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The blink reflex is a reflex which is designed to naturally protect the eyes. Most animals with eyes have some form of this reflex, and the reflex is present from the time that an animal first opens its eyes. Abnormalities in this reflex can indicate that there is a neurological problem and may increase the risk of incurring eye damage because the eyes are no longer able to protect themselves reflexively.
Reflexes are involuntary actions. They allow the body to react very quickly to potentially dangerous situations by taking defensive action of some form. In the case of the blink reflex, the blink is often over before someone is consciously aware that there was a risk. There are many other reflexes which appear at different stages of development.
Several different stimuli can trigger this reflex. Anything which touches the cornea will cause someone to blink, and people usually blink when objects appear to be on a collision course with the eye, as for example when something is thrown in the direction of someone's head. Very bright light also stimulates the blink reflex, as do loud noises.
Multiple cranial nerves are involved in the process of blinking, as is the brainstem. In a healthy person, the blink reflex appears in both eyes. If both eyes fail to blink, it can mean that there is a problem with one of the cranial nerves or the brainstem which is inhibiting the normal processing of the reflex. If someone fails to respond at all to a stimulus which should elicit a blink, or responds slowly, it is also a sign of a neurological issue.
Neurological exams can include a quick assessment of the blink reflex to see how a patient responds to stimuli. This can be part of the process of checking on a patient who is believed to be nonresponsive or in a coma as well, because the reflex should kick in unless a patient has severe damage to the brainstem. It is also possible to assess the blink reflex in a test which involves placing electrodes around the eyes and providing very mild electrical stimulation while recording the responses.
Teachers introducing children to reflexes in biology classes often use the blink reflex as an example because it is easy to demonstrate. One way to demonstrate the reflex is to position a student behind a clear barrier and to throw a lightweight soft object like a cotton ball or a wad of paper at the barrier. The student will usually blink even though she or he is safe behind the barrier because the eye perceives the approaching object as a threat.