[Mne_analysis] Query over medial wall activity

Krieger, Donald N. kriegerd at upmc.edu
Wed Feb 6 17:53:06 EST 2013
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Although it's reasonable that the majority of detectable MEG sources are due to population post synaptic currents, it also seems reasonable that transient longitudinal currents in axon bundles due to synchronized volleys of action potentials could also produce detectable fields.  Somatosensory evoked responses from the hand typically produce a sharp response in the MEG recordings at about 25 msec post-stimulus (n20) and an opposite going response a few msec later.  This latter response usually localizes well using equivalent current dipole localization to the primary somatosensory cortex with the current's direction pointed posterior consistent with the orientation of the cortex.  Additional waves over the next 20 msec or so typically also localize close by and with the current direction nearly exactly parallel to that of the first p wave, suggesting that they too are cortical.

But the n20 wave, while it also localizes nearby, shows a non-parallel orientation.  Now this wave is known to derive from thalamo-cortical projections.  It is preserved in humans in electrical recordings in the face of profound cortical ischemia due to carotid cross-clamp.  So I speculate that at least in that case, the n20 is due to passage of volleys of action potentials through axons perhaps terminating in the somatosensory cortex.  And given the timing, nearby localization, and the orientation of the equivalent current solution, it seems to originate from the bundles where they curve to enter the cortex.  

Perhaps you are seeing something comparable in the auditory system.

Don

Don Krieger, Ph.D.
Department of Neurological Surgery
University of Pittsburgh


________________________________________
From: mne_analysis-bounces at nmr.mgh.harvard.edu [mne_analysis-bounces at nmr.mgh.harvard.edu] on behalf of dgw [dgwakeman at gmail.com]
Sent: Wednesday, February 06, 2013 5:29 PM
To: A.C.G. Thwaites
Cc: mne_analysis at nmr.mgh.harvard.edu
Subject: Re: [Mne_analysis] Query over medial wall activity

Generally, I am dubious about the likelihood of there being sufficient
pyramidal dendrites in these regions to provide the necessary
architecture for generating signal. I have considered removing these
locations from my forward solution. I am also curious about others'
thoughts.

D

On Wed, Feb 6, 2013 at 5:01 PM, A.C.G. Thwaites <acgt2 at cam.ac.uk> wrote:
> 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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