Modulation of short- and long-interval intracortical inhibition with increasing motor evoked potential amplitude in a human hand muscle

• Intracortical inhibition assessed with paired-pulse transcranial magnetic stimulation (TMS) is influenced by test TMS intensity.
• Using two different experimental approaches, we also show that intracortical inhibition can vary with the amplitude of the test motor evoked potential (MEP) when expressed relative to the maximal compound muscle action potential (%Mmax).
• We suggest that the test MEP should be normalised to the maximal muscle response when assessing intracortical inhibition.

ObjectiveThe aim of the current study was to investigate the effect of increasing test motor evoked potential (MEP) amplitude on short- (SICI) and long-interval intracortical inhibition (LICI) at rest and during activation of the first dorsal interosseous (FDI) muscle.MethodsIn 22 young subjects, a conditioning-test transcranial magnetic stimulation (TMS) paradigm was used to assess SICI and LICI at 5 different test TMS intensities (110–150% motor threshold) in resting and active FDI. In 9 additional subjects, SICI and LICI data were quantified when the test MEP amplitude represented specific proportions of the maximal compound muscle action potential (Mmax) in each subject.ResultsTest TMS intensity influenced SICI and LICI in rest and active FDI muscle. The normalised test MEP amplitude (%Mmax) did not influence SICI at rest, whereas there was a decrease in LICI at rest and an increase in SICI in active FDI with an increased normalised test MEP amplitude (%Mmax).ConclusionsOur results demonstrate differential effects of normalised test MEP amplitude (%Mmax) on SICI and LICI in resting and active FDI muscle.SignificanceEstimation of SICI and LICI under some circumstances may be influenced by the normalised test MEP amplitude in subject populations with different Mmax characteristics.