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

[Cedric Cannard]

Hi Makoto,

This is basically what I was referring to yes. Doesn't this paragraph you sent support this recommendation? 

"As the number of electrodes increases the error in the approximation is expected to decrease. [...] 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)."

However, I should have provided a more nuanced response. Both average and REST/ininfity references are both considered superior to all other known references, especially with 64+ channels. However, I now tend to recommend and use REST because, while it faces the same limitations caused by low electrode coverage and density (i.e., spherical harmonics degree < 7 with less than 128 channels; Srinivasan et al. 1998), the error can be reduced as the head model improves. See here for great discussion on this: https://urldefense.com/v3/__https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2967671/__;!!Mih3wA!GHxYARW27MxFaJ2eDEyohyAmJb8T3foIM3Ee70lzdOc35_OV2YZVDGWwy4ZnfyPRe7E06ijETkbzKxADMMhzKOjCew$ 

"idealized simulations (Marzetti et al, 2007; Qin et al, 2010) where the head model used to estimate G is very similar to model used to estimate errors, REST outperforms AVE. From this argument we see that the choice between AVE and REST largely boils down to questions of genuine head model accuracy [...]. The most popular head model consists of 3 or 4 concentric spherical shells representing, brain, CSF, skull, and scalp tissue (Rush and Driscoll, 1969; Nunez, 1981; Nunez and Srinivasan, 2006). The spherical symmetry allows for relatively simple analytic solutions to the forward problem. On the other hand [...], despite these limitations, simple head models can be extremely useful, typically by proving that many EEG analysis methods proposed over the past 50 years or so will NOT work. For example, the so-called quiet reference myth is easily discredited with simple models (Rush and Driscoll, 1969; Nunez, 1981; Nunez and Westdorp, 1994; Nunez and Srinivasan, 2006). Or, distortion by reference and volume conduction is shown to produce very large errors in scalp coherence estimates (Nunez et al, 1997, 1999; Srinivasan et al, 1996, 1998; Marzetti et al, 2007). Moderate head model inaccuracy does not change the central conclusions of these studies. Simulations using simple head models then provide a critical “filter” through which mathematical methods must first pass to be considered further by serious scientists. We must be continually reminded that fancy mathematics can never trump physical principles. The Qin et al (2010) study has passed this important first test by showing that REST works with simple head models and certain assumed source distributions, but its accuracy with read heads and other source distributions is unknown. For this reason, I suggest that REST and AVE be adopted as reference partners, at least until better information becomes available."

In conclusion, they are similar, but REST seems to be the most promising in the long term (along with Surface Laplacian methods), as the head models improve. It already performs slightly better than AVE (e.g., https://urldefense.com/v3/__https://www.sciencedirect.com/science/article/abs/pii/S1388245710004153__;!!Mih3wA!GHxYARW27MxFaJ2eDEyohyAmJb8T3foIM3Ee70lzdOc35_OV2YZVDGWwy4ZnfyPRe7E06ijETkbzKxADMMhgoEdbtw$  and https://urldefense.com/v3/__https://www.frontiersin.org/articles/10.3389/fnins.2017.00205/full?ref=https:**Agithubhelp.com__;Ly8!!Mih3wA!GHxYARW27MxFaJ2eDEyohyAmJb8T3foIM3Ee70lzdOc35_OV2YZVDGWwy4ZnfyPRe7E06ijETkbzKxADMMg1UjXsBA$ ), and will keep improving over the years. That's why I tend to use it and recommend it now. 

Additionally, REST (and especially the new regularized REST) may present new advantages (e.g., the effective rank deficiency issue although this is pretty much solved now with the recent solution, data recorded with monopolar reference, etc.). See:
https://urldefense.com/v3/__https://link.springer.com/article/10.1007/s10548-019-00706-y__;!!Mih3wA!GHxYARW27MxFaJ2eDEyohyAmJb8T3foIM3Ee70lzdOc35_OV2YZVDGWwy4ZnfyPRe7E06ijETkbzKxADMMjinh8EZg$ 
https://urldefense.com/v3/__https://iopscience.iop.org/article/10.1088/1741-2552/aaa13f__;!!Mih3wA!GHxYARW27MxFaJ2eDEyohyAmJb8T3foIM3Ee70lzdOc35_OV2YZVDGWwy4ZnfyPRe7E06ijETkbzKxADMMhHPAIx-Q$ 
https://urldefense.com/v3/__https://www.sciencedirect.com/science/article/pii/S1388245723005941__;!!Mih3wA!GHxYARW27MxFaJ2eDEyohyAmJb8T3foIM3Ee70lzdOc35_OV2YZVDGWwy4ZnfyPRe7E06ijETkbzKxADMMiRFwJb5w$ 

Note that the EEG reference conversation will go to infinity ;)

Cedric


Sent with Proton Mail secure email.

------- Original Message -------
On Thursday, June 15th, 2023 at 2:25 PM, Makoto Miyakoshi via eeglablist <eeglablist at sccn.ucsd.edu> wrote:


> 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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