I am new to using spm8 and am curious about the contrasts. I have found some seeming inconsistencies with what spm users that I know have said to use as contrasts and would like some clarification.
Example 1 (single subject analysis):
We are making post-treatment vs. pre-treatment comparisons and have 2 experimental conditions of interest. Pretend we have 1 run and so the contrast for looking at
experimental condition 1 vs. exp cond 2 is
1 -1 0 0 0 0 0 0 0
for each post and pre session. We've been told the contrast for
post (exp cond 1 vs. exp cond 2) vs. pre (exp cond 1 vs. exp cond 2) should be
1 -1 0 0 0 0 0 0 0 -1 1 0 0 0 0 0 0 0.
However, this will also be the contrast for
pre (exp cond 2 vs. exp cond 1) vs. post (exp cond 2 vs. exp cond 1).
How can we be sure we're looking at post-pre for exp cond 1 vs. exp cond 2 rather than pre-post for exp cond 2 vs. exp cond 1?
Example 2 (group analysis):
In a random effects analysis, say we have exp cond 1 and exp cond 2 again. Con1 is exp cond 1 vs. exp cond 2 and Con2 is exp cond 2 vs. exp cond 1. We've been told that for a simple t-test, the contrast would be 1 for activations and -1 for deactivations such that if we are looking at Con1, a contrast of 1 would be activations for exp cond 1 vs. exp cond 2 and a contrast of -1 would be deactivations for exp cond 1 vs. exp cond 2. However, it seems that "deactivations" is just the same as activations for Con2 (i.e., exp cond 2 vs. exp cond 1).
Am I making any sense? I want to make sure I'm looking at the right thing and as of late, I'm getting very confused.
Thanks in advance for any help you can give,