[Eeglablist] Question about AVG referencing

[Joseph Dien]

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$