James,
It will show you all regions where patients are greater than controls
irrespective of the betas. Thus -1 1 is the contrast you want as the
patients are less than the controls.
The t-statistic doesn't change for one and two sided tests. The only
thing that changes is the p-value. That said, the maps contain both
positive and negative t-statistics; however, SPM results viewer uses a
one-sided test and only shows the positive values of the map.
On Friday, April 22, 2011, Ellis, James <[log in to unmask]> wrote:
> Hi All, We’re struggling with some two-sample results. Our metric here are beta-weights, and both groups have negative average values for a certain voxel/region. Now, when you run the patients as group 1 and healthy controls as group 2, set up a [1,-1] contrast (patient > HC), will this voxel/region show up if the patients have a more negative value? If the average in our VOI/ROI is, say, -3.5 for patients and -1.5 for HC, which contrast will have this area pop out? Related: is the default for two-sample ttests in SPM5 two-tailed or one-? Thanks! James
--
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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