[Freesurfer] primary diffusion direction & surface normal vector

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Hi Freesurfer experts,

After DTI tensor fitting, usually we obtain primary, secondary, and tertiary eigenvectors (V1, V2, V3) and eigenvalues (L1, L2, L3). So what do we use to describe the primary diffusion direction, FA or AD(=L1)? I read the term "surface normal vector" from a paper and the formula r=[Vn * e1|. V presents the surface normal vector, which can be computed using the cross-product between edges. Where can I find this information? The paper named: "Surface-based analysis of diffusion orientation for identifying architectonic domains in the in vivo human cortex" Thank you so much!

Best, Qi