Dear SPM experts,
I have a quick question on the difference between using con images as opposed to beta images for
the second-level analysis in SPM2. Let’s say I have 4 conditions A, B, C and D, and I’m interested in
the effect AB > CD. Using the con images, I would create the linear contrast [1 1 -1 -1] on the
subject-specific first level, and would then take the resulting con images from all my subjects to the
second level, calculating a 1-sample T test. Alternatively, using the beta images that result for each
condition from the GLM (bA, bB, bC, bD), I would set up a 2nd level repeated-measures ANOVA with
the 4-condition beta images as my repeated measures. To get at AB > CD, I would then set up the
contrast [1 1 -1 -1]. Having done the analysis both ways, I get very similar results, but they are not
identical. It would be great if you could help me understand what the exact difference is between
these two approaches.
Any help would be appreciated.
Cheers,
Bernhard
|