A design, for approximately 6-7 weeks in each condition. Speech recognition, in both quiet (nonsense syllables) and noise (nonsense syllables and sentences), was measured at the end of each trial period. It was also measured at several time points when frequency compression was enabled in order to investigate perceptual learning. The opportunity was taken to gather preliminary self-report data (Glasgow Hearing Aid Benefit Profile and Speech, Spatial and Qualities of Hearing Scale) in order to identify possible trends on which to base future research. The results demonstrate that mean benefit on the speech recognition measures was greater when frequency compression was enabled. 9 out of 12 listeners obtained higher scores on the majority of outcome measures. There was no obvious difference on self report measures. Upon initial exposure to NLFC, there was an increase in confusions of some high frequency phonemes, especially of /f/ and /? / with /s/ (in both quiet and in noise); however, these confusions were less frequent after 6 weeks of NLFC hearing aid use. Limited evidence of perceptual learning of speech in noise was observed. In agreement with the findings of previous studies, large individual differences in benefit were evident. The relation between sentence in noise recognition (with and without NLFC enabled) and a variety of audiological (high frequency hearing loss and presence of dead regions) and cognitive factors (performance in the reading span and trail making tests) was examined. Audiological factors were shown to be predictive both of speech in noise recognition and of additional benefit obtained from NLFC. Listeners with the greatest high frequency loss derived the most benefit from NLFC. Once the effect of hearing loss had been partialled out, no other predictor correlated significantly with benefit from NLFC. However, the results support previous findings that cognitive functioning is predictive of benefit without NLFC, but suggest that this is primarily due to the influence of executive function rather than working memory span.The novel findings of the study (particularly those relating to speech in noise perception, acclimatisation to NLFC, and predictors of benefit from amplification) may help influence clinical practice, in relation to the assessment of candidacy for NLFC hearing aids and subsequent counselling offered to device users.