[Eeglablist] re-refeference? /correcting for the polar average reference effect

[Zara Bergström]

Hi Andrew and list members,

what you are describing sounds like a bias in the average reference 
transform known as the polar average reference effect (PARE, Junghöfer 
et al.,1999, see abstract below). Junghöfer et al recommend a particular 
method for correcting this bias, described in their paper. I also have a 
very large PARE in 64 channel data, and would like to use their method 
for correcting it, but I'm a little bit confused about how to implement 
that correction in practice.
So, I was hoping that someone on this list might be able to please 
explain it?

In their paper, Junghöfer et al. describe how one can correct for the 
PARE by using the scalp electrodes to estimate the potential on the 
surface not covered by electrodes using a spherical spline 
interpolation, and then computing the average reference for the entire 
surface using the results from the interpolation.

How exactly can I implement this in practice? I'm guessing that it would 
involve creating a new "channel" representing the non-sampled area, and 
then including that channel in the average reference (so, PARE-corrected 
average reference = average of (all scalp channels + interpolated new 
channel)). But how can I compute an interpolated channel without 
inputting the coordinates of the channel to be computed? And how do I 
update the EEGLAB data structure to take into account this new channel 
when re-referencing? Will the re-referencing work without channel 
coordinates for the new channel?

Or even better, does anyone know of an already existing EEGLAB function 
or plugin (or a MATLAB script) that implements a PARE correction when 
re-referencing to an average reference?

Any comments will be much appreciated!

Thank you very much in advance,

Zara


Abstract from Junghöfer et al., The polar average reference effect: a 
bias in estimating the head surface integral in EEG recording. Clin 
Neurophysiol. 1999 110(6):1149-55

"A reference-independent measure of potential is helpful for studying 
the multichannel EEG. The potentials integrated over the surface of
the body is a constant, i.e. inactive across time, regardless of the 
activity and distribution of brain electric sources. Therefore, the average
reference, the mean of all recording channels at each time point, may be 
used to approximate an inactive reference. However, this
approximation is valid only with accurate spatial sampling of the scalp 
fields. Accurate sampling requires a sufficient electrode density
and full coverage of the head's surface. If electrodes are concentrated 
in one region of the surface, such as just on the scalp, then the average is
biased toward that region. Differences from the average will then be 
smaller in the center of the region, e.g. the vertex, than at the periphery.
In this paper, we illustrate how this polar average reference effect 
(PARE) may be created by both the inadequate density and the uneven
distribution of EEG electrodes. The greater the coverage of the surface 
of the volume conductor, the more the average reference approaches
the ideal inactive reference."



[Eeglablist] re-refeference?
Andrew Hill andrewhill at ucla.edu
Tue Jun 16 19:01:59 PDT 2009

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hello folks,

i've got a question on data referencing on some .CNT files.

it seems that using reref to avgref state still shows an inverse 
relationship between the (vertex) reference and all (64) electrodes, 
getting larger as they get further away from the original common 
average reference.

this data was from a 64-channel recording on a Synamps2 (32-bit).
creating epochs/averages from this data either with linked ears re-ref 
or average reference, i still "see" the influence of the reference 
near Cz.

is there any way to correct for this when performing a compute average 
reference in EEGLab?

thanks,
andrew




-- 
Dr. Zara Bergström

Postdoctoral research fellow
Memory and consciousness research group
Department of Neurology
Faculty of Medicine
Otto von Guericke University Magdeburg
Leipziger Str. 44
39120 Magdeburg
Germany

Tel. +49 (0)391 6117544