Dear SPM Experts,

I have a somewhat basic question regarding T and F contrasts in SPM 8.

Specifically, I have a first level design with 12 conditions, and would like to simply compare the first 6 to the last 6 (since the first 6 are obtained under one drug, and the remaining 6 under another).

At the first level, I've run both a T test with contrast matrix: 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 (positive effect of the first drug) and an F test with contrast matrix of the exact same form: 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 (main effect of the drug). At the second level, I am just running a one-sample T test (t test with contrast matrix 1 or -1) on con/ess images coming from the first level (one image per subject).

I always had the impression that if I run both directions of the T test, either at the first, or at the second level (that is, either run T test with contrast matrix:1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 at the 1st level followed first by one sample T test with contrast 1 at the 2nd level and then by one sample T test with contrast:-1 at the second level, or first run T test with contrast matrix:1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 and  T test with contrast matrix: -1 -1 -1 -1 -1 -1 1 1 1 1 1 1 at the first level, both followed by one sample T test with contrast matrix: 1 at the 2nd level), the significant differences should sum up to significant differences found by running the F test at the first level and a (one sample) T  test with contrast 1 at the second.

However, this does not seem to be the case. The results obtained using two directions of the T test do not sum up to the results obtained using the F test. Please see the attached image. I am getting significant differences by using the F test, as well as by using one direction of the T test, but not the other. In addition, 2nd level significant differences found using the T test at the first level do not match the differences found using the F test at the first level.

Could you please tell me why this would be the case?

Thank you.

Best,

Sanja