Changes in Cortical Activation by Transcranial Magnetic Stimulation Due to Coil Rotation Are Not Attributable to...

Area of Science:

• Neuroscience
• Biomedical Engineering
• Signal Processing

Background:

• Transcranial magnetic stimulation combined with electroencephalography (TMS-EEG) is vital for investigating brain dynamics in various conditions.
• Cranial muscle activation during TMS-EEG can obscure true neural signals, necessitating advanced preprocessing.
• The impact of coil rotation, a method to minimize muscle artifacts, on TMS-EEG signal integrity remains unclear.

Purpose of the Study:

• To compare TMS-EEG data acquired using a standard coil orientation versus an orientation optimized to minimize muscle activation.
• To evaluate the effects of coil rotation on key TMS-EEG metrics: TMS-evoked potentials (TEPs), TMS-related spectral perturbation (TRSP), and intertrial phase clustering (ITPC).
• To assess the consistency of these effects across two distinct preprocessing pipelines (ICA and SSP-SIR).

Main Methods:

• Two coil orientations were used: standard (M1_standard) for optimal motor evoked potentials and rotated (M1_emg) to reduce electromyography (EMG) artifacts.
• TEPs, TRSP, and ITPC were computed for both conditions.
• Data were processed using independent component analysis (ICA) and signal-space projection with source-informed reconstruction (SSP-SIR) pipelines.
• Concordance correlation coefficient (CCC) was used to quantify signal similarity between conditions.

Main Results:

• Significant reductions in TEPs, TRSP, and ITPC were observed in the M1_emg condition compared to M1_standard.
• A lower-than-expected concordance correlation coefficient (CCC) indicated notable differences between the two conditions.
• These findings were consistent across both ICA and SSP-SIR preprocessing pipelines.

Conclusions:

• While rotating the TMS coil can effectively reduce cranial muscle artifacts, it significantly alters the measured TMS-EEG signals.
• Researchers must carefully consider the trade-offs and tailor coil positioning strategies to their specific experimental goals and preprocessing methods.
• The choice of coil orientation impacts the interpretation of brain dynamics derived from TMS-EEG.