[Mne_analysis] question about FIFF constants and CTF data

[Gustavo Sudre]

hey all

I'm working with CTF data after converting them to FIFF. I used a simple
mne_ctf2fiff call, and my data has no digitized points nor EEG. After
looking at raw.info['dig'] I was a bit unsure about the location of the 3
electrodes that were used (LPA, RPA, nasion), so I decided to check a
sample dataset just to verify it wasn't something weird with my data.

So, I converted Brainstorm's sample raw .ds to FIFF (sorry, I didn't find a
sample .ds by the MNE-python gang ;) ), and looked at the dig structure.
Here's what I see:

In [8]: raw.info['dig']
Out[8]:
[{'coord_frame': 4,
  'ident': 3,
  'kind': 2,
  'r': array([  7.45058060e-09,   8.64083171e-02,   0.00000000e+00],
dtype=float32)},
 {'coord_frame': 4,
  'ident': 1,
  'kind': 2,
  'r': array([ -6.84481487e-02,  -5.76255843e-09,   0.00000000e+00],
dtype=float32)},
 {'coord_frame': 4,
  'ident': 2,
  'kind': 2,
  'r': array([  7.05333501e-02,   5.87897375e-09,   0.00000000e+00],
dtype=float32)},
 {'coord_frame': 4,
  'ident': 1,
  'kind': 3,
  'r': array([-0.02922045, -0.09704589,  0.0735818 ], dtype=float32)},
 {'coord_frame': 4,
  'ident': 2,
  'kind': 3,
  'r': array([ 0.03363838, -0.09628744,  0.0735818 ], dtype=float32)}]

The two last points are not present in my actual data, but the output above
can be used to illustrate my question. Based on the coordinates in head
space, shouldn't the first point be nasion (FIFFV_POINT_NASION, ident=2)?

Maybe I'm misreading something in the manual, but it's something that has
been bugging me for a while, and I was wondering if you guys could quickly
point out my mistake!

Thanks,

G


-- 
Gustavo Sudre
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