[Eeglablist] What makes a bad channel?

[Matthew Stief]

Sorry to barrage the list so much lately!

I was wondering if anyone would like to provide some more general guidance
on channel rejection than has been discussed on the list in the past.

As has been noted several times before, the automatic channel rejection
sometimes does not perform well, identifying acceptable electrodes and
ignoring comically bad ones.  Given the absence of comically bad ones, in
most of my data it seems to identify electrodes that are not visibly any
different from the others at least in scroll plot.  For example:
http://s1153.photobucket.com/albums/p512/mstief/EEG%20Issues/?action=view&current=channelrejection1.jpg

And just to take one channel identified in this way, it seems perfectly
reasonable under channel properties as far as I can tell and no different
from the adjacent one, here's a screenshot of that:
http://s1153.photobucket.com/albums/p512/mstief/EEG%20Issues/?action=view&current=channelrejection2.jpg

Given this dearth of automatic guidance, I am left with the question of
whether or not these channels are worth removing at all, and what are the
hallmarks of a channel worth removing.  Perhaps I'm just worrying too much
and have admirable data, but I am too paranoid to think so.  So, searching
for ways in which to distinguish channels from one another at all, I
experimented a little, and zooming out to a broader view it becomes clear
that there are some bands of electrodes that have some extra high frequency
noise in them that was not taken care of by the filter.  You can see that
here:
http://s1153.photobucket.com/albums/p512/mstief/EEG%20Issues/?action=view&current=channelrejection3.jpg

Now though these channels have high frequency noise it's clear they've also
got a lot of good neural information in them, and it seems to me that the
ICA may be able to handle them admirably.  Here is an example of one such
channel:
http://s1153.photobucket.com/albums/p512/mstief/EEG%20Issues/?action=view&current=channelrejection4.jpg

So first definite question: are such channels with high frequency noise
likely to destabilize the ICA decomposition, especially if there are a lot
of them, or will the noise be nicely separated out into a distinct
component, leaving the underlying neural activity?

Second question: if these are the only visibly different channels I am
dealing with, am I otherwise safe and can continue without rejecting any
channels?  I can always select parameters in automatic channel rejection to
detect just a small handful of channels, but they never seem any different
than the others so I'm reluctant to remove them.

Third question: If the general idea is that I should just use the automatic
channel rejection function in some more refined way, is there still any
guidelines on just what makes a channel bad, especially for the purposes of
ICA?  It may be helpful to know that I am not using ICA for artifact
rejection but rather to isolate the visual P1.  With that in mind should I
be more or less draconian with occipital channels (i.e. do I want to be
sure to save them to keep occipital activity, or remove them to avoid
contaminating occipital ICs)?


-- 
_________________________________________________________________
Matthew Stief
Human Development | Sex & Gender Lab | Cornell University
http://www.human.cornell.edu/HD/sexgender


Heterosexuality isn't normal, it's just common.
-Dorothy Parker
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