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A couple of quick questions, then some thoughts.
(1) Are the seeds in the same voxel in all subjects?
(2) How big are the seeds?

The analysis that you have done is not the typical analysis that others have
reported. If you notice, almost all papers use Z-scores as the second level
input; rather than the con_ images. To get the Z, you need to convert the
t-statistic image to an r-value and then use the fisher Z-transform to get a
Z-score.

However, it is probably useful also to do the analysis with the con images,
although there could be scaling issues. In the past, although not published,
I converted to percent signal change to make all the voxels on the same
magnitude scale.

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 Thu, May 12, 2011 at 11:31 AM, Cornelia McCormick <
[log in to unmask]> wrote:

> Dear SPM users,
>
> I am analyzing resting state data of a group of healthy individuals
> with a seed-based approach. In my regression model, I added the
> extracted timeseries of the seed voxel as well as a few other
> regressors (motion parameters, CSF, WM). The first level analysis
> (t-test) shows strong activation in the common default mode areas.
> However, when I set up the second level random effects model (all con
> files combined in a one-sample t-test), there is no activation left.
> No even the seed voxel lights up anymore. There is clearly some
> mistake in setting up the second level analysis but I don't know what.
> Does anybody have an idea? Any help would be aprreciated!
>
> Thanks a lot,
> Cornelia
>