[Eeglablist] ERP analyses and average referencing

Sat Oct 25 12:36:50 PDT 2008

I appreciate the kind things folks have said about my '98 paper.  I'd  
like to add one further thought about average reference visa vis mean  
mastoids.  As I describe in my paper, standard windowed ANOVAs are  
implicitly contrasts between the electrode site being tested and the  
reference site.  This means that mean mastoids yields stronger  
statistical power for vertically oriented ERP components (like the  
P300) since the test benefits from both the positive voltages at Pz as  
well as the negative voltages at the mastoids (the two ends of the  
dipolar field).  However, mean mastoids will conversely hurt  
statistical power of any ERP components whose peak is on the lateral  
surface of the head near the mastoids.  Past of my preference for  
average reference is that it does not result in favoring midline ERP  
components over lateral ERP components, a situation that has led the  
ERP field to systematically neglect lateral ERP effects.  Of course,  
there is no need to necessarily do ANOVAs in this manner.  Ideally,  
whatever reference one uses, one should be using an explicit contrast  
between the two poles of the ERP component (if they are known in  
advance).  Then the reference site would not mater as much.  As I  
concluded in my '98 article, I do echo Steve's bottom line  
recommendation to look at things both ways as long as the ERP montage  
is appropriate for an average reference.  In general I stick to the  
average reference since the vertically oriented dipoles that I study,  
like the P300, are pretty strong and don't need the extra boost to  
statistical power whereas the lateral ERPs that I study are harder to  
detect.

Cheers!

Joe

On Oct 23, 2008, at 1:29 PM, Steve Luck wrote:

> 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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--------------------------------------------------------------------------------

Joseph Dien, Ph.D.
Birth Defects Center
Davidson Hall, Room 314A
Belknap Campus
University of Louisville
Louisville, KY 40292

Office: Davidson Hall 314d (Belknap)
E-mail: jdien07 at mac.com
Phone: 502-852-2512
Fax: 502-852-4702
http://homepage.mac.com/jdien07/

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