THE INFLUENCE OF PERIPHERAL LOAD ON RESETTING VOLUNTARY MOVEMENT BY CORTICAL STIMULATION - IMPORTANCE OF THE INDUCED TWITCH

We studied the effects of changes in loading torque on the effectivene ss of magnetic cortical stimulation in evoking phase resetting of volu ntary wrist movement. Nine normal subjects were studied (five on two o ccasions), while making rhythmical movements of the right wrist, at th eir preferred rate, against extension torque loads of 0.35 Nm, 0.26 Nm and 0.18 Nm, flexion torque loads of 0.09 Nm and 0.18 Nm and without external load. The position records of individual trials were used to measure the effectiveness of resetting (resetting index: the slope of the phase-response curve) and the ''null phase'', the phase to which t he trials were being reset. The loading torque had a strong influence upon both the resetting index and the null phase, generated by a const ant intensity of cortical stimulation such that the largest resetting indices were obtained for movements made against the largest extension torque load (mean resetting index 0.72). The degree of resetting and null phase were related to the mean amplitude and direction of the fir st poststimulus position peak, which in turn was largely determined by the twitch induced by the cortical shock. The timings of the averaged post-stimulus position peaks following the first were simple multiple s of the prestimulus movement period. Our results indicate that loadin g conditions profoundly influence the effectiveness of magnetic cortic al stimulation in resetting a voluntary movement and that these effect s appear to be largely explicable by the changes in the muscle twitch evoked by the stimulus with the different loads. We suggest that the m agnetic shock is therefore unlikely to reset voluntary movement by an action directly upon the motor programme. We propose that the main met hod by which magnetic cortical stimulation resets repetitive wrist mov ement is indirect: normal generation of repetitive wrist flexion and e xtension is disrupted by the cortical shock, following which afferent information related to the twitch induced is able to reset the movemen t.