Dear list,
I've run into trouble with a pretty basic fMRI paradigm. We measured 10 patients
suffering from a neuropsychiatric disorder and 10 controls. Both groups performed three
types of tasks plus a rest condition (A-Rest-B-Rest-C-Rest...3x). When contrasting the
tasks on the second level, there are meaningful differences across groups. So far, so
good.
Now, however, we also acquired behavioral measurements according to the speed
subjects performed the tasks with (difficulty C > B > A). I am interested in activations
correlating with the speed measurement, therefore I imagined a parametric design would
be in order. Here's what I did: first I classified each task condition as being "fast",
"medium" or "slow" as compared to the according group performance quantiles. Then I
designed EVs for each of the three categories and entered them into the first level design
matrix.
Unfortunately, some subjects only exhibited "fast" speeds, leaving "slow" and "medium"
regressors empty, thus resulting in a rank deficient design matrix. Even omitting the
"medium" EV did not help, obviously. Others had two different speeds, no one had three
types.
How can I specify a design matrix of this type, preserving all three EVs for a higher level
analysis? Or should I assign "slow", "medium" and "fast" to subjects on the second level?
Thanks for your help - any comment is appreciated!
Cheers
Cornelius
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