Mark:
I'm told that "one always centers covariates" so I would center it.
As far as the global normalization, I would say yes. The issue is
different for random effects analyses because by using the con**.img
these are already normalized. In this case you are putting back a
mean corrected covariate into the analysis, but are then using the
same set of [non-globally-normalized] images so I would do this.
For the second stage, the parameter maps would not need to be normalized.
hth,
Darren
>Hi Daren,
>
>----- Original Message -----
>> Dear Russ:
>>
>> It's not clear why you're torturing yourself slighty with doing these
>> correlations in this manner. Why not 1) extract a voxel time course
>> using spm_regions. 2) Use the first eigenvariate saved by spm_regions
>> in another analysis as a covariate "of interest." using a simple
>> correlation. 3) The resulting parameter maps for this correlation for
>> each subject can then be entered into a random effects analysis.
>>
>I am trying to do something similar to this myself at present. To save me
>"re-inventing the wheel" it would be very useful to have the benefit of your
>experience with this approach. The obvious questions I have are to do with
>centering of these eigenvariables and global normalization.
>
>Any tips gratefully received,
>
>Mark
>========================
>Dr. Mark Daglish
>Clinical Research Fellow, Psychopharmacology Unit, University of Bristol
>Visiting Clinical Fellow, MRC Cyclotron Unit, Hammersmith Hospital, London
>
>Tel: +44 (0) 117 - 925 3066
>Fax: +44 (0) 117 - 927 7057
>email: [log in to unmask]
>========================
--
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Darren R. Gitelman, M.D.
Cognitive Neurology and Alzheimer's Disease Center
E-mail: [log in to unmask] WWW:
http://www.brain.northwestern.edu
Voice: (312) 908-9023 Fax: (312) 908-8789
Northwestern Univ., 320 E. Superior St., Searle 11-470, Chicago, IL 60611
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