Pupillary response is a physiological response that varies the size of the pupil, via the optic and oculomotor cranial nerve. This response results in either constriction (miosis),<ref>Ellis CJ (November 1981). "The pupillary light reflex in normal subjects" (PDF). Br J Ophthalmol 65 (11): 754–9. </ref> narrowing the pupil, or dilation (mydriasis), widening the pupil. Dilation of the pupil occurs when the smooth cells of the radial muscle, controlled by the sympathetic nervous system (SNS), contract. Constriction of the pupil occurs when the circular muscle, controlled by the parasympathetic nervous system (PNS), contracts.

The responses can have a variety of causes, from an involuntary reflex reaction to exposure or inexposure to light — in low light conditions a dilated pupil lets more light into the eye — or it may indicate interest in the subject of attention, or sexual stimulation<ref>"Pupil Size as Related to Interest Value of Visual Stimuli", Science 132 (3423), 5 August 1960: 349–50,</ref>. The pupils contract immediately before someone falls asleep<ref>"Pupillary Movements During Acute and Chronic Fatigue: A New Test for the Objective Evaluation of Tiredness" (PDF), Investigative Ophthalmology (St. Louis: C.V. Mosby Company) 2 (2), April 1963: 138–157</ref>. A pupillary response can be intentionally conditioned as a Pavlovian response to some stimuli<ref>Baker, Lynn Erland (1938). "The Pupillary Response Conditioned to Subliminal Auditory Stimuli". Ohio State University.</ref>

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! Muscular mechanism

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! Cause in pupillary light reflex