As we know, Anisotropic range is from -1 to +1
From 0 -> +1, usually, the anisotropic orientation is vertical and. from 0 -> -1, is horizontal.
So, in my opinion, black should be horizontal and white vertical. And, it's so.
But, from RGB [0,0,0] until RGB [64,64,64] nothing change!!! And, from [0,0,0] until [64,64,64] the anisotropy is perfectly horizontal O_O
Also, we have different noise quality in the preview at [64,64,64]