This article details a method for conducting real-time EEG-synchronized transcranial magnetic stimulation (TMS) experiments. The process involves monitoring brain activity and muscle responses to understand cortical excitability.
Begin with a human participant wearing an EEG cap, secured with a mesh cap.
EMG electrodes were placed on the abductor pollicis brevis muscle for signal acquisition.
Use a reflective head tracker to monitor head position in real-time.
Next, position the transcranial magnetic stimulation or TMS coil over the motor cortex and deliver magnetic pulses to activate cortical neurons.
This activation evokes muscle contractions, captured as motor-evoked potentials or MEPs by the EMG electrodes.
Identify the cortical site producing the strongest MEPs and fix the coil in place.
Adjust the stimulation intensity to determine the motor threshold--the lowest intensity that reliably evokes MEPs.
Then, set the system to trigger TMS during the negative phase of the EEG signal, when cortical excitability is highest.
Initiate the EEG-synchronized TMS sequence.
EEG electrodes record brain activity, while a real-time processor detects negative phases and triggers TMS, producing consistent muscle responses.
To perform a realtime EEG-synchronized TMS experiment first determine the exact location where the TMS of the motor cortex evokes the strongest motor response from the hand muscles. Then mark this hotspot and coil position in the neuro-navigation software. Next fix the head of the subject with a vacuum pillow and fix the coil in the hotspot location with a mechanical arm.
To determine the threshold stimulation intensity gradually adjust the stimulation intensity until 50% of the TMS pulses result in a motor response. Here the intensity has been set to 110% of the threshold intensity. To configure the realtime system to combine multiple EEG channels to extract a specific oscillation, use a five channel laplacian spatial filter centered on electrode C3 to extract sensory motor rhythm.
To trigger TMS at either the positive or negative peak of this oscillation, set the phase trigger condition to phase zero or phase pi randomly for each trial before arming the realtime device and setting the sequence to be repeated on a loop every two seconds. Then run the experiment for about 10 minutes to acquire a sufficient number of trials to differentiate phase specific stimulation effects. During the experiment the coil position will be monitored on the neuro-navigation system and the EEG and EMG signals will be monitored on the EEG system.
The raw data as well as pre-stimulus EEG and the post-stimulus muscle response for each condition are also displayed on the EEG system. The realtime device will perform spatial filtering to target the brain region of interest and band pass filtering to isolate the oscillation of interest, estimating the instantaneous amplitude and phase using autoregressive forward prediction and the Hilbert transform. This signal is then compared to the trigger condition.
If the power threshold and phase conditions are met the stimulator is triggered. Using the displayed online running averages, the accuracy of the phase targeting and the effect of phase on muscle response can be estimated during the experiment.
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