Modulation of motor cortical output to distal muscles during contralateral sequential finger movement: a study with transcranial magnetic stimulation

In nine normal subjects, we evaluated cortical motor evoked potential-, (MEP) from left abductor pollici brevis (APR) and biceps brachii (MM; to trancranial electric (TES) and magnetic (TMS) stimulation of the righi hemisphere during execution of three types of right finger movement and compared them with those obtained at rest. The experimental conditions were (a) repetitive opposition movement (thumb against the 3th finger); (b) isolated finger movements in an usual sequence (thumb against fingers 2.3.4.5) and (c) in an unusual sequence (thumb against lingers 3,5,2.4). All sequential finger movements were externally paced at 2H/ frequency rate using a metronome. Before performing TMS. .subjects were trained until the disappearance of the F.MG synchinetic activity in left APH that was recorded in 8 out of 9 subjects during the execution of paradigms (b) and (e). In all subjects MEP amplitude from ABP to TMS. but not to TES. was enhanced during paradigms (b) and even more during (c). while it remained unchanged during paradigm (a). In 2 subjects undergone to overtraining, this MEP amplitude increase disappeared. MEP amplitude from BB was not affected by any motor paradigm. These results documents an increased excitability of cortical motor outputs targeting the unmoving hand muscles during contracterai sequential finger movements which disappears with overtraining. These. findings suggest that learning of unimanual fine motor skills reduces ihu imcrhemispheric transfer of informations between homologous rnoioiareas leading to a lateralized control of the movement.