Hello all,
my question may be very trivial to you but I would very much appreciate
your help with the problem I encountered when trying to analyze my blocked
design fMRI data with FSL. In my design, there are two types of tasks, L
and C, and each task has two "situations", L1 and L2, and C1 and C2.
I intended to find out whether there would be differences in brain
activations during the two different tasks and/or situations. So I tried
to do this by using FEAT fMRI Analysis and Full model setup. I wished to
model each of my four conditions separately, so I made a custom file
of each of the conditions (made of zeros and ones) and set each EV according to
these text files. By now I got 4 EVs: L1, L2, C1 and C2. My intention was
to use contrasts to find the differences between conditions. But here
comes the problem: when I tried to view the design, I received the error
message: "Problem with processing the model: At least one EV is (close to)
a linear combination of the others. You should probably alter your
design."
I was quite puzzled since I donīt really see how these 4 conditions can be
combinations of each other. They are separated in time, one comes after
another, so to my understanding they all should be independent from each
other, not combinations or mirror images. Can you help me to understand
this issue, and if the problem really is in the design, are there any
means to counteract this without having to change the design?
I noticed that FSL gives the opportunity to force an EV to be orthogonal
to other EVs. I tried this option, and when all EVs were orthogonalized to
the lower number EVs, the program proceeded to process the data without
complaining. I still feel that this kind of forcing is not the solution to
my problem. What kind of criteria does the program have to "decide"
whether some EVs are combinations of each other or not?
I would also be interested to run an ANOVA on my data and study the main
effects and interactions. If I understood the instructions of the manual
correctly, to study an interaction, I should make an EV to represent an
expected interaction (with zeros and ones). So in my design there would be
several of these interaction EVs. How to avoid the problem presented in
the beginning of this message?
The text files that I used are attached here in case that you want to
take a look at them. It seems that I have more than one question here.
Anyhow, all help is warmly welcome and very much appreciated.
Virve Vuontela
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