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

Anne Hauswald*, Anne Keitel, Ya‐ping Chen, Sebastian Rösch, Nathan Weisz

*Corresponding author for this work

Research output: Contribution to journalArticle

Abstract

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.
Original languageEnglish
JournalEuropean Journal of Neuroscience
Early online date20 Jul 2020
DOIs
Publication statusE-pub ahead of print - 20 Jul 2020

Bibliographical note

© 2020 The Authors. European Journal of Neuroscience published by Federation of European Neuroscience Societies and John Wiley & Sons Ltd.

Fields of Science and Technology Classification 2012

  • 501 Psychology

Keywords

  • alpha power
  • continuous speech
  • degraded speech
  • low frequency speech tracking
  • MEG

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