Based on what you wrote, why do you think that you did anything wrong?
Suppose that a given region shows a significant response to P1 alone,
but not P2 alone. That doesn't mean that a direct contrast of P1-P2 (or
P2-P1) will necessarily yield anything in that region. e.g., If a
region is just above threshold for significance for P1, and just below
threshold for P2, then it is quite likely that the difference of P1-P2
will be non-significant.
That distinction is why one tests contrasts directly, rather than
"guessing" about differences between two conditions based on the
appearance of their activation maps. (Dr. Smith has commented on this
previously in one of his papers, although I don't know off the top of my
head which one).
On Wed, 2010-10-27 at 20:20 +0100, Aditya Kumar Kasinadhuni wrote:
> Hi all,
> I have run into a problem and request assistance. I have setup two contrasts on my model P1 and P2. Then I have gone ahead to set another two contrasts to look for regions having a a dominant effect of P1 and regions having a dominant effect of P2 indicating P1-P2 and P2-P1 as [1 -1] and [-1 1]. The results show me the regions involved with P1 and P2 solely very well, however the other two contrasts that I have mentioned above show nothing.
> Can anyone suggest what I could have done wrong?