Hi Russ,
Thanks for the suggestion! Unfortunately, I forgot to mention
to the SPM list that for each of my subjects I've already included the
motion parameters for each session as covariates in the analysis. Could
ring-like deactivations in the white matter due to motion persist even
after using the motion parameters as covariates in the analysis?
Daniel Weissman, PhD
Center for Cognitive Neuroscience
Duke University
Durham, NC 27708
> try running the analysis with motion parameters as covariates- I bet it's
> task-correlated motion.
> rp
>
> Daniel Weissman wrote:
>
> > Dear SPMers,
> >
> > I've been noticing some fairly extensive ring-like
> > white-matter deactivations in a fast-rate event-related fMRI paradigm. In the
> > study, there are 7 trial types, which are presented in a randomized
> > fashion. So far, I have 15 subjects and am analyzing the data with
> > random effects analyses on the con* images.
> >
> > When I contrast events in which a target stimulus appears to
> > events in which no target appears, I find activations in expected areas
> > such as the visual and motor cortices. However, ring-like deactivations
> > in the whitematter (and deactivations in other areas as well) occur when
> > I contrast non-target events to target events.
> >
> > The extensive ring-like deactivations in the white matter for
> > non-target events led me to test whether they might be an artifact of
> > global proportional scaling which I had performed during estimation of
> > the general linear model for each individual subject. As others have
> > noted, when a particular volume contains highly activated brain regions,
> > global proportional scaling reduces activation in all brain areas within
> > that volume to make the volume's mean activation equal to that of all
> > other volumes in the series. Unfortunately, this procedure can sometimes
> > render inactive regions within a highly active volume (e.g., the white
> > matter) deactivated.
> >
> > Thinking along these lines, I re-estimated the model for each
> > individual subject, this time selecting no global scaling during the
> > procedure. However, the ring-like deactivations remain.
> >
> > If I understand them correctly, recent discussions on the SPM list have
> > stated that SPM scales all volumes to a mean of 100 by default and
> > that selecting global scaling during model estimation makes the
> > scaling proportional. Is this true? If so, then would anyone be able to
> > advise me on how to turn off global scaling entirely? Alternatively,
> > would anyone have any other ideas about what might be causing the anomalous
> > rings of deactivation I am observing?
> >
> > Any advice would be greatly appreciated.
> >
> > Thanks and Cheers!,
> >
> > Daniel Weissman, PhD
> > Center for Cognitive Neuroscience
> > Duke University
> > Durham, NC 27708
>
>
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