There are three workpackages in the main project funded by the Medical Research Council and there are also related work packages which, although not part of the grant, will be conducted in close collaboration with those involved in the project. It is hoped that the number of collaborative projects related to the issue of hidden hearing loss will continue to develop and grow as the programme of research matures.
WP1: Human Electrophysiology and Psychophysics
The Audiology and Deafness Research Group from the School of Psychological Sciences at the University of Manchester, will be responsible for this work package, which aims to characterise the perceptual changes and underlying neural functioning which is associated with hidden hearing loss. 180 participants with normal pure tone audiograms will be recruited and will complete 12 hours of testing. Groups of low, medium and high exposure will be identified with 60 individuals in each.
The initial phase of testing uses a series of questionnaires to quantify how an individual rates their hearing in everyday situations and also quantifies the amount of noise the individual has been exposed to in their lifetime. One session is devoted to acquiring Auditory Brainstem Responses (ABRs) and Frequency Following Responses (FFRs) which are electrical signals generated by subcortical neural sources in response to transient and sustained sounds respectively. Four subsequent testing sessions will evaluate a participant's ability to discriminate between different types of sounds by making judgements related to loudness, pitch and location. There are also a range of tasks involving speech and music stimuli to determine how individuals perform in different "real world" listening environments.
The proposed protocol collects the results of a battery of tests and these responses will be interpreted in the context of the noise ratings acquired in session 1. It is predicted that certain subsets of these tests will be sensitive to hidden hearing loss, and as such individuals that have a greater level of noise exposure will show impaired performance relative to those with lower levels of exposure. This work will lead to the development of a diagnostic test for hidden loss based on the measures acquired in WP1 and will be used in work packages 2 and 3.
WP2: Epidemiological Measures
An estimate of the prevalence of hidden loss will be obtained by focussing on a representative sample of 500 young people from the north-west of England. The initial diagnostic test developed over the course of WP1 will be administered along with a standard audiogram and a series of questionnaires, including potential risk factors such as attendance at music events and the use of personal music players.
500 participants will be recruited for a 2 hour session and it is predcited that both the diagnostic test and also the amount of noise exposure in keys areas (such as going to nightclubs, regularly attending concerts etc) will be a reliable predictor of the extent of hidden loss. Furthermore this work package will allow some estimation of the prevalence of hidden hearing loss due to noise exposure.
WP3: Human Neuroimaging
Conducted in parallell with WP2, the objective of this phase is to use Functional Magnetic Resonance Imaging (fMRI) in conjunction with ABR measures to study the extent to which the ascending auditory pathway is affected by noise exposure. 90 participants with normal audiograms will be recruited and the diagnostic test developed in WP1 and used in WP2 will be administered.
Our current understanding of the causes of hidden hearing loss come predominantly from animal models and the effect of this loss could be subtle. The use of fMRI provides an opportunity to consider a number of different parts of the human auditory system and determine to what extent neuronal activity, and thus the BOLD signal, is altered in individuals with a high level of exposure. In particular, the work package will investiage if hidden loss might be a cause of tinnitus, or "ringing in the ears".