Wrong alignment between layout biosemi64 and MRI template

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Dear mne users,

I am analyzing an EEG dataset collected with biosemi (64 channels). I don't have the digitized position of the sensors. I want to do source reconstruction using the biosemi layout and the anatomical template ('fsaverage'). However, when I try to plot the alignment between the EEG electrodes position and the MRI, I get a mismatch between between the nasion positions and inaccurate sensor positions (especially the frontal channels look inside the head space).

Here the code I am using:

montage = mne.channels.make_standard_montage('biosemi64')

raw.set_montage(montage)

# mne.viz.plot_montage(montage)

# Download fsaverage files

fs_dir = fetch_fsaverage(verbose=True)

subjects_dir = op.dirname(fs_dir)

# The files live in:

subject = 'fsaverage'

trans = 'fsaverage' # MNE has a built-in fsaverage transformation

src = op.join(fs_dir, 'bem', 'fsaverage-ico-5-src.fif')

bem = op.join(fs_dir, 'bem', 'fsaverage-5120-5120-5120-bem-sol.fif')

# Check that the locations of EEG electrodes is correct with respect to MRI

mne.viz.plot_alignment(

    raw.info, src=src, eeg=['original','projected'], trans=trans,

    show_axes=True, mri_fiducials=True, dig='fiducials')

And the result I obtain:

[cid:de90d7c0-cf67-4539-8dc7-daf0a831ca1a]

Does anyone have any suggestion on what am I doing wrong?

Thanks!

Silvia
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your plot looks good to me

Alex
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Thanks for your quick response!

So isn't it a problem that the big green sphere (the should be the digitized nasion), does not match with the small green sphere (mri fiducial)? LPA and RPA are matching.

Thanks again!

Silvia

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no it's not a problem.

your electrodes are well positioned. It's all what matters.

Alex
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