Authors: Craig Sinclair Geoffrey R Hammond
Publish Date: 2007/12/12
Volume: 186, Issue: 3, Pages: 385-392
Abstract
Reaction time RT is shortened when the response signal is preceded by a warning signal a finding that has been attributed to response preparation during the foreperiod between the warning and response signals Research suggests an increased excitability of cortical movement representations associated with response preparation during the foreperiod of a warned RT task Davranche et al in Eur J Neurosci 253766–3774 2007 However when the foreperiod duration is short and constant the motor evoked potential MEP amplitude elicited by transcranial magnetic stimulation TMS during the foreperiod is suppressed Touge et al in Clin Neurophysiol 1111216–1226 1998 suggesting a competing inhibitory process Three experiments measured MEP amplitude and intracortical inhibition during the foreperiod of a warned RT task in which the response was a flexion of the right index finger Experiments 1 and 2 measured shortinterval intracortical inhibition SICI with paired TMS pulses separated by interstimulus intervals ISIs of 3 SICI3 and 15 ms SICI15 respectively Experiment 3 measured longinterval intracortical inhibition LICI with paired TMS pulses with an ISI of 100 ms LICI100 In all experiments MEP amplitude was smaller in the warned condition than in the unwarned condition There was less SICI3 in the warned condition than in the unwarned condition Experiment 1 whereas SICI15 was similar in both conditions Experiment 2 There was less LICI100 in the warned condition than in the unwarned condition Experiment 3 The intracortical inhibitory processes measured here cannot explain the suppression of MEP amplitude in the warned condition We propose that the suppression of MEP amplitude is the result of an inhibitory mechanism which acts on primary motor cortex to prevent premature response during the foreperiod
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