[Mne_analysis] Query over medial wall activity

Wed Feb 6 18:16:10 EST 2013

Hi MNEers,

Thanks for the interesting question and discussion.

About 5 years ago, I too encountered this problem of additional sources
(typically on the medial wall) being reconstructed by MNE, from a dataset
consisting of a simulated source in the primary auditory cortex. We since
resorted to calling them ghost sources. Since then, I have also
encountered sources in other lateral cortical regions 'ghosting' to the
medial wall, especially in the context of resting-state analysis.

I haven't seen a general treatment of this problem, except (very elegant)
case-by-case analysis based on neuroanatomical constraints (such as Donald
Krieger's discussion for S1).

One approach which I meant to try a long time ago, but never did, was to
generate an atlas of "ghost sources" for every simulated source on a
surface vertex, for the MNE parameters in question. Essentially this atlas
would be a point-spread function for each source location. Such an atlas
could either be used to compensate surface current estimates directly (by
incorporating them into some kind of Bayesian priors), or then at least
use alongside our MNE estimate to 'interpret with caution'.

With more groups getting interested in the precise spatiotemporal
localization of scene- and face-processing regions, I would imagine that
ghosting would be an important signal analysis issue to resolve. So thanks
again for the question. Look forward to others' views.

Kind regards,
Pavan

> 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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-- 
Pavan Ramkumar
Brain Research Unit
MEG Core
O.V.Lounasmaa Laboratory
School of Science
Aalto University
Espoo, Finland

e: pavan at neuro.hut.fi
w: http://neuro.hut.fi/~pavan/home
S: pavan_ramkumar
g: pavan.ramkumar at gmail.com
t: +358 50 344 2721