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To Whom it May Concern,

I have a question regarding the use of contrasts when generating
subtraction images.  Basically, we have mutiple subjects with
different conditions.  Here is the question:

-  Say for subject 1 we have two scans: A,B
-  In SPM's selection window I select the order: A=1, B=2 (A-B)
-  Then I select two contrasts, the first contrast being "1 -1"
   and the second being "-1 1"
-  Then does contrast 1 correspond to: 

   1. pixels significantly higher in condition A vs. B
   2. pixels significantly lower in condition A vs. B
   3. pixels significantly higher in condition B vs. A
   4. pixels significantly lower in condition B vs. A

Thank you very much for your time, and I look forward to hearing
from you soon.  

Nathan White
College of Medicine
Department of Anesthesiology
University of California, Irvine.  

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Nathan S. White
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