Pupillary response

A constriction response (miosis),[2] is the narrowing of the pupil, which may be caused by scleral buckles or drugs such as opiates/opioids or anti-hypertension medications.

A dilation response (mydriasis), is the widening of the pupil and may be caused by adrenaline; anticholinergic agents; stimulant drugs such as MDMA, cocaine, and amphetamines; and some hallucinogenics (e.g.

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 arousal, sexual stimulation,[4] uncertainty,[5] decision conflict,[6] errors,[7] physical activity[8] or increasing cognitive load[9] or demand.

The responses correlate strongly with activity in the locus coeruleus neurotransmitter system.

[14] Some humans have the ability to exert direct and voluntary control over their iris sphincter muscles and dilator muscles, granting them the ability to dilate and constrict their pupils on command, regardless of lighting condition and/or eye accommodation state.

Dilation and constriction of the pupil
A human adult exhibiting voluntary control over his iris muscles, where he can cause his pupil to dilate and constrict on command
Constricted (left) and dilated (right) pupil of the same eye
Scheme showing sympathetic and parasympathetic innervation of the pupil and sites of lesion in a Horner's syndrome
Sympathetic connections of the ciliary and superior cervical ganglia