Dear SPM developers,
I´m writing to you because one of the reviewers of my manuscript is asking to include the time since injury as regressor in my model and I was hoping you could help me to address this point correctly.
My dataset consists of T1 images at 3 time points (tp) and 2 groups (gr) from patients with traumatic brain injury. I ran a longitudinal VBM analysis and specified a flexible factorial to examine groupxtime interactions with the ICV as nuisance regressor. In this analysis, I found some significant effects of groupxtime for the grey matter volume. Here one example of a contrast vector (T contrast) for the interaction (gr1>gr2 x Tp2>Tp1) is: [-1 1 0 1 -1 0].
In the new design, I also included the time since injury in the model as a covariate and got almost identical results. Then I wanted to have a look at the effect of the time since injury as a covariate of interest in this design.
Could you please suggest some contrast vector options for this in this model? And what effects would I be assessing exactly with such contrasts so that I can make correct inferences from my results?
Many thanks in advance for your time.
Kind regards,
Noelia
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