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

[Hari Bharadwaj]

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


-- 
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