Table 3

TMS-EEG outcome measures

Global mean field powerReflects global brain activation, calculated as the root mean-squared value of the signal across all electrodes
Event-related spectral perturbationThe modulation of amplitude elicited by a specific event (e.g., TMS pulse) relative to a baseline (e.g., pre-stimulus). The power spectra values of the EEG recorded after TMS are computed for each trial. Single-trial values are then averaged across all trials and the average baseline power is subtracted from the whole trial. These are conducted for the relevant EEG channels in the frequency bands of interest. Event-related spectral perturbation values are independent of the phase of the evoked or induced activity
Intertrial coherenceA measure of phase-locking of the TMS-evoked oscillations across trials; it is independent of signal amplitude. This measure reflects the degree to which a stimulus induces phase synchrony (or resetting) of ongoing oscillations at specific frequencies. Intertrial coherence values range from 0 (no intertrial coherence) to 1 (perfect phase alignment across trials)
Natural frequencyRefers to the intrinsically generated dominant frequency that different brain regions oscillate at when perturbed. Can be calculated as the frequency bin with the largest cumulated event-related spectral perturbation over time (e.g., 20~300 ms post-stimulus).
Relative spectral powerThe power spectra expressed as the percent of power in a given frequency
  • EEG = electroencephalography; TMS = transcranial magnetic stimulation.