In medicine, pupil constriction (also known as the pupillary reflex) is reduction of pupil size. It is the normal response of the pupil of the eye to light and dependent on the function of the optic nerves and oculomotor nerves. It is also important in accommodation (when the eyes focus on something close).

Emergency room physicians often assess the pupillary reflex because it is useful for gauging brain stem function. Normally, pupils react (constrict) equally (or consenually). Lack of the pupillary reflex or an abnormal pupillary reflex can be caused by optic nerve damage, oculomotor nerve damage, brain death and things that mimic brain death, such as barbiturates. The optic nerve is responsible for the afferent limb of the pupillary reflex, or in other words, senses the incoming light. The oculomotor nerve is responsible for the efferent limb of the pupillary reflex; in other words, it drives the muscles that constrict the pupil. Dependent on how the pupils constrict or do not constrict one can determine which of the cranial nerves is damaged.

If the optic nerve is damaged on one side: the pupil on that side will constrict if light is shone into the other eye. However, it will not constrict if light is shone in that eye. If the oculomotor nerve is damaged on one side the pupil will not constrict on that side. However, if light is shone into that eye (and the afferent limb of the reflex is intact) the other eye will constrict.

Retinal ganglion cells convey information from the photoreceptors up the optic nerve to the pretectal nucleus of the high midbrain, bypassing the lateral geniculate nucleus and the primary visual cortex. Pretectal nucleus neurons send axons to neurons of the Edinger-Westphal nucleus whose axons run along the left and right oculomotor nerves. Oculomotor nerve axons synapse on ciliary ganglion neurons whose axons innervate the constrictor muscle of the iris. Here is a nice animation (http://medstat.med.utah.edu/kw/hyperbrain/anim/reflex.html) of this pathway.

Normally it is difficult to make the pupil constrict arbitrarily, i.e., at a person's will, without a natural reason (lighting or viewed distance change). However it is possible, after some practice. Some hypnotists use the knack of quick constriction and dilation of their pupils in their trade.