this is an excellent question and observation you’re reporting here.
I recently worked on some montage-related stuff in the MNE code, and it turned out that even in the core development team, there was (and probably still is, to some extent) quite some confusion or at least surprise in regards to how we deal with those “standard montages”.
Here is the gist:
Choice of built-in montages
standard_* montages are based on idealized electrode placement but fitted to the
fsaverage head. Use these montages if you intend to perform source localization.
- Other montages, like
biosemi64 in your example, are template montages provided by EEG cap manufacturers (in this case, Biosemi) and are usually placed on a perfect sphere (i.e., not on an actual or average head).
2D-visualization of montages
Because we store 3D coordinates for each EEG electrode, visualizing a montage in 2D is complicated, as there is not just one way to do it.
By default, MNE makes it such that the head circle represents the plane that goes through Nasion and Inion. This is why in the plot for your Biosemi montage, all electrodes are placed inside the head circle – which is a bit of an uncommon sight, at least for users coming from other EEG toolboxes like EEGLAB.
I believe the reason why this seems to be different for
standard_1020 is that the electrodes there are placed on
fsaverage. So be careful when directly comparing
standard_* with the other built-in montages.
In MNE-Python 1.1 (to be released very soon), in all functions that produce topographic plots – including
montage.plot() – you will be able to pass the parameter
sphere='eeglab', which will make it such that the head circle is on the plane through Fpz and Oz. Please see the updated tutorial here:
Taken from this tutorial, this is an EEG montage plotted with the default settings:
And here with
Again, this will only become available with MNE-Python 1.1.
I hope this helped clarify things a little bit!