[Eeglablist] ERP analyses and average referencing

[arno delorme]

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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