Network Dynamics Mediating Ipsilateral Motor Cortex Activity during Unimanual Actions

■ Executing difficult actions with the left hand results in bilat-eral activity of motor areas along the precentral gyrus. Using TMS and fMRI, we explored the functional relationship between primary (M1) and premotor areas during unimanual actions, fo-cusing on M1 activity in the ipsilateral hemisphere. Single-pulse TMS revealed that the amplitude of motor-evoked potentials (MEPs), elicited in the stationary right-hand muscles following left M1 stimulation, fluctuated with the state of homologous muscles in the moving left hand. This ipsilateral excitability was pronounced when the left-hand movements were more complex. We used fMRI to visualize the cortical dynamics dur-ing unimanual actions. Trial-by-trial fluctuations in ipsilateral M1 activity were correlated with contralateral M1 responses and this correlation increased with movement complexity. Consis-tent with previous studies, the left caudal precentral premotor area (pcPM) was engaged during movements of either hand. Following low-frequency rTMS over left pcPM, the correlation between the activity level in the two M1s increased. This finding indicates that left pcPM may regulate the unintentional mirror-ing of motor commands in M1 during unilateral movement.