[Mne_analysis] Large mean voltages in BEM EEG forward model.

[Yury Petrov]

As Matti said, the average reference only matters when you compute the  
inverse. The forward solution is computed for each hypothetical dipole  
assuming, for example, that the dipole potential at infinity is 0  
(which is the usual choice of the potential reference in  
electrostatics). The forward solution has nothing to do with the  
actual measurement, except for the positions of the electrodes on the  
scalp.

On Sep 30, 2008, at Sep 30, 2008 | 5:12 PM, Justin Ales wrote:

> I'm confused. I don't understand the hypothetical potential
> distribution in your example. I thought the  forward operator should
> take into account the reference of the electrode measurement montage.
> The sum of the potentials over the electrodes in the recorded data
> would be 0 because of the explicit average referencing.
>
> Justin
>
>
> On Tue, Sep 30, 2008 at 1:18 PM, Yury Petrov <y.petrov at neu.edu> wrote:
>> I think that the MNE forward matrix is just the distribution of  
>> potentials
>> on your electrodes produced by a current dipole at a given location  
>> times
>> all such locations. There is no reason for the sum of potentials  
>> over the
>> electrodes to be always 0. For example, if the electrodes cover  
>> mostly the
>> area of the scalp to where one pole of the dipole points, then the  
>> averaged
>> potential will be quite different from zero. You could add a constant
>> potential to turn the average to zero, but this would only work for  
>> one
>> dipole out of many.
>>
>> On Sep 30, 2008, at Sep 30, 2008 | 3:54 PM, Justin Ales wrote:
>>
>>> Hi,
>>>
>>> I've been looking at the forward computations generated by MNE.   
>>> I've
>>> been generating my fif files from data I have in matlab. I have an
>>> average reference montage, and when I look at the forward model  
>>> there
>>> are large variations in the average across channels.  Since it is an
>>> average reference montage I expected the forward model to have zero
>>> mean. I'm not sure if I don't understand the forward model output  
>>> from
>>> MNE, or if I'm not importing my data correctly.
>>>
>>> Thanks,
>>> Justin Ales
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>>> Mne_analysis at nmr.mgh.harvard.edu
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>>
>>
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