Not only are the eyes the windows to the soul, they also offer a glimpse into the mind and the internal state of the body. The pupils, the black circles in the centre of the eyes, respond to light entering the eyes. In low light they dilate or widen to collect more light, while in bright light they constrict or become smaller. Pupil size is also modulated for reasons other than light, for example thoughts, emotions or mental effort.
Certain prescription medications can cause the pupils to become dilated by interfering with the chemicals that transmit messages from the brain to the eyes, and recreational drugs, such as hallucinogens and stimulants have a similar effect. Neurological conditions of the nerves which go to the eye can interfere with constriction and dilation of the pupils and are often accompanied by other vision symptoms. Injury to the eye may damage the nerves controlling the pupils or the iris, disrupting normal pupillary response. Brain injury as a result of trauma, a stroke or a tumour can cause increased intracranial pressure which can affect the eyes and the reaction of the pupils. Whether it's caused by external or internal factors, pupil dilation is an involuntary nervous system response. In other words, we can't control it.
The visual cortex processes and interprets the images transmitted by the eyes to the brain. A different part of the nervous system, the autonomic nervous system, controls the involuntary functioning of the body, functions over which we have no control, such as heart rate, perspiration and pupil dilation. Stimulation of the autonomic nervous system's sympathetic branch, known for triggering "fight or flight" responses when the body is under stress, induces pupil dilation, whereas stimulation of the parasympathetic system, known for "rest and digest" functions, causes constriction. The size of the pupils at any given time reflects the balance of these forces acting simultaneously.
For over a century, scientists have reported that the pupils dilate for reasons unrelated to the levels of light in the environment, betraying mental and emotional states within the body. Pupillometry is a valuable tool for psychological research because the eyes are easy to observe and provide a sensitive indicator of cognitive, emotional and sensory response. It has been used to assess everything from sleepiness, introversion and sexual interest to race bias, moral judgement and depression. Pupil dilation can indicate attraction or surprise, as well as feelings of anger and fear. One researcher has reported that the pupils are able to reflect mental effort very precisely, while another has shown that pupil size increases in proportion to the difficulty of a task. Pupillometry became popular in the advertising industry during the 1970s as a way to test consumers' responses to television commercials.
Sounds can evoke pupil dilations, depending on their emotional content. Highly arousing sounds, such as loud voices arguing lead to larger pupil dilations than neutral sounds such as background office noise. When people are listening to music, their emotional reactions to the music are reflected in their pupil size. New research has introduced an innovative hearing test that may help people who are unable to respond directly, due to a physical disability for example. The test relies on measurements of pupil dilation rather than a response such as pressing a button.
Pupil dilation may be performed by optometrists during a comprehensive eye examination to purposely increase the size of the pupil and force it to remain open and not constrict in response to light. Special drops are inserted into the eyes to dilate the pupils, allowing the optometrist a closer look at the retina, the blood vessels and the optic nerve at the back of the eye. As well as giving information about the health of the eyes, this examination may reveal the early stages of conditions such as diabetes before symptoms are apparent. While your eyes are dilated, your vision will be blurry, you will have trouble focusing on close objects and you will be extra sensitive to bright light. Sunglasses will help with the glare and light sensitivity which usually lasts 4 to 6 hours.