[Mne_analysis] Query over medial wall activity

acgt2 at cam.ac.uk acgt2 at cam.ac.uk
Thu Feb 7 14:23:43 EST 2013
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Hi everyone,

I have done the reconstruction for a simulation of Heshls Gyrus activity and
I end up with Figure 1 (attached), which confirms the findings of Pavan,
namely that sources simulated in Heshls Gyrus can end up (very strongly) on
characteristic parts of the medial wall [1]. I can confirm that the
positioning of this activity is identical to where I find my medial wall
matches. From the comments so far, this seems to be a wide spread
phenomenon.

As Don suggests, this doesn't necessarily mean that all medial wall activity
found is erroneous, but I feel it more than likely that, in my particular
case, it seems misleading to portray my medial wall matches as correct,
given that I have strong prior reasons from the literature to believe that
HG activity is correct.

As I see it, there are a couple of options open to those that find strong
spread from HG (or other areas) onto the medial wall. Please tell me if you
don't agree with these, or if you think there are other options :-)

1.	Exclude 'unknown' and other medial regions from the final analysis,
saying that the reconstruction mislocalises 'real' activity to this area.
But this is a bit difficult - who is to say which area is prone to
mislocalisation and which isn't? The medial wall isn't the only place that
simulated activity mislocalises too - one could use the same logic to cut
out any regions one didn't want to see activity in. As I mention above, I
might be OK claiming this, but only because I have strong evidence from the
literature that tells me HG is correct (and thus that HG is mislocalising to
'unknown', and not the other way around).

2.	Do what Dan suggests and exclude this area from the forward
solution. This would have the added advantage that it would presumably make
the resulting reconstruction more accurate. But one would need to be pretty
sure that the sensors aren't (ever) picking up medial wall activity (or at
least, only very weakly), because once it's gone, true medial activity is
then localised to the lateral surface. As Dan points out, perhaps we might
justify this by assuming there aren't sufficient pyramidal dendrites in this
region, but Don has cautioned that sensors might pick up medial transient
longitudinal currents. I would be very interested to hear any further views
on this.

3.	Further priors of some sort, perhaps related to physiology of the
medial wall. But this would require some sort of agreement of number 2.

Hope his is helpful, thanks for all your thoughts so far,

Andy

[1] the 'no depth' option was present in this reconstruction.

-----Original Message-----
From: mne_analysis-bounces at nmr.mgh.harvard.edu
[mailto:mne_analysis-bounces at nmr.mgh.harvard.edu] On Behalf Of
acgt2 at cam.ac.uk
Sent: 07 February 2013 11:23
To: mne_analysis at nmr.mgh.harvard.edu
Subject: Re: [Mne_analysis] Query over medial wall activity

Hi everyone,

Many thanks for all your thoughts so far. Sheraz - will test this and post
my results to this list, hopefully later today.  As I understand it from his
email, Pavan's ghosting appeared when simulating a source in auditory cortex
- so, as a first step, let's see if I can replicate that. I'll try and look
at a small cross-section of lateral vertices.

Perhaps also worth mentioning, as I didn't put it in the original email - I
also did some MEG-only and EEG-only reconstructions to see which information
is contributing greatest to this (real or unreal) 'ghosting effect', and it
seems to be EEG. This doesn't seem to me to invalidate one view or another -
as I understand it, EEG is better at picking up deep sources, with some
groups using EEG to record brainstem responses (although this effect
requires thousands of trials to become reliable); on the other hand, 'EEG is
better at localising deep sources' is another way of saying 'MEG is worse at
localising deep sources',  so if this effect is a mislocalisation to the
medial wall then perhaps it is not surprising that MEG is not contributing
as much to it. 

Anyway, will report back later with results.

Andy


-----Original Message-----
From: sheraz at nmr.mgh.harvard.edu [mailto:sheraz at nmr.mgh.harvard.edu]
Sent: 06 February 2013 23:20
To: A.C.G. Thwaites
Cc: mne_analysis at nmr.mgh.harvard.edu
Subject: Re: [Mne_analysis] Query over medial wall activity

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
>
>
> _______________________________________________
> Mne_analysis mailing list
> Mne_analysis at nmr.mgh.harvard.edu
> https://mail.nmr.mgh.harvard.edu/mailman/listinfo/mne_analysis


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