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

[Thomas Ferree]

Dear Zara,

The following paper states exactly how to do this, and demonstratesnumerically
the advantages of this way of estimating average reference.
Furthermore, in my own studies I have seen that it improves the
accuracy of topographic maps, which was your original question.

There is not a new channel per se, so your question on that point need
not be addressed.

I have Matlab code for doing this that I might be willing to share. It is
set up to work with EEGLAB data structures, but I have not gotten around
to making a plug-in for it.  Please contact me in person about that.

Best regards,
Tom.

Spherical splines and average referencing in scalp
electroencephalography.<http://www.ncbi.nlm.nih.gov/pubmed/17019635?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DefaultReportPanel.Pubmed_RVDocSum>

Ferree TC.

Brain Topogr. 2006 Winter;19(1-2):43-52.

2009/7/2 Zara Bergström <zara.bergstroem at med.ovgu.de>

> 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
>
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-- 
Thomas Ferree, PhD
Department of Radiology
UT Southwestern Medical Center
Email: tom.ferree at gmail.com
Voice: (214) 648-9767
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