Recruitment of the motor system in the perception of handwritten and typed characters

Many different functional roles have been ascribed to the motor system due to its prevalent recruitment in perceptual and cognitive tasks other than motor production. We discuss findings that suggest the motor system might take on multiple roles that vary with context and the brain networks involved. Using single-pulse TMS, we measured the corticospinal excitability of the FDI muscle in primary motor cortex as participants viewed words that were either typed or handwritten. We observed consistent facilitation of corticospinal excitability during reading of handwritten text. Although we observed facilitation in corticospinal excitability during the presentation of typed text, this effect decreased with repetitive presentations of stimuli. We suggest that the facilitation during presentation of typed words is a case of action simulation, and that the diminishing facilitation in the case of typed stimuli is representative of sensory prediction by the motor system. These findings suggest that we should consider multiple roles for motor recruitment during the observation of visual stimuli, taking context into consideration.