[Freesurfer] Jacobian Questions

Hello,

Since the jacobian files contain only positive numbers, is there any way to make a distinction between a brain area that needs to be stretched bigger to fit the template versus a brain area that needs to be squished smaller?

fsaverage is comprised of 40 subjects, but I wonder if the jacobian is most accurate only for my subjects that have the same age as the fsaverage mean? If so, then the younger subjects would need to be squished and the older subjects would need to be stretched. But since the jacobian is always positive, then on a scatterplot of jacobian versus age, this would look like a hump centered on the fsaverage mean age. Am I on the right track with this?

Thank you, Jeff