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All procedures involving human participants have been performed in compliance with the institutional, national, and international guidelines for human welfare and have been reviewed by the local institutional review board.
- Finding the motor "hotspot" and determination of the resting motor threshold
- Find the motor "hotspot" (i.e. the stimulation location over which single-pulse TMS elicits well-shaped MEPs of a comparably consistent amplitude across trials) and save the corresponding coil position (including coil orientation and angulation) in the neuronavigation system.
- Find the resting motor threshold (RMT) by applying single TMS pulses over the motor cortex at gradually increasing stimulation intensities until the elicited MEPs have peak-to-peak amplitudes greater than 50 µV in more than 50% of the trials.
- If available, use an automated script for parameter estimation by sequential testing (PEST), for instance, following a maximum likelihood strategy which also provides an online estimate of the confidence interval of RMT based on the observed variability of single responses and which typically requires ca. 30 test pulses of adaptively varying intensity to obtain a robust RMT estimate.
- If this is not the first experimental session, compare the coil position with the previous position and compare the obtained RMT with the previous RMT to validate consistency.
- If required, determine stimulation intensities for the active motor threshold (AMT) or for the 1-mV peak-to-peak MEP amplitude using standard procedures.
- Final participant preparation
- Optionally, immobilize the head of the subject using a vacuum pillow.
- Optionally, deliver a masking noise through earplugs (when planning to analyze TMS-evoked EEG potentials). Otherwise, provide the subject with earplugs and headphones for hearing protection.
- Optionally, align and fix the coil at the desired position using a mechanical arm.
- Pre-experiment data quality validation
- Check that the real-time processor is receiving data from the EEG system.
- Check the signal obtained from the desired EEG spatial filter (e.g., C3-centered Laplacian montage) for obvious artifacts.
- Visually confirm the EEG signal quality, check for bad electrodes, excessive line noise, and muscle artifacts, and adjust the time window and amplitude scaling on the EEG system software for ongoing visual inspection during the experiment.
- Main experimental session
- Unless the stimulator intensity is remote-controlled in the experimental script, manually set the stimulation intensity to the desired value (e.g., 110% of the RMT).
- Start the experimental script to apply pulses at different phases of the target oscillation in a randomized order.
- During the experiment, monitor the trigger condition thresholds (artifact-detection threshold, pre-innervation threshold, minimum power, etc.).
NOTE: Stimuli will be triggered at irregular intervals, as the real-time processor is waiting for the trigger conditions to occur. However, the conditions should be set such that most stimuli occur within a predictable interval (e.g., 2-3 s after the previous pulse), and long pauses (e.g., in this case, >5 s) are avoided as these would lead to larger evoked responses due to novelty.- Alternatively, use post hoc stratification to remove trials following overly long intervals.
- To achieve sufficient statistical power to differentiate phase-specific stimulation effects, acquire a sufficient number of trials
NOTE: We typically chose 80-120 interleaved trials per condition. - Document the start and end times of the various sessions and keep a record of any unusual occurrences.