Fundamentals of human hearing: from basic cochlear processing to cortical representations

Hearing loss is a prevalent and growing health problem that can cause considerable life burden, inhibiting social and work activities and lowering mood, and has recently been identified as a major risk factor for developing degenerative brain diseases, such as Alzheimer's, in later life.

Hearing loss is diagnosed by a reduction in the patient's ability to hear faint sounds. However, in everyday life, inability to hear faint sounds is not usually the patient's primary problem. Rather, patients usually complain about difficulties hearing in noisy environments (e.g., following a conversation in a noisy pub), and these difficulties persist even when wearing a hearing aid. In addition, some patients also complain about experiencing certain everyday sounds as excruciatingly loud, and many hearing-impaired patients experience a continuous, and often irritating, ringing or hissing sound in the ear or head, referred to as "tinnitus". These problems are currently untreatable and their causes remain largely unknown.

In this research programme, we will investigate the idea that they arise as a result of changes in the way in which sounds are processed and represented within the brain. We will focus on the highest-level, most sophisticated part of the brain referred to as "cortex", which is particularly well-developed in humans. Using cutting-edge brain imaging technologies, we will develop methods for mapping cortical sound responses at a sub-millimetre spatial scale, and tracing their temporal dynamics with millisecond accuracy. Using these methods, we will then investigate the exact mechanisms by which cortical sound processing is altered as a result of hearing loss.

Insights gained from this programme may lead to the development of better, more individualized treatment options for hearing impairment, and help to understand how hearing loss impacts brain health.

Neurophysiological research has suggested that, in animals, peripheral hearing loss can lead to profound changes in cortical auditory processing, causing large-scale reorganization in primary cortical tonotopic maps and down-regulating contextual and time-dependent suppression of cortical responses through lateral inhibition and neuronal adaptation. Such changes, if they also occur in humans, might explain common, but as yet untreatable consequences of hearing loss, such as hearing difficulty in noisy environments, hyperacusis and tinnitus. This research programme is aimed at testing the extent to which these changes occur in humans, and developing efficient and unbiased tools for measuring them in individual patients. In order to measure cortical tonotopic reorganization, we will first formulate an integrated model of human auditory cortex organization based on functional and structural MRI data measured at ultra-high field strengths (7T). Based on this model, we will develop a set of operational tools for delineating auditory areal borders in individual living brains (in-vivo parcellation). Hearing loss-related changes in contextual and time-dependent suppression will be measured with EEG. For that, we will develop a new method of estimating EEG sources from different cortical areas defined by tonotopic fMRI mapping, which will reduce inter-individual variability and establish a direct relationship between EEG and MRI results.

Audiological patients may present with disparate problems, even though their audiometric, or hearing, thresholds are similar: some may suffer from severe tinnitus, whilst others hardly notice any tinnitus; some may feel that their hearing has become excessively loud, whereas others may feel that they don't hear enough. This research aims to uncover the neurological mechanisms by which these differences might come about. This will help audiological patients and their health professionals appreciate and cope with their problems better, and, in the longer run, may lead to the development of better treatment options.