Its unclear what was subtracted - single subject contrasts values or
the the group level T-maps of A>B and C>D. You should not subtract
T-maps nor use T-maps in second level or third level analysis. Only
con images should be used.
I wouldn't use imcalc as its more time consuming, but in theory will
produce the same results.
I'd use the contrast manager:
A>B is 1 -1 0 0
(A>B) > (C>D) is 1 -1 -1 1
The enter the conimages into the second-level analysis. If D>C, then
the contrast will show more than A>B.
Best Regards, Donald McLaren
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General
Hospital and Harvard Medical School
Office: (773) 406-2464
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On Tue, Feb 8, 2011 at 11:59 AM, Soha Saleh <[log in to unmask]> wrote:
> I have 4 conditions in my experiment (A, B, C, D).. I am interested in finding the neuro activity that is higher in A than in B, and also the areas where the difference in activity between A and B, are higher than between C and D. Simply it is (A>B) - (C>D). I did each of the contrasts (A>B) and (C>D) in SPM8 and then used imcalc to subtract them to get (A>B)-(C>D). I got huge activity and T value that is very high which made me suspicious about this procedure.
> My questions are: Is this a valid procedure? Is there any precautions or limitations I need to be aware of?
> Thank you, I appreciate any assistance or advice you give.
> Kind regards,