[Eeglablist] Question about AVG referencing

Wed Jul 29 14:13:49 PDT 2026

Thank you all for your precious replies.
For standardisation purposes across datasets, then, I might want to
calculate the average using only the "common electrodes" (often, the famous
19 electrodes available in every clinical eeg)

If instead I have consistent electrodes sets for every patient, I would add
the non cephalic reference to the average,  as well as other non cephalic
electrodes.

This was mostly my practice, but your explanations and details have
significantly cleared my mind!

Best,
Enrico

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

On Wed, Jul 29, 2026, 23:04 Makoto Miyakoshi via eeglablist <
eeglablist at sccn.ucsd.edu> wrote:

> Hi Enrico,
>
> Joe is a real specialist in this topic. My answer should be the same as
> his.
>
> 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?
>
> Yes, calculate average referenced potentials including the non-cephalic
> initial reference electrode, which you may remove subsequently.
>
> Can still I make pooled analysis on the shared electrodes (e.g. O2) in the
> AVG
> montage?
>
> If you are dealing with > 128ch data and only a few number channels are
> inconsistent between the two datasets, the practical difference would be
> negligible. You would still want to run a simple simulation to show the
> scale of the expected errors to convince readers and reviewers that this
> small violation of the good practice cannot be a serious problem in this
> particular case. Otherwise, you'll want to stick to the basics and use the
> same set of electrodes between the two datasets. It seems possible that
> your scalp channels can be upsampled so that two datasets have the same set
> of electrodes. But interpolating electrodes in the edge of a montage yields
> poor estimates and is generally not recommended. Thus, this upsampling
> approach is case dependent.
>
> Hi Joe,
>
> 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.
>
>
> Your 1998 paper refers to a chapter of the 1st edition of EFB (Nunez, 1981)
> written by Katznelson. The Chapter 7 of the EFB 2nd edition has probably
> the same description. I'll compare the descriptions when I have a chance. I
> also heard this explanation directly from Paul.
>
> Makoto
>
> On Wed, Jul 29, 2026 at 2:17 AM kj via eeglablist <
> eeglablist at sccn.ucsd.edu>
> wrote:
>
> >
> >
> >
> >
> > ---- Replied Message ----
> > | From | Enrico Fratto via eeglablist<eeglablist at sccn.ucsd.edu> |
> > | Date | 07/24/2026 03:51 |
> > | To | Joseph Dien<jdien07 at mac.com> |
> > | Cc | eeglablist at sccn.ucsd.edu |
> > | Subject | Re: [Eeglablist] Question about AVG referencing |
> > 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$
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