SPM produces both positives and negatives, but only displays the positives. If you want to view the negatives, either create a contrast that is negative, or use a viewer that can display both sides (e.g. OrthoView -- http://nmr.mgh.harvard.edu/harvardagingbrain/People/AaronSchultz/OrthoView.html)

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 Mon, Jan 30, 2012 at 7:10 PM, sam <[log in to unmask]> wrote:
Dear SPM experts,
I have a quick question about obtaining negative t values with one sample and two sample t test in SPM.
Firstly I have run Granger causality analysis using coefficients to analyse resting state fMRI data with a matlab tool box. My second level analysis require making group comparisons using t tests, which I did with spm. The expected outcome for this type of analysis is that the time course of an roi either predicts positive or negative correlation with the time course of other brain areas.
 Surprisingly I have obtained only positive t values, indicating only positive correlations. I was wondering the reason why my results are not showing any negative t values and whether I need to run a specific function. Given that I have very limited experience with spm any hint would be really appreciated.
Thank you for your attention,
sam