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

Tue Jul 25 22:02:02 PDT 2023

Dear Kyle,

This is a video that compares the different references.

https://urldefense.com/v3/__https://www.youtube.com/watch?v=ioIETUX4G4k__;!!Mih3wA!D97F7LI26rSgo7FYUU_7QJk_aYPBL1yLuG73xrrZru6n329pvG1vRsIKvMQGmn4dKeg3esjgML_fGjnpf_5xkDqh$ 

Arno

> On Jul 24, 2023, at 4:10 AM, Kyle Lepage via eeglablist <eeglablist at sccn.ucsd.edu> wrote:
> 
> Hi, you might try:
> 
> @article{lepage2014statistically,
>  title={A statistically robust EEG re-referencing procedure to
> mitigate reference effect},
>  author={Lepage, Kyle Q and Kramer, Mark A and Chu, Catherine J},
>  journal={Journal of neuroscience methods},
>  volume={235},
>  pages={101--116},
>  year={2014},
>  publisher={Elsevier}
> }
> 
> https://urldefense.com/v3/__https://www.mathworks.com/matlabcentral/fileexchange/64643-apply-rcar/?s_tid=LandingPageTabfx__;!!Mih3wA!DnegK98ehdTXFqLwanApFI6uX5CRADC5cc0HYGAMKvej1yK8Dq8xPj0PAXwHyWmXP_RdnbAgdJjXQmHz3Yx67WF9Pw$ 
> 
> Best,
> 
>  Kyle
> 
> 
> On Fri, Jun 16, 2023 at 12:09 PM Cedric Cannard via eeglablist <
> eeglablist at sccn.ucsd.edu> wrote:
> 
>> 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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