The black dot in the centre of the eye, the pupil, is not colourful like the iris, but it has an important function. The pupil regulates the amount of light that reaches the retina. It expands, or gets larger, and contracts, or gets smaller, to accommodate environmental light conditions.
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What is a Pupil?
The pupil of the eye is in the centre of the iris, which is the coloured part of the eye. The purpose of the pupil is to let light into the eye. Pupils enlarge or contract based upon the amount of light, getting bigger in lowlight environments to let more light in, and getting smaller when the light is bright. This is why when an optometrist dilates eyes (enlarges the pupils) the patient must wear sunglasses to block out the light since the pupils cannot contract.
Unequal Pupil Sizes
Pupils are the same size, varying by at most 1 millimetre, when conditions are normal. The pupils may be temporarily different sizes, such as if one side of the face is in the light while the other side is shadowed, but they return to normal when back in similar light. Unequal pupil sizes which do not regulate can be symptoms of a medical problem. Causes include migraines, tumours, glaucoma or head injuries. Some medicines which get in the eye can also change pupil size. You should have a doctor check out an unexplained change in pupil size that does not go away.
Uses of Measuring Pupil Size
Pupil size measurements can be useful in several instances. Pupil size can affect how well a patient will react to LASIK surgery. LASIK can cause difficulty with night vision, when pupils are largest, so people who have large pupils may not be good candidates for the procedure as their night vision may be too greatly affected. Some police officers also use pupil reactiveness to determine if a person is under the influence of drugs. Pupil size stability can also help identify people with sleep deprivation, as the pupil may change size, even if light conditions haven't changed, if the muscles in the eye are tired.
Pupil Size and Emotions
Several studies have been conducted regarding the reaction of pupils to emotional stimuli. The enlarging or contracting of the pupils may indicate happiness, sadness, anger or other types of arousal. Change in pupil size can also indicate how much a person is thinking about, whether their minds are very active or more restful.
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