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Hi Peter, thanks for the reply, I have to take a look a closer look at the data. At the moment I do not know what causes these differences. But could you clarify what you mean by modeling the sources? That is, how would I model them in the DCM framework? greetings Am Mo., 5. Nov. 2018 um 15:56 Uhr schrieb Zeidman, Peter < [log in to unmask]>: > Dear David > > I don’t think this is an SPM related question… The amplitude of the > response will be determined by the physics of the acquisition as well as > the generation of the underlying signal. > > > > If you look at your functional images, are some areas brighter than > others? I.e. are they heavily affected by bias (a slow gradient in signal > amplitude)? You can often find the areas closer to the head coil are > brighter than deeper areas. You might want to consider increasing the bias > correction in your preprocessing if so. > > > > As for the effect on DCM… yes I think it might make a difference that some > areas have greater scaling than others. All timeseries will be rescaled > during DCM estimation (to have maximum variance of 4% if I remember > correctly). So the scientific question for you to consider is – what are > you hypotheses for the source of this difference in scaling? And do you > want to try to model those sources in order to address the question? > > > > Best > > Peter > > > > *From:* David Hofmann [mailto:[log in to unmask]] > *Sent:* 24 October 2018 23:09 > *To:* spm <[log in to unmask]> > *Cc:* Zeidman, Peter <[log in to unmask]> > *Subject:* VOI time course shows large differences in magnitude of > principal components > > > > Hi all, > > > > I used the VOI tool to extract the time course of two different ROIs. > Strangely, the magnitude of the principal components are very different > between the ROIs (see attached). That is, the first ROI (blue) has a much > higher magnitude. Can anybody explain to me how such large differences come > about? > > > > I also wonder, if this might cause a problem in the estimation of a > resting state DCM later on? > > > > greetings > > > > David > > >