The role of rhythm has been found to be a key factor in language development. We asked, how do the rhythmic features of the speech signal map on to the rhythmic activity of the brain? And are there any differences between children with dyslexia and controls? Based on our prior theoretical work, we predicted that the brain activity in children with dyslexia will show deficits in latching onto (or “tracking”) the low-frequency (or slower rhythmic) prosodic aspects of the speech signal.
In this study, we recorded brain activity using magnetoencephalography (MEG) while children (8-9 years old) with dyslexia and without dyslexia listened to a children’s story. MEG allows us to measure changes in the brain’s activity with millisecond precision. In the children’s story, we were able to identify linguistic features in the story corresponding to prosodic aspects, such as stressed and unstressed syllable rate, phoneme (individual speech sound) rate etc. We identified a network of brain regions —spanning 3 main areas, temporal, parietal, and frontal—that tracked the speech signal's rhythmic aspects. We then demonstrated that this “speech tracking network” in children with dyslexia (relative to typical readers) is less efficient at integrating information.
With our novel analysis, we showed that children with dyslexia do indeed have difficulties in tracking prosodic and syllable rate information in natural speech. From a developmental perspective of reading and language acquisition, the study has significant implications. Firstly, they converge with prior data that dyslexia begins as a speech processing problem: the dyslexic brain processes speech prosody differently from other brains. From our work with infants, we know that the prosodic aspects of the speech signal provide a ‘scaffold’ for early language acquisition. If an infant or child has difficulties in processing prosodic information, then it can have downstream effects on how the sound inventory (i.e., phonology) of a language is represented in the brain. Secondly, if the child’s phonological ‘representations’ for words are less accurately encoded, this is likely to affect their subsequent ability to decode written words back to speech, as seen in children with dyslexia.
Mandke, K., Flanagan, S., Macfarlane, A., Gabrielczyk, F. C., Wilson, A. M., Gross, J., Goswami, U. (2022). Neural sampling of the speech signal at different timescales by children with dyslexia. Neuroimage, 253, 119077. https://doi.org/10.1016/j.neuroimage.2022.119077