Re: [Marsbar-users] combining ROIs

[Todd G. <tg...@ps...>]

I haven't tried this with marsbar, but one would expect different =
results - with different meanings, depending on the nature of the ROIs. =
That is, if they are all identical in volume (e.g., a uniform sphere =
surrounding 6 different peaks), then the average of the averages should =
be the same as the average as the combination of all 6 in 1 - the =
variances of course would be quite different. If the volumes are =
different sizes then only a weighted average (e.g., first dividing each =
average by the # voxels) would give the same results - otherwise the =
means would differ because smaller and larger ROIs would contribute more =
and less, respectively, to the average. Make sense? I'm guessing from =
the fact that the 6 regions encompass the full PFC that they are =
anatomically defined and thus of different volumes. You might want to =
think about what 'meaningfulness' you are after as well. Given that you =
took the trouble to label the ROIs, I assume regional differences are of =
some use (vs. just defining a single PFC ROI) -? But perhaps you are =
also interested in a global measure of PFC activation? (for its own =
purpose or for standardizing the relative regional activations) - If so, =
I'd recommend the approach that you first suggested: combine ROIs and =
run a new extraction.
=20
-Todd
=20
*****************************
Todd A. Girard, Ph.D.
Department of Psychology
Ryerson University
350 Victoria St.
Toronto, ON, Canada
M5B 2K3
Ph: +1-416-979-5000 x2646
FAX: +1-416-979-5273

________________________________

From: mar...@li... on behalf of Carolyn =
L. Fort
Sent: Wed 18/07/2007 5:00 PM
To: mar...@li...
Subject: [Marsbar-users] combining ROIs



Hello.

We've created six ROIs within the pre-frontal cortex (which combined =
comprise the PFC in its entirety) and have looked individually at the =
extracted data. Now we're interested in the PFC as a whole. I'm assuming =
the appropriate step is to combine my six ROIs and run a new extraction, =
yes? Would averaging or summing the data from the six individual ROIs =
tell me anything meaningful about this region?

Carolyn

Carolyn L. Fort
Research Specialist, Sr.
Department of Psychiatry
University of Arizona
PO Box 245002
Tucson, AZ 85724-5002
v: 520-626-8568
f: 520-626-6050


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