Hi, I have performed a longitudinal analysis using the lme module in FreeSurfer, with this model:
*intercept(random effect) + centered age + group + group x centered age + sex* I tested the effect of time with this contrast vector [ 0 1 0 0 0 ]. Dependent variable is area. Here, mapping the second beta means mapping the effect size for (change over) time. In the beta map, I find values from 0 to 0.004. I would interpret that to mean that local area shrinks by at most 0.004 mm² per year in the reference group. But I'm not 100% sure about the biological (or geometrical) meaning of that. Can I interpret this literally as the mean yearly shrinkage of the three triangles surrounding a given vertex, the average of whose area comprises the area score of the vertex, being 4/1000 mm? Of course, these maps are smoothed with 30mm, so the real spatial resolution is nowhere near this.... Thank you! -- yours, Lars M. Rimol, PhD St. Olavs Hospital Trondheim, Norway
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