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Subject:

Re: cognitive covariate interpretation in VBM

From:

Saemann Philipp <[log in to unmask]>

Reply-To:

Saemann Philipp <[log in to unmask]>

Date:

Fri, 3 Aug 2007 11:05:21 +0200

Content-Type:

text/plain

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Hello Susie,

Approach 1 indeed disregards the higher power of the parametric statistics,
and correlation within the patient group between SPMs and performance
variables seems
more advantageous. In any case, it is advisable to check correlation plots, as
also in SPM outliers can easily produce spurious effects.

Ad 2) Comparing the monovariate effects of each cognitive variable
visually, that is
interpreting the patterns of significant clusters/voxels between two different
cognitive measures, can give you a first hint, if different cortex areas
are supporting these different domains (well rather the other way round in
neurodegeneration: if decline of grey matter volumes go along with decline
in performance).

It should be noted that results are more locally specific if total volumes,
that may also
be affected by the process, are included as nuisance variable - some people
however prefer total
intracranial volume as covariate of no interest, as in diffuse atrophic
processes entering
total GM may indeed wipe out any regional effect. If both variables
influence the performance,
you could extract regional volumes you found by SPM, combine them with
total GM and estimate
the contributions in an 'offline' analysis.

Ad 3) You can directly compare slopes between the two regressors in a
t-test that opposes
e. g. performance score 1 to performance score 2 (-1 1 or 1 -1), all
adjusted for the other
nuisance covariates. A problem arising here is may be collinearity, so
ideally,
the cognitive score regressors are orthogonal on each other or at least not
strongly correlated.
It may be worth starting with the least correlated scores, or orthogonalize
one score on the other.

If just entered as nuisance variable, "global" cognitive scores may be
entered to make cognitive-brain-relationships
more specific for the task of interest (I have seen this in papers on
Alzheimer disease, but could not
give you the precise source at the moment).

Another method to analyse combined effects is conjunction analysis,
however, I have scarce experience herein.

Any other comments are appreciated,

best wishes,
Philipp

Max Planck Institute of Psychiatry
NMR Research Group
Kraepelinsr. 2-10
80804 Munich
Mail: [log in to unmask]
Phone: 0049-89-30622-413

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