Dear Paolo,
> We have a question regarding contrast for fmri study. we have two
> activation
> conditions 1 2 1 2 1 2. we think that the contrast would be like that:
>
> C1: 1 0 to see effects of activation 1 against activation 2
> C2: 0 1 to see effects of activation 2 against activation 1
>
> is that correct?
>
Not really. If you have two active conditions (1 and 2) and an
implicitly modelled baseline, C1 (1 0) will show main effects for condition
1 (i.e., significantly greater than baseline for a t-contrast, significantly
different from zero for an F-contrast) and C2 will show likewise main
effects for condition 2. To compare 1 and 2, specify (1 -1) for 1>2 and (-1
1) for 2>1 (both t-contrasts). Also note that SPM models each
session/subject separately so that with multiple sessions/subjects
comparisons for single subjects/sessions would be 1 -1, 0 0 1 -1, 0 0 0 0 1
-1 (cond1>2 for session 1,2,3); across sessions 1 -1 1 -1 1 -1 ....etc.
> Please excuse me for such basic questions
>
No problem
Dick Veltman
> Paolo Vegetti
>
>
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