Pupil dilation

Pupillary response is a physiological response that varies the size of the pupil, via the optic and oculomotor cranial nerve. A constriction response (miosis), is the narrowing 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 anticholinergic agents or drugs such as MDMA, cocaine and amphetamines. Dilation of the pupil occurs when the smooth cells of the radial muscle, controlled by the sympathetic nervous system (SNS), contract. Constriction of the pupil occurs when the circular muscle, controlled by the parasympathetic nervous system (PSNS), contracts.

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. The pupils contract immediately before REM sleep begins. A pupillary response can be intentionally conditioned as a Pavlovian response to some stimuli.

The latency of pupillary response (the time in which it takes to occur) increases with age. Use of central nervous system stimulant drugs and some hallucinogenic drugs can cause dilation of the pupil.

In ophthalmology, intensive studies of pupillary response are conducted via videopupillometry.

Anisocoria is the condition of one pupil being more dilated than the other.

...
Wikipedia