Motor cortical excitability during preparation for a speeded response assessed by transcranial magnetic stimulation

[Truncated abstract] Human behavior is characterized by the use of advance information to prepare efficient and appropriate responses to stimuli in the environment. Response preparation in humans has been studied using the warned reaction time (RT) task paradigm, in which a response signal is preceded on some trials by a warning signal. The ability to prepare some or all of the response during the foreperiod between the warning signal and response signal is thought to underly the shorter RT on warned trials than on unwarned trials (Rosenbaum, 1980). While the warned RT task paradigm has proven fruitful in developing models of response preparation, the neural mechanisms by which these processes occur are still unclear. Transcranial magnetic stimulation (TMS) delivered over the primary motor cortex (M1) can elicit a motor evoked potential (MEP) in a contralateral muscle, the amplitude of which reflects the momentary excitability of the corticospinal system underlying contraction of that muscle. Research using TMS during the warned RT task has shown a suppression of corticospinal excitability during the foreperiod between the warning and response signals (Hasbroucq, Kaneko, Akamatsu, & Possamaï, 1999; Touge, Taylor, & Rothwell, 1998); this finding is unexpected given that response preparation has been associated with increased cortical activity in motor preparatory areas (Deiber, Ibanez, Sadato, & Hallett, 1996). ... In a series of experiments, subjects made a ballistic flexion of their right index finger as soon as possible after a response signal which was preceded by a warning signal on some trials. Single TMS pulses delivered 200 ms in advance of the response signal elicited smaller amplitude motor evoked potentials (MEPs) during the foreperiod on warned trials compared to unwarned trials. A control experiment indicated that the suppression of MEP amplitude observed during the foreperiod could not be attributed to increased predictability of the TMS pulse on warned trials. When different warning signals informed subjects of different probabilities of response signal delivery on the current trial, the suppression of MEP amplitude was sensitive to the expectancy of response signal delivery when the different types of warning signals were delivered in separate "pure" blocks, but not when they were interspersed in mixed blocks. In some experiments the excitability of short- (SICI) and long- (LICI) latency intracortical inhibitory circuits within M1 were measured with a paired-pulse TMS protocol. Both SICI and LICI circuits were found to have reduced excitability during the foreperiod on warned trials compared to unwarned 3 trials, suggesting a reduction of ICI associated with response preparation. The suppression of corticospinal excitability during the foreperiod of the warned RT task thus could not be explained by increased ICI within M1. It was instead proposed that the suppression of corticospinal excitability during the foreperiod was the result of an extrinsic inhibitory mechanism from outside M1, engaged to suppress descending M1 output below the threshold for triggering a premature response during the foreperiod (Boulinguez et al., 2008; Davranche et al., 2007; Jaffard, Benraiss, Longcamp, Velay, & Boulinguez, 2007). Furthermore, because ICI is known to occur within M1 (T. Kujirai et al., 1993; J. Valls-Solé, Pascual-Leone, Wassermann, & Hallett, 1992), its reduction during the foreperiod, well in advance of movement onset, suggests that M1 is engaged in response preparation.