[Eeglablist] Comments on EEG and ERP reference

[Ramesh Srinivasan]

I am not claiming there is no leakage.  Only that it must be small and the
resistance in the neck must be very high. The heart makes potentials orders
of magnitude larger than the brain.  Yet EKG artifacts in EEG are quite
small in the same range as EEG and not observed in every subject.  This is
only possible if the resistance in the neck is very high.

All methods are based on models of the data. Luck does not seem to
understand that. The REST method is one reasonable model.  The average
reference is based on another reasonable model.  Each provides advantages
and disadvantages as has been discussed by the commenters.  I especially
appreciate that Dezhong is honest about his views and how he sees the
strengths and weaknesses.  It allows space for a good debate


Ramesh Srinivasan
Professor
Cognitive Sciences
Biomedical Engineering

On Sun, Dec 28, 2025, 11:00 PM Евгений Машеров <emasherov at yandex.ru> wrote:

>
>
> > Hi Dezhong,
> >
> > Id like to comment first on your quote from Luck's book and then about
> your
> > method REST.
> >
> > First, Luck is apparently unaware of the argument for the average
> > reference, or of the physical principles underlying them and is
> > intellectually lazy by focusing on whether the models used to test these
> > ideas are spherical or realistic. That has nothing to do with it.
> >
> > The critical issue is whether the head is a closed object, not whether it
> > is a sphere. In other words, is there evidence that current flows out of
> > head into neck and vice versa. The preponderance of evidence suggests
> > that very little current generated by sources in the brain travels down
> the
> > neck into the body. How do we know this? Well, the simplest evidence is
> > that we can record EEG at all. If there was a significant current path
> > through the neck, EEG recordings would be overwhelmed by EKG which is
> > orders of magnitude larger signal. Hence it is reasonable to assume that
> > the neck is like a giant resistor limiting current flow in both
> directions.
> > From the point of view of current flow from brain sources the head is
> > mostly a closed object. Indeed your REST method hinges on this being true
> > because you dont use a volume conduction model of the whole body, just
> the
> > head. So, this quotation from Luck is just wrong, and is inconsistent
> > with your views and methods.
> >
>
> Unfortunately, leakage from extracerebral sources does occur. In
> particular, the ECG produces one of the most devastating artifacts. On the
> one hand, QRS complexes appear as sharp potentials on the EEG, which
> confuses epileptologists. On the other hand, neither filtering nor changing
> the patient or electrode position helps. Some time ago, I attempted to
> develop a method to combat this using a single-lead ECG recording, which is
> often used specifically to distinguish intracerebral sharp potentials from
> ECG artifact. Regression of the EEG signal onto the ECG signal allowed us
> to remove 60% to 90% of the artifact, with the remainder being averaged
> using the QRS complex as a trigger. Complete elimination was not achieved,
> but the residual component amounted to a few percent of the original
> artifact.
> Attempting to localize the source of this averaged potential, they found a
> dipole at the base of the brain. But whether this was a real source or a
> fictitious dipole at the boundary of a region of different conductivity
> could not be determined.
> The potential generated by the heart is indeed approximately two orders of
> magnitude higher than that generated by intracerebral sources. However, the
> distance from the heart to the electroencephalographic electrodes is more
> than an order of magnitude greater than that from the intracerebral sources
> to the electrodes, and since the dipole potential decays inversely with the
> square of the distance, artifacts from the heart are approximately the same
> order of magnitude as those from the EEG.
>
> > If current is contained with the head, then the potentials on the head
> > should sum to zero. But we can never measure the underside of the head,
> so
> > the average of the potentials we measure indeed must have some error.
> > Thus, your papers make a good argument for REST. I would only caution
> that
> > the head model is approximate and not exact and is thus a potential
> source
> > of error as well.
>
> Well, actually, they tried measuring the potential from the undersurface.
> Nasal derivation, with an electrode inserted into the nasal canal, produced
> such a signal. However, it was extremely inconvenient for the patient, and
> the information yield was lower than expected. Nevertheless, it is
> technically feasible.
>
> >
> > Ramesh Srinivasan
> > Professor
> > Department of Cognitive Sciences
> > Department of Biomedical Engineering
> >
> > On Sat, Dec 27, 2025 at 7:55 PM dyao--- via eeglablist <
> > eeglablist at sccn.ucsd.edu> wrote:
> >
> >> Hello everyone,
> >> Since the discover of EEG in 1924, EEG reference is continuously a
> >> debate issue, which one is the best?
> >> 1. Prof Arnaud Delorme, the first author of the EEGLAb, has a video on
> >> "What is the best EEG reference?"
> >>
> >>
> https://urldefense.com/v3/__https://www.youtube.com/watch?v=ioIETUX4G4k__;!!Mih3wA!H4eUzNmw2tOSr4Yu5olqnIPXoeADBK0vmXkMOIBvk_d6LqSb0l_YqNxv43Ug-3I3FuirE3dNqJiL7VkJeJYlMQ$
> >> , the answer is REST
> >> 2. Prof Steven Luck, in his new textbook(Applied Event-Related
> >> Potential Data Analysis,2022), in section 6.1 "I’d like to point out
> using
> >> the average across sites as the reference in order to approximate the
> >> absolute voltage assumes that the surface of the head sums to zero, but
> >> this is only true for spheres. I have yet to meet someone with a
> spherical
> >> head. And no neck. Fortunately, there is a way to estimate the true
> zero,
> >> called the Reference Electrode Standardization Technique (REST), and
> there
> >> is an EEGLAB plugin that implements it (Dong et al., 2017). I haven’t
> tried
> >> it myself or looked at the math, so I don’t have an opinion about
> whether
> >> it’s useful and robust. But if you really want to get an estimate of the
> >> absolute voltage, REST seems like the best current approach"
> >> 3. in 2014, Lepage etal annouanced that they designed an updated
> >> average reference (robust common average reference(rCAR))being better
> than
> >> REST, and since then rCAR were adopted by a few following work.
> however, a
> >> recent comment on rCAR,attached here, confirmed that rCAR is not an EEG
> >> reference method but an "noise removement method on various artificial
> >> data" as the all illustrative examples adopted in rCAR paper were not
> EEG
> >> data (assumed, not generated by sources inside a brain). In this comment
> >> paper, on Data generated by sources inside a head model, REST is much
> >> better than rCAR. (Lepage, K.Q., Kramer, M.A., Chu, C.J., 2014. A
> >> statistically robust EEG re-referencing procedure to mitigate reference
> >> effect. J. Neurosci. Methods 235, 101–116).
> >> In general, REST and average reference(AR) are the best two as both of
> >> them are based on EEG physics. REST is based on the equivalent
> >> distributed-sources principle of the scalp potential, it depends on the
> >> 'equivalence between the unknown neural sources in the brain and the
> >> reconstructed equivalent sources in the brain', various simulations
> >> confirmed the equivalence depends on the cover range and density of the
> >> scalp electrode array. AR is based that the whole surface potential
> >> integral is zero if the head a sphere, apparently, the weakness is that
> "
> >> our head is not a sphere, and the measurment is usually limited to the
> uper
> >> surface, not and impossible being the whole surface as we have the
> neck",
> >> various simulations confirmed AR depends on the cover range and density
> of
> >> the scalp electrode array,too. The conducted comparative studies showed
> >> that REST is usually better than AR especially when electrode number>20.
> >> For both methods, the most important factor is the cover range, then is
> the
> >> density of the electrode density ,or say, the number and distribution of
> >> electrodes.
> >> wish the above message is meaningful for your work in EEG and ERP.
> >> Best wishes
> >> -----------------------------
> >> Dezhong Yao, PhD, CheungKong Professor
> >> AIMBE Fellow，Cuba Academico Correspondiente，CSBME Fellow
> >> Director, Brain-Apparatus Communication Institute
> >> Editor-in-Chief，Brain-Apparatus Communication, Taylor & Francis Group
> >> University of Electronic Science and technology of China, 611731,
> Chengdu,
> >> China
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