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Dear Andreas,

The paired t-test in SPM will give you exactly what you want as a function of the contrast you use to implement your hypothesis. If the first image in a pair is your pre and the second image is your post condition, the contrast [-1 1] will test for a greater effect post compared to pre and vice versa, using a one-tailed p-threshold at the positive tail of the null t distribution. 

An alternative way of implementing your question is to create contrast images at the first level of the difference between your two conditions using the contrast above and perform a one-sample t-test at the second level with a 1 or -1 contrast, respectively. 

I hope this helps 

Martin

On 03 Feb 2016, at 16:32, Andreas Jell Heinrich-Heine-Universität Düsseldorf <[log in to unmask]> wrote:

Dear SPM community,

I have a problem with a specific graphic representation of contrast images in the SPM glass brain. The contrast images represent the differences between pre- and post- conditions after the second level analysis by a paired t–test. Because in SPM the paired t–test is two-tailed, both directions of differences are considered and therefore those brain areas involving increased and decreased brain activations are displayed in the SPM glass brain. I would like to separate both directions graphically and to show after the paired t–test analysis one time only brain regions, where activation is increased in the post-condition and another time only brain regions, where activation is decreased in the post-condition. I can’t find any possible programming option in SPM or MATLAB how to separate images showing increased from images showing decreased activation in the SPM glass brain after a paired t–test analysis?

Kind regards and many thanks for helping me!

 

Dipl.-Ing. Andreas Jell

Heinrich-Heine-Universität
Institut für Experimentelle Biologische Psychologie Gebäude 23.03
Universitätsstraße 1
D-40225 Düsseldorf

[fon] +49 (0)211 81-13958
[fax] +49 (0)211 81-14522

[email] [log in to unmask]