[Mne_analysis] averaging forward solutions for meg and eeg

[Matti Hamalainen]

hi,

On Sep 26, 2008, at 11:15 AM, Daniel Goldenholz wrote:

> Hi
>
> I have reservations about the concept of averaging MEG forward  
> solutions. You will effectively spatially blur data that does not  
> need to be blurred. You lose information about the brain this way.
>
> The forward solution for MEG is based on head structural  
> information and position relative to the MEG sensors.
> If you assume the subject did not move between sessions, then you  
> should gain nothing by averaging forward solutions.
> On the other hand, if the subject's head DID move between runs, a  
> more precise solution would be to solve the forward solution per  
> run, compute source space inverse models of each run, THEN do  
> averaging across runs, because then you are in a common coordinate  
> frame, and you are not spatially blurring anything.
> On the other hand, if the head moved DURING runs, you are required  
> to at least consider correcting for that continuously using the SSS  
> technique, which could achieve the same thing for you - i.e.  
> getting all your data into a common coordinate system.

This is true but still averaging the forward solutions is a good  
first-order approximation as discussed in

Uutela K, Taulu S, and Hamalainen M, Detecting and correcting for  
head movements in neuromagnetic measurements. Neuroimage, 14:  
1424-31, 2001.

Also,

Wehner DT, Hamalainen MS, Mody M, and Ahlfors SP, Head movements of  
children in MEG: quantification, effects on source estimation, and  
compensation. Neuroimage, 40: 541-50, 2008.

indicates that even the within-run movement might not be such a bad  
problem as we are lead to believe.

> In the EEG forward solution, head movement is not going to change  
> your forward solution at all. It is based entirely on head  
> structural information and EEG electrode position information,  
> which does not change. The exception to this is if you are  
> accounting for changes in conductivities of the tissue layers - and  
> those may change dynamically. (typically we simply assume constant  
> values from the literature)
> So actually, I don't think you would gain anything at all by  
> averaging EEG forward solutions. You also shouldn't lose anything,  
> since across runs the EEG forward solutions should be identical.

This is exactly true if the runs were within the same session. Across  
sessions, the electrode positions may be a little bit different.

> This is just my perspective. Matti (or anybody else!)- please  
> correct me if I am mistaken in any of the above.

- Matti