[Eeglablist] Question about AVG referencing

Thu Jul 23 12:09:20 PDT 2026

You should be consistent across your dataset.

Joe

On 7/23/26 04:52, Enrico Fratto wrote:
> 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) 
>     <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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-- 
--------------------------------------------------------------------------------

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!F1Nw24agdYmPxRPsDJNBHhqU5fBEVmPxTRQgqtbKn7xseOsqb6OMqycnEOzfQxKEmU6zwWzhvosCQPLXeEk$ 