[Eeglablist] ERP analyses and average referencing

[Steve Luck]

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
>
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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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