[Mne_analysis] Discrete Source Space source localization

[Luke Bloy]

Thanks for the response Eric.

While I agree with everything you are saying, I think something else is
going on.

Take a look at the attached.
What I did was to first select only the right sensors and then I fit the
following.
1) a cortical sheet dSPM, with a functional label.
2) a fixed location dipole (mne.fit_dipole(pos=...))
3) a dspm with a single source volume source space

the dipole location and the source space location are the same.

As you can see from the time course plots, the cortical sheet dSPM and
fix_location dipole are very similar however the single source dSPM is
quite different.

This doesn't make sense to me. Both the dipole and the single source volume
source space should have the same lead field and should be trying to
explain the same data. granted there is regularization in the dspm, but I'm
not sure why that would move the peaks around in time?


On Thu, Feb 22, 2018 at 12:42 PM Eric Larson <larson.eric.d at gmail.com>
wrote:

> What I am trying to do is to fit auditory data by placing anatomically
>> defined sources in left and right auditory cortex. The issue I'm having is
>> that the time courses aren't matching up with time courses generated using
>> a similar approach in BESA.
>>
>
> Are you doing the equivalent computation in BESA, i.e. a minimum norm
> estimate with dSPM noise normalization, regularization parameter = 1. / 9.,
> and so forth?
>
> More concerning, the time courses don't match what I get if I do a full
>> cortical sheet source localization and extract the time courses from a
>> functional label.
>>
>
> If I understand your source space correctly -- that it is just a handful
> of dipoles (or one dipole) in left and right auditory cortices, or a
> similar small subset -- this is not too surprising to me (though I have
> never tried it). Minimum norm will try to account for all observed sensor
> data using activations in exactly the set of source points provided. So if
> you only provide auditory cortex sources in the source space, MNE will try
> to explain all sensor activity using only these sources. This means that if
> there is any activation outside of auditory cortex -- or even sensor or
> environmental noise -- it will show up in "auditory cortex" anyway (as it
> is the *only* place it can show up). If you have a full cortical source
> space on the other hand, activity from other areas can be accounted for by
> those other source points, and sensor/environmental noise sources can be
> distributed across other sources.
>
> Thinking about the limiting case of only a few locations, activations in
> tiny source space might approach something like the time course of the
> global field power (although dSPM normalization might change this a bit).
>
> There might be other things going on, too -- I'm not sure how nicely the
> minimum norm and dSPM code behave if there are *fewer* source points than
> sensors, for example, though I'd expect it to be okay -- but this is at
> least what first came to mind.
>
> Eric
>
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