The pupil is the large black circle that is in the center of each eye. It widens and narrows naturally in response to changes in the light. Pupils become larger in dim light to let in more of the image and smaller in bright light to limit the amount of light that is received. Optometrists and ophthalmologists typically use medicinal drops to dilate the pupils so that they can investigate the health of the eye, report doctors at the National Eye Institute. When pupils dilate for no apparent reason, there typically are underlying causes.
Researchers at the American Council for Drug Education caution parents to watch for dilated pupils to tell if their children are using drugs. Illegal substance abuse can be spotted by dilated pupils in those who are using stimulants such as methamphetamines, cocaine and hallucinogens like mescaline and peyote. The drugs excite the central nervous system and mix signals from the brain, which also cause the euphoria associated with the drugs in addition to dilated pupils, increased heart rate, irrational behavior and mood swings.
According to the University of Maryland Medical Center, dilated pupils can be caused by seizure disorders, including epilepsy. As other symptoms occur, such as muscle spasm, unconsciousness and staring, the pupils become wide and dilated and do not respond to light or any other stimuli. Hyperexcitable nerve cells can misfire and send inappropriate signals to the brain, which lead to seizures.
A concussion can cause dilated pupils as the condition interferes with the message delivery system in the brain. Following a concussion, caused by a fall or hit on the head, patients who remain conscious typically have dilated pupils. The dilation will cause the patient to be more sensitive to light and may cause additional vision problems.
In his book, "Zen Sex: The Way of Making Love", author and Zen master Philip Toshio Sudo says that the eyes are the portals that allow all light to enter; organs that reflect the stimulation they receive. Unconsciously, the brain sends stimulating references to the eyes, and the pupils widen to take in the complete picture of that which is causing the arousal, whether it's a loved one or a provocative picture. Sudo reports that when men are shown pictures of beautiful women, their pupils can dilate as much as 30 percent. Zen practitioners refer to it as the "Way of the Eyes".