Dear Vladimir,
I would like to compare source wave forms across subjects (e.g. GrandMean,
statistics...). However, I realized that sign/axis of the first eigenvariate
may be different across subjects. Now, I could look at individual time
courses and try to align signs/axis. This is impossible, however, for
subjects or regions that have a lower SNR. Hence, what I would need is an
objective way to know how to align signs across subjects, because otherwise
any across-subjects statistics wouldn't make sense. Do you have any
suggestion on how to do that?
Best,
Anette
Ps.: I realized that the first eigenvariate's sign depends on whether I
extract time courses of only one source (e.g. V1) or two sources (e.g. V1
and A1). I get your point that you cannot unequivocally determine the first
eigenvariate's sign, but why does the sign depend on how many sources you
extract? Similarly, the eigenvariate's sign also depends on the order of
extraction (i.e. (1) V1 & A1 or (2) A1 & V1)
-----Original Message-----
From: Vladimir Litvak [mailto:[log in to unmask]]
Sent: Tuesday, January 31, 2012 8:50 PM
To: Anette Giani
Cc: [log in to unmask]
Subject: Re: [SPM] M/EEG: problems with sourcewave extraction
I don't think there is a way to define uniquely the sign of the first
eigenvariate of a group of dipoles with different orientations. There is
some meaning to the sign of one dipole but not when you summarize a group.
So you can flip the sign if it suits you.
Vladimir
On Tue, Jan 31, 2012 at 4:17 PM, Anette Giani
<[log in to unmask]> wrote:
> Hi Vladimir,
>
> Thanks for having a look at the data. I will try using a fine mesh as
> well as COH or IID and let you know.
> One last question, however: Do you have any idea why the field
> intensities seem to be flipped/inverted depending on which radius you
> chose? (see dropbox folder: SourceExtractionVladimir.pptx, slide 4 and 5).
>
> Best,
> Anette
>
>
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