State-Guided TMS-EEG for N100 Enhancement Study Based on Whole-Brain EEG Microstates

Transcranial magnetic stimulation-electroencephalography (TMS-EEG) enables the non-invasive assessment of cortical excitability and inhibition. However, the N100 component-a key marker of cortical inhibition-often exhibits poor trial-to-trial stability, limiting its utility as a reliable readout. The aim of this study is to propose a whole-brain, state-guided TMS-EEG protocol for enhancing TMS-evoked N100.

TMS-EEG data were acquired from 19 healthy adults. After standard preprocessing, single trials were sorted into five datasets according to the microstate present at the time of stimulation. For each microstate-specific dataset, global and local mean field amplitudes (GMFA/LMFA) and TMS-evoked potentials (TEPs) were computed to compare N100 characteristics across microstates.

TMS delivered during the S4 microstate produced the numerically largest N100 values in the GMFA and LMFA. However, these differences did not reach statistical significance and should therefore be interpreted as exploratory descriptive findings. The clearest statistically significant microstate effect was observed for the TEPs. The N100 component of S4 group (-2.38 μV; |N100| = 2.38 ± 0.33 μV) was significantly higher than S2 group (p_adj = 0.030), and S1 also elicited a larger response than S2 (p_adj = 0.012). This pattern was most clearly retained after moderate spatial downsampling to 32 channels, whereas the S4-related evidence became weaker and was no longer statistically retained at lower electrode densities. At 64 channels, S4 showed a significantly larger absolute N100 amplitude than S2 (p_adj = 0.030), and S1 was also significantly larger than S2 (p_adj = 0.013). At 32 channels, both contrasts remained significant, with S4 exceeding S2 (p_adj = 0.034) and S1 exceeding S2 (p_adj = 0.013). At 21 channels, only the S1 versus S2 contrast survived correction (p_adj = 0.025), whereas no S4-related pairwise contrast survived. At 9 channels, no pairwise comparison survived correction.

Based on offline analysis, we established and validated a whole-brain, state-guided TMS-EEG framework for stabilizing and amplifying the N100 component. Stimulation delivered during the S4 microstate produced a larger local TEP N100 response, whereas GMFA and LMFA were analyzed as exploratory descriptive outcomes and showed S4-related numerical patterns. Moreover, the S4-related N100 enhancement remained statistically supported after downsampling from 64 to 32 channels, but was not retained in the 21- or 9-channel montages. These findings indicated that microstate-based state monitoring may offer a viable framework for future state-guided neuromodulation in both research and clinical applications.