On 8/11/2020 11:35 AM, Purcell, Juliann wrote:

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Hello FreeSurfer experts,

I’m planning to run an analysis using mri_glmfit in the command line to look at relationships between cortical gray matter volume and indices of adolescent substance use. Do I need to include brain/head size as a covariate in these analyses even though they are performed in the conformed space of fsaverage and not in native space?

By default, the analysis is not done in fsaverage space. "conformed" just means that the volume is 1mm3, 256^3.

I suspect the answer is yes, but in looking through the archive many of the questions pertained specifically to subcortical analyses or those performed in QDEC so I wanted to clarify for cortical volumetric analyses. Thank you for your help!

Yes, for cortical volume you need to account for eTIV by either dividing by it or by including it as a nuisance regressor.

Best,

Juliann Purcell, MSc Doctoral Candidate Medical/Clinical Psychology Doctoral Program University of Alabama at Birmingham Department of Psychology juliannp@uab.edu mailto:juliannp@uab.edu

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