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
>
>