FC14.4 Short-latency intracortical inhibition preceding movement: Different time-course of modulation during simple and choice reaction time

Abstract

Background: Motor preparation is associated with excitability changes along the corticospinal tract. In simple reaction time task (SRT), the motor-evoked potentials (MEP) to transcranial magnetic stimulation (TMS) are facilitated before onset of EMGactivity (EMGonset), and the short-latency intracortical inhibition (SICI) is reduced preceding the facilitatory changes.We examined the timing of those neural processes in choice reaction time task (CRT). Aim: To identify potential differences in SICI during SRT and CRT paradigms. Patients/Methods: Five subjects were requested to perform a hand pinch in response to a visual stimulus in the context of SRT/CRT paradigms. MEPs were recorded from the first dorsal interosseous. TMS was applied over the contralateral hand area at an intensity of 0.95 active motor threshold for conditioning stimuli and 1.2 resting motor threshold for test stimuli. We obtained sets of responses combining single and paired (interstimulus interval 2.5 ms) stimuli with control and movement trials. Test stimuli were applied at 100, 75, 50 and 25 ms with respect to EMGonset. We used ANOVA to determine the differences between control and test trials. Results: During SRT, significant reduction of SICI with respect to baseline values occurred at 50 ms before EMGonset, while MEP facilitation started 25 ms later. Reduction of SICI occurred significantly later during CRT than during SRT (p 0.05). Discussion: The time course of intracortical excitability changes before EMGonset is different in SRT and CRT. In SRT, with direct stimulus/response mapping, SICI reduction precedes MEP facilitation. In CRT, characterized by uncertain stimulus/response mapping, reduction of SICI and MEP facilitation occur in parallel. Conclusion: The timing of premovement modulation of SICI with respect to MEP facilitation before EMGonset depends on the behavioral context.