Abstract
Social difficulty is a core phenotype of autism spectrum disorder (ASD), yet its mechanisms remain poorly understood due to insufficient integration of interpersonal neural dynamics with real-world behavioral and linguistic impairments. This study elucidated this integrated mechanism by investigating inter-brain synchrony (IBS) networks between 44 children with ASD and 48 typically developing children and their caregivers. Using functional near-infrared spectroscopy during puzzle-solving and free-talking tasks, an interpretable graph neural network identified ASD-specific atypical IBS subnetworks. ASD dyads exhibited task-dependent, modular atypical IBS patterns: abnormal coupling between caregivers' frontal eye field (FEF)/dorsolateral prefrontal cortex (DLPFC) and children's motor regions during puzzle solving, and defective synchronization between children's DLPFC and caregivers' FEF during free talking. Concurrently, ASD children exhibited significant behavioral impairments and atypical linguistic features. Finally, atypical IBS subnetworks significantly predicted autism-related symptoms, including restricted and repetitive behaviors, socialization, communication, and executive function. These findings reveal that social difficulty in ASD is underpinned by an integrated neural-behavioral-linguistic mechanism, advancing our understanding and laying a foundation for personalized interventions targeting dyadic social interaction.