The pupillary light reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retina of the eye, thereby assisting in adaptation to various levels of darkness and light, in addition to retinal sensitivity. Greater intensity light causes the pupil to become smaller (allowing less light in), whereas lower intensity light causes the pupil to become larger (allowing more light in). Thus, the pupillary light reflex regulates the intensity of light entering the eye.
... The pupillary light reflex is mediated by the pretectum ... This reflex is responsible for the constriction of the pupils upon light entering the eye ... proceeds to relay the command to constrict the pupils to the pupillary sphincter via the ciliary ganglion ...
... In addition to controlling the amount of light that enters the eye, the pupillary light reflex provides a useful diagnostic tool ... of both eyes respond identically to a light stimulus, regardless of which eye is being stimulated ... Light entering one eye produces a constriction of the pupil of that eye, the direct response, as well as a constriction of the pupil of the unstimulated eye, the consensual response ...
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