For BioSemi EEG systems you have to re-reference the EEG signal via subtracting one mastoid channel “EXG5” from the other “EXG6”. I intend to do this via the mne.channels.combine_channels function, but I can’t quite work out how to write a lambda function to accomplish the subtraction of one channel from the other. Apologies for the seemingly easy question, but any advice would be really useful!
@julesneuro subtracting one channel from another will not have any impact on your remaining channels, because they are not connected in any way. All you’ll end up with is a new channel that is the difference of EXG5 and EXG6. For that, the method that Richard proposed will work, regardless of the channel type.
Perhaps you intended to afterwards re-reference all your remaining channels with respect to the newly created “subtraction channel” (EXG5 - EXG6)?
Note that re-referencing EEG channels according to some EOG channel (subtracted or not) does not really make sense, because the EOG is a different kind of signal (dipoles of the eyeballs, not brain activity)