CMS/DRL Location relative to Reference

[Stefan.Berteau]

I apologize if this has an obvious answer that I have simply missed, but in educating myself on the difference between CMS/DRL and the Reference, I read the FAQ page on CMS/DRL (unfortunately I cannot include a link since this is my first post, but it is currently the top Google result for the query "CMS DRL Reference"). It seems to imply that the CMS electrode should be located near the reference, by saying "When the CMS electrode is located near Cz (which it usually is) the extra subtraction step in the example above (choosing Cz as a Reference) won't have much influence on the EEG part of the signals. However it will decrease the CM interference part of the signals."

I am using an averaged mastoid reference, for purposes of comparison with a previously published work. My first thought was that this meant the CMS electrode (and possibly the DRL as well?) should be located next to one the mastoid reference electrodes in order to prevent the extra subtraction step from influencing the EEG portion of the signal.

Then, however, I read other posts on this forum stating that CMS/DRL placement does not affect the signals measured between the other electrodes. I am having trouble reconciling this with the above sentence from the FAQ, and am unsure how to proceed. Any clarification would be greatly appreciated.

Regards,
Stefan

[Coen]

It is correct that the locations of the CMS and DRL do not affect the voltages measured between the other electrodes. The CMS is best located in the center of all other electrodes (not necessarily close to the reference), to achieve a minimal Common Mode voltage. The DRL can be located anywhere (but not to close the the CMS).

Best regards, Coen (BioSemi)

[Stefan.Berteau]

Thanks for the fast response, and for clearing that up!

- Stefan