Dear Yuhong:
At 08:43 AM 7/25/2001 +0100, you wrote:
>Dear SPM group,
>
>I just started to do random contrast. Here is my design:
>
>12 subjects, four conditions (A B C D). I'd like to find out whether D is
>significantly greater than C in a random effect analysis.
The easiest way to do this is for each subject to generate a contrast image
(i.e. a con_*.img file) of D vs. C. So you would specify: D-C == [0 0 -1
1]. This will create a con_*.img for each subject. Note the number of the
contrast in each subject.
Once these are done you would enter the con_*.img files into a second level
one-sample t-test analysis. This analysis tests basically if the con_*.img
at each voxel are different from 0. So you are correct to find out if the
value is greater than 0 at each voxel (which is what you want) you would
enter [1]. To see if it's less than 0 (i.e. if C > D) you would enter -1.
>I did the following:
>(1) For each subject, get a Con***.img and Con***.hdr for the contrast of my
>interest (D-C).
>(2) In SPM99, choose "Basic Models" > "one-sample t-test", and enterred each
>subject's Con***.img as the images. The computation created an output
>"SPM.mat".
See above you want the con_*.img files for the contrast of interest in each
subject.
>My question comes in the next step. The SPM program asks me to use "Results"
>to assess results. So I clicked "Results", selected "SPM.mat" that I just
>created. But then the program requires me to specify a contrast. I am
>completely at a loss as what the "contrast" means. I tried some numbers, and
>it seems to ask me to input one number (e.g., 1) rather than a true contrast
>(e.g., 0 0 1 -1, with the sum = 0). Could you please explain what "contrast"
>means in this step?
It means the same thing as it always does- a linear combination of the
parameter effects. See above.
Regards,
Darren
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Darren R. Gitelman, M.D.
Cognitive Neurology and Alzheimer¹s Disease Center
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