Action complexity modulates corticospinal excitability during action observation

Observing an action engages the same brain regions as executing an action. Activity in the motor areas of the brain can be measured indirectly using non-invasive techniques, such as single-pulse transcranial magnetic stimulation (TMS). The current study aimed to investigate the time-course of motor cortex activity during action observation of a simple and complex action. In Experiment 1 single-pulse TMS was used to investigate whether the time-course, and magnitude of change in motor cortex excitability was mediated by complexity of an observed action. Measures of motor cortex excitability acting on two intrinsic hand muscles and two forearm muscles were obtained during observation of a simple and a complex action. Experiment 1 found excitability in an intrinsic hand muscle was greater during observation of a complex action compared to a simple action, at the time-point at which the actions were kinematically different. Paired-pulse TMS can be used to measure excitability of motor cortex inhibitory processes. In Experiment 2, single and paired-pulse TMS was used to determine whether greater excitability during observation of the complex action compared to the simple action was associated with reduced motor cortical inhibition. Experiment 2 showed no significant difference in motor cortical inhibition between the complex and simple action. Findings suggest action complexity modulates changes in motor cortex excitability during action observation.