[Mne_analysis] averaging forward solutions for meg and eeg

Daniel Goldenholz daniel at nmr.mgh.harvard.edu
Fri Sep 26 11:42:40 EDT 2008
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I defer to Matti on the relative value of doing what I proposed.


I stand by the following more restricted statements:
Procedure A: average forward models, then average MEG data in sensor space
and use common forward/inverse model
Procedure B: use individual forward models, compute inverse for each
dataset, then average in brain source space

- It is less precise to do procedure A than procedure B between runs (or
sessions) provided that it is true that you have minimal movement within run
(or session). The level of precision difference will depend on many factors,
including differences in head position (in MEG) and differences in EEG
electrode placement (in EEG)
- If there is head movement within a run or session, averaging forward
models (only in MEG) between adjacent runs (not sessions) may have an
advantage of crudely correcting for the movement. SSS, if you trust the
method, would represent a finer correction because it accounts for head
position continuously.

Doing procedure B also is computationally slower.

D

On Fri, Sep 26, 2008 at 11:22 AM, Matti Hamalainen
<msh at nmr.mgh.harvard.edu>wrote:

>
> 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
>
>
>


-- 
Daniel Goldenholz MD, PhD
--------------------------------------------------------
http://www.nmr.mgh.harvard.edu/~daniel
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