Central and peripheral auditory changes and cognitive decline in ageing

Hearing loss (HL) is a common disorder of the elderly, and is associated with communication difficulties and social isolation. Most recently, HL has been inconsistently linked to cognitive decline and it has been postulated that HL may be an independent risk factor for dementia. The aim of this thesis is to determine whether HL is associated with cognitive decline during normal and/or pathological ageing by specifically investigating contributions from peripheral HL, central auditory processing and the psychosocial pathway. Results show that central auditory processing was linked with cognition during normal and pathological ageing, but there was no independent association with peripheral hearing levels or psychosocial factors. The prevalence of HL was not significantly higher in the patient sample compared with matched controls, nor did HL influence the cognitive profiles in normal ageing or neurodegeneration. There was, however, a statistically significant interaction between HL and decline in executive function only in participants with HL who were high performers at baseline, thereby suggesting that peripheral HL is not an independent risk factor for cognitive decline. Higher scores on central auditory processing were associated with better performance on a test of visual long term memory, after controlling for the effects of peripheral HL, and could predict decline in cognitive performance over time on the Short Cognitive Evaluation battery. In keeping with this, the severity of pathological cognitive impairment was closely related to central auditory processing performance, and patients with mild cognitive impairment recruited the right hemisphere for linguistic processing, which was corroborated with findings of increased grey matter in the right auditory association areas. In summary, it was concluded that the inconsistent findings in the literature may be due to differing influence of HL on normal and pathological cognitive ageing. Peripheral and central hearing changes may be a marker of impending neuronal decline or vulnerability to dementia in people with pathological cognitive impairment, but during healthy ageing, HL does not influence cognitive performance or increase the risk of developing dementia.