Hi,
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).
Regards,
Christian
____________________________________________________________________________
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]
http://dbm.neuro.uni-jena.de
On Wed, 26 May 2010 16:04:02 +0200, Marko Wilke <[log in to unmask]> wrote:
>Hi,
>
>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,
>Marko
>
>
>김지현 wrote:
>> Dear Gaser and other experts
>>
>> I've just tried the beta version of vbm8(r335), according to the vbm8
>> manual.
>> Preprocessing steps worked well with no errors, and the final modulated,
>> normalized GM (m0wrp1.nii) images are looked so fine without any
>> misalignment.
>> 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.
>>
>> Kim
>
>--
>=====================================================================
>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
>=====================================================================
|