Dear SPM users,
There have been questions similar to this posted in the list before, but I have doubts on a particular point I hope someone might clarify further.
I have a group of subjects where I want to compare activation in response to tactile stimulation before and after a treatment. Thus I have two identical sessions per subject. In each session there is only one condition (stimulation) which is contrasted to baseline. The design is a simple block design with blocks of 32 seconds.
I want to enter the two sessions in the same 1st level design for fixed effect analysis. When I do so I get a matrix where the first two columns represent a session each (see attachment). How then would I indicate when doing the contrast that I want to look at that effect (stimulation dependent activation) for that session? In other words
(stimulation pre - baseline pre) - (stimulatin post - baseline post)
Is that what I am getting if I enter the vectors 1 -1 0 0 ? For a single session I would just use [1] as vector for that contrast (stimulation - baseline). In this case however, that within-session contrast is not specified.
For the sake of clarification, let us say I wanted to look not at activation but deactivation during stimulation. For a sinlge session the vector would be just [-1], right?. How then would I compare deactivation between sessions?
Would it make sense to enter the rest periods as another condition? In that case, what would the contrast vectors look like?
Many thanks in advance.
Mario Gatica
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