Search as I might, I can't find a succint answer to this rather simple question.
I have two identical fMRI sessions in a single subject.
The paradigm is a simple AB for each session.
I prescribe the number of sessions as two in the SPM99 design matrix.
This results in a design matrix having 4 columns. The first two columns contain the condition effect matrix for the two sessions, offset in time. The last two columns contain white bars representing session effects for each session, again offset in time.
In oder to compute a t contrast for the specified condition, do I specify the contast as [1 1 0 0]?
I have processed this set of data two ways. When I analyze the data as two sessions, I get what appears to be reasonable activation for the n-back task that we are using. When I process the data collectively as one session, at the same level of significance, there seems to be global activation. The activation patterns are probably similar, but when I lump the data into one session, I have to dramatically lower the p value to see the pattern. What would explain this behavior.
When I realigned and coregistered the data, I treated it as one subject, two sessions, thus all of the data is essentially realigned to the first scan of the first session. When setting up the models, data were all "scaled", thus I would expect the data to be globally scaled to the same value (100?).
Jerry Allison, Ph.D.
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