LAUSR

Introduction Corticospinal excitability is transiently suppressed during motor preparation (Greenhouse et al., 2015 ) and when cancelling a prepared movement (Hoshiyama et al., 1997 ). Because MEPs are suppressed even in task-irrelevant muscles Greenhouse et al., 2015 , Hoshiyama et al., 1997 ), it has been proposed that there is a “global” inhibitory signal (Greenhouse et al., 2015) . Objectives We tested the whether the inhibition is global or specific to different excitatory interneuron circuits in primary motor cortex (M1). Materials & methods A controllable pulse parameter TMS (cTMS; Rogue Resolutions Ltd.) device was used to activate distinct interneuron circuits in M1 by changing the coil orientation (posterior–anterior, PA; anterior–posterior, AP) and TMS pulse width (30–120 μs). Motor evoked potentials (MEPs) were elicited in the first dorsal interosseous muscle(s) using short AP- (AP30) and long PA-directed (PA120) pulses at various stages during: (1) a left/right choice reaction time task (CRTT) in which a non-informative warning cue appeared 0.5 s prior to the imperative signal indicated the required response; and (2) a Go/No-Go task in which a non-informative warning cue appeared 2 s prior to a Go or No-Go signal indicating to respond or not respond. Because TMS pulse specificity is greatest at low stimulus intensities, experiments were conducted with weak voluntary contraction (5–10% maximum) to reduce stimulus threshold. Results AP30 pulses evoked MEPs with a longer latency than PA120 pulses, confirming that different excitatory interneurons were recruited by the different pulse. [RH1] In the CRTT, the amplitude of AP30-evoked MEPs was suppressed at the time of the imperative stimulus in both responding and non-responding hands, whereas responses to PA120 stimuli were unaffected (Fig. 1A) Download : Download high-res image (366KB) Download : Download full-size image .[RH2] In the Go/No-Go task, AP30 and PA120 MEPs were suppressed to a similar extent in the No-Go condition near to the expected onset of movement (Fig. 1B). Conclusion During motor preparation for the CRTT there was a selective suppression of one interneuron circuit in M1. In contrast, a global inhibition of corticospinal excitability was observed following a decision not to move. The implication is that the mechanisms underlying each form of inhibition are at least partially distinct.