Pre-movement facilitation of motor-evoked potentials in man during transcranial stimulation of the central motor pathways

Motor evoked potentials (MEPs) following trans-cranial stimulation (TCS) through unifocal electric or magnetic impulses have been evaluated in the pre-movement period in 8 healthy volunteers. By utilizing a simple reaction time paradigm, progressive amplitude increments and latency decrements of MEPs have been demonstrated in the 100 ms preceding the onset of EMG activity in the muscle examined. By employing surface and depth recordings from various muscles of hand and forearm contralateral to the TCS, it was observed that in the 'early' period of pre-EMG facilitation (100-60 ms before EMG onset) TCS solely recruited the same low- threshold motor units which are fired first during self-paced contractions. In the 'middle and late' epochs of pre-EMG facilitation. TCS recruited additional motor units with larger amplitudes and shorter latencies. A similar rank order in motor units recruitment was observed when MEPs were recorded from a relaxed muscle, during TCS of progressively higher intensity. Multiple muscle recordings showed that pre-EMG facilitation was remarkably limited to the muscular group of the hand primarily involved in the intended movement.