Hi Tori, There is no way to investigate the interaction of two continuous effects with the contrast vector alone, if the design matrix itself only involves main effect terms. The interaction itself needs to be coded into the design matrix.
Note that it can become tricky as to the proper coefficients to use for the contrast vector when you have both main effect and interaction terms in your design matrix. (There are in fact different "types" of statistics that can be estimated, and you want your contrast vector to be an "estimable" function. Sophisticated statistical packages check that this estimability constraint is satisfied, but I suspect that mri_glmfit does not).
e.g., http://support.sas.com/onlinedoc/913/docMainpage.jsp
cheers, Mike H.
On Wed, 2009-08-05 at 15:48 -0400, Victoria Williams wrote:
Hello,
I apologize if this is a very simple question, but how would you set up a contrast matrix to look at the interaction of 2 variables on a measure? For example, my fsgd file contains:
class=1 variables=2 Input Subjid class var1 var2
When I tried using a contrast of 0_1_1 the significance map looks very similar to the glm run only containing var1 (contrast_0_1). I thought 0_1_1 was correct, but now I'm not so sure and would like to double check. What I am interested in is how the interaction of var1 and var2 is correlated with the brain measure per voxel. Is this possible with mri_glmfit?
I've checked the tutorials and scoured the internet as not to ask any silly questions, but I'm having trouble finding anything helpful.
Thanks in advance! Tori _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer