Dear all,
I have a group of subjects that performed a task where the stimuli 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 to look at regions where activation both increases with the size of parameter 1 and decreases with the size of parameter 2 since I am interested in regions that respond to both these characteristics but in opposite directions.
I now wanted to look, among these regions that respond differently to these two characteristics, at the regions where the slope of increased activation with p.regressor1 (c1) is smaller than the slope of decreased activation with p.regressor2 (c2).
I am not sure if the correct way to do this to compare contrasts 1 and 2 using a mask with the regions obtained in the conjunction analysis, or how to set these comparisons. Should I do a 2nd-level analysis, and if yes how should I set up the contrasts?
I appreciate your help.
Best regards,
Ann
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