[Eeglablist] Average reference questions.

[Arnaud Delorme]

Dear Andy,

>My name is Andrew Prokofiev, I'm  studying in the Moscow City University of Psychology and teacher's training .
>I've a question to those, who use average reference in their EEG researches.
>
>1. What type of electrodes you include in average references? Do you include HEOG and VEOG electrodes?
>  
>
No. In most scalp montages, these electrodes are recorded using a 
different reference than scalp electrodes. The "average reference" 
assumption is that, if you average electrical potential from a large 
number of electrodes (using the same reference) distributed evenly on 
the scalp, the net potential is 0. Of course, using standard caps, this 
is just an approximation since usually electrode do not cover the whole 
head...

>2. Which advantages and disadvantages average references method has?
>  
>
I am not an expert on this topic. I would say that the advantage of 
using average reference is that you recover the signal from the 
reference channel (thus you have one extra channel even if it is 
linearly dependent on the activity of the other channels). Also, average 
reference is necessary for dipole fitting.

Applying an average reference transformation before applying ICA should 
not change the ICA decomposition (since it is a linear operation). 
However, we have experienced better quality decompositions if we run ICA 
prior to applying average reference, so we advise to apply 
average-reference transformations after running ICA (the re-referencing 
will be also applied to the ICA weigth matrix).

Best

Arno

-- 

* Arnaud Delorme <http://www.sccn.ucsd.edu/%7Earno>, Ph.D.* , SCCN, 
UCSD, San Diego, USA

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