Andreas -
that's right. I calculate the global grey/whote/csf value for each image
using that routine and then enter it instead of the default global
calculation when specifying my model.
I take it that you have some experience of this? I think that regarding the
issue of proportional scaling, that in VBM where the 'effect' we are
studying is effectively schizophrenia and we know that this effect covaries
with the global image value (i.e. in VBM's case total gm volume/wm
volume/csf volume) then following Russ Poldracks' response proportional
scaling may be counterproductive and inappropriate.
Any thoughts appreciated.
Regards,
Toby.
-----Original Message-----
From: [log in to unmask]
To: Toby Mackinnon
Sent: 05/02/2002 21:31
Subject: Re: Proportional scaling in modulated VBM with global voxels.
Dear Tony,
> Additionally I am using the global voxel calculation algorithm given
by
> John Ashburner previously in this mailing list. Will this global
> calculation interact differently with prop scaling than the default
global
> voxel calculation?
Are you refering to
http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind0010&L=spm&P=R36678
and do you just enter this value as the global for scaling?
TIA-
Andreas
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Dr. Andreas J. Bartsch phone: +49
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Division of Neuroradiology,
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BJMU Wuerzburg pager:
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Germany
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