Hi all,
I've recently run a 2x2x2 ANOVA, where I find very significant results through
SPM, but have very confusing results when extracting values for significant
voxels. The ANOVA (see attached design matrix) has 2 within subject factors,
and 2 groups of subjects (N=17 and 16). I'm trying to look at the effect of
group and specified the following contrast vector:
ones(1,17)/17 -ones(1,16)/16 1 -1 0 0 0 0 .5 .5 -.5 -.5 0 0 0 0 .5 .5 -.5 -.5
Doing so gives me a pattern of results that are echoed in effects of group
when I look at ANOVAs across only one within subject factor (ie 1/2 of the
current ANOVA), as well as when I look at t-tests between groups across one
level of a single factor....so that all makes sense. The strange part is that if I
extract signal from a peak voxel or cluster in the group2>group1 contrast,
using the eigenvariate (VOI) function, I get data that makes a lot of sense (4
values per subject, significant ANOVA results when put into SPSS, etc). When
I extract, however, a highly significant peak voxel or cluster from the
group1>group2 contrast, in the same way as I did the reverse, the data no
longer make any sense....there's no longer any significance at the t-test or
ANOVA level when put into SPSS.
It seems I must be doing something wrong, but not sure what it is. Any and all
thoughts welcome!!
thanks,
Amit
|