[Eeglablist] get_chanlocs - a new 3-D image-based electrode position digitizer app

[Scott Makeig]

*To achieve high-resolution EEG source imaging requires (a) an accurate
electrical head model, and (b) accurate co-registration of the scalp
electrode positions to the head model. Several packages are available for
fashioning a geometrically accurate head model from an anatomic MR head
image. We use Zeynep Akalin Acar's Neuromagnetic Forward problem Toolbox
(NFT), which she is now coupling to the first non-invasive, universally
applicable method (SCALE) for estimating individual skull conductivity from
EEG data (Akalin Acar et al., 2016; more news of this soon!). *


*Without a subject MR head image, equivalent dipole models for independent
component brain sources need to use a (good) template head model. Zeynep
has shown that the dipole localization process is more accurate when the
template head is warped to fit the recorded 3-D positions of the electrodes
-- IF these are recorded accurately. *

*For too long, it has been both expensive and time consuming (for both
experimenter and subject) to record the 3-D positions of the scalp
electrodes for each subject. Magnetic and ultrasound wand systems to do
this require the subject to sit patiently while the experimenter clicks on
the 3-D position of each electrode. In recent years, however, cameras
capable of recording images in 3-D have appeared and are now becoming
cheaper and more prevalent. *
Robert Oostenveld, originator of the FieldTrip toolbox, alerted us that he
and his students in Nijmegen had added functions to FieldTrip to compute
the 3-D positions of scalp electrodes from scanned 3-D images acquired by
one such (suitable and quite easy to use) camera, the Structure scanner (
structure.io) mounted to an Apple iPad. The 3-D head scanning process
greatly minimizes (to seconds) the time spent by the subject snd
experimenter on electrode position recording, while also minimizing the
position-measuring system cost (currently to near $1000).

Because of the importance of widespread adoption of electrode position
recording to advancing EEG source imaging, I asked Clement Lee at SCCN to
build and test a gui-based EEGLAB plug-in incorporating this new approach.
The new plug-in, *get_chanlocs*, is now available through the EEGLAB
Extension Manager. A wiki page (sccn.ucsd.edu/wiki/get_chanlocs) gives an
overview, and a linked pdf user guide describes the process of using
*get_chanlocs*.

In brief, the process is as follows: a 3-D head image (3-D head ‘scan’) of
the subject wearing the electrode cap is acquired using the Structure
scanner; this image acquisition typically requires less than a minute to
perform. The resulting 3-D .obj image file is then stored with the data



*Later, when the data are to be analyzed, the get_chanlocs plug-in, called
from the Matlab command line or EEGLAB menu, guides the data analyst
through the process of loading the 3-D image and then clicking on each of
the electrodes in the image (in the suggested order) to  compute and store
their relative 3-D positions. (Note: in future this step will likely be
automated using machine vision methods). The electrode labels and their 3-D
positions relative to three skull landmarks (‘fiducial points’) are then
written directly into the dataset EEG.chanlocs structure. A montage
template created for each montage used in a laboratory can be used by
get_chanlocs as a reference during this process to speed and minimize human
error during the electrode location measurement process. Once the electrode
positions have been stored in the dataset, further processes can use their
positions for source localization and electric field plotting. We thank
Robert Oostenveld and the FieldTrip team for creating and sharing the basic
functions used in get_chanlocs, and urge that forward-looking EEG
laboratories strongly consider adopting 3-D electrode montage image
scanning when acquiring their data so as to enable high-resolution source
imaging of the data -- in the near term and/or in future.Scott
MakeigClement Leep.s. As always, we are interested in feedback concerning
any bugs or feature suggestions. It is best to use the EEGLAB Bugzilla
facility for this.*

-- 
Scott Makeig, Research Scientist and Director, Swartz Center for
Computational Neuroscience, Institute for Neural Computation, University of
California San Diego, La Jolla CA 92093-0961, http://sccn.ucsd.edu/~scott



-- 
Scott Makeig, Research Scientist and Director, Swartz Center for
Computational Neuroscience, Institute for Neural Computation, University of
California San Diego, La Jolla CA 92093-0961, http://sccn.ucsd.edu/~scott
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