Unveiling the alterations of action processing and mu rhythm in Williams Syndrome.

Williams syndrome (WS) is a neurodevelopmental disorder marked by social difficulties, which may stem from atypical action processing. We investigated whether individuals with WS show impaired action processing as indexed by electroencephalographic (EEG) mu suppression, a biomarker of action processing. 17 individuals with WS, 17 healthy controls (HC) and 17 individuals with intellectual and developmental disability (IDD) participated in the study. During EEG recording, participants performed a task requiring to predict actions under perceptual ambiguity. In a preceding learning phase, the probability of actions co-occurring with contextual cues was manipulated to establish varying association strengths: high informativeness from very frequent or rare pairings, and moderate informativeness from intermediate ones. A control task required to use contextual cues for predicting moving shapes. HC and WS groups, but not the IDD group, utilized contextual cues to predict action/shape unfolding. EEG data revealed distinct patterns of mu event-related desynchronization (mu-ERD) across groups. In the HC group, mu-ERD was stronger during action than shape prediction and varied with the cue probability, with greater mu-ERD in low vs. high probability trials. In WS and IDD, mu-ERD was attenuated compared to HC. Notably, WS participants exhibited greater mu-ERD for low- than high-probability actions in moderately informative contexts; no modulation was observed in highly informative contexts in either task. In IDD, mu-ERD was not modulated by task or cues predictability. The attenuation and distinct contextual modulation of mu-ERD in WS may reflect anomalies in perception-action mechanisms, potentially linked to impaired simulation of observed actions.