With anxiety as an explanatory variable, your model is
brain response = a + b * anxiety + e
In the output of 3dttest++, you would get two sets of results:
1) the association between anxiety and brain response (value 'b' in the above model);
2) brain response (value 'a' in the above model) when anxiety is controlled at a center value. Read the centering part in the 3dttest++ help:
3dttest++ -help | less
and the following:
[
afni.nimh.nih.gov]
Gang
