The first question is whether you are using PET or fMRI.
With PET, and arterial blood sampling, it is possible to acquire an estimate
of regional cerebral blood flow. Without blood sampling, and without
performing global scaling, you get a more inaccurate estimate of blood flow,
since variations in hemodynamics and injection will affect the actual PET
counts. If you then do use global normalization, you at least can look at
relative changes in resting blood flow, but the interpretation becomes
difficult; changes in one brain area could be due to an opposite change in the
remaining areas.
In fMRI, where I have less experience, I believe it is not meaningful to look
at differences in baseline, because there are so many sources of variation for
the "resting' BOLD signal. Maybe perfusion MRI will solve this problem -
maybe not.
Hope this helps.
-George Wittenberg
Neurology/Rehabilitation
Wake Forest Univ.
Kris Boksman wrote:
> Hello Friends and Colleagues,
>
> I'm hoping someone might be able to advise about the global scaling
> feature of SPM99. I'm looking at comparing the activation elicited by a
> cognitive task between a clinical and a nonclinical group of study
> participants. I've set up the design to subtract the baseline task from
> the "activation" task, etc., and I am clear on how to perform the
> subsequent analyses.
>
> One task my advisor has suggested is to compare the baseline conditions to
> examine whether or not activation in the more "restful" conditions differ
> between the two groups of participants, and to determine if any particular
> areas of the brain are more active in one group than in the other when the
> brains are at relative "rest". It just occurred to me that perhaps by
> using the global scaling and modeling the baseline condition explicitly
> (instead of implicitly as SPM99 can do for you) any comparison I perform
> between the two sets of baselines could be uninformative, since I've
> globally scaled them in order to do the first analysis. (task A - task B)
>
> So my question is, would it be advisable to conduct a new analysis that
> includes only the scans for the baseline but this time to avoid using the
> global scaling? Intuitively this seems right (to me!), but I'm really not
> sure if it is the most appropriate method.
>
> Advise would be greatly appreciated, (and thanks in advance!)
>
> Kristine
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Kristine Boksman, B.Math The University of Western Ontario
> M.A.2 Clinical Psychology London, Ontario, CANADA
>
> There are two ways to live your life.
> One is as though nothing is a miracle.
> The other is as though everything is a miracle.
> Albert Einstein (1879-1955)
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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