[Mne_analysis] Interpretation of zero/signs in source current estimations

[acgt2 at cam.ac.uk]

Hi Dennis and Dan - sorry, to be clear - I don't actually get zero :-)  I
meant to ask how zero should be interpreted conceptually.

Many thanks for your thoughts Hari - very helpful. As you say, most people
are interested in the fluctuations in current (regardless of what it is
'relative' to) not the literal interpretation of the values. And I hadn't
considered that there may be DC over and above the AC estimated by MNE.

Thanks again,

Andy

-----Original Message-----
From: mne_analysis-bounces at nmr.mgh.harvard.edu
[mailto:mne_analysis-bounces at nmr.mgh.harvard.edu] On Behalf Of Hari
Bharadwaj
Sent: 01 October 2014 19:43
To: Discussion and support forum for the users of MNE Software
Subject: Re: [Mne_analysis] Interpretation of zero/signs in source current
estimations

Hi Andy,
   In my understanding, the estimated currents are absolute in the sense
that there is no standard reference current.

However, note that

(1) the MEG sensors used may not have sensitivity to DC fields (and hence DC
currents).. See http://www.ncbi.nlm.nih.gov/pubmed/16012676 .. though it may
be possible to recover them
http://www.ncbi.nlm.nih.gov/pubmed/16012635

(2) there is typically processing that makes things mean zero (e.g.,
something like baseline correction) and commonly what is of interest is the
fluctuations.. The size of the these fluctuations are indeed in absolute
units.. The sign of the MNE when the orientation is fixed (or if you are
talking about a component along some reference axis) is the direction of
current flow along the axis (parallel or anti-parallel) of the source.

HTH,
Hari


On Wed, October 1, 2014 1:40 pm, acgt2 at cam.ac.uk wrote:
> Dear MNE users
>
> This might have come up before, but I have a bit of an interpretation 
> question - when the source current is estimated, what would 'zero' in 
> the .stcs mean? Should it be interpreted as an absolute value, where 
> no current is moving, on average, in or out of the scalp at that 
> source, or it is relative to some reference current?
>
> I was under the impression that it was some sort of absolute value (ie 
> 'on average, the net movement of positive ions moving towards or away 
> from the scalp is zero, in this source'). Then a positive current 
> value (in a particular source) should be interpreted as the average 
> charge flowing one way (into the scalp? out?) and negative current 
> values meant it was, on average, flowing the other direction. But a 
> colleague today suggested that the zero was relative to a reference: 
> that, due, to neural physiology, there is always an average positive 
> current inwards/outwards, and it is to this 'baseline' rate that the 
> 'zero' is aligned to.
>
> Does anyone have any thoughts on which is the correct interpretation?
>
> Thanks,
>
> Andy
>
>
>
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--
Hari Bharadwaj
Post-doctoral Associate,
Center for Computational Neuroscience
  and Neural Technology (CompNet),
Boston University
677 Beacon St.,
Boston, MA 02215

Martinos Center for Biomedical Imaging,
Massachusetts General Hospital
149 Thirteenth Street,
Charlestown, MA 02129

hari at nmr.mgh.harvard.edu
Ph: 734-883-5954
www.haribharadwaj.com

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