Interhemispheric inhibition modifies reaction time of the index finger

Interhemispheric inhibition (IHI) acts between hemispheres to decrease motor cortical excitability. IHI changes during movement preparation, but it is unknown whether altering IHI, independent of other factors, alters movement initiation. For the index finger abductor muscle (first dorsal interosseous; FDI), IHI is weaker during contralateral index abduction than adduction. Thus, this study aimed to modulate IHI through contralateral contraction and measure resultant changes in reaction time. Fifteen healthy participants (age 19-39 years) completed a reaction-time task requiring brief left index finger abduction. Prior to reactions, participants started either a sustained isometric abduction or adduction contraction of the right index finger. Single- or paired-pulse transcranial magnetic stimulation (TMS) elicited motor evoked potentials (MEPs) from left FDI in the late pre-movement phase. For each contralateral contraction direction, unconditioned and conditioned MEPs (preceded by suprathreshold TMS over the other hemisphere at 10 or 40 ms) were recorded. Conditioned MEPs, expressed relative to unconditioned MEPs, provided measures of short-interval IHI (SIHI) and long-interval IHI (LIHI). Left index finger reaction time was also measured. Linear mixed models showed that reaction time was 9 ± 9 ms (6%) slower during right adduction than abduction (F_1,1875 = 30.7, p < 0.001). Unconditioned MEPs in left FDI were 1.1 ± 2.0 mV (37%) smaller (F_1,270 = 8.82, p = 0.003) and SIHI 11 ± 16% stronger during right adduction (F_1,279 = 15.15, p < 0.001). However, LIHI was not different between right contraction conditions (F_1,272 = 0.410, p = 0.522). These results suggest that IHI can alter reaction time by influencing corticospinal excitability. Stronger SIHI during right adduction likely delayed pre-movement increases in corticospinal excitability, slowing reaction time.