Abstract:
A central component of effective motor control is the ability to cancel pre-planned movement, termed response inhibition. When stopping is unexpected, response inhibition is reactive and associated with a non-selective reduction in corticomotor excitability. If forewarned to stop, response inhibition can be proactive and excitability may be selectively decreased. The modulation of GABAergic inhibitory networks within the primary motor cortex during proactive versus reactive response inhibition is unclear. The present study investigated the modulation of long and short-interval intracortical inhibition using paired-pulse TMS in eighteen right-handed participants performing reactive and proactive response inhibition tasks. Measures of long and short interval intracortical inhibition (LICI and SICI respectively) were obtained preceding responses from motor evoked potentials in task-relevant and task-irrelevant intrinsic hand muscles. When one response had to stop, the continued response was delayed to a greater extent in reactive versus proactively cued trials. LICI was reduced in both muscles during all types of response inhibition task compared with “pre-task” baseline. Additionally, on reactive trials, the extent of LICI for task-relevant LICI positively correlated with behavioral response times of the right hand when responses were inhibited on the left side. Task-relevant SICI was reduced when proactive cues indicated responding was highly likely but was unchanged when response conditions were uncertain. These novel findings indicate that GABAB receptor-mediated pathways may be involved in setting inhibitory tone according to task expectations whereas GABAA receptor -mediated pathways may be recruited proactively with response certainty.