| Summary: | Listening effort is the deliberate allocation of mental resources to carry out a listening task. Spending effort can be rewarding if it yields desired results but to invest listening effort constantly will lead to negative consequences such as fatigue, difficulties in speech recall, and social disengagement, especially for hearing-impaired individuals.
Listening effort can be measured subjectively via questionnaire and/or objectively via behavioural and physiological measures. One of the most used measures for listening effort objectively is electroencephalography (EEG). EEG is a brain-imaging modality which provides excellent temporal resolution to study brain oscillations. EEG picks up electrical activities on the head (scalp EEG) or from the ears (ear-EEG) for investigating the brain in different cognitive tasks. Alpha power is one of the features that can be extracted from the EEG signal and has been widely used for measuring auditory and non-auditory effort. In this thesis, EEG will be recorded during effortful listening tasks, and alpha power will be extracted to investigate listening effort.
One of the downsides of the studies on listening effort is controlling (or not controlling) different parameters in the experimental environment which reduces generalisability to real-life scenarios. The aim of the current thesis is to measure listening effort in settings which are more ecologically valid compared to traditional laboratory scenarios. To increase ecological validity, motivation of individuals will be manipulated (through monetary reward) to account for personal factors, and different rooms will be simulated (through characterisation of reverberation time) to account for environmental factors in effortful tasks which involve listening to speech in noise. Single sentences or continuous discourse will be presented as stimuli speech to cover more realistic conversation in everyday life. Additionally, as a new and wearable technology, ear-EEG will be used in one study which has the potential to be used as an ambulatory EEG measurement in hearing aids.
The main hypothesis of the thesis is that alpha power increases with increased listening effort. The overall results of five different studies in ecologically valid situations showed that the pattern of alpha power can be opposite when listeners are presented with single-sentence stimuli or continuous discourse. In single-sentence paradigm, in line with the hypothesis, alpha power reflected listening effort. However, in continuous speech alpha power indicated performance of the individuals rather than expenditure of resources in the brain. These results suggest that applying a one-measure-for-all-scenarios approach when measuring listening effort is not reliable in a real-life setting.
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