[Eeglablist] ERP analyses and average referencing

Thu Oct 23 10:29:35 PDT 2008

I'd like to make one last set of comments about the choice of the  
reference electrode:

1) Alex is right that there is no truly correct reference.  It is  
always a matter of balancing the advantages and disadvantages of  
different reference montages.

2) He is also correct that there may be statistical power advantages  
to using the average reference.

3) The major advantage of mastoids (and earlobes, etc.) is that they  
are widely used in ERP research (although not in all subareas).

4) The most important thing is to recognize that your data are always  
influenced by your choice of reference.  As long as you don't forget  
this (and make sure that others don't forget it as well), you will be  
fine.

5) For experiments with broad, dense, and uniform distributions of  
electrodes, the wisest choice is usually to look at the data _both_  
with an average reference and with a mastoid (or earlobe, etc.)  
reference.  That gives you the best of both worlds.

6) For experiments with a limited or nonuniform distribution of  
electrodes, avoid using the average reference (unless you are being  
extremely careful).

Steve Luck

>> From: "Alexander J. Shackman" <shackman at wisc.edu>
>> Date: October 21, 2008 12:22:47 PM PDT
>> To: eeglablist at sccn.ucsd.edu
>> Subject: Re: [Eeglablist] ERP analyses and average referencing
>> Reply-To: ajshackman at gmail.com
>>
>>
>> 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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