[Mne_analysis] Query over medial wall activity

sheraz at nmr.mgh.harvard.edu sheraz at nmr.mgh.harvard.edu
Wed Feb 6 18:19:49 EST 2013
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Hi Andy,

Best solution to find out, your activity on medial wall is real or not,
put a simulated source on Heshl's Gyrus of apropriate size, multiply it
with the forward operator and then add some empty room noise to it. Solve
again the inverse solution to find the spread.

This can be done easily in mne-matlab or mne-python.

Sheraz




> Hi MNE-ers
>
>
> I am working with auditory data, running my analysis on source estimations
> reconstructed from MEG and EEG sensor recordings.
>
>
> My analysis takes the form of pattern matching over the estimated activity
> of each of the vertices in a source space, and as such, is reliant on the
> reconstruction being of good quality. I am very pleased with the quality
> of the results using MNE - my pattern matching technique should locate
> those vertices along Heshl's Gyrus, and indeed it does - an indication,
> presumably, of the high quality of the reconstruction. (so a big thank you
> to everybody involved with constructing and maintaining MNE!)
>
>
> However, I did want to ask this mailing list about one concern: my pattern
> matching technique also picks up vertices directly 'under' HG - on the
> medial wall in the 'unknown' label of the Destrieux Atlas
> (aparc.a2009s.annot) (see figure 1 attached). It seems pretty clear why:
> the inverse solutions given by MNE give both these regions similar evoked
> responses (figure 2 of the attached), which is why my pattern matching
> technique flags both areas up. While it is possible that these results may
> be correct (the auditory thalamus is in this area, and so might plausibly
> causing this medial activity) I wanted to poll this mailing list to get a
> feel for how likely you think this activity is being correctly estimated
> here, or if you feel it is a simple case of mislocalisation from the
> auditory cortex (and if so, whether it can be fixed). I'm not really sure
> what my grounds for suspicion are, except that the affected vertices on
> the medial wall are directly under HG - implying the HG source activity
> might be 'seeping' through to these more medial sources during
> reconstruction.
>
>
> I have observed this phenomenon in two independent experiments. And
> although I can't do my pattern matching on the MNE example 'audvis' data,
> this too seems to show the same phenomenon (figure 3).
>
>
> I have tried pretty much every flag and option MNE offers - depth on/off,
> sLORETA vs. MNE vs. DSPM, different SNRs, pick_normal on/off, different
> looseness's - all end up with pretty much identical results (which is
> good, I guess, as it means the reconstruction is pretty robust).
>
>
> I appreciate that for many people this isn't an issue if they are doing
> analysis only in predetermined regions of interest (I can't imagine that
> many people are looking for results in a label called 'unknown'). But as
> my analysis works by searching vertex-by-vertex, I want to say truthfully
> that I looked through all vertices the reconstruction gave back, or at
> least give a reason why I excluded vertices in the `unknown' label from my
> analysis.
>
>
> Anyway, I don't know if it is a common occurrence, or is something I have
> done wrong (although the fact that we see the 'audvis' data behave in the
> same way is evidence against this). Or maybe you think it is correct - a
> number of my co-authors have suggested we take it as correct, and say it
> is evidence of a cortico-Thalamic loop.
>
>
> I attach some figures that demonstrate the phenomenon.
>
>
> Thanks in advance for any thoughts.
>
>
> Andy
>
>
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