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Technical tips for dense array EEG-TMS

Transcranial Magnetic Stimulation (TMS) in combination with EEG is becoming increasingly popular as researchers seek to monitor the effects of TMS.
Here we discuss the best techniques to maximize both the quality of the TMS and the dense array EEG during a session when the two data types are recorded concurrently.
First, for EEG-TMS, it is best to use low profile electrodes, such as in EGI’s MicroCel Geodesic Sensor Net. This allows the TMS coil to be as close as possible to the head when the participant also has EEG electrodes applied. These Nets use Redux gel for shorter recordings, of around 2 hours, or Elefix gel for longer recordings, providing you with more  than 2 hours for your EEG-TMS study.
Second, select the sampling rate option of “1000 Hz s/s Fast Recovery”. Available from Net Station 5.3 onwards, this sample rate allows for the EEG filters to recover quickly from TMS induced artifact, therefore minimizing the length of time that your EEG is masked. This is achieved by switching the anti-aliasing filter of the NA 400 series amplifier from being software driven, as with all other sample rates, to being hardware controlled.
The amplifier’s analog-to-digital converter (ADC) built-in hardware anti-alias filter settles within three samples (3 ms at 1000 samples/sec) after the TMS pulse, allowing you to immediately view and record the real-time effects on EEG after the TMS stimulation. There is a cost to this advantage however, and this change will reduce the effective bandwidth of your recording. Rather than the resolvable bandwidth being the traditional ½ the sampling rate, it will be roughly ¼ the sampling rate instead. This means that for the “1000 Hz s/s Fast Recover” sampling rate option, the highest analyzable frequency is 250 Hz.
Third, maintain as low an impedance as possible ( ≤10 kOhm) for EEG-TMS recordings.  Having impedances at or below 10 kOhms in the EEG-TMS recording environment reduces the capacitive effect (charging) of the electrolyte-skin interface.  High charge at the electrolyte-skin interface appears as artifact in the EEG (and is induced by the TMS stimulation). The higher the charge, the longer it takes to dissipate. Once the charge is dissipated, the EEG can be seen again. Therefore, keeping the impedances low is also important for fast recovery from the TMS induced artifact. 
You might notice that this is different than EGI's recommendation for normal EEG recording environments. Since the NA 400 has a high input impedance of ≥ 1.0 GΩ, high quality data can be collected with impedances near 100 kOhms in normal recording environments.
To get impedances in this range, you will need to prepare the skin before placing the Net. For example, you can mix some NuPrep (skin exfoliant used in clinical EEG) with the baby shampoo and wash the patient's head to gently exfoliate dead skin and oils from the scalp. Rinse the head and hair thoroughly to remove any residue from the NePrep. Other skin preparation methods include brushing the scalp to remove dead skin and then washing the head with dandruff shampoo. Your  patient's head and hair should be dry before applying the Net, so you'll want to keep a hair dryer handy.
Please make sure to remember that the skin preparation method you choose should not break the skin. The EGI cleaning protocol allows for disinfection of the Nets, but it is not a high level disinfection or sterilization, and therefore not sufficient to render an electrode safe after contact with blood.
After the data is collected, several other things to consider include that most concurrent EEG-TMS studies with the goal of evaluating an Event-Related Potential (ERP) or Evoked Potential (EP) record 200 or more trials in order to achieve a robust trial average.  If you are replicating an EEG-TMS study, keep in mind that the recording reference in the Geodesic Sensor Net is the Cz/Vertex electrode, and you may therefore need to re-montage the data to compare your results with findings in other studies.
Finally, just like with any experiment, it’s good to have a method for validating your technique. The motor evoked potential (MEP) can be a great way to do this, since it is well described in the literature and therefore the latency of your MEP can be used as feedback on how successfully your data was cleaned and processed. Since we know the filter recovery after the stimulation artifact will affect the latency of when the evoked potential is seen in the EEG, you can assess whether the resulting average waveform of all your trials reveals the MEP at 40 ms or earlier.  

Additional Info

  • Product Type: GES 400
  • Information Type: Application Advice
Last modified on Friday, 05 March 2021 16:53

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