Phase retardation per unit depth (PR/UD, proportional to birefringence) measurements along a 360° circular sweep around the ONH. PR/UD measurements (ring or radial) in each eye are plotted with a unique symbol. The black squares in both figures represent the average PR/UD of combined ring and radial scans for each angular section region that was used to determine neurotubule density. (A)Two different radial area and single ring scan for subject 52704. (B) Area scan and ring scan of 1853 OS. N, nasal; T, temporal; S, superior; I, inferior