Degradation levels of continuous speech affect neural speech tracking and alpha power differently. (7th August 2020)

• Record Type: Journal Article
• Authors: Hauswald, Anne; Keitel, Anne; Chen, Ya‐Ping; Rösch, Sebastian; Weisz, Nathan
• Other Names: Keitel Christian guestEditor.; Ruxxoli Manuela guestEditor.; Dugué Laura guestEditor.; Busch Niko A. guestEditor.; Benwell Christopher SY guestEditor.
• Is Part Of: European journal of neuroscience. Volume 55:Number 11/12(2022)
• Page Start: 3288
• Page End: 3302
• Publication Date: 2020-08-07
• Subjects: alpha power -- continuous speech -- degraded speech -- low‐frequency speech tracking -- MEG; Nervous system -- Periodicals; 612.8
• DOI: 10.1111/ejn.14912
• Languages: English
• ISSNs: 0953-816X

Abstract

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 and 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. Abstract : Recording magnetoencephalography while presenting continuous speech under different levels of degradation shows that alpha power and speech tracking are differently affected. While alpha power declines with declining clarity speech tracking follows an inverted U‐shape. Linear mixed models suggest that a combination of alpha power and tracking contribute to subjective speech understanding.