Timing-dependent inhibitory effects of peripheral somatosensory inputs on the primary motor cortex: A transcranial...

Short-latency afferent inhibition (SAI) is a transient suppression of corticomotor excitability elicited by peripheral electrical stimulation (ES). When the interstimulus interval (ISI) between ES and transcranial magnetic stimulation (TMS) was set to 2 or 5 ms after the estimated arrival of somatosensory input in the primary somatosensory cortex, motor-evoked potentials (MEPs) were suppressed compared with single-pulse TMS. Previous studies have shown that the inhibitory activity depends on the timing between ES and TMS. However, whether subtle differences in afferent timing modulate TMS-evoked cortical responses remains unclear. To address this, we examined whether subtle ISI differences modulate cortical responses by SAI using TMS-electroencephalography (TMS-EEG). Thirty healthy adults (21.9 ± 4.4 years) participated in the current study. The ISI was determined relative to the N20 latency of somatosensory-evoked potential. MEPs were measured under three conditions: single-pulse TMS (TMS-alone), SAI with an ISI of N20 + 2 ms (SAI_N20+2), SAI with an ISI of N20 + 5 ms (SAI_N20+5). Subsequently, TMS-evoked potentials (TEPs) were recorded under the same three conditions and the single-pulse ES condition. MEP amplitudes were decreased in both SAI_N20+2 and SAI_N20+5 compared with TMS-alone (all P < 0.001), with greater inhibition in SAI_N20+2 than in SAI_N20+5 (P < 0.001). In contrast, the N45 component of the TEP was specifically modulated in SAI_N20+5 compared with TMS-alone (P < 0.05), whereas no change was observed in SAI_N20+2. These findings suggest that, in contrast to MEP suppression, cortical inhibition is differently modulated by subtle ISI differences, potentially reflecting distinct timing-dependent mechanisms underlying SAI.