How did you define the VOI(s)? If they are in MNI space as your second level implies, then you're applying one VOI to all the subjects... and this will unsurprisingly have the same volume in all.
I don't know what kind of data you have - it sounds like fMRI. An approximation of individual volumetrics would be to backtransform your standard space VOIs into individual space - if you've used Unified Segmentation for your MRI spatial normalisation, then the *inv_seg_sn.mat file can accomplish this.
Such a procedure's accuracy will be limited by 1) the quality and provenience of your standard space VOI (see our recent thread on single-subject vs multi-subject atlases) and 2) the low-ish number of degrees of freedom of the transformation for standard normalisation and Unified Segmentation. Point #2 should be improve-able if you use DARTEL.
The gold standard for just one particular area would be to devise a protocol, check the reliability of results and ideally their veracity, and then outline those areas by hand in your subjects.
Hope this helps,
Alexander Hammers, MD PhD
Chair in Functional Neuroimaging
CERMEP – Imagerie du Vivant
Hôpital Neurologique Pierre Wertheimer
59 Boulevard Pinel, 69003 Lyon, France
Telephone +33-(0)4-72 68 86 34
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Visiting Reader; Honorary Consultant Neurologist
Division of Neuroscience and Mental Health, Faculty of Medicine
Imperial College London, UK
Honorary Reader in Neurology; Honorary Consultant Neurologist
Department of Clinical and Experimental Epilepsy
National Hospital for Neurology and Neurosurgery/ Institute of Neurology, University College London, UK
On 28 Feb 2011, at 10:11, sarika cherodath wrote:
> Hi SPMers,
> I have been trying to perform VOI analysis to obtain signal changes at a particular area for individual subjects in the dataset, so as to make correlations with behavioral scores. When i tried to calculate the values, SPM returns the same value for all subjects! Can anybody explain why this might be? The dataset has been analysed at second level and then moved to another directory (along with first level data). Is it possible that SPM is not able to access the data because of the path change??
> Thanks in advance,
> Sarika Cherodath
> Graduate Student
> National Brain Research Centre
> Manesar, Gurgaon -122050