Hello,
We have got some differences on MTLobe when comparing MCI and AD patient
groups, besides controls, on an encoding paradigm based on voxel-by-voxel
statistics for fMRI. Volumetry of hippocampus also showed significant
differences among all groups. A reviewer suggested us to introduce volumetric
measures as a covariate to study structure-function relationship. However, we
have some problems to understand how suitable or how to interpret our results.
Why? Because when we introduce left+right hipocampal volume as a single
covariate (in a contrast defined as nuisance [0 1 -1 0 0]at ANCOVA RFX level),
differences among all groups (mainly between MCI and AD groups) increased
spectacularly.
Is it correct to introduce that covariate? And, then, may we consider that once
variance on fMRI data explaned by hippocampus volume fMRI increased differences
may be attributed to functional activity?
On the other hand, how may I introduce two covariates in an ANCOVA? Since we
were controlling for sex differences previously and we would like to control
for both covariates (sex and volumetry).
Thanks you all for any advice.
Al
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