Handling duplicate electrode names in 10–20 system

[scott-huberty]

I would expect this to run without error out of the box but it doesn't:

import mne
import numpy as np

standard_montage = mne.channels.make_standard_montage("standard_1020")
data = np.random.randn(len(standard_montage.ch_names))
info = mne.create_info(ch_names=standard_montage.ch_names, sfreq=1000, ch_types="eeg")
info.set_montage(standard_montage)
mne.viz.plot_topomap(data, pos=info)

File ~/devel/repos/mne-python/mne/channels/layout.py:974, in _auto_topomap_coords(info, picks, ignore_overlap, to_sphere, sphere)
    968 if len(locs3d) > 1 and np.min(dist) < 1e-10 and not ignore_overlap:
    969     problematic_electrodes = [
    970         chs[elec_i]["ch_name"]
    971         for elec_i in squareform(dist < 1e-10).any(axis=0).nonzero()[0]
    972     ]
--> 974     raise ValueError(
    975         "The following electrodes have overlapping positions,"
    976         " which causes problems during visualization:\n"
    977         + ", ".join(problematic_electrodes)
    978     )
    980 if to_sphere:
    981     # translate to sphere origin, transform/flatten Z, translate back
    982     locs3d -= sphere[:3]

ValueError: The following electrodes have overlapping positions, which causes problems during visualization:
T7, T8, P7, P8, T3, T5, T4, T6

My understanding is that there are essentially two flavors of the 10–20 system (ref) that differ slightly in electrode naming:

the MCN system renames four electrodes of the 10–20 system:
T3 is now T7
T4 is now T8
T5 is now P7
T6 is now P8

MNE’s standard_1020 montage includes both sets of names, which is usually fine, because acquisition systems will stick to one convention or the other.

But when I used MNE's interpolate_to function to map an EGI net to the 10-20 montage, I ended up with both versions (e.g. T3 and T7), which triggered the same error I demonstrated above.

After speaking with @britta-wstnr @drammock and @larsoner , we agreed that in this specific case, it would be helpful to raise a clearer error message, something like:

"You have duplicate electrodes (T3, T7), please use drop_channels to drop one of them"

[britta-wstnr]

I might even advocate for keeping things simpler and telling users directly which four channels to drop (i.e., we decide on the set). On the odd chance that a user wants one specific set, they will probably already understand what's going on anyway.