Channel position unit conversion

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1

Hello,

I have a dataset were the channel positions are given in millimeters instead of meters. Is there a quick fix for such cases or is it necessary to perform an ugly hack:

for ch in range(len(raw.info['chs'])):
    raw.info['chs'][ch]['loc']*=1e-3

followed by

montage = raw.get_montage()
raw.set_montage(montage)

EDIT: I thought I needed to convert units for interpolate_bads due to the warning Estimated head size (85000.0 mm) exceeded 99th percentile for adult head size. But it seems that the interpolation is not affected by the scale.

2

Yes, this is to be expected, as the underlying mathematical function performs the exact same thing, just scaled by a factor of 1000 when it comes to distances :slight_smile: Think of a recipe for cookies; the recipe says to use a certain amount of ingredients to produce 100 cookies, but you recon you’ll only want to eat 50. So you simply take 50% of the volume / mass of all listed ingredients. :slight_smile: But the produced cookies are just the same :cookie:

3

I would create a proper transformation and apply it to the montage. Something like this:

import numpy as np
mm_to_m_trans = np.eye(4)
mm_to_m_trans[:3, :3] *= 1e-3
mm_to_m_trans = mne.Transform(
    fro='head',
    to='head',
    trans=mm_to_m_trans
)

montage = raw.get_montage()
montage_scaled = montage.copy().apply_trans(mm_to_m_trans)
raw.set_montage(montage_scaled)
1 Like
4

That’s a neat solution.

1 Like