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 effectiveness of magnetic cortical stimulation in evoking phase resetting of voluntary wrist movement. Nine normal subjects were studied (five on two occasions), while making rhythmical movements of the right wrist, at their 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 the trials were being reset. The loading torque had a strong influence upon both the resetting index and the null phase, generated by a constant 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 first poststimulus position peak, which in turn was largely determined by the twitch induced by the cortical shock. The timings of the averaged poststimulus position peaks following the first were simple multiples of the prestimulus movement period. Our results indicate that loading conditions profoundly influence the effectiveness of magnetic cortical stimulation in resetting a voluntary movement and that these effects appear to be largely explicable by the changes in the muscle twitch evoked by the stimulus with the different loads. We suggest that the magnetic shock is therefore unlikely to reset voluntary movement by an action directly upon the motor programme. We propose that the main method by which magnetic cortical stimulation resets repetitive wrist movement is indirect: normal generation of repetitive wrist flexion and extension is disrupted by the cortical shock, following which afferent information related to the twitch induced is able to reset the movement.
Wagener
D S
DS
Department of Neurology and School of Medicine, Prince of Wales Hospital, Sydney, NSW, Australia.
Colebatch
J G
JG
D000328:Adult; D004574:Electromagnetic Fields; D004576:Electromyography; D005260:Female; D005542:Forearm / Q000502:physiology; D006801:Humans; D008297:Male; D008959:Models, Neurological; D009044:Motor Cortex / Q000502:physiology*; D009068:Movement / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D014892:Weight Perception / Q000502:physiology*; D014953:Wrist / Q000502:physiology