TMS and EEG recordings are used more and more in neuroscience research and clinical settings. The technical requirements are pretty demanding, but if they are handled correctly; new scientific experiments can be carried out and new results can be found. But before we dive into it, let’s talk basics.
Transcranial Magnetic Stimulation (TMS) is used to magnetically stimulate certain regions of the brain to study and treat the patient. This is often combined with EEG recordings to see the reaction of the brain to the magnetic stimulation. A combination of TMS and BCI technology can be used e.g. in neurorehabilitation settings.
You can use either a series of single TMS pulses and measure the TMS induced evoked potentials or you can do repetitive TMS and look for EEG and ERP changes after the treatment or for diagnostic purposes.
So called TMS coils are placed over certain brain regions that should be stimulated and then the magnetic field is applied. At the same time EEG electrodes are recording the brain function of the human brain with biosignal amplifiers. Then the data is transmitted to the computer and is visualized and stored for later analysis or it is analyzed in real-time.
For the EEG recordings it is important to use pretty flat EEG electrodes in order to enable the TMS coil to be as close as possible to the head in order to maximize the magnetic pulse to a certain brain region. Typically, the magnetic fields cause some stimulation artifacts in the EEG data that can be minimized with proper filters and signal processing techniques in the EEG system.
Very important is the proper positioning of the TMS coil over a certain brain region. First of all, the magnetic field has to stimulate the region and secondly it must be ensured that the coil stays at the proper position. Therefore, the users are looking e.g. at motor responses to see if the magnetic pulse is hitting a motor region or people are investigating EEG responses to understand if the magnetic stimulation is effective. The coil needs to be placed:
A further important consideration is the sampling frequency that the recording should have.
You can use the wireless g.Nautilus EEG headsets, the g.USBamp or g.HIamp amplifiers for the EEG recordings. Here is a list of g.tec’s EEG amplifiers that can be configured in the online product configurator and that can be used for simultaneous TMS recordings:
Depending on your application you can chose the proper equipment. The g.Nautilus wearable EEG headsets can sample up to 500 Hz, while g.USBamp and g.HIamp biosignal amplifiers can sample up to 38.4 kHz and this determines the highest EEG signal components that can be recorded. For short lasting evoked potentials, g.USBamp and g.HIamp are required. All of g.tec’s biosignal amplifiers allow EEG/EMG/ECG recordings with passive or active EEG electrodes.
Passive electrodes don’t have a pre-amplifier inside the electrode and therefore require abrasive gel to bring the electrode impedance down. The advantage is that the TMS stimulation artifact is smaller because the electrode is just smaller.
If active electrodes like g.LADYbird active or g.SCARABEO active EEG are used, then the pre-amplifier is inside the electrode and this allows a quick assembling. But the electrodes are a bigger and therefore the TMS artifact is longer.
The wireless g.Nautilus on the other hand, is fixed on the EEG cap on the subject’s head and therefore much closer to the TMS coil. If the TMS coils is activated close to the g.Nautilus wireless EEG headset, the device is switched off for security reasons
The TMS coil is positioned e.g. over the left sensorimotor cortex to produce a right finger movement like illustrated below. Then, the TMS pulse is switched on and at the same time the EEG is recorded from 16 positions according to the international 10/20 system. In this case, the TMS coil is just positioned over electrode C3 to produce the left finger movement. The blue line shows the EP below the motor threshold and we don’t observe a finger movement, but we can see the reaction in the EEG data. The purple line shows the EP for a TMS pulse above the motor threshold where a finger movement in the patient can be seen.
The black rectangle shows, when the TMS pulse is applied. Typically, the EEG signal shows a huge artefact around and after the TMS pulse. Electrode C3 shows the highest TMS artifact because the coil is closest to it. All the other electrodes will show a smaller artifact that is logarithmical reduced with the distance to the coil.
One trick is to record mostly from EEG electrodes that are far away from the coil to get artifact free data. Otherwise, it is possible to perform e.g. source derivations to kill the artifact.
Another trick is to use the right hardware: g.HIamp EEG amplifier has a special 64 Channel Passive Electrode Connector Box TMS with highly effective TMS filters inside that kill most of the magnetic artifact before the artifact is recorded by the system. This is important because the TMS pulse has pretty high frequencies and this can produce aliasing artifacts when the EEG signal is sampled. Together with g.LADYbird passive EEG electrodes, the stimulation artifact is shorter than 1 ms (compared to g.SCARABEO active EEG electrode system where the stimulation artifact is shorter than 4 ms).
Nevertheless, the magnetic pulse is so big that it will overlay the EEG signal. So, what else can you do?
g.BSanalyze Offline Biosignal Analysis software has a special artifact removal algorithm that can kill such transient events to be able to record artifact free data after a very short interval. To make the algorithm effective, it is important to sample the data with 19.2 kHz. The EEG electrodes are measured against the ground electrode on the forehead. Then a common average reference is calculated, a baseline correction is performed, the trial mean is calculated and the artifact is removed.
Generally, it helps to reduce the impedance as much as possible because then the magnetic pulse artefact is smaller.
Check out the online product configurator to get an EEG amplifier, suitable EEG electrodes and a software environment:
g.tec offers a very powerful software: g.HIsys Professional is a rapid prototyping environment to quickly realize new experiments. The software environment has dedicated Simulink models for TMS recordings that include all necessary steps to record clean EEG data while TMS is applied. The model reads in the data from 64-256 EEG channels with the g.HIamp block. Then a CAR source derivation is calculated to remove common mode noise like power line interference and a highpass and lowpass filter is applied. In the TMS processing block, evoked potentials are calculated and the EP blocks show the EPs.
The TMS block allows to specify EOG channels to remove ocular artifacts, to specify the EP length and the pre-stimulus interval. Furthermore, an artifact rejection can be activated to remove trials with too much contamination.
g.Recorder Professional has automatic TMS artifact rejection and reduction tools included and it does not require MATLAB for the recording.