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

M Zaaroor,Hillel Pratt,A Starr

doi:10.1016/s1388-2457(01)00516-8

Abstract

Objective: The time course of the right motor cortex excitability in relation to a task-related voluntary right thumb twitch was studied using sub-threshold transcranial magnetic stimulation (TMS) to the right motor cortex. Methods: Motor excitability was studied in 8 adult subjects who made a brief right thumb twitch to the predictable omission of every fifth tone in a series of tones 2.5 s apart. This paradigm avoided an overt sensory cue, while allowing experimental control of TMS timing relative to both movement and the cue to move. Motor excitability was characterized by several measures of motor evoked potentials (MEPs) recorded from the left thenar eminence in response to TMS over the right scalp with a 9 cm coil: probability of eliciting 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 excitability were observed before the response: two subjects showed excitation, beginning about 500 ms before response until 300 ms after it, followed by the post-movement suppression; 6 subjects displayed pre-movement suppression, beginning 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 response. These findings are compatible with two separate processes, inhibitory and excitatory, which interact to determine motor excitability ipsilateral to the responding hand.

Highlights

Since the early days of transcranial magnetic stimulation (TMS) the inhibitory and excitatory inuences on motor excitability in humans have been the focus of numerous studies
All 8 subjects displayed suppression of right motor cortex excitability after a response in the right thenar eminence, but two distinctly opposite patterns of results preceding this response: two of the subjects showed an excitatory effect before the homologous response, while 6 subjects showed a suppressive effect
The average probability for evoking motor evoked potentials (MEPs) in subjects with an excitatory effect rose from 3% when no response was required, to 14% when the subjects responded

Summary

Introduction

Since the early days of transcranial magnetic stimulation (TMS) the inhibitory and excitatory inuences on motor excitability in humans have been the focus of numerous studies. MEP facilitation by voluntary contraction of muscles ipsilateral to TMS (contralateral to the recorded muscle) has been reported (Hess et al, 1986; Zwarts, 1992; Meyer et al, 1995; Stedman et al., 1998; Tinazzi and Zanette, 1998; Muellbacher et al, 2000). All studies on the excitatory inuence of homologous muscle activation used supra-threshold TMS intensities and it has been suggested that only intensities in excess of 120% of MEP threshold can reveal the facilitatory effect (Muellbacher et al, 2000), and that at lower intensities the effect can be missed (Chiappa et al, 1991; Samii et al, 1997). Observing the facilitatory effect is dependent on forceful contraction of the homologous muscle (Chiappa et al, 1991; Samii et al, 1997; Muellbacher et al, 2000)

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