Dear SPM experts
Please, may we have some information to these questions ?
We have recently performed a blocked-designed fMRI study which comprised
2 sessions and 2 conditions per sessions: A1 and B1 for the session one,
and A2 and B2 for the session 2. Each session comprised 6 epochs with 3
conditions A and 3 conditions B, each of them separated from each other by
a rest condition (R). Thus, one can have, for instance, for one subject :
Session 1: R A1 R B1 R A1 R A1 R B1 R B1
Session 2: R A2 R B2 R A2 R A2 R B2 R B2
The same stimuli were used for both sessions, but a different task was used
in the second session.
We want to extract specific differences between the conditions A and B by
performing the following contrasts :
A1-B1 and B1-A1
A2-B2 and B2-A2
A1-A2 and A2-A1
B1-B2 and B2-B1
In the 'fMRI models' specifications, we have defined 2 sessions and 2
conditions per session. However, in the 'Results' specifications, to define
the contrasts, we must indicate 6 values, and not 4 values as expected.
Therefore, we have provided the following contrasts:
A1-B1 and B1-A1, so 1 -1 0 0 0 0 and -1 1 0 0 0 0, respectively
A2-B2 and B2-A2, so 0 0 1 -1 0 0 and 0 0 -1 1 0 0, respectively
A1-A2 and A2-A1, so 1 0 -1 0 0 0 and -1 0 1 0 0 0, respectively
B1-B2 and B2-B1, so 0 1 0 -1 0 0 0 and 0 -1 0 1 0 0 0, respectively
Our first question is: what is the meaning of the last two values ?
Second, we also want to compare, at least in a first time, the different
conditions with the rest condition. How must we define the contrasts to
compare, for instance:
A1-R
B1-R
A2-R
B2-R
A1A2-R
B1B2-R
Thanks in advance
Dr. ROYET, J.-P.
Laboratoire de Neurosciences et Systèmes Sensoriels
Batiment 404
Universite Claude-Bernard LYON 1
43, Boulevard du 11 novembre 1918
69622 VILLEURBANNE Cedex
FRANCE
Phone: +33 (0) 4 72 44 85 82
Fax : +33 (0) 4 78 94 95 85
email: [log in to unmask]
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