BILATERAL ACTIVATION OF THE HUMAN SOMATOMOTOR CORTEX BY DISTAL HAND MOVEMENTS

We recorded cortical magnetic signals, simultaneously over the whole s calp, from 6 healthy subjects during 3 motor tasks to track the varyin g proportion of contra- vs. ipsilateral activation. The subjects perfo rmed self-paced index finger flexions, simultaneous flexion of 4 finge rs, and a sequence of rapid digit movements in different sessions. Ind ex finger and 4-finger movements were associated with phasic bilateral dampening of spontaneous 10 and 20 Hz rhythms along the central sulcu s, starting approximately 1 sec before the movement in the contralater al hemisphere. A rebound occurred within 1 sec after the index finger and 4-finger flexions; the rapid finger movements resulted in a persis tent blocking of the rhythms. Averaging with respect to movement onset showed a slow bilateral frontal readiness field starting about 0.5 se c prior to motion onset. It was followed, within 200 msec after moveme nt onset, by phasic movement-evoked fields (MEFs) which were bilateral during the tasks involving several fingers. The contra- vs. ipsilater al MEF amplitude ratio C/I decreased from 4.0 during index finger move ments to 0.6 during rapid finger flexions, reflecting the enhanced act ivation of the ipsilateral primary somatomotor cortex with increasing complexity of movement.