Dear Anette,
Since the sign is not meaningful, shouldn't you perhaps take the
absolute value or square of the individual waveforms and then average?
I don't know why you get differences depending on order etc. It
shouldn't be the case from brief look at the code. If you give me an
example I can look into it.
Vladimir
On Fri, Mar 2, 2012 at 4:26 PM, Anette Giani
<[log in to unmask]> wrote:
> 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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