[Eeglablist] Comments on EEG and ERP reference

[Arnaud Delorme]

In theory, the REST reference is attractive because it approximates a zero potential at infinity. However, it critically depends on the accuracy of the head model used to estimate the transfer matrix. In EEGLAB, using the REST_fieldtrip plugin in combination with DIPFIT is advisable, as it allows explicit control over the forward head model. In practice, all head models are approximations, so REST does not guarantee an improvement over simpler references, although it could offer measurable gains when the model and sensor geometry are reasonably accurate.

The robust average reference implemented via a RANSAC-based approach instead estimates an average reference after iteratively identifying and excluding poorly predictable or artifactual channels (https://urldefense.com/v3/__https://github.com/VisLab/EEG-Clean-Tools/blob/master/PrepPipeline/utilities/robustReference.m__;!!Mih3wA!FQLutu7i9N7nGpzmZk8lCEhdbxpr8ZWd57iqxuYOlNcGQfIOitgFVsvwQ0ztmJ4a8GwckbML4oHGJmr8xDIhONI0$ ). This method does not rely on a head model and aims to produce a stable average reference in the presence of bad channels, rather than approximating a physical reference at infinity.

Arno

> 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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