Adjusted or unadjusted CT values and interpretation of volume values in ocn.dat from group analysis
Hello Freesurfer experts,
I have performed a group level analysis of volume and cortical thickness using mri_glmfit and performed multiple comparisons correction with mri_glmfit-sim. I used the contrast matrix 0 1 0 to analyze 1 group with 2 covariates, along the 1st covariate (education) with the 2nd covariate being age.
I get 2 significant clusters (after mult. comp. corr.) for CT and 2 for volume. When I look at the values in ./cache.th30.abs.y.ocn.dat, the average CT values within my sample are 2.57 and 2.50 for the 2 regions.
*My first question is: *are the CT values outputted to ocn.dat adjusted for my 2nd covariate (age) or are they unadjusted CT values within the sig. cluster?
The volume values contained within the ocn.dat files are also in the same range as the CT values, with averages of 2.50 and 1.87 for left superior frontal and right inferior parietal volume, respectively.
*My second question is:* What is the metric for volume reported in ocn.dat? These values seem too low to be total cluster volume for each subject expressed as mm^2. The volumes reported in ./cache.th30.abs.sig.cluster.summary are more like what we I expect for volume (283.24 mm^2). Could these values be the average volume surrounding each vertex in the inflated mesh (volume contained within the surrounding vertices)?
Thank you,
Elizabeth
On 09/20/2017 01:54 PM, Elizabeth Bartlett wrote:
Hello Freesurfer experts,
I have performed a group level analysis of volume and cortical thickness using mri_glmfit and performed multiple comparisons correction with mri_glmfit-sim. I used the contrast matrix 0 1 0 to analyze 1 group with 2 covariates, along the 1st covariate (education) with the 2nd covariate being age.
I get 2 significant clusters (after mult. comp. corr.) for CT and 2 for volume. When I look at the values in ./cache.th30.abs.y.ocn.dat, the average CT values within my sample are 2.57 and 2.50 for the 2 regions.
_My first question is: _are the CT values outputted to ocn.dat adjusted for my 2nd covariate (age) or are they unadjusted CT values within the sig. cluster?
unadjusted
The volume values contained within the ocn.dat files are also in the same range as the CT values, with averages of 2.50 and 1.87 for left superior frontal and right inferior parietal volume, respectively.
_My second question is:_ What is the metric for volume reported in ocn.dat? These values seem too low to be total cluster volume for each subject expressed as mm^2. The volumes reported in ./cache.th30.abs.sig.cluster.summary are more like what we I expect for volume (283.24 mm^2). Could these values be the average volume surrounding each vertex in the inflated mesh (volume contained within the surrounding vertices)?
Yes, the average. You can multiply it by the number of vertices in the cluster to get total volume
Thank you,
Elizabeth
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Elizabeth Bartlett