Methodological choices matter: A systematic comparison of TMS-EEG studies targeting the primary motor cortex

Background Transcranial magnetic stimulation (TMS) triggers time-locked cortical activity that can be recorded with electroencephalography (EEG). Transcranial evoked potentials (TEPs) are widely used to probe brain responses to TMS. Methods Here, we systematically reviewed 137 published experiments that studied TEPs elicited from TMS to the human primary motor cortex (M1) in healthy individuals to investigate the impact of methodological choices. We scrutinized prevalent methodological choices and assessed how consistently they were reported in published papers. We extracted amplitudes and latencies from reported TEPs and compared total cortical activation and specific TEP peaks and components. Results Reporting of methodological details was overall sufficient, but some relevant information regarding the TMS settings and the recording and pre-processing of EEG data were missing in more than 25% of the included experiments. The published TEP latencies and amplitudes confirm the ’prototypical’ TEP waveform of M1, comprising distinct N15, P30, N45, P60, N100, and P180 peaks. However, variations in amplitude and latencies were evident across studies. Higher stimulation intensities were associated with overall larger TEP amplitudes. Active noise masking during TMS generally resulted in lower TEP amplitudes compared to no or passive masking but did not specifically impact those TEP peaks linked to long-latency sensory processing. Studies implementing independent component analysis (ICA) for artifact removal generally reported lower TEP amplitudes. Conclusion Some aspects of reporting practices could be improved in TEP studies to enable replication. Methodological choices, including TMS intensity and the use of noise masking or ICA, introduce systematic differences in reported TEP amplitudes. Further investigation into the significance of these and other methodological factors and their interactions is warranted.