[Eeglablist] Question about AVG referencing

[Enrico Fratto]

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


-- 
Enrico Fratto, MD
Neuroscience Research Center, Department of Medical and Surgical Sciences
Institute of Neurology, Department of Medical and Surgical Sciences
University Magna Graecia,
Viale Europa, 88100, Catanzaro, Italy