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→Pupillary responce
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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> and is influanced by oxytocine level<ref>Leknes, S., Wessberg, J., Ellingsen, D.-M., Chelnokova, O., Olausson, H., & Laeng, B. (2012). Oxytocin enhances pupil dilation and sensitivity to “hidden”emotional expressions. Social Cognitive and Affective Neuroscience, nss062.</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>.
The latency of pupillary response (the time in which it takes to occur) increases with age<ref>Latency of pupillary reflex to light stimulation and its relationship to aging (PDF), Federal Aviation Agency, Office of Aviation Medicine, Georgetown Clinical Research Institute, September 1965, p. 12,</ref>. Use of central nervous system stimulant drugs and some hallucinogenic drugs can cause dilation of the pupil<ref>Jaanus SD (1992), "Ocular side effects of selected systemic drugs", Optom Clin 2 (4): 73–96,</ref>.