Dear Karl
I thank you very much for your help.
If I use these covariate will the contrast [1 -1] for cases in which the
counts are higher during the traumatic event repetition than durin the
baseline, but the counts are decreasing from one repetition to the next?
If I want to use these covariates to compare between the group should I
use the same covariates but within the Multi group*covariate model?
Thank You
Yoram Louzoun
On Mon, 13 Mar 2000, Karl Friston wrote:
> Dear Yoram
>
> > I am conducting a research on the response of PTSD and non PTSD patient
> > to the repetition of their traumatic event.We have 2 group one of
> > PTSD patients and one of non PTSD patients.For each patient we have 3
> > baselines and 4 repetition of the traumatic story.
> >
> > We want to check the difference in the reaction to the traumatic script
> > between the PTSD and control group.
> >
> > We use three type of covariates
> >
> > constant response - 0 0 0 1 1 1 1
> > rise during the repetition - 0 0 0 1 2 3 4
> > decrease during the repetition - 0 0 0 3 2 1 0
> >
> > These covariates are not orthogonal. If I want to check in what region
> > of the brain the response correspond to each of the covariate how
> > should I define my contrasts?
>
> The repetition-dependent increases and decreases are modelling the same
> effect and you only need to specify one regressor:
>
> main effect of traumatic script - -1 -1 -1 1 1 1 1
> script x [linear] time interaction - 0 0 0 -3 -1 1 3
>
> Note that these regressors are orthogonal (and orthogonal to the
> constant term) and can be tested with contrasts [1 0], [-1 0], [0 1]
> and [0 -1].The last two give you increases and decreases
> respectively.
>
> I hope this helps - Karl
>
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