Hi Maria
> I am using VBM5 to look for gray matter differences between two groups using
> a two sample t-test. I would like to control for total intracranial volume
> (TIV) but I am not sure where I should enter the TIVs when I am specifying
> my design in SPM5. Should I enter the TIVs as a user specified global
> calculation or should I add it as a covariate?
Typically you would enter it as a covariate. For every voxel, this
will remove any variance in GM that could be attributed to TIV,
meaning that the effects of your t-test should reflect GM differences
*not* attributable to TIV.
One thing to keep in mind is that total GM volume may also have an
impact on your results. For example, if you are comparing patients
with some neurodegenerative condition vs. healthy controls, the
patients are likely to have reduced total GM, as well as some regional
specificity to their loss. Because skull shape isn't affected by
disease, these global differences will not be reflected in TIV.
Hope this helps,
Jonathan
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