Modulation of Motor Cortex Plasticity by Repetitive Paired-Pulse TMS at Late I-Wave Intervals Is Influenced by Intracortical Excitability

Abstract

The late indirect (I)-waves recruited by transcranial magnetic stimulation (TMS) over primary motor cortex (M1) can be modulated using I-wave periodicity repetitive TMS (iTMS). The purpose of this study was to determine if the response to iTMS is influenced by different interstimulus intervals (ISIs) targeting late I-waves, and whether these responses were associated with individual variations in intracortical excitability. Seventeen young (27.2 ± 6.4 years, 12 females) healthy adults received iTMS at late I-wave intervals (4.0, 4.5, and 5.0 ms) in three separate sessions. Changes due to each intervention were examined with motor evoked potential (MEP) amplitudes and short-interval intracortical facilitation (SICF) using both posterior-anterior (PA) and anterior-posterior (AP) TMS current directions. Changes in MEP amplitude and SICF were influenced by iTMS ISI, with the greatest facilitation for ISIs at 4 and 5 ms with PA TMS, and 4 ms with AP TMS. Maximum SICF at baseline (irrespective of ISI) was associated with increased iTMS response, but only for PA stimulation. These results suggest that modifying iTMS parameters targeting late I-waves can influence M1 plasticity. They also suggest that maximum SICF may be a means by which responders to iTMS targeting the late I-waves could be identified.

Cerebellar transcranial direct current stimulation disrupts neuroplasticity of intracortical motor circuits

“…Second, this pattern of responsesimulatenuously suggesting that late I-waves were (MEPPA) and were not (MEPAP) responsive to iTMScan only be consistent with the more contemporary interpretation that different current directions recruit from different intracortical populations (for review, see; [54]). As previous work has shown that AP MEPs are potentiated by iTMS in isolation [44], our results therefore suggest that tDCSCB influenced (reduced) the neuroplastic response of intracortical circuits activated by AP stimulation. Despite this, given the delayed onset latency of MEPAP suggests that these responses primarily involved late I-waves (Table 3), it is unclear if tDCSCB is also able to infleunce the neuroplastic response of early I-waves recruited with AP stimulation.…”

“…Previous work applying iTMS in isolation reported increased SICF post-intervention [21, 44]. Specifically, iTMS 1.5 has been demonstrated to potentiate SICF at ISIs of 1.5 ms and 4.5 ms [21], whereas iTMS 4.5 has been shown to potentiate SICF at ISIs of 4 - 5 ms [44].…”

“…As neuroplastic changes in MEP 1mV and SICF (i.e., measures recorded with PA stimulation) were also absent, it may appear counterintuitive to suggest that tDCS CB targets AP circuits. However, although different current directions can be expected to selectively target different interneuronal networks at sufficiently low stimulus intensities, there will be greater overlap between recruited populations as stimulus intensity increases [44]. Given that MEP 1mV and SICF were recorded with higher stimulus intensities than were used for MEP PA , we could therefore speculate that the differential response between these meaures was driven by MEP 1mV and SICF having a greater relative contribution from intracortical circuits that are AP-sensitive at low intensity, but able to be recruited by PA currents as intensity increases.…”

“…Specifically, iTMS1.5 has been demonstrated to potentiate SICF at ISIs of 1.5 ms and 4.5 ms [21], whereas iTMS4.5 has been shown to potentiate SICF at ISIs of 4 -5 ms [44]. Given that SICF is thought to index excitability within the intracortical circuits responsible for I-wave generation [22], these findings have been suggested to reflect neuroplastic effects of iTMS within these circuits.…”

“…However, although different current directions can be expected to selectively target different interneuronal networks at sufficiently low stimulus intensities, there will be greater overlap between recruited populations as stimulus intensity increases [44]. Given that MEP1mV and SICF were recorded with higher stimulus intensities than were used for MEPPA, we could therefore speculate that the differential response between these meaures was driven by…”