I'm comparing patients and controls on a simple contrast of a cognitive
task. Region A is hypoactivated in patients, however I want to make sure
that this hypoactivation is not due to the fact that patients are
slower. For this purpose I want to covary out the effect of response
times by modelling them at the second level (mean response time for each
subject). I'm using the multiple regression model in SPM2, but I have
- should I model the 2 groups with 1 covariate (i.e. 1 1... -1 -1) and
do a contrast of  OR with 2 covariates (i.e. 1 1... 0 0 and 0 0... 1
1) and then do a contrast of [1 -1] ?
- same question for the RTs: should I model the RTs of all subjects
(controls and patients) in 1 covariate or in 2 separate covariates ?
I have tried these different ways and observed that the results were
pretty different, but I'm not expert enough in statistics to decide
whether one of them is "more correct". Moreover I found conflicting
comments on the SPM list...
Any help will be greatly appreciated.
Thank you very much.
Guillaume Sescousse, PhD student
'Reward and decision making' group
Centre de Neuroscience Cognitive
CNRS UMR5229 - UCB Lyon 1
67 Bd Pinel, 69675 Bron, France
tel: 00 33 (0)4 37 91 12 44
fax: 00 33 (0)4 37 91 12 10