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> From [log in to unmask] Tue Apr 6 18:43:08 1999 > Date: Tue, 6 Apr 1999 12:44:39 -0500 (CDT) > Reply-To: leann helka kinnunen <[log in to unmask]> Dear Leann, > I still have not heard back from anyone regarding my message from March > 30, so I thought I would reiterate my problem and ask if anyone else > has ever tried viewing the images which SPM performs its statistics on > and whether or not they found the same random patterns of zeros as I > did. I tried for the first time recently to use the "generate > normalized image data" option in SPM which should allow me to generate > and save the images resulting from the application of the global > normalization, scaling and gray matter threshold (in the statistics > portion of SPM). I used a multi-subject, different conditions (PET) > design type with 6 subjects and 2 conditions each (a drug and a > placebo). I used proportional scaling with a gray matter threshold of > 0.8 and an uncorrected F threshold of 0.99. > Upon viewing the resulting PET images, I discovered that they showed a > random pattern of voxels throughout the brain with values of zero, > having no relation to white or gray matter areas. These patterns were > the same for every one of the images generated (even across subjects). > The realigned, normalized and smoothed (at FWHM of 12x12x12) PET images > I entered into my design matrix had no such pattern/problem. Once again > I ask if anyone has observed such a phenomenon before and whether > anyone would know the cause. I am concerned as I am not sure why such > patterns would be generated and how SPM statistics would handle these > random zeros when performing its analyses. I would be more than happy > to ftp my pre and/or post statistical analysis images to anyone who > would be willing to take a look at them. I have to confess I am not sure what the "generate normalized image data" option is. I cannot find this option in SPM96. It may simply reflect the omission of voxels that fail to reach the low F threshold specified by p = 0.99. SPM96 and previous versions only retains voxels at which the F value was significant. SPM99 includes all [intracranial] voxels. With best wishes - Karl %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%