Dear Paul,
I'm not sure if you are working with PET or fMRI. Certainly with PET
paradigms and most fMRI paradigms, contrasts across conditions within a
group, need to total zero; i.e. they need to be balanced. It appears from
what you have written that you have three conditions a, b, and ab. If so you
need to specify contrasts across these conditions that add up to zero. So to
test for areas that are more active in b than a, you would need to enter: -1
1 0. For areas more active in ab than a AND b: -1 -1 2; which I think
answers the latter part of your query.
I hope that others will contribute if I have misled you here.
Cheers.
Alex.
-----Original Message-----
From: [log in to unmask]
To: SPM
Sent: 14/07/00 16:50
Subject: contrasts
Hello everyone,
I sent out a question early this week and have not received any replies.
So, I am reposting the question.
I have 3 conditions a, b, and ab. I set my contrasts -1 -1 1 0. Does
this show areas where ab>a+b. If not, what does this contrast represent?
Thanks,
Paul Laurienti
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