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>. Emotional pictures arouse the SNS which cause mydriasis<ref>[http://europepmc.org/articles/pmc3612940 Bradley, M. M., Miccoli, L., Escrig, M. A., & Lang, P. J. (2008). The pupil as a measure of emotional arousal and autonomic activation. Psychophysiology, 45(4), 602–607.]</ref>.