Dear Sir or Madam, 

I'm working with a repeated measures design with two participant groups and two visits corresponding to two conditions (drug/placebo). I have read in previous threads that by entering the two visits as two sessions (sharing the same structural scan), I can specify two conditions (drug and placebo) and perform interaction analysis as a - 1 1 / -1 1 contrast (for participant groups and conditions) in the results section. 

However, I would prefer to assign a different structural scan to each functional scan (so one structural and one functional scan per visit). By doing so, I seem to be unable to specify the two drug conditions in the 'conditions' section (error message: subject ... does not have a scan associated with condition ...) . Therefore, I am wondering if there is another way to specify an interaction effect analysis. 

Thank you very much for any help, 
Kind regards

Charlotte