Dawes, P., Munro, K., Kalluri, S., & Edwards, B. (2014). Acclimatization to hearing aids. Ear and Hearing, Published Ahead-of-Print.
This editorial discusses the clinical implications of an independent research study and does not represent the opinions of the original authors.
New patients frequently report that their new hearing aids sound tinny, metallic, loud, or unnatural. The clinical audiologist recognizes that these comments will decrease in frequency with time. A process often described as acclimatization: a reaction to new hearing aids that occurs because the patient has adjusted to hearing sound filtered by their hearing loss. When amplification is introduced, the subsequent increase in audibility and loudness perception is unfamiliar and therefore unnatural.
A smooth transition to hearing aid use can be achieved through counseling prior to fitting, preparing the individual for a period of unnatural sound quality. At the fitting, the instruments can be set below prescribed target, allowing the listener a more comfortable period of adjustment. Most individuals will accept increased gains, approaching prescribed target over 2 or 3 months. Some patients, however, require a much longer period of acclimatization of one to two years (Keidser, et al., 2008).
In addition to changes in the preferred gain of new hearing aid users, other improvements due to acclimatization have been proposed: speech discrimination over time (Bentler, et al.1993a, Gatehouse, 1992), subjective benefit and sound quality over time (Bentler, et al., 1993b; Ovegard, et al., 1997) and loudness perception and intensity discrimination over time (Olsen, et al., 1999; Philibert et al., 2002). Most of these studies reported small but significant acclimatization effects; while others found no significant differences between new and experienced hearing aid users (Smeds et al, 2006a, 2006b).
Ultimately, there is little agreement on the definition of this effect and even less agreement in the methods that quantify these changes. A high degree of response variability is usually noted, indicating that several factors (degree, etiology, and configuration of hearing loss) may contribute to the adjustment that is experienced by new hearing aid users.
Dawes and his colleagues outlined a number of goals for their study: First, they hoped to determine if there is an acclimatization effect for aided speech recognition with current, nonlinear hearing aids and if there is a difference between unilateral and bilateral fittings. Second, they wanted to know if new hearing aid users’ self-reports would indicate a period of acclimatization. Third, they sought to determine if acclimatization could be predicted by the degree of hearing loss, prior hearing aid use or cognitive capacity.
Forty-nine subjects participated in the study, recruited from four audiology clinics. There were 16 new unilateral hearing aid users, 16 new bilateral users and 17 experienced users, including 8 bilateral and 9 unilateral users. Experienced subjects used their own hearing aids and new users were fitted with BTE or CIC instruments with comparable circuit technology. New instruments were fitted to NAL-NL1 targets and verified with real-ear measurements. Newly-fitted subjects had a few days of hearing aid use prior to commencement of the study and were allowed gain adjustments only if necessary due to discomfort with prescribed gain levels.
To measure speech recognition, a 4-alternative forced-choice procedure was used, in which listeners were asked to select one word from a closed set of four rhyming words, in response to the prompt, “Can you hear the word X clearly?” In addition to the speech recognition test, subjects completed the Spatial, Speech and Qualities of Hearing Questionnaire – Difference version (SSQ-D; Gatehouse & Noble, 2004), as well as two measures of cognitive processing. The SSQ-D was administered after 12 weeks and allowed the subjects to judge their own changes in performance and listening effort with the hearing aids over the course of the study.
Two cognitive tests were administered. The first, a visual reaction time task, required participants to watch digits presented on a computer monitor and press the corresponding numbers on a keypad as quickly as possible. Responses were scored as correct or incorrect and response times were measured in milliseconds. Working memory was also evaluated, using the Digits Backwards subtest from the Weschler Adult Intelligence Scale – III (WAIS-III; Wechsler, 1997). Subjects listened to lists of digits and were asked to repeat them in reverse order. Lists increased in length as the test progressed and responses were correct if all digits were repeated in the correct order.
In all test conditions, variability was high and a small improvement was noted over time, likely due to practice effects. The mean SNR required to achieve 50% performance did not differ between new unilateral and new bilateral hearing aid users, but experienced users required significantly more favorable SNRs to achieve this level of performance, compared to new users. This was attributed to the older average age and poorer hearing thresholds of the experienced user group.
For the new user groups, if acclimatization occurred it was expected that performance would improve in aided conditions over time. Instead there were small trends of improvements in unaided and aided conditions. For unilateral users, the trend was noted in the fitted ear, whereas for bilateral users, small improvements were noted for both ears. Of all the variables studied, the only one to have a significant effect on performance was time, which yielded a small consistent improvement across groups and listening conditions. When place, manner and voicing errors were analyzed, there was no significant difference for type of error, nor was there a significant interaction with the other variables of group, aiding, ear or presentation level.
Because of the high variability in responses, correlations were measured for effects of hearing aid usage, degree of hearing loss, cognitive capacity, and a change in audibility referred to as “stimulus novelty”. For new hearing aid users, there was no significant correlation between the change speech recognition scores, severity of hearing loss, cognitive test score, or hearing aid variables. Older age was only correlated with slower reaction time scores and a higher amount of time spent in quiet conditions. There were no significant correlations for SSQ-D scores and change in aided performance in any of the listening conditions. Disparate SSQ-D scores did indicate that new hearing aid users perceived improvements over the course of the study, whereas experienced users did not.
Though there were small increases in speech recognition performance over time in all conditions, this was consistent with a practice effect and was not taken as evidence for acclimatization. Self-reports from the SSQ-D showed that new users experienced improvements with amplification that were significantly greater than those reported for experienced users. It is not surprising that SSQ-D scores might still show improvement, as the SSQ-D probes subjective perceptions of performance, including listening effort and sound quality. These elements may well improve with consistent use of new hearing aids even if actual speech recognition has not changed significantly. Improved audibility may allow the listener to function well in everyday environments with significantly less effort, making a positive impression on the listener, more so than small but measurable improvements in word recognition.
Another potential explanation for the lack of agreement between objective and subjective measures in this study could be related to the actual comparison that was made by the subjects when the responded to the SSQ-D items. Because new users probably experienced noticeable benefits from the hearing aids, they may have had trouble comparing their performance immediately post-fitting versus 12 weeks later and may have inadvertently compared pre-fitting and post-fitting performance, yielding a larger SSQ-D score.
Though the results of this study did not support an acclimatization effect for speech recognition, they do not rule out the existence of acclimatization altogether. Preferred gain, perceived listening effort, and sound quality improvements, among other effects, may well occur for most new hearing aid users, to varying degrees based on degree of hearing loss, duration of prior hearing loss and prior experience with hearing aids.
The subjects in this study were fitted with either BTE or CIC hearing aids but the hearing aid style was not examined with regard to acclimatization. CIC users often experience occlusion and adjustment to their own voices in the early days of hearing aid use; much more so than BTE users who probably have less occlusion than commonly found with CIC hearing aids. Whether this could have an impact on speech recognition acclimatization is questionable, but it could have affect subjective reports. Similarly, individuals using hearing aid features such as frequency-lowering or wireless routing of signal may demonstrate other perceptual learning or acclimatization effects.
Perhaps the greatest finding of this study was the contrast between measurable outcomes in the domain of subjective spatial perception and traditional measures of speech recognition. Many failed attempts to document acclimatization have focused on speech recognition or loudness perception rather than probing the patient’s perception of their acoustic environment—something achieved with the SSQ-D. The apparent sensitivity of this measure should direct future experimental design in this area. For the practicing clinician, this contrast can aid in developing counseling approaches: it’s clear that speech recognition won’t change over time, but the complexity or overwhelming nature of the acoustic environment may become simpler with time.
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