Investigation of low-sound-level auditory processing deficits after chronic exposure to very high noise levels.

Aims
Hearing loss is usually defined as a loss of sensitivity to quiet sounds. However, it reflects the accumulation of damage due to factors such as genetics, or lifestyle such as diet, smoking, exercise and exposure to noise. Consequently, damage to the hearing system is likely to be present in multiple sites, the precise pattern being almost unique, like a "fingerprint". Besides the loss of sensitivity, recent work has shown that exposure to very high sound levels can lead to reduced ability to detect low-amplitude fluctuations in sounds. Fluctuations in sounds are important for communicating information, such as in speech, or melody in music. The aim of this work is to measure the impact of this newly identified deficit in people with hearing loss, a population where the disease is likely to be much further progressed than in the younger populations studied to date.

Importance
Although noise-at-work regulations have reduced noise exposure in the workplace, exposure to high sound levels "recreational" settings has increased markedly. In the UK, the prevalence of this contribution to noise exposure was estimated to have tripled between 1980 and 1994 (Smith et al., 2000). Deficient processing of fluctuations was only demonstrated in regular attendees at nightclubs and rock concerts. The new deficit was measurable in 21-year old males, but only in a comparatively small range of their hearing, so may only produce a small handicap. By age 29, the deficit is more obvious, and co-exists alongside the traditional loss of sensitivity. Even in this older population, clinical measures classify this loss as within "normal" bounds, and unlikely to benefit from hearing aids. Underneath this, a "hidden" hearing loss is developing.

Increased recreational exposure, alongside an ageing population, implies future increases in this deficit. This deficit could mean that subjects will receive less benefit than expected from a hearing aid. There is a need to provide better diagnosis of this hidden loss with the intention of producing hearing aid fittings tailored to the patient's unique hearing loss.

Methods
Using hearing-impaired subjects with a history of high-level noise exposure, we will measure how much the newly reported deficit has spread compared to that seen in the previously tested, younger, populations. This process requires the use of specialist test signals to which the subject is asked to respond. We also aim to co-develop techniques that require no conscious input from the subject, by use of measuring brain activity in response to sounds.

We then will correlate these measures against performance on speech-in-noise tests when wearing hearing aids. We will compare results from this group against a group whose hearing impairment does not arise from noise exposure. Finally, we will assess if varying the speed of signal processing in the hearing aid interacts with the measured deficit so as to improve outcomes for the wearer.

Benefits
Better definition of the contributions to hearing loss will lead to the generation of more realistic expectations to be communicated to the patient. Tuning the hearing aid to the particular loss should lead to greater effectiveness of aiding and hence perceived benefit, and, hopefully, greater adherence. Apart from improving remediation, the work should also identify a suitable test that can detect the early stages of the deficit in at-risk populations, so that behavioural modification can be applied.

Noise exposure produced by nightclubs and rock concerts, where sound levels exceed 95 to 100 dB(A), has been shown to produce deficits in processing of signal envelopes by humans. These deficits typically appear with signals presented at low sensation levels (SL) (Stone et al., 2008; Stone and Moore, in revision 2014). Since they have only been identified so far in younger listeners, the intention is to recruit an older group of hearing impaired (HI) listeners, in order to characterise a more mature manifestation of the deficit. This older group will be sub-divided according to self-report into those with a history of high or low exposure to these recreational noises.

The project aims to improve diagnosis of the extent of the deficit previously identified in the processing of low-SL modulations. We propose to measure auditory processing at low SLs in narrow frequency regions in subjects with different aetiologies to their hearing loss. Specifically we will use psychophysical and electro-physiological measures to characterize frequency-specific auditory abilities as well as a signal processing technique to selectively degrade only low-level portions of target speech. We will also investigate the effects of varying the speed of dynamic range compression to modify the extent of hearing-aid-induced distortions to the temporal envelopes of signals. Apart from remediation, the work has the potential to identify early signs of hearing damage so as to advise on behavioural changes.

Future benefits of this work are likely to roll out over several years, but with different timescales for different groups.

(1) The study employs a hearing-impaired population, nearer the end of their noise exposure than the start, so results will affect management of health issues arising from a currently irreparable hearing loss. By specifically studying the efficacy of aiding in this population, recommendations can be made to manufacturers to include design guidelines into their devices. The timescale for this is in a 3-5 year window.

(2) Campaign groups representing hearing impaired people will have access to a greater evidence base to support their work. The timescale for this is also a 3-5 year time window.

(3) Greater accessibility and exposure to high-sound-level reproduction equipment than even 20-30 years ago would imply a possible future disease burden that has yet to reach its peak. The use here of a cross-sectional study of an older population, with self-report measures of accumulated noise exposure only reflects historic exposures. The correlation of these accumulations with the measured deficits, allows estimates to be generated of the progress of the deficit as a function of exposure. This should allow generation of estimates of the expected prevalence in the current younger population, and hence associated potential future costs. The impact will then be seen through modified guidance to those regulating noise exposures. The timescale for this is in a 5-10 year window.

(4) The development of a sensitive test to provide early detection of this deficit, even in a population with normal hearing, would permit early warnings to be given to at-risk populations, permitting modified behaviours and reduce future disease burden. The timescale for a test impacting on behaviour is a 5-10 year time window.

The UK population is experiencing increasing longevity, so delaying the mean age of onset of debilitating hearing loss is of great importance in reducing future healthcare costs.