Dear freesurfer experts,
I have some questions about the statistical model in freesurfer. In my
experiments, I would like to compare the cortical thickness of two groups
(schizophrenia patients and healthy controls) with age and sex as covariates.
I want to know that the variable in fsgd file must be continuous covariate?
Could the fsgd file like this (attached fsgd1.png):
There are two classes and two columns of age and sex variables in the fsgd file.
Now, there are six regressors. Regressor 1: ones for subjects in Group SCH, 0
otherwise; Regressor 2: ones for subjects in Group HC, 0 otherwise; Regressor
3: age for subjects in Group SCH , 0 otherwise; Regressor 4: age for subjects
in Group HC, 0 otherwise. Regressor 5: sex for subjects in Group SCH, 0
otherwise. Regressor 6: sex for subjects in Group HC, 0 otherwise.
If I want to compare the cortical thickness of two groups and regress out the
effect of age and sex, the contrast is 1 -1 0 0 0 0.
Or I must put it in this way (attached fsgd2.png):
There are four classes and one column of age variable in the fsgd file.
Now, there are eight regressors. Regressor 1: ones for SCHmale subjects, 0
otherwise. Regressor 2: ones for SCHfemale subjects, 0 otherwise. Regressor 3:
ones for HCmale subjects, 0 otherwise. Regressor 4: ones for HCfemale subjects,
0 otherwise. Regressor 5: age for SCHmale subjects, 0 otherwise. Regressor 6:
age for SCHfemale subjects, 0 otherwise. Regressor 7: age for HCmale subjects,
0 otherwise. Regressor 8: age for HCfemale subjects, 0 otherwise.
If I want to compare the cortical thickness of two groups and regress out the
effect of age and sex, the contrast is 0.5 0.5 -0.5 -0.5 0 0 0 0.
Which one is right, or both of them are right? If just the latter is right,
what is the rationale behind this? Why treat continuous and discrete variables
differently here. In the general linear model Y = XB+U, X is a matrix that
might be a design matrix, B is a matrix containing parameters and U is a matrix
containing errors or noise. Why we cannot treat the age and sex equally in the
GLM of freesurfer software?
All the best,
Feng
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