Subjects have reported awareness of moving 80-90 ms before voluntary muscle activation (Libet et al. 1983). When transcranial magnetic stimulation (TMS) of motor cortex was used to elicit forearm muscle contractions, some subjects reported a sense of movement even when movement was abolished by ischaemic block (Amassian et al. 1989; Brasil-Neto et al. 1993). These findings support the idea of a corollary discharge or efference copy. Our experiments were devised to resolve whether TMS evokes a sensation of movement directly in the brain or as a result of sensory feedback.
Local ethical approval and informed consent was obtained for the following procedures carried out on six normal subjects (aged 23 to 56). Twitch contractions evoking left and right finger extensions were elicited by electrical stimulation of muscles. In another series, finger extension of one hand was elicited by TMS (MagStim 200, 9 cm round coil) of the contralateral motor cortex and on the other side by muscle stimulation. Left and right finger movements were matched in amplitude using accelerometer recordings. The time interval between left and right stimuli was varied randomly (range ± 90 ms in 15 ms steps) from trial to trial. Allowance was made for the extra conduction time with TMS. Subjects were asked to report whether they sensed that the left or the right movement occurred first, or if they could detect no difference.
Sequence discrimination plots (percentage correct responses against time interval) indicated that left and right movements evoked by dual electrical stimulation were sensed as near simultaneous when there was zero delay between them. The sense of movement for the six subjects ranged from right preceding left by 10 ms to left preceding right by 5.5 ms, for simultaneous movements. When TMS was combined with muscle stimulation the cortically evoked movement was felt on average 20 ms after the movement evoked by muscle stimulation. The sense of movement in response to TMS ranged from 7 ms preceding to 53 ms following the electrically induced movement.
Since the TMS response was felt late rather than early, the results do not support the notion of corollary discharge or efference copy being elicited by TMS but favour sensory feedback as the source of the sense of movement. The delayed perception of the TMS-induced movement may reflect cortical suppression of the sense of movement.
This work was supported by Alberta Heritage Foundation for Medical Research and Wellcome Trust.