Is it true that pupils dilate when we see something we like?

Your eyesight is one of your most essential senses. Almost 80% of what we perceive comes through our sense of sight. Its complex parts and functions make us perceive everything around us.

When you try to look at the mirror, you will see a dark circle in the middle of your eye- it is your pupil. The pupils are the opening in the center of the iris where light enters. Iris is the structure that gives color to your eyes. The pupil allows light to enter the eye to focus on the retina to begin the sight. Typically, the pupils appear perfectly round, equal in size and black in color. The black color is caused by the light that passes through the pupil. It is received by the retina and reflected (in standard lighting).

When the pupil is cloudy, or pale in color, usually because the lens of the eye has become opaque due to the formation of a cataract, when the cloudy lens is replaced by a clear intraocular lens (IOL) during cataract surgery, the usual black appearance of the pupil is restored.

When a friend takes a photo of you with the camera’s flash function, depending on your direction of gaze when the picture is taken, your pupils might appear bright red. It is due to the intense light from the flash being reflected by the red color of the retina.

Besides changing colors due to light reflection, do you know that your pupils can vary in size? Your pupils become bigger or smaller, depending on the amount of light in your surroundings. In low light, your pupils open up or dilate, to let in more light. When it’s bright, they get smaller or constrict, to let in less light.

The standard pupil size in adults varies from 2 to 4 mm in diameter in bright light to 4 to 8 mm in the dark. The pupils are generally equal in size. They constrict to direct illumination and the illumination of the opposite eye. The pupil dilates in the darkness. Both pupils constrict when the eye focuses on a near object. The pupil is abnormal if it fails to enlarge in the dark or constricts in the light.

The pupils respond to more than changes in light. They also reflect mental and emotional experiences.

Pupil dilation is also linked with arousal so consistently. Researchers use pupil size, or pupillometry, to investigate a wide range of psychological phenomena.

According to Stuart Steinhauer, director of the Biometrics Research Lab at the University Of Pittsburgh School Of Medicine, the dilations are by-products of the nervous system’s processing of necessary information. The visual cortex found at the back of the brain assembles the actual images we see. But a different, older part of the nervous system—the autonomic—manages the continuous tuning of pupil size (along with other involuntary functions such as heart rate and perspiration).

The stimulation of the “fight or flight” responses when the body is experiencing stress induces dilation of the pupils while “rest and digest” functions of the parasympathetic system causes constriction. The discomfort of the latter system can, therefore, cause dilation.

Emotional and cognitive events also trigger the response of the pupils.

Eckhard Hess, a psychologist at the University of Chicago, conducted a pioneering study. He coined the term pupillometry in 1975 for the study of pupil size as an indicator of emotions. One night, Hess was looking at photos of animals, and his wife noticed that his pupils expanded. She asked him if the light was not enough, and he said there was good enough light. This instance struck him as interesting enough to embark on research about what caused pupils to dilate.

Another psychologist, Daniel Kahneman from Princeton University, said that pupil size increases in proportion to the difficulty of a task at hand. Calculate 9 times 13, and your pupil dilate slightly. Try 29 times 13, and they will widen further and remain dilated until you get the answer and stop calculating at mind. In his book entitled “Thinking Fast and Slow,” he said that he could determine when someone gave up on a multiplication problem by merely watching for pupil contraction during the experiment.

Pupils dilate when people see or hear something interesting. In one study, the subjects listened to three book excerpts. The first book was erotic, the second book involved mutilation, and the third book was neutral. There was an observation that the subjects’ pupils become more prominent for all three passages. However, in the first two – the erotic, and the one involving mutilation, the pupils remained wide.

The opposite also applies. Pupils constrict when people see something disgusting. In a study, subjects were shown pictures of injured children. The results showed that subjects’ pupils became more prominent while looking at the photos at first but (as their interest was piqued), but they constricted very soon because they were disgusting. Unlike in the neutral case wherein the pupils become normal again due to the object not continuing to hold one’s interest, the opposite happens when the object has an adverse interest.

Additional reading:
Pupillometry (Wikipedia)