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?
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?
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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).
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
> 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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>> is
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(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.
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
[mailto:mne_analysis-bounces at nmr.mgh.harvard.edu] On Behalf Of Hari
Bharadwaj
I