[Eeglablist] ERP analyses and average referencing

[Alexander J. Shackman]

Arno prefaced his comments by noting that "average referencing is always
incorrect." But as Joe Dien notes in his excellent '98 paper, it would be
equally appropriate to say that "mastoids montages are always incorrect" or
"ALL referencing schemes are always incorrect."
Steve and Arno are correct in noting, as Dien did, that the topography and
waveforms will differ across montages, making it difficult to compare
average to the more commonly used (for ERPs, at least) mastoids montage.

But there are at least two other reasons, at least with high-density
recordings, to consider using the average reference. 1) Dien suggests that
the average reference, which is employed by both dipolar and distributed
source modeling algorithms, potentially provides more insight into the
underlying cerebral generators. 2) The average reference is likely to more
psychometrically reliable (cf. S. Gudmundsson et al., Clinical Neurophys,
2007).

Alex Shackman




On Sun, Oct 19, 2008 at 2:29 PM, Steve Luck <sjluck at ucdavis.edu> wrote:

> I would like to echo and expand upon Arno's comments about average
> referencing.  Under the most optimal conditions this can be perhaps a decent
> approximation (see Dien, 1998).  However, under most conditions it is a poor
> and misleading approximation (and, as Arno pointed out, is is never
> completely correct).  Your waveforms will look completely different
> depending on what electrodes you happen to be using (see Figure 2 and the
> related text in chapter 3 of An Introduction the Event-Related Potential
> Technique).  As a result, your data may look quite different from the data
> of other researchers, even if they are also using the average of all sites
> as the reference (because they probably don't have exactly the same set of
> sites that you have).
>
> So, what to do?  Lately, my lab has been seeing the same sort of problem,
> with lots of muscle activity being picked up by mastoid reference
> electrodes.  The best thing to do is to try to get subjects to sit in a more
> neutral position so that they do not need to contract the neck muscles to
> keep the head upright. However, if you already have this noise in your
> mastoid data, you can try referencing to scalp sites that are close to the
> mastoids (e.g., P9 and P10), which may have less muscle noise.  Or, if you
> have a sufficiently dense array of electrodes, you could use the average of
> a small cluster around the mastoids on each side as the reference.
>
> The most important thing is to realize that you are _always_ looking at the
> potential between two electrode sites (or groups of sites).  There is no
> such thing as potential at a single site.
>
> Steve Luck
>
> *From: *arno delorme <arno at ucsd.edu>
> *Date: *October 18, 2008 4:15:53 AM PDT
> *To: *Yvonne Tran <Yvonne.Tran at uts.edu.au>
> *Cc: *eeglablist at sccn.ucsd.edu
> *Subject: **Re: [Eeglablist] ERP analyses and average referencing*
>
>
> Dear Yvonne,
>
> average referencing is always incorrect. The amount of current going in and
> out of the head is assumed to be 0. Using that properties, average
> referencing means that the average potential across all electrode is 0 at
> all times. However, you cannot expect that the electrode spatial
> distribution will be homogenous over the head (because first you cannot put
> any within the neck, and there is usually no electrode on the face etc...).
> It is generally assumed that the current flowing within the neck is
> negligible (because of high conductances).
>
> As an answer to your question, if your electrode repartition is relatively
> homogenous on the scalp, then you may use average reference. Nevertheless,
> average reference will not make it easy to compare between montages.
>
> Best regards,
>
> Arno
>
> On 17 sept. 08, at 04:20, Yvonne Tran wrote:
>
> Dear All
>
>
> We are currently working with spinal cord injured participants and have
> recorded some oddball data. We have been using A1 and A2 mastoid for
> reference channels, however, with this particular group we are experiencing
> increased muscle tension in this region (which cannot be prevented, as some
> participants are unaware that they are tensing up), and therefore when the
> data are re-referenced the other EEG channels become flooded with muscle
> tension noise. This can be overcome when we re-reference using average
> referencing. My question is how many electrodes (evenly distributed around
> the scalp) will be ok for average referencing for ERP analyses? We have 26
> EEG channels.
>
>
> Any suggestions/opinions appreciated!
>
>
> Thank you
>
> regards
>
> Yvonne
>
>
> --------------------------------------------------------------------
> Steven J. Luck, Ph.D.
> Professor
> Center for Mind & Brain and Department of Psychology
> University of California, Davis
> 267 Cousteau Place
> Davis, CA 95618
> (530) 297-4424
> sjluck at ucdavis.edu
> --------------------------------------------------------------------
>
>
>
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-- 
Alexander J. Shackman, PhD
Laboratory for Affective Neuroscience
Waisman Laboratory for Brain Imaging & Behavior
University of Wisconsin-Madison
1202 West Johnson Street
Madison, Wisconsin 53706

Telephone: +1 (608) 358-5025
Fax: +1 (608) 265-2875
Email: shackman at wisc.edu
http://psyphz.psych.wisc.edu/~shackman
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