Dear Mario,
I think that indeed explicit modelling of the baseline would make your
design suitable to ask your questions. Therefore for comparing activations
for pre/post effects you would have vectors
1-1 -1 1 (padded with 2 zeros for session means if not using gui)
and the reverse of the above for deactivations.
Also I think you can do the reverse on the 2nd level...
Of course the above contrasts are the simplest approach. I think you could
also model your experiments with 2X2 factorial design or run your second
level with t-paired 1st level images of comparison:
1 0 0 0
0 0 1 0
I don't know maybe there are still even better solutions.
Also I'm not sure which one is actually the best to answer these questions
HTH
Iwo
On Aug 25 2011, Mario Gatica wrote:
>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
>
--
Iwo Bohr, PhD (Torun)
University of Cambridge
Department of English and Applied Linguistics
9 West Road
Cambridge
CB3 9DP
UK
Tel: +44 1223 767 354
Fax: +44 1223 767 398
http://www.rceal.cam.ac.uk/People/Staffpages/ijb25.html
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