MOTOR CORTEX INVOLVEMENT DURING CHOICE-REACTION TIME - A TRANSCRANIALMAGNETIC STIMULATION STUDY IN MAN

It has been shown that transcranial magnetic stimulation can delay sim ple reaction time; this happens when the stimulation is delivered duri ng the reaction time and over the cortical area which commands the pri me mover of the required response. Although it is agreed that magnetic stimulation could be a useful tool for studying information processin g in man, we argue that, because of the use of simple reaction time, t he results reported so far are difficult to interpret within this theo retical framework. In the present paper, three experiments are reporte d. Six subjects participated in experiment 1 in which magnetic stimula tion was delivered, at different times, during choice reaction time. T he effects of the magnetic stimulation of the cortical area involved i n the response (induced current passing forward over the motor represe ntation of the responding hand), were compared to the effects of an el ectrical stimulation of the median nerve (K-reflex). In a first contro l experiment (experiment 2a; 5 subjects), the coil was placed over the ipsilateral motor cortex (induced current passing backward over the m otor representation of the non-responding hand) thus minimizing the pr obability to excite the same neural nets as in the first experiment. I n a second control experiment (experiment 2b; 4 subjects), the coil wa s placed a few centimeters away from the subject's scalp thus ensuring no stimulation of any neural nets. The results show that: (1) the noi se and the smarting of the skin associated with the coil discharge pro duce an intersensory facilitation thereby shortening reaction time (ex periment 2a), (2) actually, the noise produced by the stimulation is s ufficient to produce such a facilitatory effect (experiment 2b), (3) a stimulation over the area at the origin of the motor command causes a reaction time delay which counteracts this intersensory facilitation effect (experiment 1), (4) in this latter case, the closer the stimula tion to the actual overt response, the longer the delay and (5) there is no trace of correlation between the amplitude of the motor evoked p otential and the reaction time change. (C) 1997 Elsevier Science B.V.