Dear all,
I’m updating this message I sent a few weeks ago, now with a new question ….
I have a group of subjects that performed a task where the stimuli (various geometric forms) presented had different contrast to background levels and different sizes. I used SPM12 to set up a contrast (1st-level analysis) to look at regions where activation increases with increasing contrast to background (parametric regressor 1, so c1 = 0 1 0) and a contrast to look at regions where activity decreases with increasing size of stimuli (parametric regressor 2, so c2 = 0 0 -1) [the first column corresponding to task onset, the second to parametric regressor 1 and the third to parametric regressor 2].
I then performed a conjunction analysis (conjunction-null) to look at regions where activation both increases with increasing contrast to background and decreases with the size of stimuli since I am interested in regions that respond to both these characteristics but in opposite directions.
I now wanted to do the same analysis but at the group level, so using a 2nd-level analysis, but I am unsure on how to do this. I tried doing two separate t-tests, one for each 1st-level contrast, but now I can’t find a way of doing a conjunction analysis with these two contrasts because they are stored in different SPM.mat.
Which kind of design should I use in the 2nd-level analysis to be able then to do a conjunction analysis?
I appreciate your help.
Best regards,
Ann
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