Hi Adrian
> 1. Are the above contrasts correct.
Yes, they seem fine.
> 2. Why do the contrasts have to sum up to 0? I thought the F-contrast should be [zeros(4, 11) eye(4)], but SPM says "invalid contrast".
When you have columns for subject effects, it's not possible to
directly estimate the mean for certain conditions. I.e., if you look
at the indicators along the bottom of the design matrix, they will be
gray. This means you can't do the equivalent of a "[1 0...]" contrast
with these columns, and thus an eye(4) contrast is similarly invalid
(because each row contains an invalid contrast). Comparing conditions
is still fine (as you've discovered).
> 3. Can I simply multiply my contrasts by -1 to test for "negative" effects?
Yes.
Hope this helps!
Jonathan
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