[Eeglablist] Is average reference not recommended on 10-20 system?

[Makoto Miyakoshi]

Dear Jinwon and Cedric,

Let's confirm the problem first. 'Electric Fields of the Brain' by Nunez
and Srinivasan (2006) (hereafter EFB) p.295 says:

...The surface integral of the potential over a volume conductor containing
dipole sources must be zero as a consequence of current conservation
(Bertrand et al. 1985). In this case, the surface integral can be estimated
by the second term on the right-hand side of (7.10); that is, by averaging
the measured potentials and changing the sign of this average. (...) Since
we cannot measure the potentials on a closed surface surrounding the brain,
the first term on the right-hand side of (7.10) will not generally vanish.
The distribution of potential on the underside of the head (within the neck
region) cannot be measured. Furthermore, the average potential for any
group of electrode positions, given by the second term on the right-hand
side of (7.10), can only approximate the surface integral over the volume
conductor. For example, this is expected to be a very poor approximation if
applied with the standard 10/20 electrode system. As the number of
electrodes increases the error in the approximation is expected to
decrease. Thus, like any other choice of reference, the average reference
provides biased estimates of reference-independent potentials.
Nevertheless, when used in studies with large numbers of electrodes (say
128 or more), we have found that the average reference often performs
reasonably well as an estimate of reference-independent potentials in
simulation studies (Srinivasan et al. 1998).

So the problem is that the difference between the real average and
electrode-sampled average becomes worse, and it is actually very bad when
you are using 20 channels supported by 10-20 systems.

Cedric, I do not see they are against using average reference when < 128
ch. Did you find that description elsewhere from the book?

Jinwon, that said, from the linear algebraic point of view, it makes no
sense to say one choice of reference electrode, including average of
arbitrary combinations of electrodes, is 'worse' than another.When you use
average reference with low number of electrodes (say 20 or 30), you may be
criticized that your electrode average is severely deviated from the true
surface average. But using average reference does not mean you are making a
claim that your electrode average at a given frame be zero. Compared with
using Fz, Cz, Pz, or (digitally linked) mastoid/earlobe (physically liked
mastoid/earlobe is out of question) as a reference electrode, your average
potential may be still useful for certain purposes. Clarifying the merit of
using average reference in your case over other choices of reference
requires elaborated simulations which may not be easy or even realistic.
But my point is that blindly following the rule 'average reference applied
to less than 64 ch == bad' is ridiculous. For example, depending on your
targeted EEG phenomenon, if it has a dominant low spatial frequency,
relatively low spatial sampling by a relatively low number of electrodes
(which is hopefully uniformly distributed) could be more tolerable.

If your reviewer does not write much comment on your experimental design or
result interpretation but just writes this kind of general and trivial
technical things about EEG, that is not a good reviewer. Send me the
reviewer's comment in a separate email and I can assess it further for you.

Makoto




On Thu, Jun 15, 2023 at 1:35 PM 장진원 via eeglablist <eeglablist at sccn.ucsd.edu>
wrote:

> Dear all,
>
> One of my reviewers has suggested that because average reference relies on
> the assumption that the electrode coverage represents a sphere, it is not
> good to average-reference with electrodes less than 64. I have used the
> average-referencing as recommended on
>
> https://urldefense.com/v3/__https://eeglab.org/tutorials/05_Preprocess/rereferencing.html__;!!Mih3wA!D_yQ2dwBfLAhlMiue98MMVMBa2NNnbOxZRm_SDLtYScegpGg5wR7Ly9SmCRGov3uc7pP5TS6NKxNf9Zs2cdtn560dQ$
> , so I wonder
> what could be the alternative.
>
> Best Regards,
> Jinwon Chang
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