Neural processing of natural speech by adults with and without dyslexia: Evidence for atypical temporal sampling of speech information in the delta and theta EEG bands.

Neural oscillations in the delta (.5-4 Hz) and theta (4-8 Hz) bands play a key role in tracking the temporal structure of speech. According to Temporal Sampling (TS) theory, dyslexia arises from atypical oscillatory sampling of the speech signal in these bands during infancy and childhood, which is particularly disruptive regarding phonological encoding. However, studies of adults with dyslexia have rarely examined both delta and theta oscillatory sampling under naturalistic listening conditions. Using EEG, here we focused on delta and theta band cortical tracking of continuous natural speech by adults with and without dyslexia, applying a decoding analysis previously used with dyslexic children. Forty-eight English-speaking adults (24 dyslexic, 24 control) listened to a 16-min continuous spoken narrative while EEG was recorded. Neural tracking of the speech envelope was quantified using backward decoding models, implemented using the multivariate Temporal Response Function (mTRF) toolbox. Decoding analyses were applied at two levels: a between-group analysis evaluating group-level differences in neural representation patterns, and a within-participant analysis assessing individual decoding accuracy. Cerebro-acoustic coherence was also computed to provide a complementary measure of low-frequency neural-speech synchronisation. Additional complementary analyses examined band power, cross-frequency phase-amplitude coupling (PAC), and cross-frequency phase-phase coupling (PPC). Dyslexic adults exhibited less accurate delta- and theta-band decoding in the between-group analysis and reduced theta-band decoding accuracy in the within-participant analysis, alongside reduced theta-band coherence and increased delta-band power, particularly over the right temporal region. No group differences were found for PAC or PPC.