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You have two choices: 
(1) Create a flipped brain for one condition, then do a voxelwise test of the positive and negative stimuli. This assumes that the flip aligns homologous regions on each side.

(2) Draw regions in the left and right hemispheres, extract the contrast values in the ROIs, then test for an interaction between hemisphere and valance. 

You must explicitly test the hemisphere interaction in some way to draw the conclusion you have hypothesized. Seeing activation in one hemisphere and not the other doesn't allow you to say the positive/negative differences are different between hemispheres. 

Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Postdoctoral Research Fellow, GRECC, Bedford VA
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
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On Thu, Sep 11, 2014 at 12:45 PM, "Jan Müller" <[log in to unmask]> wrote:
Dear colleagues,
 
I'm new and have no experience with SPM. To introduce myself, I' study psychology in Germany. I did an fmri experiment. I had my probands view words and faces to find out about emotional valence effects. I wanted to check on valence hypothesis (positive stimuli processed in left hemisphere, negative in right). This hemisphere asymetry should be especially true for PFC. Is anyone into this matter and knows whether I have to do a Whole Brain Analysis or ROI Analysis to test valence hypothesis, and especially for PFC.
 
Many Thanks in advance.
 
Have a beautiful day.