Neural and Cognitive Effects of Hearing Loss on Speech Processing

Published articles have been reprinted with the permission of the copyright holders. All illustrations are made by the author. Abstract i Abstract Understanding speech in the presence of background noise can be difficulty and even more so if you suffer from a hearing loss. Although speech understanding depends on hearing acuity, the individual capacity to perform higher-order Working Memory (WM) processing also plays an important role (Lunner, 2003). Despite the impact of cognition on speech processing, it has yet to be determined how hearing loss influences neural signatures of WM processing. By examining the dynamic behaviour of the electroencephalogram (EEG), it is possible to investigate how hearing loss and listening condition affect the neural processing of speech. The aim of the current thesis was to investigate how hearing loss in elderly listeners influenced speech processing by examining both behavioural and neural measures of the cognitive involvement. In three studies, we tested the hypothesis that hearing loss and adverse listening conditions result in increased cognitive involvement. In the first study (Paper I), worse hearing and lower WM capacity related to poorer performance in a speech-in-noise task where the sound sources were spatially co-located. In a similar speech-in-noise task with spatially separated sounds, the performance was influenced by hearing loss, but not WM capacity. This suggests that the reading span test, used for quantifying the WM capacity, inadequately describes the dynamic nature of the WM involvement under speech processing. In relation to the theoretical Ease of Language Understanding (ELU) model, the results partly confirmed the view that WM abilities are more important during speech processing for listeners suffering from a hearing loss. As suggested by the inconsistent observations in Paper I, there is a need for more objective and dynamic measures in order to understand the relation between WM processing, speech understanding, and hearing loss. An interesting, cheap, and potentially portable technology for objectively measuring the WM involvement during speech processing is EEG. As a measure of WM involvement, the alpha oscillations (~10 Hz activity in the EEG) was examined during an auditory delayed-match-to-sample task (Paper II). Despite equal task-performance, alpha power was generally higher in listeners with worse hearing. This indicates that worse hearing is linearly related to higher cognitive load during speech processing. In the most difficult experimental condition, an alpha-power breakdown was observed for listeners with the highest degree of hearing loss. This suggests that the generation of alpha …