Response inhibition activates distinct motor cortical inhibitory processes

We routinely cancel pre-planned movements that are no longer required. If stopping is forewarned, proactive processes are engaged to selectively decrease motor cortex excitability. However, without advance information there is a non-selective reduction in motor cortical excitability. Here we examine modulation of human primary motor cortex inhibitory networks during response inhibition tasks with informative and uninformative cues using paired-pulse transcranial magnetic stimulation. Long- and short-interval intracortical inhibition (LICI and SICI), indicative of GABAB- and GABAA-receptor mediated inhibition respectively, were examined from motor evoked potentials obtained in task-relevant and task-irrelevant hand muscles when response inhibition was preceded by informative and uninformative cues. When the participants (10 male and 8 female) were cued to stop only a subcomponent of the bimanual response, the remaining response was delayed, and the extent of delay was greatest in the more reactive context, when cues were uninformative. For LICI, inhibition was reduced in both muscles during all types of response inhibition trials compared with the pre-task resting baseline. When cues were uninformative and left hand responses were suddenly cancelled, task-relevant LICI positively correlated with response times of the responding right hand. In trials where left hand responding was highly probable or known (informative cues), task-relevant SICI was reduced compared when cued to rest, revealing a motor set indicative of responding. These novel findings indicate that the GABAB-receptor mediated pathway may set a default inhibitory tone according to task context, whereas the GABAA-receptor mediated pathways are recruited proactively with response certainty.

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