There actually was a reply very recently.
See:
https://www.jiscmail.ac.uk/cgi-bin/webadmin?
A2=ind1004&L=FSL&P=R57536&1=FSL&9=A&I=-3&J=on&d=No+Match%3BMatch%
3BMatches&z=4
On Tue, 2010-05-04 at 18:59 +0100, Jonathan Peelle wrote:
> Hi Darren
>
> > I have read through the list but still have a question about FSLVBM and setting up a proper model. My understanding is that the gray matter data are modulated by the Jacobian, but that the recommendation is that a covariate for total intracranial volume is not necessary in the analysis. I'm not sure I understand why. Does the Jacobian include both affine and non-linear aspects of the normalization? If so wouldn't modulating by the affine scaling mean that size differences would not be accounted for between subjects any more?
>
> I haven't heard the recommendation that covarying for total
> intracranial volume (TIV) isn't necessary, at least in regards to the
> modulation issue. TIV is really just a global effect (1 number per
> subject), as opposed to the modulation, which is voxel-specific.
> I.e., they're controlling for different things.
>
> In my experience including TIV almost always accounts for a decent
> amount of variability; and, generally speaking, if I have an effect
> that can be explained by TIV, that's not really what I'm after. So I
> think it's generally a reasonable thing to include (at least to the
> degree that it's a reliable estimate). Although, like anything, no
> doubt it depends on the particular question being asked...
>
> All that being said, I'd be very interested to hear other people's opinions...
>
>
> > A second question is whether FSLVBM can be used on white matter? If so would it just be a matter of changing the Template specification at the top of the fslvbm_2_template script, and then making sure files were named appropriately through, e.g., WM for GM in file names?
>
> (I'll have to leave that for someone else.)
>
> Hope this helps,
> Jonathan
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