[Mne_analysis] Forward solution and inverse operator gain for deep discrete dipoles

[Andres Felipe Salazar Gomez]

Thank you Matti,

I understand that is the case when using several dipoles at the cortex and
a few at deeper structures but in my case the model has only 2 dipoles in
the cortex and four at the brain stem. Will it give me such results
regardless the number of dipoles and its locations?

Also, is it possible to find a way to scale the estimates at the cortex to
be able to compare them with the ones at deeper structures?

Thanks,

-- Andrés

>
> On Jun 3, 2009, at 3:13 PM, Andres Felipe Salazar Gomez wrote:
>
>> Hi MNE users,
>>
>> With mne_volume_source_space I got a model with 6 discrete sources,
>> 4 in
>> deep brain structures (brain stem) and 2 at the cortex. After
>> getting the
>> forward solution and the inverse operator I was able to find the
>> estimates
>> for these 6 sources but those at the cortex seem to be two orders of
>> magnitude smaller, which is not what I was expecting to get.
>>
>> My question is: when MNE calculates the forward solution and the
>> inverse
>> operator, does it take into account how deep are the dipoles? I am
>> wondering if somehow the gain in the G matrix (fwd.sol.data) or the
>> noise
>> covariance matrix (inv.noise_cov.data) is smaller for the cortical
>> dipoles
>> and therefore that is why I am seeing this differences in magnitude.
>> Any
>> idea?
>>
>> Thank you very much in advance for any help regarding this issue.
>> Sincerely,
>
> Hi,
>
> The deeper dipoles are not weighted any differently from the cortical
> ones.
>
> The difference in magnitude is because on the cortex the estimates
> have a chance (and will) spread out thus creating a weak distributed
> source in MNE whereas there are only few brain stem dipoles have to
> account for the data and will, therefore, be stronger. How to best
> deal with this is subject to further investigation.
>
> - Matti
>
>
>
>


-- 
Andres F. Salazar
Research Technologist
Neuroscience Statistics Research Laboratory
asalazar at nmr.mgh.harvard.edu
149 13th Street, Room 4005
Charlestown, MA  02129