 |
 |
Unmodulated analysis shows regions where registration error differs significantly among the populations, whereas modulated analysis attempts to show regions of volumetric difference. Everyone accepts that using a different linear model to fit data will give different results. In the same way, different pre-processing is also expected to produce different findings (as this also changes how differences among the data are modelled). Best regards, -John On 28 October 2010 12:07, Cullen, Alexis <[log in to unmask]> wrote: > > Hi SPM experts > > I have been running some analyses on a on a set of structural paediatric MRI > images using the VBM 5 toolbox in SPM 5. I have carried out the segmentation > and normalisation steps according to the VBM 5 manual with the exception > that I have segmented against a custom template produced using the > Template-O-Matic toolbox. A 2-sample t-test has been used to compare my two > groups and these analyses have identified sig differences in areas that > would be expected in the literature. However, I am concerned as the results > produced from the modulated (non-linear) and unmodulated images differ > substantially. I understand that the analysis of modulated and unmodulated > images will answer different questions and that the two techniques will give > varying results. However, I would have expected based on the literature and > my understanding of the modulation process that analyses of unmodulated data > would give differences generally in the same areas as the modulated but with > more extensive group differences (e.g., Fortino et al, 2009; Schizophrenia > Research). Or at least that group differences would be in the same direction > using both image types i.e., areas of reduced volume in both analysis types. > > In contrast I have found group differences in opposite directions using the > two images, namely that in modulated analyses my group of interest shows > predominately increased GM and WM relative to the control group, but in > unmodulated analyses the group of interest show predominately decreased GM > and WM (albeit in different areas). I have one grey matter finding that is > maintained in both analyses (interest group > control group), otherwise > there seems to be no overlap between the results. > > I have repeated the analysis several times as the sample has expanded and > the same finding has come up. I also checked my segmentation outputs and > none of them look strange and there are no real inhomogeneity issues. > > I am just wondering whether anyone has any thoughts on whether this is some > sort of error or whether the findings are possible? The areas of decreased > GM and WM are more in line with what I would expect to see in an adult > population but it is possible that in this child/adolescent population some > specific areas might be increased in volume. > I would be keen to hear if anyone has any thoughts/explanations for this. > Many thanks, Alexis > > Alexis Cullen > Research Worker/PhD Student > R&R and CHADS projects > Department of Forensic Mental Health Science (PO23) > Institute of Psychiatry, De Crespigny Park > Denmark Hill, SE5 8AF > [log in to unmask] > 020 7848 5678 > >