Changes in pupil size are involuntary. The autonomic nervous system controls pupil size. This is the same system that controls other automatic actions, such as breathing. However, other factors can also cause pupils to increase in size, or dilate. Some examples of these factors include :. In some cases, the pupils will dilate permanently. For example, someone with congenital aniridia will have larger pupils all the time, due to a deficiency of the surrounding iris.
For example, emotions such as anxiety may also cause changes in pupil size. This can be a result of the fight-or-flight response, which is involuntary. Eye contact has long been central to human interactions.
Changes in emotion might cause pupil dilation. The autonomic nervous system triggers various involuntary responses during emotions, such as fear or arousal. Some research suggests that pupil dilation is one of these involuntary responses to arousal or attraction. For example, one study recruited men and women with varying sexual orientations.
The researchers played erotic videos to the participants while monitoring their pupil size. The study found that erotic videos led to pupil dilation in the participants. The team showed that bisexual men and heterosexual women had dilated pupils in response to erotic videos that included people of either sex.
Another study, this time in the journal Evolution and Human Behavior , focused on associations between pupil dilation and sexual attraction. The researchers found that it was possible to determine sexual interest from the size and darkness of the pupils.
Pupil size also appears to respond to hormonal changes. One study recruited 14 women to view arousing pictures on a computer screen on different occasions. Half of the women were taking hormonal birth control pills. Women not taking birth control pills had dilated pupils while viewing arousing pictures during ovulation. This did not occur in women taking birth control pills.
There is also some evidence to suggest that pupil dilation may indicate trust. A study in the Proceedings of the National Academy of Sciences suggests that pupil size changes play a role in trust formation.
It remains unclear why pupil dilation might be involved in attraction. It could be a result of hormonal changes. Whereas stimulation of the parasympathetic system, known for "rest and digest" functions, causes constriction.
Inhibition of the latter system can therefore also cause dilation. The size of the pupils at any given time reflects the balance of these forces acting simultaneously.
The pupil response to cognitive and emotional events occurs on an even smaller scale than the light reflex, with changes generally less than half a millimeter. By recording subjects' eyes with infrared cameras and controlling factors that might affect pupil size, such as ambient brightness, color and distance, scientists can use pupil movements as a proxy for other processes, like mental strain.
Princeton University psychologist Daniel Kahneman showed several decades ago that pupil size increases in proportion to the difficulty of a task at hand. Calculate nine times 13 and your pupils will dilate slightly.
Try 29 times 13 and they will widen further and remain dilated until you reach the answer or stop trying. Kahneman says in his book , Thinking Fast and Slow, that he could divine when someone gave up on a multiplication problem simply by watching for pupil contraction during the experiment.
Subsequent research found that the pupils of more intelligent people as defined by their Scholastic Aptitude Test scores dilated less in response to cognitive tasks compared with those of lower-scoring participants, indicating more efficient use of brainpower.
Scientists have since used pupillometry to assess everything from sleepiness , introversion and sexual interest to race bias , schizophrenia , moral judgment , autism and depression. And whereas they haven't been reading people's thoughts per se, they've come pretty close. Participants were told to press a button at any point during a second interval, and their pupil sizes correlated with the timing of their decisions.
Dilation began about one second before they pressed the button and peaked one to two seconds after. But are pupils informative outside the lab?
Can pupil size be used to "read" a person's intentions and feelings? According to Men's Health magazine a man can tell when it is "time to make your move" by watching his date's pupils, but some skepticism is warranted. Other efforts to exploit pupil dilations for purposes beyond scientific research have failed.
The iris the coloured part of the eye plays an important role in the proper functioning of the eye, as it's primary job is to regulate how much or how little light enters the eye. This is controlled by muscles in the iris which will constrict or enlarge the pupil.
Nerve impulses travel down the optic nerve after light enters the eye, which affects the size of the pupil. The pupil changes involuntarily, which is known as the pupil reflex. However, there are a range of other emotional or physical environmental triggers which can cause your pupils to change in size which include Lighting conditions Just as a camera requires the correct amount of light to capture vivid images - your eyes also need the correct amount of light to see properly.
During the day less light is needed to see things around you, which is why the iris narrows and the pupil constricts and gets visibly smaller. Conversely, in darker conditions, the pupil will enlarge as your eye will try to let in more light to see things you otherwise would have easily seen during the daytime.
Focus When focusing your eyes on a near object your pupils will constrict to increase the depth of focus in the eye by blocking the light scattered by the periphery of the cornea. Conversely, when you when you look at something further out into the distance, the diameter of your pupil will increase and muscles will relax in order to stretch the ligaments inside your eyes. This will pull on the lens, causing it to flatten and create a thinner contour allowing you to see distant objects in focus.
Medicines A few medicines can affect the muscles that control your pupils; preventing your pupils from getting smaller when the light shines in. Similarly, in order for the eye doctor to view the back of the eye during an eye exam, dilation eye drops work by relaxing the muscles in the iris that are responsible for constricting the pupils. For example, 'Atropine' is a medication that enlarges the pupil and comes from a plant called Belladonna which is Italian for 'beautiful woman' because historically the herb was used by women to make them appear more seductive.
Drug Use Certain illegal drug can temporarily affect the ability of the iris to contract, which is why enlarged pupils can sometimes be an indication that someone has used illegal drugs. Marijuana is a good example of this, as it triggers a release of dopamine, which excites the adrenergic receptors and induces mydriasis dilation of the pupils.
On the other hand, narcotics are known to cause "pinpoint pupils".
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