Speech comprehension has been proposed to critically rely on oscillatory cortical tracking, that is, phase alignment of neural oscillations to the slow temporal modulations (envelope) of speech. Speech-brain entrainment is readjusted over time as transient events (edges) in speech lead to speech-brain phase realignment. Auditory behavioral research suggests that phonological deficits in dyslexia are linked to difficulty in discriminating speech edges. Importantly, research to date has not specifically examined neural responses to speech edges in dyslexia. In the present study, we used MEG to record brain activity from normal and dyslexic readers while they listened to speech. We computed phase locking values (PLVs) to evaluate phase entrainment between neural oscillations and the speech envelope time-locked to edge onsets. In both groups, we observed that edge onsets induced phase resets in the auditory oscillations tracking speech, thereby enhancing their entrainment to speech. Importantly, dyslexic readers showed weaker PLVs compared to normal readers in left auditory regions from ~0.15 s to ~0.65 s after edge onset. Our results indicate that the neural mechanism that adapts cortical entrainment to the speech envelope is impaired in dyslexia. These findings here are consistent with the temporal sampling theory of developmental dyslexia.
Steps to reproduce
1- Pre-processing of the MEG data: 1.1. Use Maxfilter to correct MEG for head movements and interpolate bad channels. 1.2. Apply Independent Component Analysis (ICA) (using Fieldtrip toolbox) to remove visual and heart artifacts. 2. Phase Locking Value (PLV) calculation at the MEG sensor level: 2.1. Detection of the speech edges. For each sentence (check the stimuli folder) edge onsets are detected following Gross et al., 2013. 2.2. Pre-processed MEG data is aligned to the corresponding speech edges. 2.3. Phase locking between the speech edges and MEG is calculated. Following this procedure, we obtained a PLV value for each participant (control or dyslexic), sensor (306 MEG sensors) and time point (0.4 sec before to 1 sec after the edge onset). 2.4. PLV values were compared between the two groups using a t-test statistical analysis. P-values were corrected using the false discovery rate (FDR) procedure.