[Eeglablist] Question about average reference

[Steve Luck]

Hi Alan.  The first question is whether a reference electrode was used during the recording.  With almost every EEG system (excluding BioSemi), a reference is used during recording.  It may be that whatever reference was used during recording is just fine.

As discussed in chapter 3 of my book on ERP methods (An Introduction to the Event-Related Potential Technique), there is no such thing as a perfect reference.  You are always looking at the potential (difference) between one site and another site (or set of sites).  The reference site (or sites) will have a substantial effect on the waveforms.  The only exception to this is when you use the average of the entire head as the reference (because the average across the entire head necessarily sums to zero).  You can approximate this with very large sets of electrodes (see Dien, 1998), but most people who use the average of all electrodes do not cover enough of the head to achieve a very good approximation.  Transforming to current source density is another good approach, but it also requires a large set of electrodes to work well.

So, the bottom line is that you will be looking at the difference between your active and your reference no matter what you do, and you just need to keep this in mind when interpreting your data (and comparing your waveforms to those of other people).  The best thing is usually to use what other people have used in similar research so that your waveforms will be comparable (but if most people in your area use the average reference, and you also use the average reference with a different set of electrodes, this is NOT using the same reference as other people).

Steve

> From: Alan Yi <jackm_ustc at yahoo.com.cn>
> Date: March 4, 2011 1:32:52 PM PST
> To: eeglablist at sccn.ucsd.edu
> Subject: [Eeglablist] Question about average reference
> 
> 
> Hi experts,
> 
> I am running a pilot experiment in which I only use 16 electrodes of a 64-channel cap. There is not any reference electrode on the cap, so I have to re-reference in EEGLAB. Should I just re-reference to the average of the 16 electrodes? Will this generate any problems on the result? After all, the 16 electrodes are mainly at the posterior of the head. And the paper that previously is suggested, Spherical Splines and Average Referencing in ScalpElectroencephalography, mentioned that 19 electrodes are too few. So I guess 16 should be even worse...But if I don't re-reference to the average, would that leads to other problem? Thanks!
>  
>  
> Alan

--------------------------------------------------------------------
Steven J. Luck, Ph.D.
Director, Center for Mind & Brain
Professor, Department of Psychology
University of California, Davis
Room 109
267 Cousteau Place
Davis, CA 95618
(530) 297-4424
E-Mail: sjluck at ucdavis.edu
Web: http://mindbrain.ucdavis.edu/people/sjluck
Calendar: http://www.google.com/calendar/embed?src=stevenjluck%40gmail.com&ctz=America/Los_Angeles
--------------------------------------------------------------------










-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://sccn.ucsd.edu/pipermail/eeglablist/attachments/20110306/67fdb8a7/attachment.html>