[Eeglablist] Question about AVG referencing

[Enrico Fratto]

Thank you all for the precious feedbacks. This really helps me clarifying
the assumptions behind the set of electrodes making the average.
As another question: let's say I have eegs which have been recorded with
slightly different electrode sets (i.e. I have A1 A2 in some but not in
others), so that also the AVG defining electrodes slightly differ. Can
still I make pooled analysis on the shared electrodes (e.g. O2) in the AVG
montage?
My guess would still be cautiously yes, but I wonder how this can affect
calculations in the setting of a low electrode density, as in clinically
recorded EEGs
Best,


Enrico Fratto, MD
Institute of Neurology,
Department of Medical and Surgical sciences
Università Magna Graecia,
Catanzaro

On Thu, Jul 23, 2026, 01:26 Joseph Dien via eeglablist <
eeglablist at sccn.ucsd.edu> wrote:

> The reasoning behind the average reference is the physics principle that
> if you have a voltage source in a conducting sphere (approximation of
> head shape), then the voltages over its surface will sum to zero (as you
> allude to).  The average of the entire surface is therefore the "true"
> zero.  Of course, we can't actually measure the entire surface of the
> head, but the more measurement sites we have, the better the estimate of
> the voltage surface.  The argument is that although it is a necessarily
> imperfect estimate, it is more accurate than just arbitrarily declaring
> that an electrode is zero voltage (which is what a conventional
> reference does).  We include the reference site because we do have a
> voltage measurement for it (by definition it is zero).  In the case of
> an extra-cephalic reference, all that reasoning goes out the window.  So
> typically you would want to not include it in the average and you would
> not want to generate a rereferenced waveform for that site.  That said,
> it has been reported (I don't have the citation on hand) that basically
> the entire body is equipotential with the neck (since electrically we're
> a bag of salt water, which is an excellent conductor), so one could
> argue for it being included as an estimate of the underside of the
> head.  Potentially, it could be especially helpful for average reference
> schemes like PARE or REST, which seek to correct for the undersampling
> of the bottom part of the head.  You'd need the appropriate equivalent
> coordinates in that case, so it'd take some thinking.  If you have a
> reference at another cephalic site (even if not 10-20 or 10-05 or
> whatever), it would be nonsensical to drop it.  It'd just be losing
> information.  Just treat it like any other EEG site.
>
> The key to thinking about EEG voltage information is that the
> fundamental information is not the waveforms from an electrode (they
> change with the reference scheme).  The fundamental information is the
> difference, at a given time point, between electrodes.  Voltages are
> relative measures.  That's why a reference site is of equal value to the
> other EEG sites even though it is a flat zero voltage line prior to
> rereferencing.  The real information there is its relative difference
> from each and every other electrode at a given moment in time.  That
> does not change with reference site choice or with decisions about which
> electrodes to keep.  My 1998 paper is a helpful reference for thinking
> about all this.
>
> Dien, J. (1998). Issues in the application of the average reference:
> Review, critiques, and recommendations. /Behavior Research Methods,
> Instruments, and Computers/, /30/(1), 34–43.
>
> https://urldefense.com/v3/__https://doi.org/10.3758/BF03209414__;!!Mih3wA!F_zQRsBFYq5PQs849J1FaIb_YpNn30vDpQS9f86dtATUz2VnGCt_VFDvQmFLIE0a_jUVr24CDqQwWhR6DGI$
>
> Cheers!
>
> Joe
>
>
>
> On 7/22/26 09:58, Enrico Fratto via eeglablist wrote:
> > Greetings to all the community.
> >
> > I am reaching out to ask about something which is often overlooked but
> > sometimes clicks hard to understand for me as the average reference
> process.
> > Let's say I have 22 channels + Fz which is used as the reference.
> >
> > According to Makoto's famous explanation (
> >
> https://eeglab.ucsd.edu/wiki/Makoto%27s_preprocessing_pipeline?source=post_page---------------------------&utm_source=chatgpt.com#Re-reference_the_data_to_average_(10/31/2025_Updated) 
>
> > )
> > we are supposed to add a 0 line in the AVG reference calculation to
> account
> > for the reference channel itself (corresponding to dividing the sum at
> the
> > n electrodes by n+1 to encompass the reference) when the flat channel is
> > not already present in the data.
> > In my case, it would be the sum at the 22 recording electrodes divided by
> > 23 to encompass Fz.
> > My question now is: let's say I acquired EEG with a reference I am not
> > interested in (say, extra-cephalic). Would I still have to encompass it
> in
> > the denominator while calculating the AVG in this case? Or, because it is
> > not part of the head, I treat it as if it does not belong to the
> > "zero-potential" assumption and I just estimate the average using the
> > remaining electrodes?
> > And what should I do instead if the reference is at another cephalic
> custom
> > location (not belonging to the standard electrodes)?
> >
> > Thank you in advance for your help,
> >
> > Best
> >
> >
> --
>
> --------------------------------------------------------------------------------
>
> Joseph Dien, PhD
> Senior Research Scientist
> Department of Human Development and Quantitative Methodology
> University of Maryland, College Park
>
> https://urldefense.com/v3/__http://joedien.com__;!!Mih3wA!F_zQRsBFYq5PQs849J1FaIb_YpNn30vDpQS9f86dtATUz2VnGCt_VFDvQmFLIE0a_jUVr24CDqQw3JjvVI8$
> _______________________________________________
> To unsubscribe, send an empty email to
> eeglablist-unsubscribe at sccn.ucsd.edu or visit
> https://sccn.ucsd.edu/mailman/listinfo/eeglablist .