In literature, the term is often used synonymously with Marcus Gunn phenomenon or relative afferent pupillary defect (RAPD).1 After exposure to bright light, a normal pupil constricts. In many ways, pupil responses are similar to other eye movements, such as saccades and smooth pursuit: like these other eye movements, pupil responses have properties of both reflexive and voluntary action, and are part of active visual exploration. Marcus Gunn pupil (MGP) is the term given to an abnormal pupil showing aberrant pupillary response in certain ocular disorders. Although pupil responses likely serve many functions, not all of which are fully understood, one important function is to optimize vision either for acuity (small pupils see sharper) and depth of field (small pupils see sharply at a wider range of distances), or for sensitivity (large pupils are better able to detect faint stimuli) that is, pupils change their size to optimize vision for a particular situation. Abnormal pupillary function Bilateral pupil function abnormality Bilateral pupil response abnormality Bilateral pupil response abnormality (eye condition). A constriction response (miosis), is the narrowing of the pupil, which may be caused by scleral buckles or drugs such as opiates/opioids or anti-hypertension. I also discuss the functional relevance of pupil responses, that is, how pupil responses help us to better see the world. In this review, I describe these three pupil responses, how they are related to high-level cognition, and the neural pathways that control them. Pupils respond to three distinct kinds of stimuli: they constrict in response to brightness (the pupil light response), constrict in response to near fixation (the pupil near response), and dilate in response to increases in arousal and mental effort, either triggered by an external stimulus or spontaneously.
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