Hi,
In an event-related fMRI analysis using SPM99b we want to determine
differences between events of type A, B or C. In 50% of each of these
events subjects respond (go-trial) and in 50% they do not respond (no-go
trial). We are not interested in the differences between go and no-go and
want to model them as a nuisance variable. How should this be done?
I can think of two ways, none of which really satisfies me. I could define
a fourth and fifth event type, corresponding to go and no-go respectively.
The elements within the vectors corresponding to events of these two types
would however strongly overlap with the elements contained in the vectors
corresponding to events A, B, or C. Second, perhaps I could make use of a
'user-specified regressor', using dummy variables for go (e.g. 1) and no-go
(e.g. 0) trials respectively. However this option asks for a vector equal
in length to the total number of scans, where I would have wanted to feed
in a vector equal in length to the total number of events.
Thank you very much for your help
Rik Vandenberghe, M.D., Ph.D.
Visiting research associate
Cognitive Neurology and Alzheimer's Disease Center
Northwestern University Medical School
320 East Superior Street
Suite 11-461
Chicago, Illinois 60611
phone: (312) 908-8571
fax: (312) 908-8789
http://www.brain.nwu.edu
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|