Characterizing and minimizing the contribution of sensory inputs to TMS-evoked potentials

• Published in: Brain stimulation Vol. 12; no. 6; pp. 1537 - 1552
• Main Authors: Biabani, Mana, Fornito, Alex, Mutanen, Tuomas P., Morrow, James, Rogasch, Nigel C.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.11.2019
• Subjects: Adolescent; Adult; Control condition; Electroencephalography - methods; Evoked Potentials, Motor - physiology; Female; Healthy Volunteers; Humans; Male; Motor Cortex - physiology; Offline filter; PEPs; Peripherally-evoked; Potentials; SSP-SIR; TEPs; TMS; TMS-Evoked potential; Transcranial Magnetic Stimulation - methods; Young Adult; Peripherally-evoked; Offline filter; TEPs; Potentials; Control condition; TMS; SSP-SIR; TMS-Evoked potential; PEPs
• ISSN: 1935-861X; 1876-4754
• DOI: 10.1016/j.brs.2019.07.009

Transcranial magnetic stimulation (TMS) evokes voltage deflections in electroencephalographic (EEG) recordings, known as TMS-evoked potentials (TEPs), which are increasingly used to study brain dynamics. However, the extent to which TEPs reflect activity directly evoked by magnetic rather than sensory stimulation is unclear. To characterize and minimize the contribution of sensory inputs to TEPs. Twenty-four healthy participants received TMS over the motor cortex using two different intensities (below and above cortical motor threshold) and waveforms (monophasic, biphasic). TMS was also applied over the shoulder as a multisensory control condition. Common sensory attenuation measures, including coil padding and noise masking, were adopted. We examined spatiotemporal relationships between the EEG responses to the scalp and shoulder stimulations at sensor and source levels. Furthermore, we compared three different filters (independent component analysis, signal-space projection with source informed reconstruction (SSP-SIR) and linear regression) designed to attenuate the impact of sensory inputs on TEPs. The responses to the scalp and shoulder stimulations were correlated in both temporal and spatial domains, especially after ∼60 ms, regardless of the intensity and stimuli waveform. Among the three filters, SSP-SIR showed the best trade-off between removing sensoryrelated signals while preserving data not related to the control condition. The findings demonstrate that TEPs elicited by motor cortex TMS reflect a combination of transcranially and peripherally evoked brain responses despite adopting sensory attenuation methods during experiments, thereby highlighting the importance of adopting sensory control conditions in TMS-EEG studies. Offline filters may help to isolate the transcranial component of the TEP from its peripheral component, but only if these components express different spatiotemporal patterns. More realistic control conditions may help to improve the characterization and attenuation of sensory inputs to TEPs, especially in early responses. •EEG responses to TMS over M1 (TEPs) and shoulder (PEPs) were compared.•Long latency components (>∼60 ms) were correlated between TEPs and PEPs.•Changing TMS intensities and waveforms did not alter TEP-PEP relationships.•Offline filters showed promise for isolating TMS-evoked neural activity.•More realistic control conditions may lead to more accurate comparisons.