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

[Denis-Alexander Engemann]

Another unfortunate way to produce strict zeros in STCs can result from
morphing to fsaverage with insufficient iterations. As a results vertices
remain empty.

-Denis

2014-10-01 20:23 GMT+02:00 dgw <dgwakeman at gmail.com>:

> Hi Andy,
>
> I suspect that if the value is exactly zero, that source location is
> likely excluded due to the mindist requirement when calculating your
> forward solution. Data from points like these should be excluded from
> your analysis (you can convert them to NaN s and then use the
> appropriate NaN tools to deal with them).
>
> HTH,
> D
>
> On Wed, Oct 1, 2014 at 1:45 PM, Denis-Alexander Engemann
> <denis.engemann at gmail.com> wrote:
> > Hi Andy,
> >
> > 2014-10-01 19:40 GMT+02:00 <acgt2 at cam.ac.uk>:
> >>
> >> 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?
> >
> >
> > do you mean literally zero?
> > Do you talk of individual source estimates or did you morph the
> estimates to
> > the average brain?
> >
> > -Denis
> >
> >> 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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