Dear SPM List,
We have been working on a longitudinal VBM analysis (2 groups, 3 time-points), and have come up against (what seems to us) a tricky issue of which template we should be initially generating for our analysis. We begin by realigning the the images within participant, and applying an intra-subject bias correction (as implemented in the VBM toolbox).
We ultimately want to be able to examine the Group x Time interaction.
We are intending to use the DARTEL machinery of SPM8 (the "New Segment" routine) to produce the templates and deformations which warp the chosen images to the "standard" (MNI) template.
The options we have considered are:
1) A "grand average" template derived from the complete set of data, over all participants and time-points.
We would then be comparing images warped by the parameters that deform the grand mean to the standard. The differences between the warped structurals will then reflect the "distance" between the images and the grand mean.
2) A template derived from all the time-point 1 data, across subjects.
This then leaves us examining the differences between later images and the mean of time-point 1
3) A template derived for each participant:
-Either from time-point one, or the mean of the 3 scans.
Giving the within-subject differences over time.
We are currently inclined towards this last option, i.e. producing warps for each participant that map their first scan to the standard template, and applying this to subsequent scans to examine the differences. Are there any issues that are imposed for the subsequent analysis by not using a consistent template across participants?
If anyone has any insight into which of these is the most appropriate upon which to base a VBM, and subsequently a DBM analysis, we would be extremely grateful for any thoughts or suggestions.
Thanks in advance,
Alexis Hervais-Adelman
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Alexis Hervais-Adelman, PhD
Functional Brain Mapping Lab
Departement des Neurosciences Fondamentales
University of Geneva Medical School
rue Michel Servet 1
CH-1206 Genève
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