Influence of task-related ipsilateral hand movement on motor cortex excitability

Objective: The time course of the right motor cortex excitability in relati on to a task-related voluntary right thumb twitch was studied using sub-thr eshold transcranial magnetic stimulation (TMS) to the right motor cortex. Methods: Motor excitability was studied in 8 adult subjects who made a brie f right thumb twitch to the predictable omission of every fifth tone in a s eries of tones 2.5 s apart. This paradigm avoided an overt sensory cue, whi le allowing experimental control of TMS timing relative to both movement an d the cue to move. Motor excitability was characterized by several measures of motor evoked potentials (MEPs) recorded from the left thenar eminence i n response to TMS over the right scalp with a 9 cm coil: probability of eli citing MEPs, incidence of MEPs and amplitude of MEPs. Results: All subjects showed suppression of motor excitability immediately following a voluntary right thumb twitch (ipsilateral response), and up to 1 s after it. However, two distinctly different effects on motor excitabili ty were observed before the response: two subjects showed excitation, begin ning about 500 ms before response until 300 ms after it, followed by the po st-movement suppression; 6 subjects displayed pre-movement suppression, beg inning about 600 ms before the response and persisting for the duration. Conclusions: The net effect of an ipsilateral response on motor cortex can be either inhibitory or excitatory, changing with time relative to the resp onse. These findings are compatible with two separate processes, inhibitory and excitatory, which interact to determine motor excitability ipsilateral to the responding hand. (C) 2001 Elsevier Science ireland Ltd. ALI rights reserved.