[Eeglablist] Question about AVG referencing

[ivano triggiani]

Ciao Enrico,

I think the main concern here is ICA rather than average referencing in
isolation.

As far as I understand, Makoto’s recommendation is intended to avoid an
unnecessary additional rank loss when average referencing data that do not
contain the original reference channel. This becomes particularly important
when the dataset already contains interpolated channels, because if you
interpolated before, you already reduced the effective rank.

If Fz was the original reference and you want to include it in the scalp
average, add it back as a zero-filled channel before re-referencing. For
ICA, make sure the rank accounts for any interpolated channels.
I would not include an extracranial reference in the scalp average. A
custom scalp reference can be included if you want to include that location
in the average.

So there are two separate issues: which locations define the average
reference, and what the effective rank is when ICA is computed.

Best,

Ivano
---

*A. Ivano Triggiani, Ph.D.*


________________________________________________



On Wed, Jul 22, 2026 at 4:31 PM Enrico Fratto via eeglablist <
eeglablist at sccn.ucsd.edu> 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
>
>
> --
> 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
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