it all depends on your hypothesis. If you think there are regionally
varying differences in the thickness and want to be conservative, then
overall mean is a good way to test this, and is probably a more stable
measure than ICV. It's all speculation I think as no one has really
quantified the effects of different normalizations a as far as I know
cheers
Bruce
On Thu, 8 Apr 2010, j janssen wrote:
Hi,
you could try including average cortical thickness over the entire
hemisphere as a covariate. This was Mike Harms' suggestion, and I think a
good one.
why is it a good one? for cortical thickness: i'm not so sure. my 2 cents
(and i may be wrong!):
- controlling for intracranial volume is done to remove differences due to
overall size variability.
- the relationship between thickness and size is much less strong than
between volume and size (or surface and size).
- suppose your groups differ in mean thickness, if you would like to include
mean thickness as a covariate in a mapping study (vertex-wise comparisons)
of cortical thickness you would have to show that the group-difference in
mean thickness is due to an effect that's spread out over the whole cortex
(an overall effect, not localized), otherwise the mean thickness covariate
is not a good estimator of the overall size factor that you'd like to
remove.
check Peter Kochunov's recent publications on this.
-joost
cheers
Bruce
On Thu, 8 Apr 2010, Liukarl wrote:
Hello, Freesurfer experts:
I noticed for analyzing the results from autorecon2, the intracranial
volume are controlled as covariate in some publications. Is there any
corresponding covariate for cortical results from autorecon3, cortical
thickness, surface, and volume?
In my current study, there are two group: one is a group with less optimal
developmental environment and the other is control. I noticed that without
any covariate, the r square from cortical measure and group variable was
really low, only accounting for 2%. Is any standard way to analyze the
cortical data?
Thanks!
Karl
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