Hi FS support team,

I am going to be using mri_glmfit to test 1) main effects (which regions show differences in cortical thickness between group 1 and group 2) and 2) interaction effects (does the relationship between IV and DV depend on moderator). Depending on the model, the IV is sometimes a binary categorical variable and sometimes a continuous variable. The moderator is always continuous. So, interaction effects are either continuous x continuous OR categorical x continuous.

One thing I'm trying to understand is whether the two examples below are basically two different approaches that answer the SAME question: Do people in Group 1 differ from Group 2 on cortical thickness? Version 1 is the one provided by FS and Version 2 is an analog of another setup I have been provided by colleagues. Are these models answering the same or different questions? The freesurfer link below also shows an example of how to do an interaction (Group x Age) using Version 1 setup; for Version 2 setup of an interaction, I know I would multiply the IV and Moderator before this step so that I would have an additional "interaction variable" column to add as one of the 'Variables' listed in the fsgd code. So, I'm also curious if the two different methods of testing an interaction (in addition to the first question about the main effect) will get you the identical answer OR if there is something conceptually different? Do the nuances of this setup have to do with the difference between using DODS or DOSS?

Main effect of Group on cortical thickness Version 1 (copied from FS example. link here)

GroupDescriptorFile 1

Title OSGM

Class Group1

Class Group2

Variables Age Weight

Input subject1 Group1 30 100

Input subject2 Group2 40 120

Contrasts: 1 -1 0 0 0 0 (to test main effect of group; this feels like an anova approach?)

Main effect of Group on cortical thickness Version 2

GroupDescriptorFile 1

Title OSGM

Class Subjects

Variables Group Age Weight

Input subject1 Subjects 0.5 30 100

Input subject2 Subjects -.5 40 120

contrasts: 0 1 0 0 (to test main effect of group; this feels like a linear regression approach?)

Thank you in advance for your help!

Lydia

Lydia Wu-Chung, M.A.
Doctoral candidate
BMED Lab
Department of Psychological Sciences
Rice University
6500 Main St - MS201
Houston, TX 77030
Lab Phone: 713-348-8126
Email: lydia.wu@rice.edu

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