[Mne_analysis] Query over medial wall activity

[A.C.G. Thwaites]

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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