Trying to follow a conversation in a crowded room can be a challenge, especially if you suffer from a hearing loss. Despite wearing hearing aids that ensure audibility, hearing-impaired listeners often complain that they feel fatigued from having to listen in situations with background noise. Their brains seem to work overtime. Interestingly, not only hearing acuity, but also the cognitive capacity to processes sounds influence your speech understanding in the presence of background noise. In behavioural studies, individuals with larger cognitive abilities to perform working memory processing, i.e. simultaneously processes and store information, has been found to have a larger tolerance towards background noise.
In a recent doctoral thesis from Linköping University, the effect of hearing loss on the neural processing of speech was investigated. By analysing the electrical activity of the brain (electroencephalogram/EEG) recorded during different listening tasks, it was expected that worse hearing would result in higher cognitive involvement.
Indeed, it was observed that listeners with worse hearing had a higher degree of inhibitory alpha activity (~10 Hz activity in the EEG). The presence of alpha activity during listening has been linked to the inhibition of task-irrelevant brain processes and regions in order release neural resources to solve the listening task. Hence, listeners with worse hearing involve more cognitive resources during listening to compensate for their poorer hearing. The results further indicated that there is an upper limit to how many resources the brain can release through neural inhibition and that this limit is reached at a lower level of background noise for listeners with worse hearing.
Why is it that listeners with hearing impairment shows sign of higher cognitive involvement during speech processing, despite wearing hearing aids and performing the task just as good as the normal-hearing listeners? One important factor for understanding speech is the ability to separate multiple sounds from one another and selectively attend to one of them. Previous studies have established that neural activity reflect the dynamic behaviour of the speech signals that is presented and that, for normal-hearing younger listeners, the speech being attended to is always better neurally encoded the speech they are trying to ignored. The same pattern was seen for elderly listeners with normal hearing, however as the degree of hearing loss increased, so did the neural tracking of the to-be-ignored speech. Oppose to their normal-hearing peers, hearing-impaired listeners are unable to neurally supress, or ignored, disturbing speech potentially causing the increased need for cognitive resources during listening.
The results of the thesis “Neural and Cognitive Effects of Hearing Loss on Speech Processing” provides some of the first evidence that the neural activity induced by processing speech is affected by the degree of hearing loss. That is, despite providing the listeners with hearing aids to compensate for the loss of audibility, hearing loss still sends your brain on overtime.
Petersen EB, Wöstmann M, Obleser J, Stenfelt S, Lunner T (2015). Hearing loss impacts neural alpha oscillations under adverse listening conditions. Frontiers in Psychology, 6:177. doi: 10.3389/fpsyg.2015.00177
Petersen EB, Wöstmann M, Obleser J, Lunner T (2017). Neural tracking of attended versus ignored speech differentially affected by hearing loss. Journal of Neurophysiology, 117(1), pp. 18–27. doi: 10.1152/jn.00527.2016
Petersen EB (2017). Neural and Cognitive Effects of Hearing Loss onSpeech Processing. PhD Dissertation, Linköping, Sweden. doi: 10.3384/diss-diva-134255