[Mne_analysis] averaging forward solutions for meg and eeg

[Daniel Goldenholz]

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.

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

Daniel

On Fri, Sep 26, 2008 at 10:58 AM, <ssteinv at nmr.mgh.harvard.edu> wrote:

> Hi,
>
> we collected meg and eeg data in a task with several runs. I am wondering
> how to average the forward solution of these runs combining the meg and
> eeg data? mne_average_forward_solutions only averages meg forward
> solutions but not any eeg data.
>
> thanks, Sarah
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-- 
Daniel Goldenholz MD, PhD
--------------------------------------------------------
http://www.nmr.mgh.harvard.edu/~daniel
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