Introduction: Short intracortical inhibition (SICI) is a GABAa-mediated phenomenon argued to mediate motor selectivity. SICI is markedly reduced, producing motor disinhibition, in the sub-acute period following cortical (CORT), but not subcortical (SC), stroke. Previous work suggests SICI may normalize as part of the natural course of recovery. Importantly, SICI is typically measured at rest complicating our understanding of its role in motor control and recovery following stroke. Here we investigated task-dependent differences in SICI in chronic stroke survivors.
Hypothesis: We hypothesized: i) differences between SICI measured at rest and during voluntary movement and ii) SICI during voluntary movement would reveal persistent impairments following CORT stroke.
Methods: We tested 13 adults (65±8.5 yr, 11 male) with chronic (72.8±52.6 mo) stroke (5 CORT, 8 SC) and 7 controls (CON)(58±6.9 yr, 4 male) using paired-pulse transcranial magnetic stimulation during 3 tasks: rest, grip, box & blocks (B&B). Motor evoked responses (MEPs) were measured from the first dorsal interosseous of the paretic and non-dominant hands of stroke and CON, respectively. SICI was induced by conditioning the test MEP at 0.8 resting or 0.7 active, motor threshold at the interstimulus interval producing maximal SICI at R (3 ms CON, 3.7 ms Stroke) and quantified as the ratio of conditioned/unconditioned MEParea. Stimulation intensity was adjusted to maintain test MEP amplitude at 1mV pk-pk across tasks.
Results: At rest, SICI was similar between groups (p > 0.5). However during grip, CORT revealed significantly less SICI than CON or SC (p’s .05). During B&B, CORT again revealed less SICI than CON (p =.04); however SICI was similar between CORT and SC (p >.05). Across tasks, SICI produced by CON and SC was similar (p >.05) while CORT revealed less SICI during both grip and B&B (p’s <.005) compared to rest.
Conclusions: Measured at rest, SICI is similar across groups suggesting GABAa circuits are normalized as part of the natural course of stroke recovery. However, SICI measured during voluntary motor tasks reveals persistent impairments, which are more widespread following CORT stroke and may interfere with rehabilitation involving voluntary movement.