I have a question, particularly for Dr. Friston, on AnCOVA/correlation
(could the responder please respond directly in addition to the mail group,
as I'm having trouble getting mail from the list!).

I have the following PET design: 2 groups, 4 conditions (in 4 scans), and a
subject characteristic measured outside the scanner, on which the groups
happen to differ. I noted a discrepancy in some results that I don't
understand.

First, in order to make a condition, I used the means utility to create
proportionally scaled images for the subjects.  Then I used the image
calculator to subract condition 1 from condition 2 (I don't know if there is
an easier way to do this, but since I didn't have replication scans, I could
not get *con.imgs for subjects from a sub x condition interaction).

Then, I used a basic correlation model to correlate these subtraction
(condition) images with my external characteristic (co-variate; design
matrix= covariate mu).
Finally, I used AnCOVA to compare the two groups, adjusting for differences
in the co-variate (design matrix= group1 group2 mu covariate).

At this point I assume that in the correlation analysis, a contrast of [1 0]
tells me the areas where my co-variate correlates with the condition effect
represented by the image subtraction (Condition1 - Condition2).

I also assume that in the AnCOVA, the contrast [0 0 0 1] should tell me the
identical information as the above contrast for the correlation.  However,
it doesn't.  The results are similar in some areas, and dramatically
different in others.

Do I misunderstand AnCOVA?

I also assume that the contrast [1 -1 0 0] tells me were group 1 is greater
than group 2, with this difference having variance due to the covariate
already subtracted out. Is this correct?

A quick incidental question: Assuming that I have made the condition images
correctly (mean utility, image calculator), how can I apply the regular mask
to exclude areas outside the brain?

Thanks for your patience and help.

________________________________
David A. Kareken, Ph.D.
Assistant Professor & Director
Neuropsychology Section, Department of Neurology
RI 1773
Indiana University School of Medicine
Indianapolis, IN 46202
(317) 274-7327 (voice)
(317) 274-1337 (fax)