[Eeglablist] Registering EGI 128 Channel GSN Electrode Locations to BESA/BEM Coordinates

[James Jones-Rounds]

Hi Michael,

I've had this problem in the past, too, using BioSemi's 128-channel system,
which uses the ABCD montage (not 10-20). I had to do some manual warping,
in the end, and I tried about a dozen set of parameters (x-, y-, and
z-shift, pitch, roll and yaw, and radius changes in each dimension as
well). For "pop_dipfit_settings", I use the standard MNI template brain
image provided in EEGLAB's BEM folder, the BEM "standard_vol_SCCN.mat", and
the "standard_alphabetic.elc" coordinate file, and these co-registration
parameters:

'coord_transform', [0 -12  5  -0.1  0 -1.5708 97.88 92 97.8781]

This both looks qualitatively like the electrodes are all on the scalp, and
out of the dozen or so parameters I tried, is the one that maximizes the
number of components with residual variances under 25%, which I take as a
general sign of accuracy and good-fit. Nevertheless, most of my dipoles are
always stuffed a bit further into the brain than I think is accurate
(assuming that the cortex is what is primarily responsible for the EEG
signal in the first place).

I am soon going to be working on a project that has structural MR scans for
each participant, which I hope will help improve the localization from the
dipole-fitting process.

Hope that helps!

James

On Thu, Sep 17, 2015 at 3:00 PM, <eeglablist-request at sccn.ucsd.edu> wrote:

>
>
>    1. Registering EGI 128 Channel GSN Electrode Locations to
>       BESA/BEM Coordinates (Michael Boyle)
>    2. Re: Processing Startle in EEGlab (Tara Miskovich)
>
>
> ---------- Forwarded message ----------
> From: Michael Boyle <mrboyle at live.unc.edu>
> To: EEGLAB Discussion Mailing List <eeglablist at sccn.ucsd.edu>
> Cc:
> Date: Wed, 16 Sep 2015 22:36:47 +0000
> Subject: [Eeglablist] Registering EGI 128 Channel GSN Electrode Locations
> to BESA/BEM Coordinates
> Hey all,
>
> I have been trying to get a good registration between the sensor locations
> provided by EGI for their 128 channel geodesic sensor net and the default
> BESA and BEM coordinate systems that DIPFIT provides, but the results are
> quite unsatisfactory with the standard warping methods (no manual warping).
> I have tried using the automatic warp when aligning just the 10-20 sensor
> locations, the 10-10 sensor locations that EGI reports to be within 1cm of
> an EGI GSN electrode (
> ftp://ftp.egi.com/pub/documentation/technotes/HydroCelGSN_10-20.pdf),
> both with and without fiducials, and even just assessing the correspondence
> visually shows that the fits are quite poor. I was wondering if anyone has
> had success getting a good registration particularly with the BESA
> coordinates, since that is the dipole fitting method I've converged on. The
> most noticeable issue with my registrations is with the frontal electrodes
> in either model (the Fz's for example are clearly not well coregistered).
> Possibly as a result, I have had, in a few cases, DIPFIT place the
> well-fitting dipole for eye blink components slightly within the brain and
> thus not automatically exclude them.
>
> I can send pictures of my best attempts at registration to anyone
> interested.
>
> Thanks for your help!
> Michael
>
>
> --
James Jones-Rounds
Laboratory Manager
Human Development EEG and Psychophysiology (HEP) Laboratory,
Department of Human Development,
--------------------------------------------
Cornell University | Ithaca, NY
607-255-9883
eeg at cornell.edu
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