Timing of mirror system activation when inferring the intentions of others

Neuroimaging studies have shown mirror system (MS) activation when participants infer internal states e.g. emotions, intentions or beliefs (known as 'mentalizing') from others' actions. However, the exact role of the MS in mentalizing tasks is unknown. Dysfunctional MS activation may underlie mentalizing deficits experienced by adults with autism spectrum disorder (ASD). This study investigated the timing of MS activity when inferring intentions in order to delineate between existing models of MS involvement. Single-pulse transcranial magnetic stimulation (TMS) was applied to the primary motor cortex at different time points during the observation of hand actions whilst participants inferred intentions (mentalizing task) and performed a non-mentalizing task. Electromyographic activity in the contralateral hand was used as an indirect measure of MS activity. Greater corticospinal activity was found during the mentalizing task than the non-mentalizing task, but only at the end of observed actions, suggesting late MS involvement in processing intentions. Enhanced corticospinal activity was not related to autistic traits or behavioural performance suggesting the MS has a more automatic role in processing others' intentions, irrespective of mentalizing ability. Our results extend current knowledge of MS activation when mentalizing, allowing initial delineation between different models of MS involvement in mentalizing.