"The goal is to recieve differences between the gray matter density and a total score of a questionnaire." This analysis doesn't make any sense, but from the description, it sounds like you want the correlation (not the differences) between gray matter and total score.
If you are interested in the correlation, then the design matrix looks fine. I'm not sure why you thought it was weird.
For the contrast, you need to tell SPM to test the second beta value - the slope of the covariate. Instead of using 1, use [0 1] or [0 -1] as that will test whether the second beta is different from zero and will be the test of the correlation between gray matter and total score.
Best Regards, Donald McLaren ================= D.G. McLaren, Ph.D. Research Fellow, Department of Neurology, Massachusetts General Hospital and Harvard Medical School Postdoctoral Research Fellow, GRECC, Bedford VA
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