 |
 |
Dear SPMers, 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 several concerns: - should I model the 2 groups with 1 covariate (i.e. 1 1... -1 -1) and do a contrast of [1] 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 -- ___________________________________ 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 http://www.cnc.isc.cnrs.fr http://www.isc.cnrs.fr/dre ___________________________________