[Eeglablist] Eye movement artifact in ICA - missing coordinates for EOG electrodes

Scott Makeig smakeig at gmail.com
Wed Sep 8 19:26:52 PDT 2010


If the EOG electrodes are bipolar -- that is, not linked to the same
reference electrode as the rest of the channels -- then their values in the
IC map are 'floating' with respect to the other channels (i.e. have some DC
offset) and their values in IC maps should be ignored when submitting the IC
maps to source localization (either formal or 'by eye') . But since activity
in the EOG channel should be linearly related to the other channels (if well
recorded), it may be included in ICA decomposition; the relevant source
activities will also contribute to the EOG channel.

If important or useful, it should be possible to estimate the true values
for the EOG electrodes in the IC maps by making a source model for activity
projected to the other common-reference channels, projecting it to the two
EOG electrode locations, and then minimizing the difference of their
projected difference from the decomposed IC EOG-channel value (i.e.,
removing the floating EOG-channel offset to make the resulting IC map as
smooth as possible).

Scott Makeig

On Sun, Sep 5, 2010 at 7:15 AM, Evans, Ian D. <ievans at une.edu.au> wrote:

>  Shall endeavor to find out this week, if there's time amongst the
> residential schools. But from what I can discern the EOG electrodes do seem
> to make a difference - when I ran an ICA on the eye calibration task used to
> measure artifact without any EOG electrodes in the mix, around 40 of the 68
> components highlighted the F8 region as a strong source, and this in a task
> purely involving eye movement, no other processing involved other than
> possibly recognizing the shapes used to indicate where the subject should
> look. Tried running it again but included some estimated positions of
> EOG electrodes at the front, and there were still 40+ components all
> indicating activity at the frontal lobe, but now the activity was around the
> face instead of F8. If nothing else, the EOG information should assist ICA
> by isolating changes in multiple electrodes with correlate with the EOG data
> and rendering it completely independent from the underlying actual data.
>
> As for labels, well I couldn't find any consistent EOG labeling systems for
> MATLAB, so why not start gathering labels? Let me know what labels you use
> for the eye channels and I can build the .loc file - David Groppe (many
> thanks again!) sent me a set of coordinates for the ocular electrodes, but
> there is no problem with having multiple names for the same set of
> coordinates - EEGLab detects what we're using anyway. Worth a shot.
>
>
> Ian Evans
> Cognitive and Affective Neuroscience
> University of New England.
> ievans at une.edu.au
>
>  *From:* Baris Demiral <demiral.007 at googlemail.com>
> *Sent:* Sunday, September 05, 2010 8:08 PM
> *To:* Evans, Ian D. <ievans at une.edu.au>
> *Cc:* eeglablist at sccn.ucsd.edu
> *Subject:* Re: [Eeglablist] Eye movement artifact in ICA - missing
> coordinates for EOG electrodes
>
> Hi Ian,
>
> I also made a similar query few months ago. I use 64 electrode 10-20 system
> plus 4 EOGs. I asked whether there are specific labels for such EOG
> electrodes in the standard-10-5-Cap385.sfp so that I can map these
> electrodes on the fly. Are there labels for those, or can we decide and add
> those labels and coordinates as a community together?
>
> Actually, I find something more important: How much really does this
> influence ICA related artifact detection and correction? Is there a huge
> change when you include horizontal and vertical EOGs in the ICA based ERP
> calculations? Are there others over there who compared  ICA based
> computations with and without including the EOG electrodes?
>
> Baris
>
> On Fri, Sep 3, 2010 at 7:42 AM, Evans, Ian D. <ievans at une.edu.au> wrote:
>
>> Greetings all! I'm trying to isolate eye movement artifact with ICA in an
>> attempt to make EOG correction more accurate and hopefully remove less of
>> the genuine underlying data that regression methods might otherwise
>> remove,
>> however I am having trouble tracking down a satisfactory scalp map that
>> includes co-ordinates (Cartesian, 2D polar, 3D cylindrical) for the EOG
>> electrodes on the face. Neither Neuroscan nor any setup or .loc files in
>> EEGLab seem to provide these details, and all attempts to isolate eye
>> movement in ICA without the location of the eye movement electrodes have
>> been met with mocking laughter.
>>
>> The recordings have already been made on a 64-channel cap with six EOG
>> electrodes (one above and below each eye, another beside each eye for
>> horizontal eye movement) using the traditional 10-20 system. All
>> co-ordinates for the scalp are already at hand, just the facial electrodes
>> are needed. If anyone can make such a coordinate map available it would be
>> most helpful, thank you kindly!
>>
>> Ian Evans
>> Cognitive and Affective Neuroscience
>> University of New England.
>>
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>
>
>
> --
> SB Demiral, PhD.
> Department of Psychology
> 7 George Square
> The University of Edinburgh
> Edinburgh, EH8 9JZ
> UK
> Phone: +44 (0131) 6503063
>
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-- 
Scott Makeig, Research Scientist and Director, Swartz Center for
Computational Neuroscience, Institute for Neural Computation, University of
California San Diego, La Jolla CA 92093-0559,
http://sccn.ucsd.edu/~scott<http://sccn.ucsd.edu/%7Escott>
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