[Eeglablist] Drift(?) IC in Biosemi EEG systems?

[Jan R. Wessel]

Hey fellow EEGLAB'ers,

After exclusively using BrainProducts EEG Hardware for the last couple 
of years, I have recently started looking at a colleagues dataset that 
was acquired on a BioSemi System.
He keeps getting components like the following one (see download link), 
that have very "non-dipolar" topographic maps, but explain a pretty good 
part of the data:
http://img15.imageshack.us/img15/2390/component11.png
Can anybody tell me what process / artifact (or what have you) these 
components represent? It keeps occurring pretty independently of how I 
preprocess his data.

Looking at the component ERPimage (non-baseline corrected epoched data), 
I suspected it might be a drift component, since I think BioSemi doesn't 
use DC cutoff during recording.

However, two things seem to countermand this interpretation: a) HP 
filters as drastic as 2Hz did not prevent these components from being 
present in the ICA model (the screenshot is from 2Hz HP-filtered 
(two-way FIR) data), and b) there does seem to be some event-related 
activity in that component as well (there's a visual event at 1.5 secs 
in the dataset underlying this IC).

Now the latter point could in principle be due to an under-fitting ICA 
model, since the data dimensionality was very low to begin with (only 34 
electrodes were recorded), and overall data quality w.r.t. 
spurious/non-stereotyped artifacts was sub-standard (even though I've 
been pretty conservative on the pre-ICA data rejection). But that still 
leaves me curious as to what this component actually is.

I'm pretty sure that this might be a pretty standard thing that has to 
do with the BioSemi that I am missing here, but I could be mistaken. In 
an case, I would appreciate any help in this matter, because I'm just 
curious w.r.t. what I am looking at.

Thanks very much!!
Jan

-- 
Dr. Jan R. Wessel
Department of Psychology
University of California, San Diego
&
Cognitive Neurology Research Group
Max Planck Institute for Neurological Research, Cologne
http://www.nf.mpg.de/cv/jan-wessel.html