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Hi everyone,

   I have some problems in two sessions analysis. I want to compare two
activation maps from the same subject in two different conditions. I built
the design matrix with two sessions (using first level analysis) so I ended
up with four columns as you can see from the first figure of the attached
file. I then did inference analysis with contrast vector of [1 -1 0 0] and I
suppose I shoud get a map with the significative differences between the two
conditions.
my problem is that if I implement this same approach with maps that are
significantly different for sure (right ankle dorsiflexion and left ankle
dorsiflexion) I get a "difference map" that is not at all as expected. so as
you can see what I'm talking about, the second figure of the attached file
is the result I got.
yellow map: right ankle dorsiflexion
green map: left ankle dorsiflexion
red map: difference map
Is the approch wrong? any suggestions?

I would really appreciate any help
thanks a lot!!
marta