Importance CMS/DRL for EEG
The CMS is the reference channel, compared to which all the EEG signals are measured.
The DRL is responsible for bringing the potential of the subject as close as possible to the "zero" of the electrical system. The DRL/CMS contact is very important for high-quality measurements, because it reduces the noise, mainly the 50Hz, and the drift of the EEG signals.
It is very important that the electrodes that work as a reference system are in the correct position to obtain a good EEG signal and optimum impedance values (between 0-10 kΩ). Being in a correct position means that the contact surface of the electrodes really has contact with the patient skin (either for the case in which they are positioned in the ear lobe (EARCLIP electrodes) or in the mastoid (STICKTRODE electrodes)). To achieve this, we recommend the Sticktrode since they already contain gel and are adhesives (they are very easily fixed). In case you are using Earclips, make sure that the contact surface contains gel and that they are completely embedded in the ear lobe. .
Importance CMS/DRL stimulation
In theory, the sum of the intensities of the electrodes that work only for stimulation/return must be 0 so that all the current that comes in comes out. In reality, this does not happen, leaving a minimum percentage, so we use the DRL electrode. This electrode makes sure that the current that remains to exit (practically null) comes out. That is, it works like a return electrode. For this reason, it needs a good impedance, which in the end is a good contact. For stimulation channels, the CMS/DRL contact is used to perform the impedance check of the electrodes.
You can find more information about the electrodes below:
For more information about how to place the electrodes correctly, visit the following videos:
sticktrodes: https://www.youtube.com/watch?v=8PcOkocLg8k