[Eeglablist] re-referencing

Joseph Dien jdien07 at mac.com
Fri Oct 17 09:12:57 PDT 2014

You may also find my average reference paper to be helpful as it goes into the assumptions and practical consequences of rereferencing.  As you will see, reference choice definitely makes a difference for bottom line results, depending on the reference choice and the scalp topography of the EEG activity (the paper focuses on ERPs but is relevant for EEG more generally).

Dien J. 1998. Issues in the application of the average reference: Review, critiques, and recommendations. Behavior Research Methods, Instruments, and Computers 30: 34-43.


Joseph Dien, PhD
Research Associate
Cognitive Neurology
The Johns Hopkins University School of Medicine

Lab E-mail: jdien1 at jhmi.edu
Private E-mail: jdien07 at mac.com
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On Oct 4, 2014, at 15:51, Nicholas Rosseinsky <rosseinsky.nicholas.m at gmail.com> wrote:

> Oops - sorry: hit send waaaaaay too early ... 
> 1. Eric asked about references concerning re-referencing.
> Here are some, not specifically concerned with IC decomposition though:
> Nunez, P. L. (2010). REST: a good idea but not the gold standard. Clinical neurophysiology: official journal of the International Federation of Clinical Neurophysiology, 121(12), 2177
> Qin, Y., Xu, P., & Yao, D. (2010). A comparative study of different references for EEG default mode network: the use of the infinity reference. Clinical neurophysiology, 121(12), 1981-1991.
> Hu, S., Stead, M., Dai, Q., & Worrell, G. A. (2010). On the recording reference contribution to EEG correlation, phase synchorony, and coherence. Systems, Man, and Cybernetics, Part B: Cybernetics, IEEE Transactions on, 40(5), 1294-1304.
> 2. I may well be wrong, but it seems to me that the reference question could be discussed a little more, generally. Notably, the average reference seems to be though of as "harmless" or "default", but obviously it subtracts from the data the average fluctuation of brain activity relative to e.g. mastoid potential! In an idealized case in which e.g. mastoid electrodes are already "at" some idealized "nominal ground" (Nunez 2010), this means that average referencing is subtracting out the common electrophysiologically-relevant activity captured in channel-potentials-relative-to-mastoid. (I'm aware of the unrealizability of this idealized case, and of the problems concerned with the potential of the body-as-battery ... I'm just sayin').
> Notably, try visualizing (making  a topoplot movie of) alpha-band continuous EEG before and after average referencing. There are stereotypical and large spatiotemporal travelling patterns present in non-averaged data that disappear after averaging (disclaimer: in the data I am currently looking at, which has significant quality and scalp-coverage limitations). IC decomposition of non-averaged data certainly seems to have a spatially "ugly" component that captures e.g. common alpha activity - but to throw this out because it doesn't "look dipolar" is surely the wrong criterion. The question is whether this kind of activity is relevant to analyses. If alpha travelling waves affect task performance (Patten et al., 2012), and if you are interested in analysing or identifying such effects ... take care with choosing average reference.
> As I understand it (please, educate me, listers!) average reference is theoretically innocuous only if electrodes completely cover the spherical head; the further away one is from this (again, practically-unrealizable) ideal, the more potentially "impactful" average-referencing might be . And in other cases (my data may be particularly poor  in this regard), it's going to be a judgment call best guided by:
> 1. visualizing your data before and after preprocessing to know what effects your pre-processing steps are having; and,
> 2. making an intelligent, context-dependent, analysis of how any preprocessing effects are going to affect your subsequent analyses.
> I don't think there are general principles here, and: that doesn't mean that the choice doesn't matter, i.e. "no general principles" doesn't mean "choosing average reference won't affect my analyses".
> Patten, T. M., Rennie, C. J., Robinson, P. A., & Gong, P. (2012). Human cortical traveling waves: dynamical properties and correlations with responses.PloS one, 7(6), e38392.
> Hope that helps some, and please, if musings under 2 are nonsense - help me!
> Thanks
> Nicholas
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