Dear SPM and John:
I have been examining the issue of including a global covariate in VBM
analyses and the meaning of results that include it, and I am a bit
confused. I give some background below but the basic question pertains to
understanding the differences between proportional scaling and ANCOVA and
whether a global should be included at all.. For example, in an example
given below a propsca global was included in a VBM of males vs. females.
However, wouldn't including a global based on proportional scaling be
saying that males have more gray matter per voxel? Might it not be the case
that because of their bigger heads males have more voxels with gray matter
in them and not a greater gray matter density?
What happens in the case of disease states such as a degenerative dementia?
Why would proportional scaling be better than ancova in this case or should
a global not be included as there could be a correlation between regional
and global changes in gray matter, i.e., large regional changes affecting
the global covariate. I've reviewed the explanation of propsca vs. ancova
differences in HBF-1 but I don't think it makes these issues clearer.
Any thoughts on these issues would be appreciated. Info from background
list emails are below.
Darren
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In an email from 1/24/03 KJF and JA recommend proportional scaling as
perhaps the most meaningful, with the suggestion that normalization to the
total gray matter volume may be best. JA gave an example of comparision
between males and females. In this case one would want to remove the
confound of males having a bigger head size.
In an email from 5/1/2003, JA suggests that either ANCOVA or proportional
scaling may be used. " There doesn't necessarily need to be a single global
correction if the ANCOVA approach is used. There could be several, and the
choice of "globals" depends on the hypothesis you have. If you simply
want to look at any absolute volumetric difference, then you wouldn't use
any globals. If you want to see regional GM differences that cannot be
explained by differences in whole brain volume, then you would use whole
brain volume. The alternative to covarying out the effects of some
"global(s)", would be to proportionally scale the data. This involves
dividing the values by e.g.whole brain volume, such that the intensities
are normalised to whole-brain volume."
A July 22 posting makes reference to some of the above emails and a
statement that either proportional scaling or ANCOVA could be used
depending on goals of the analysis, but no further explanation is given.
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