my first guess was also to set the absolute threshold. I have checked the manual and in the examples for the statistical analysis an absolute threshold of 0.1 is always suggested. This is a good starting point where no interesting effects should be suppressed. For the final analysis I increase the absolute threshold up to 0.2..0.25. This will also reduce the search volume which utilizes the FWE correction for multiple comparisons (less voxels to correct for).
Christian Gaser, Ph.D.
Assistant Professor of Computational Neuroscience
Department of Psychiatry
Friedrich-Schiller-University of Jena
Jahnstrasse 3, D-07743 Jena, Germany
Tel: ++49-3641-934752 Fax: ++49-3641-934755
e-mail: [log in to unmask]
On Wed, 26 May 2010 16:04:02 +0200, Marko Wilke <[log in to unmask]> wrote:
>could this be due to not using an absolute threshold when specifying
>your model? For VBM studies, it is generally a good idea to exclude
>voxels below a certain threshold, say, .1 (with the new segmentations,
>likely higher). Otherwise, you may end up comparing noise with other
>noise. You could also try an explicit mask. Another alternative would be
>one image that did not process correctly, but Christian's tool "check
>sample homogeneity" might help you there.
>Hope this helps,
>> Dear Gaser and other experts
>> I've just tried the beta version of vbm8(r335), according to the vbm8
>> Preprocessing steps worked well with no errors, and the final modulated,
>> normalized GM (m0wrp1.nii) images are looked so fine without any
>> However, when comparing GM images between 2 groups (N=19 each, smoothing
>> with 10-mm kernel), I found many significant clusters just outside
>> skull(pls see the attched figure).
>> I cannot figure out why this happens. I'm used at vbm2/5 toolbox and
>> have never seen such an error.
>> Any hints on my problem would be greatly appreciated.
>> Thanks you in advance for your kindness.
>Marko Wilke (Dr.med./M.D.)
> [log in to unmask]
>Universitäts-Kinderklinik University Children's Hospital
>Abt. III (Neuropädiatrie) Dept. III (Pediatric neurology)
> Hoppe-Seyler-Str. 1, D - 72076 Tübingen
>Tel.: (+49) 07071 29-83416 Fax: (+49) 07071 29-5473