Dear SPM group:
I want to make sure I am doing a random effects analysis correctly. I am
using fMRI with block design task. There are 12 subjects and 2 tasks each
with 2 conditions.
The baseline condition of both tasks is basically the same, the active
conditions differ- basically AB and A'B.
Step 1) collapse images for each condition for each subject.
Convolve HRF with boxcars: yes
High pass filter: yes
Global covariate: yes proportional scaling.
Thus I end up with 4 images per subject.
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I then put this into a multi-study different conditions design and said
there were 2 studies, 12 subjects per study and 2 conditions per study.
Contrasts
-1 1 0 0 For main effect of B vs. A
0 0 -1 1 For main effect of C vs. A'
-1 1 1 -1 For interaction of (B - A) - (C - A)
1 -1 -1 1 For interaction of (A - B) - (A - C)
QUESTIONS:
I tried also putting the data through multi-subject different conditions to
look at the main effects but the activations were much more restricted than
in the multi-study analysis and missed areas of activation that clearly
showed up in individual subjects. Do I have to use this analysis for main
effects?
If I had an anatomically restricted hypothesis about the interaction I
assume it would be OK to threshold the map at p = .01?
thanks,
darren
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Darren R. Gitelman, M.D. E-mail: [log in to unmask]
Cognitive Neurology and Voice: (312) 908-9023
the Alzheimer's Disease Center Fax: (312) 908-8789
http://www.brain.nwu.edu
Northwestern Univ., 320 E. Superior St., Searle 11-470, Chicago, IL 60611
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