Hi Mat,
Could the difference be due to the possibility that by selecting three
contrasts to be used as masks in the manner you describe (ctrl+click) in the
GUI method, that SPM implements an AND function rather than the OR that is
implemented in the equation? (I know at first glance the equation doesn't
seem like it should be implementing an OR masking function, but Rik and I
tested it and found it to be so!). Just a thought...
~Melina
P.S. As far as I know, Ged, that equation should be specifying one-tailed
thresholds. Rik, any thoughts?
*****************************************
Melina R. Uncapher
Functional NeuroImaging of Memory Group
Center for the Neurobiology of Learning and Memory
Department of Neurobiology and Behavior
University of California at Irvine
http://fnim.bio.uci.edu/
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-----Original Message-----
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On
Behalf Of Matthew Whalley
Sent: Tuesday, December 12, 2006 7:43 AM
To: [log in to unmask]
Subject: [SPM] multiple masks
Hi All,
Can anyone tell me how SPM implements multiple masks? I've tried two
methods which I'm told should produce equivalent results:
The first method is to select the main contrast, then select three
other contrasts as masks in the spm GUI (by ctrl+clicking), and
exclusively masking them at p=0.05
The second method has been to use imcalc to create one big exclusive
mask using the formula:
i1.*((i2<threshold)&(i3<threshold)&(i4<threshold))
with the two-tailed t-threshold of 2.04 (df=31, so p=0.05)
Then to use this big mask to exclusively mask the main contrast
These two methods actually give slightly different answers - slightly
different cluster locations (some identical, some similar), cluster
sizes approx 20% bigger for method 1.
Which method would you go with?
Best wishes,
Matthew
--
Dr Matthew Whalley
Research Fellow
Department of Psychology
University College London
WC1E 6BT
Tel: 020 7679 5365
Email: [log in to unmask]
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