Motor control in simple bimanual movements: a transcranial magnetic stimulation and reaction time study

Objective: Simple reaction rime (RT) can be influenced by transcranial magn etic stimulation (TMS) to the motor cortex. Since TMS differentially affect s RT of ipsilateral and contralateral muscles a combined RT and TMS investi gation sheds light on cortical motor control of bimanual movements. Methods: Ten normal subjects and one subject with congenital mirror movemen ts (MM) were investigated with a RT paradigm in which they had to move one or both hands in response to a visual go-signal. Suprathreshold TMS was app lied to the motor cortex ipsilateral or contralateral to the moving hand at various interstimulus intervals (ISIs) after presentation of the go-signal . EMG recordings from the thenar muscles of both hands were used to determi ne the RT. Results: TMS applied to the ipsilateral motor cortex shortened RT when TMS was delivered simultaneously with the go-signal. With increasing ISI betwee n TMS and go-signal the RT was progressively delayed. This delay was more p ronounced if TMS was applied contralateral to the moving hand. When normal subjects performed bimanual movements the TMS-induced changes in RT were es sentially the same as if they had used the hand in an unimanual task. In th e subject with MM, TMS given at the time of the go-signal facilitated both the voluntary and the MM. With increasing ISI, however, RT for voluntary mo vements and MM increased in parallel. Conclusions: Ipsilateral TMS affects the timing of hand movements to the sa me extent regardless of whether the hand is engaged in an unimanual or a bi manual movement. It can be concluded, therefore, that in normal subjects si mple bimanual movements are controlled by each motor cortex independently. The results obtained in the subject with MM are consistent with the hypothe sis that mirror movements originate from uncrossed corticospinal fibres. Th e alternative hypothesis that a deficit in transcallosal inhibition leads t o MM in the contralateral motor cortex is not compatible with the presented data, because TMS applied to the motor cortex ipsilateral to a voluntary m oved hand affected voluntary movements and MM to the same extent. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.