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
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|