Early non-specific modulation of corticospinal excitability during action observation

Activity of the primary motor cortex (M1) during action observation is thought to reflect motor resonance. Here, we conducted three studies using transcranial magnetic stimulation (TMS)-induced motor-evoked potentials (MEPs) of the first dorsal interosseus muscle (FDI) during action observation to determine: (i) the time course of M1 corticospinal excitability during the observation of a simple finger movement; (ii) the specificity of M1 modulation in terms of type of movement and muscle; and (iii) the relationship between M1 activity and measures of empathy and autistic traits. In a first study, we administered single-pulse TMS at 30-ms intervals during the observation of simple finger movements. Results showed enhanced corticospinal excitability occurring between 60 and 90 ms after movement onset. In a second experiment, TMS-induced MEPs were recorded from the FDI and abductor digiti minimi muscles while pulses were delivered 90 ms after movement onset during observation of simple finger movement and dot movement. Increased corticospinal excitability was restricted to finger movement and was present in both muscles. Finally, in an exploratory experiment, single-pulse TMS was administered at 30, 90 and 150 ms after movement onset, and participants were asked to complete the Empathy Quotient (EQ) and the Autism Spectrum Quotient (AQ). Correlational analysis revealed a significant link between motor facilitation at 90 ms and the EQ and AQ scores. These results suggest that corticospinal excitability modulation seen at M1 during action observation is the result of a rapid and crude automatic process, which may be related to social functioning.