TMS is a magnetic method to non-invasively stimulate the brain. This technology has been used to investigate the connectivity between the motor cortex and corresponding muscle groups for patients with brain damage.
In addition, TMS can inhibit or excite the certain areas of the cortex and has been applied to induce neuroplasticity for therapeutic purposes. Several review and analysis articles have argued that this brain stimulation will be used by more researchers and therapists.
TMS has also been combined with Electroencephalogram (EEG) for motor rehabilitation and other clinical research applications. Therefore, we tested different complete EEG systems with a TMS device from a German company (MAG & More, Munich) for compatibility and functionality.
Test with a dummy
g.GAMMAcap was put on a dummy, a watermelon that’s about the same size as a human head, to test the durability of the entire EEG system, including testing safety up to the maximum power and duration possible for the TMS system.
Picture 1 and 2: Positioning of TMS coil for this test. The magnetic stimulation coil was placed over electrode Cz.
The tests were performed with g.USBamp and with g.HIamp using passive and active g.LADYbird electrodes as well as with the wireless amplifier, g.Nautilus with active electrodes. Biphasic single pulses, repetitive, and theta-burst stimulation were applied to the dummy model.
TMS artefact is one of main concerns from EEG users, because it causes huge unwanted signals for a long period of time and may damage electrodes and amplifiers. Therefore, the duration of stimulation artefact was calculated after averaging the EEG, and the results are summarized in the Table 1 below.
|Power: 30%||0.5 [ms]||6 [ms]||1 [ms]||5 [ms]|
|Power: 100%||1 [ms]||10 [ms]||1.5 [ms]||7 [ms]|
Table 1. Duration of TMS artefact after averaging EEG (sampled at 38.4kHz) in milliseconds.
Active electrodes showed longer artefact duration than passive electrodes due to the characteristics of the preamplifier. Higher TMS system power (100%) also caused longer artefacts than lower power. But the artefact duration is very small in all cases.
The artefacts produced through the g.Nautilus (wireless amplifier) with a g.LADYbird (active electrode) were also measured at its maximum sampling rate, 500 Hz. The temperature of one active electrode was recorded before and immediately after running the stimulation sequence with the highest energy output of the system. One g.LADYbird electrode was positioned as shown in Figure 1 and was magnetically stimulated at the maximum power of 100% until the system shut down because of the magnetic coil overheated (about 1.5 minutes). The temperature went up only about 1°C after maximum power at the maximum duration the electrode was not damaged and was thus used afterward for other measurements.
Test with healthy subjects
Single pulse TMS was also applied to a healthy subject to induce motor evoked potentials (MEP) of the thumb muscle (abductor pollicis brevis). The TTL trigger output of the TMS device was connected to the digital input of g.USBamp and the time-stamp of the magnetic stimulation was recorded using a g.HIsys Simulink model as seen in Figure 2.
Figure 1. g.USBamp Simulink Highspeed Online Processing model in Simulink to acquire EEG data and the time point of the TMS stimulation. The model acquires the data with double precision with 38.4 kHz, downsamples the data to 4.800 Hz and stores/visualizes the EEG data and TTL triggers.
The data of one subject is shown in Figure 3. The MEP was already produced at 50% power.
Figure 2. Motor evoked potentials for 16 EEG electrodes and 1 EMG channel to record the finger movement (most right EP). The TMS coil was positioned above electrode C3. The red vertical line indicates the on-set of the TMS pulse. A bigger artifact is shown in electrodes close to the TMS coil. The TMS artifact itself is just a few ms long, and the MEP can be seen afterwards.
The measurements showed that it is possible to use a TMS coil very close to active and passive electrodes acquired with g.USBamp, g.HIamp and g.Nautilus. If the TMS coil pulse is applied to the g.Nautilus device itself, then the device shuts off to avoid any damage. Passive electrodes show a shorter artifact then active electrodes, but in both cases the artifact is short. If an EEG signal is required within 1.5 ms, then passive electrodes should be used.