[Mne_analysis] Advantages of using "average EEG reference" in mne_browse_raw?

Matti Hamalainen msh at nmr.mgh.harvard.edu
Thu Mar 5 19:23:09 EST 2009
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Hi,

I agree with both Yury and Daniel, just a couple of additional comments:

(1) Actually, clinical neurophysiologists not only look at different  
references but also derivations, i.e., differences or more complex  
linear combinations (surface Laplacian) between the EEG channels.  
Effectively, this is like constructing a coarse inverse solution from  
EEG data. I am always very impressed how well-informed predictions  
about the sources underlying, e.g.,  epileptic activity a trained  
physician can make with this simple method.

(2) Theoretically, the choice of the the reference electrode is  
immaterial. However, with the average EEG reference the value  
predicted by the forward model at the reference electrode does not  
affect the solution, which may be important if the reference electrode  
location is such that the forward modeling error is larger than for  
other electrodes. I do not know whether this really counts in practice.

(3) The MNE inverse operator calculation enforces the average EEG  
reference to be used.

- Matti



On Mar 5, 2009, at 5:48 PM, Daniel Goldenholz wrote:

> People that read EEG "by hand" use multiple references because it  
> gives different views of the data.
> That is because humans can't see the physics.
>
> Daniel Goldenholz MD, PhD
> --------------------------------------------------------
> http://www.nmr.mgh.harvard.edu/~daniel
>
>
> On Thu, Mar 5, 2009 at 5:40 PM, Yury Petrov <y.petrov at neu.edu> wrote:
> Hi Andres,
>
> In physics the reference point for electric potentials is usually  
> chosen to be in the infinity, which is a nice choice, because it is  
> unique and makes sense otherwise. However, choosing reference point  
> on the head surface is necessary to remove electrical environmental  
> noise from EEG. To simplify matters, let's assume that EEG activity  
> recorded on the scalp reflects only the neuronal activity in the  
> (small) underlying part of cortex. By using referencing you  
> effectively compare the activity under a given electrode with  
> activity elsewhere in cortex. What is better for your purpose: to  
> compare it with average cortical activity, or with cortical activity  
> in the vicinity of your reference electrode(s)? It's up to you to  
> decide, really. In practical terms, referencing to one or two  
> electrodes means that signal on nearby electrodes will be weaker  
> and, thus, less informative than on remote electrodes. Also, keep in  
> mind that the particular choice of reference is immaterial for  
> source localization: the result will be the same.
>
> Best,
> Yury



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