Brain responses to handwritten and printed letters differentially depend on the activation state of the primary motor cortex

Previous studies demonstrated that visual perception of handwritten letters activates the left primary motor cortex more strongly than printed letters. Here, we used EEG to record cortical responses evoked by single letters to directly test if their visual processing is actually influenced by their motor content. We manipulated the "motor familiarity" of letters that we considered high for letters written by the observers themselves, medium for letters written by other individuals, and low for printed, machine designed letters. In order to relate the effects of motor familiarity to the activation of the primary motor cortex, we also directly manipulated its availability during the visual task: we computed Event-Related Potentials (ERPs) over the posterior cortex during a dual task where participants had to observe the letters while performing unrelated self-paced brief movements of the right hand approximately every 5 s (allowing the primary motor cortex to successively activate and “idle”). At 300-350 ms and 500-600 ms after stimulus onset, the amplitude of the ERP components markedly reflected the level of motor familiarity of the observed letter. Nonetheless, this was true only when the primary motor cortex was in an “idling” state: when the motor cortex was in an activation state, this motor familiarity effect was dropped. This clearly indicates that, at these latencies, the motor information embedded in letters is processed in the brain and that this processing depends on the activation state of the left primary motor cortex.