Dear all,
I conducted an event related fMRI study, where people had to press
visually cued buttons and were to evaluate the (visual) result by another
button press, which could be either the expected one (c for congruent) or
an unexpected one (ic for incongruent). Sometimes, there were null events
interspersed (fixation cross only). (Actually, there was one more factor,
which is of no interest right now).
During analysis, I was interested in the hemodynamic response to the
visual presentation of both the expected result, the unexpected result and
the difference between both. Therefore, I created a "three-columns" design
matrix with the actual presentation times (in seconds) of the respective
event, the actual duration (in s) and simply "1" for the weighting factor.
My contrasts were specified as follows:
c ic null
1 0 -1 Main Effect Congruent
0 1 -1 Main Effect Incongruent
-1 1 0 Differential Activation (ic > c)
All preprocessing options were left on default (except for slice timing,
which was turned on).
The problem now is that there is NOTHING coming up in the results on the
single subjects level - no higher level visual cortices, simply nil. When
leaving the data unthresholded, some brainstem lights up, which doesn't
help too much...
Now, this really makes me wonder, as the same analysis performed with SPM2
yields the expected results with not too weak t-values even in the
differential contrast. So I suppose I did something wrong with the design
matrix!?! Or are there any other typical pitfalls a beginner might stumble
into which I am not aware of??
I'd greatly appreciate your help with this!
Best regards,
Cornelius Werner
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Cornelius Werner
Institut fuer Medizin (IME)
AG Kognitive Neurologie
Forschungszentrum Juelich
52425 Juelich
Germany
Tel. +49-(0)2461-61-8609
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