[Eeglablist] removing eye channels before ICA?

[Baris Demiral]

>From my previous experience, and from the previous e-mail exchanges in this
mailing list (as well as the algorithms proposed in some of the EEGLAB
toolboxes, such as ADJUST) I see no problem of not including the EOGs in the
ICA based artifact rejection. That being said, I only use EEG channels in
ICA based correction, with their locations clearly defined, and I do not
include EOGs for the very similar reasons you proposed.

The EEG channels will be good enough (or maybe I should say 'strong enough')
to capture the eye movement sources. You can use EOG channels if you further
want to examine some other aspects of the data if needed. Maybe, for
instance, calculating the absolute value of the differences between the
horizontal electrodes, and the value of the vertical EOG during the epoch to
determine wheher there were significantly more or less eye movement-induced
potentials between the conditions. In some paradigms people may move/blink
their eyes more often in one condition than the other condition. But at the
end, artifact correction should not bias the data towards one condition
under such circumstances.

Best,
Baris

On Tue, Jun 7, 2011 at 10:14 PM, Chang Gu <chang.gu at vanderbilt.edu> wrote:

> Dear colleagues,
>
> I'm using ICA to clean the data, but I'm wondering that if we could remove
> the eye channels before conducting ICA.
>
> We're using 128-channel EGI recording system and there're 8 eye-channels on
> the face picking up eye signals like blink and movement.
> (1). For these 8 channels, they're usually recording huge potentials
> comparing with regular channels on the sculpt: the signal amplitude from eye
> channels could be 100 times to channels close to vertex before average
> re-reference.* If I run ICA on all channels, is it possible that the big
> variability in the eye channels influence the decomposition result?* From
> our data, it seems that signals from eye channels draw most of the attention
> of ICA and ICA decomposes many components of eye artifacts. However, we're
> interested in decomposing brain-signal rather than eye-signals. Also, is it
> possible that data-scale differences from eye channels (around 100 micro
> volts) and regular channels (less than 3 micro) may influence the
> performance of ICA algorithm?
>
> (2). Since the eye-channels are on face, very often they have poor contact
> with skin and dry out easily. From the ICA 2D topographic results, we often
> found maps with a single red/blue dot on eye channel, suggesting the
> distinct temporal character of the signal from this channel.* I'm
> wondering that if this 'bad' channel may affect the overall ICA deposition.
> *
>
> We tried removing these 8 eye-channels: other channels nearby also record
> eye potentials (but not as strong as eye-channels for sure) and ICA are able
> to separate eye-components from the EEG signal...
> I'm wondering that, is there any theoretical concerns about removing
> eye-channels?
>
> Thanks in advance!
>
> Chang
>
> --
> Chang Gu
> Psychology & Human Development
> Vanderbilt University
> Nashville, TN
>
>
>
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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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