> I have performed one VBM analysis using VBM8 toolbox and SPM version
> (8.4010) with a Dartel template made based on my group. (And modulation:
> non-linear only)
>
> For the statistics (a simpel two-sample t-test) I get some group differences
> which survive the Voxel and cluster FWE correction. (For statistics I add
> age as a covariate of not interest)
>
> Now, I performed the same analysis but this time without the use of VBM8
> toolbox (subjects are the same) on a newer SPM version (8.4667).
> Meaning performing , new Segment ---> Dartel template creation --->
> Normalize to MNI (with Modulation)
>
> I used the code in the mailing list to calculate the Total Gray mater, WM
> and TIV from the C[13]* images of this analysis.
>
> Question 1)
> The global numbers that I have calculated here, differ from the global
> measures which the VBM8 toolbox gives as an output (rawvolumes.txt). Why??
I'm not actually sure how the values in rawvolumes.txt are computed,
so can't say much here. The segmentation approaches differ between
the one in SPM and the one in VBM8, so if rawvolumes is computed by
summing over the tissue classes, the resulting volume measurements are
likely to differ slightly.
>
> Question2)
> Now to perform the same statistics (two-sample t-test), I have added the
> calculated TIV once as a covariate and once as a global measure with option:
> Normalize : proportional. In both cases I get completely different results
> of the group differences in compare to the VBM8 results. No significant
> Voxel or cluster survives the corrections and even the uncorrected results
> are not so similar. One may be able to say that they have the same trend but
> it is far from being significant. What could cause this differences and what
> is the correct way for me to proceed?
The linear models you formulated are quite different from each other,
and express distinct questions about the data. If one group has a
systematically different TIV from the other, then I'd expect to obtain
findings that are especially different.
If I understand correctly, the "modulation" in the VBM8 toolbox does
not incorporate the volume change from the affine transformation. For
this reason, you will obtain different findings.
Best regards,
-John
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