Degradation levels of continuous speech affect neural speech tracking and alpha power differently

Hauswald, A., Keitel, A. , Chen, Y.-P., Rösch, S. and Weisz, N. (2020) Degradation levels of continuous speech affect neural speech tracking and alpha power differently. European Journal of Neuroscience, (doi: 10.1111/ejn.14912) (PMID:32687616) (Early Online Publication)

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Making sense of a poor auditory signal can pose a challenge. Previous attempts to quantify speech intelligibility in neural terms have usually focused on one of two measures, namely low-frequency speech-brain synchronization or alpha power modulations. However, reports have been mixed concerning the modulation of these measures, an issue aggravated by the fact that they have normally been studied separately. We present two MEG studies analyzing both measures. In study 1, participants listened to unimodal auditory speech with three different levels of degradation (original, 7-channel and 3-channel vocoding). Intelligibility declined with declining clarity, but speech was still intelligible to some extent even for the lowest clarity level (3-channel vocoding). Low-frequency (1-7 Hz) speech tracking suggested a u-shaped relationship with strongest effects for the medium degraded speech (7-channel) in bilateral auditory and left frontal regions. To follow up on this finding, we implemented three additional vocoding levels (5-channel, 2-channel, 1-channel) in a second MEG study. Using this wider range of degradation, the speech-brain synchronization showed a similar pattern as in study 1 but further showed that when speech becomes unintelligible, synchronization declines again. The relationship differed for alpha power, which continued to decrease across vocoding levels reaching a floor effect for 5-channel vocoding. Predicting subjective intelligibility based on models either combining both measures or each measure alone, showed superiority of the combined model. Our findings underline that speech tracking and alpha power are modified differently by the degree of degradation of continuous speech but together contribute to the subjective speech understanding.

Item Type:Articles
Keywords:MEG, alpha power, continuous speech, degraded speech, low frequency speech tracking.
Status:Early Online Publication
Glasgow Author(s) Enlighten ID:Keitel, Dr Anne
Authors: Hauswald, A., Keitel, A., Chen, Y.-P., Rösch, S., and Weisz, N.
College/School:College of Medical Veterinary and Life Sciences > School of Psychology & Neuroscience
Journal Name:European Journal of Neuroscience
ISSN (Online):1460-9568
Published Online:20 June 2020
Copyright Holders:Copyright © 2020 The Authors
First Published:First published in European Journal of Neuroscience 2020
Publisher Policy:Reproduced under a Creative Commons License
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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
173707Institutional Strategic Support Fund (2016)Anna DominiczakWellcome Trust (WELLCOTR)204820/Z/16/ZInstitute of Cardiovascular & Medical Sciences