Dear SPMers,
sometimes it is desirable to subtract two conditions from one condition,
e.g. A - (B + C). However, the resulting contrast does not add up to
zero:
A B C
1 -1 -1
If I change this to 2 -1 -1 or 1 -0.5 -0.5 then I am comparing A to the
mean of B and C instad to the sum of B and C, which is not desired.
Now I had this idea: Is it possible to create an empty image (filled
with zeros), one for each participant, and then use this dummy image
during first-level contrast generation?
Then I would have (Dummy + A) - (B + C)
Dummy A B C
1 1 -1 -1
As the dummy contains only zeros, (dummy + A) should be identical to A
alone.
I would then use the resulting con*.img files (one for each participant)
for the second-level group statistics.
Is this possible or am I missing something?
A closely related question: I once read that each task can be decomposed
into a task-specific effect and a task unspecific effect. In the above
example, I would subtract the task-unspecific effect twice from A, which
is not correct. Therefore, it was suggested to add the resting baseline
as an estimator for the task-unspecific effect to A, i.e. (Base + A) -
(B + C):
Baseline A B C
1 1 -1 -1
This is an interaction contrast and makes sense in my case[*]. However,
I would need a reference to bolster this assumption of a task-unspecific
base effect which has to be added, but I can't find the reference. I may
be wrong, but the faint recollection I have suggests a Karl Friston
paper from roughly the mid 1990s.
Thanks a lot for your help &
Kind regards,
Andre
[*] If you assume a 2x2 factorial design: A would be level 1 for both
factors, B would be level 1 for factor 1 and level 0 for factor 2, C
would be level 0 for factor 1 and level 1 for factor 2, and Baseline
would be level 0 for both factors. As a 2x2 matrix:
A B
C BASE
Then the interaction is (A-B) - (C-Base) is equivalent to (A+Base) -
(B+C)
--
______________________________
Dr. Andre J. Szameitat
Department Psychologie
Neuro-Cognitive Psychology
Ludwig-Maximilians Universität
Leopoldstrasse 13
80802 München, Germany
Tel. +49-(0)89-2180 6778
Fax. +49-(0)89-2180 4866
www.psy.uni-muenchen.de/ncp
Office: Martiusstr. 4, Room 6
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