Hi Paul,
I think volume you can normalize with eTIV, but how to normalize thickness is an open question. You could argue it should be eTIV^(1/3), but I know at least one HBM abstract a couple of years ago from the MNI showing that you shouldn't normalize at all. If your effects are focal then normalizing by avg thickness is reasonable, or maybe the average of some region that you expect to be unaffected?
cheers, Bruce
On Mon, 7 Jan 2008, Paul Greenberg wrote:
Dear Freesurfers,
When comparing cortical gray matter volumes between groups it is appropriate to normalize subject's mean cortical volume in each cortical parcellation by their eTIV. Is there a corresponding value appropriate for normalizing cortical thickness? Would mean cortical thickness for a single hemisphere be an appropriate normalization factor? We are interested in comparing cortical thickness between a control group and a group of people with a degenerative brain disease.
Thanks very much, Paul