Atypical low-frequency and high-frequency neural entrainment to rhythmic audiovisual speech in adults with dyslexia

Developmental dyslexia has been linked to atypical neural processing of the temporal dynamics of speech, but there has been disagreement concerning whether faster or slower dynamics are impaired. According to the temporal sampling (TS) theory, dyslexia arises from impaired entrainment of low-frequency neural oscillations-particularly in the delta (1-4 Hz) and theta (4-8 Hz) bands-to the rhythmic modulations of speech. This hypothesis was tested for adults with and without dyslexia using electroencephalography (EEG) and employing a rhythmic audiovisual speech paradigm previously used with dyslexic children. Participants viewed a "talking head" repeating the syllable "ba" at a 2-Hz rate. Measures were neural phase entrainment and band power in the delta, theta, beta (15-25 Hz), and low gamma (25-40 Hz) bands. Phase-amplitude coupling (PAC) and phase-phase coupling (PPC) were also assessed for delta-theta, delta-beta, theta-beta, delta-gamma, and theta-gamma interactions. Both groups exhibited significant delta- and theta-band phase entrainment; however, the two groups differed significantly in the preferred phase for the theta band. While the control group showed consistent beta- and low gamma-band phase entrainment, this was not observed for the dyslexic group. There was significantly greater delta-band power for the dyslexic group across the whole brain and in the right temporal region. These findings indicate that temporal sampling deficits appear to persist into adulthood and show a multi-timescale pattern, with altered low-frequency processing and atypical higher-frequency entrainment. The data also generate developmental hypotheses about possible compensation or reorganisation mechanisms that could be examined in future longitudinal research.