[Mne_analysis] mne.create_info: Differences in info[ 'dig'] and info['chs']

[Eric Larson]

        External Email - Use Caution        

info['dig'] can have more than just channel positions. The first three
inf['dig'][ii]['kind'] values are fiducials (FIFFV_POINT_CARDINAL == 1) and
the rest are EEG (FIFFV_POINT_EEG == 3). All ch['kind'] values should be
EEG (FIFFV_EEG_CH == 2).

Eric


On Tue, Sep 18, 2018 at 2:48 PM Gladia Hotan <gladiach at gmail.com> wrote:

>         External Email - Use Caution
>
> Hi,
>
> I'm experimenting with creating info for a built-in montage:
>
> montage = mne.channels.read_montage(kind='biosemi128')
> fake_info = mne.create_info(ch_names=montage.ch_names, sfreq=250,
> ch_types='eeg', montage=montage)
>
> However, in the created info object, there are some differences between
> info['chs'] and info['dig']:
>
>    - There are 3 more channels in 'dig' than in 'chs'
>    - The channel kinds are different in 'dig' than in 'chs'. For 'dig',
>    the first 3 channels have kind '1' and the rest have kind '3'. For 'chs',
>    the channels all have kind '2', and are identified as EEG channels by
>    mne.io.pick.channel_type.
>    - The coordinates in info['dig'][ind]['r'] don't match the coordinates
>    in info['chs'][ind]['loc'][0:3].
>
> Could someone explain why the channels kinds and locations don't match in
> 'chs' and 'dig'?
>
> Thanks and Best,
> Gladia
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