No electrode named 'Ref2' in 10_20 system nomenclature

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1

I am trying to compute the alpha band power by using the compute_psd() method, however, the channel locations are not available for it. I used the built-in montage’s 10_20 system to label the channel locations, but my eeg file channel includes a channel named 'Ref2’which is basically the reference electrode, and that is not included in the standard nomenclature. With the help of my limited knowledge and a little bit of internet study, I have come to the conclusion, that the reference electrodes are generally placed on the lobes and the ear bones…but I am confused about the electrode which I should rename as the ‘Ref 2’ electrode. I am working with mne version 1.4.2 in Pycharm

Here is the code for naming the channels:

channel_loc = mne.channels.make_standard_montage("standard_1020")
print(channel_loc)
channel_loc.plot(show_names=True,sphere='eeglab')

keep_channels = ['Fp1', 'Fp2', 'F7', 'F3', 'Fz', 'F4', 'F8', 'T7', 'C3', 'Cz', 'C4', 'T8', 'P7', 'P3', 'Pz', 'P4', 'P8', 'O1', 'O2', 'EOG1', 'EOG2', 'EOG3', 'Ref2']
ind = [i for (i, channel) in enumerate(channel_loc.ch_names) if channel in keep_channels]
loc_new = channel_loc.copy()
loc_new.ch_names = [channel_loc.ch_names[x] for x in ind]
keep_channel_info = [channel_loc.dig[x+3] for x in ind]

loc_new.dig = channel_loc.dig[0:3]+keep_channel_info
channel_loc.plot(

eeg_notch.compute_psd(fmin=8.5,fmax=12.5).plot();#computing Alpha power frequency

calculating and plotting the above code was giving me this error: Channel locations not available. Disabling spatial colours

hence, I wanted to name the channel locations, and would really appreciate some help regarding this.

2

Reference electrodes are not always placed on the lobes (A1, A2) or on the earbones (M1, M2). They can also be placed on the center line, many system have them in CPz, Fz, AFz, Iz locations. You first need to figure out where exactly your reference was placed on the participant head (it usually depends on the system used to measure the EEG) and you can then rename this channel with raw.rename_channels({"Ref2": "NAME"}).

Mathieu

2 Likes
3

Hi, according to the reference channel diagram, the ref. electrodes are placed on the bony points behind the ears, I can now rename them accordingly! Thank you very much.