[Eeglablist] Question about AVG referencing
Joseph Dien
jdien07 at mac.com
Thu Jul 23 12:25:36 PDT 2026
Reference choice influences the waveforms so it influences the
measurements, both the signal and the noise (Dien, 1998; 2017). A1 and
A2 are on the underside of the head, which is undersampled, so
inclusion/exclusion would have more effect. It also depends on the
orientation of the EEG fields that you are studying. If they are
vertically oriented, then A1/A2 would have more influence. Having some
sessions different from others is especially a concern for a clinical
study where you are likely examining individual differences, as it is
potentially a source of confounds. You should either drop A1 and A2
from the rest of them, or add A1/A2 if possible to the missing ones if
possible, although with a sparse montage channel replacement via
interpolation or regression probably would not work well.
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!H6p5LX0QTQlRXqRhvAdUJlcZtd2fjQYMm3zRwO09Br6Cd3sA8EOfxHzwwUI59dicSgNEw8DVuC_Nep4yDZc$
Dien, J. (2017). Best practices for repeated measures ANOVAs of ERP
data: Reference, regional channels, and robust ANOVAs. /International
Journal of Psychophysiology/, /111/(1), 42–56.
https://urldefense.com/v3/__https://doi.org/10.1016/j.ijpsycho.2016.09.006__;!!Mih3wA!H6p5LX0QTQlRXqRhvAdUJlcZtd2fjQYMm3zRwO09Br6Cd3sA8EOfxHzwwUI59dicSgNEw8DVuC_N_5Fm0zU$
Cheers!
Joe
On 7/23/26 15:09, Joseph Dien wrote:
>
> 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!H6p5LX0QTQlRXqRhvAdUJlcZtd2fjQYMm3zRwO09Br6Cd3sA8EOfxHzwwUI59dicSgNEw8DVuC_Nd958CkM$
--
--------------------------------------------------------------------------------
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!H6p5LX0QTQlRXqRhvAdUJlcZtd2fjQYMm3zRwO09Br6Cd3sA8EOfxHzwwUI59dicSgNEw8DVuC_Nd958CkM$
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