Since the structure of a physiologic system often controls and predicts its function, without an exact working relationship between the two, the meaning of structural variations can on its own be unclear. Thus, functional assessment sometimes takes the primary role in patient care especially because it is usually the function of the organ that is what's clinically relevant. Glaucoma physicians are accustomed to this for posterior segment evaluation. Visual fields are a functional assessment of the structure (retinal ganglion cells as assessed by histology or OCT). Visual fields and defects therein are what a glaucoma patient actually experiences, as opposed to the absolute number of ganglion cells or thickness of the retinal nerve fiber layer on an OCT scan. Thus for glaucoma diagnosis and management, functional assessment of vision with visual fields historically preceded that of more structural evaluations that eventually became prominent today. Recent improvements in posterior segment structure and function relationship was required for this to happen [41–43].