[Mne_analysis] Lead field spikes using sample BEM - bug or explainable?

[Eric Larson]

        External Email - Use Caution        

>
> The curious thing is that, when I view the co-registration with the
> 'head-dense' surface shown, it appears as though there are no collisions
> with the outer skin surface.
>

The BEM is made from a smoothed, downsampled version of this surface. If
you use `plot_alignment` and use `surfaces='head'` or
`surfaces='outer_skin'` it might show you that one instead.


> Do you know of a decent way to detect any sensor locations that are inside
> the outer skin BEM surface to avoid problems like this in the future?
>

You can use some private functions/classes
<https://github.com/mne-tools/mne-python/blob/master/mne/surface.py#L546-L590>
from `mne.surface` for this, with the caveat that the API is not guaranteed
(subject to change at any time without maintaining backward compatibility).


> Does the outer skin surface include things like the ears?
>

Yes it will but these will be smoothed in some fashion for the BEM.

Eric
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