Perspective on Vision: The Visual System as a Black Box

Vision is a complex phenomenon that can be addressed from different points of view. Input to the visual system consists of visual stimuli, the final output is visually guided behavior, while visual perceptions are an interme-diate product. Clinicians often start by considering the input-related aspects that can be addressed by medical and surgical means. Patients, on the other hand, may be most concerned about the output aspects, that is, the ef-fect on their daily activities. The relations between these 2 points of view are often misunderstood, which may lead to miscommunication. This perspective-based on more than 4 decades devoted to vision rehabilitation-aims at exploring these differences to bridge the communications gap. Seemingly similar tests may actually assess different as-pects. One example is the relationship between letter chart acuity and reading ability, as demonstrated by the difference between Jaeger's and Snellen's tests. Clinical applications require assessment of individuals. Societal applications deal with groups of people; they include research, public health statistics, and eligibility rules for privileges or bene-fits. Such applications often rely on the application of for -mulas to input measurements to estimate the consequences on the output side. The implications of such simplifications should be understood. (Am J Ophthalmol 2021;224:66-73. (c) 2020 Elsevier Inc. All rights reserved.)