[Eeglablist] dipfitting for egi: how to align channels

[Tarik S Bel-Bahar]

hi lucy, some ways and throughts below that you might find useful.
I've been working with egi sensors and have similar issues and questions.

1. I assume below you have already done searches for similar information in
the eeglab tutorial (for example, the special appendix on channel locations
which contains some solutions to questions like ours), and the eeglab list
archives (which you can download
zipped and search through for your topic. When dealing with individual
subject electrode locations, the issues are different and depend of course
on what kind of analyses your are interested in.

2.if you don't have polhemus locations per individual subject, then you are
probably using the hydrocel or earlier model,
and have probably loaded up these locations from the egi folder with
locations for their sensor ranges and
here's the ftp folder and note I got from the egi support team, just in case
  ftp://ftp.egi.com/pub/support/3rdPartySoftwareSupport/BESA/
   You will find the HCGSN sensor position files in the
"HCGSNSensorPositionFiles" folder, and the GSN sensor position files in the
"BESA_sensor_position_files" folder.
I'll assume that you don't have single-subject electrode locations below.

3. One could try to "optimize head center" in the channel locations eeglab
gui. Play around with that, it should spread the trodes
out. I think you can do so by using all channels, some channels, or a
specific channel. Try selecting Cz as the channel
to use for optimization.

4. Either via #2 or #3 above you should get to a point where you trodes look
close in shape to the DIPFIT head when you
are doing the electrode co-registration. At this point, you can manually
play with the size and rotation via the manual controls.
Using this method you can get near perfect alignment in under 30 minutes,
and use the finalized it for the rest of your subject files
Alternatively, you can also save the locations and then load them up later.
Check the eeglab tutorial on manual co-registration.

5. Renaming labels requires a little bit of matlab dexterity. Along the
lines that Arno suggested, we can
probably relabel some of our channels as10-20 labels from  egi 's 128
-channel 10-20 estimates.
I have not tried it yet, but this should allow almost perfect alignment.

6. For warping by fiducials, the following has worked for me: clearing any
existing or automatically detected pairs,
pairing three landmarks, such as Nz,
then making sure that no other pairs exist, or are automatically detected -
if so -relabelling these channels
so they are not automatically paired, and thus making sure that the pairing
is only on landmarks of choice.
Anyway, aligning by a few landmarks (or electrodes near landmarks) is
probably not as good
as using 20 or so locations across the head to warp in a way that requires
very little manual warping or none at all.

7. please followup on this eeglab thread when and if you you get through
this impasse and others related
to channel locations. it would be great to hear of any solutions you come up
with.

8. If you're handy with Matlab you can bypass or just hack through some of
these issues or potential pitfalls.




On Wed, Jul 20, 2011 at 6:55 PM, Lucy McGarry <lucy.mcgarry at gmail.com>wrote:

> Hi all,
>
> I am having a problem properly aligning my channels when I try to use the
> dipfitting procedure for my 128 channel EGI cap.
>
> I have automatically located channels for EGI using "warp montage" and the
> MNI model, but after doing this, all the green channels end up clustered
> around CZ.  Aligning fucidials does not seem to change anything.  Has anyone
> else had this problem?
>
> I'm not sure what entries to use in the fields at the bottom of the
> coregistration window, that will optimally make the green electrode icons
> correctly circle the head.  I tried entering numbers that made the image
> look as though the electrodes were circling the head, but then when I plot
> my dipoles, they end up all clustered across the centre of the head.  Does
> anyone have any suggestions?
>
> Thanks very much,
> Lucy
>
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