Blinking is the rapid closing and opening of the eyelid. It is an essential function of the eye that helps spread tears across and remove irritants from the surface of the cornea and conjunctiva. Blink speed can be affected by elements such as fatigue, eye injury, medication, and disease. The blinking rate is determined by the "blinking center", but it can also be affected by external stimulus. When an animal (usually human) chooses to blink only one eye as a signal to another in a social setting (a form of body language), it is known as winking. Some animals (for example, tortoises and hamsters) blink their eyes independently of each other.
Blinking provides moisture to the eye by irrigation using tears and a lubricant the eyes secrete. The eyelid provides suction across the eye from the tear duct to the entire eyeball to keep it from drying out. Blinking also protects the eye from irritants. Eyelashes are hairs attached to the upper and lower eyelids that create a line of defense against dust and other elements to the eye. The eyelashes catch most of these irritants before they reach the eyeball.
There are multiple muscles that control reflexes of blinking. The main muscles, in the upper eyelid, that control the opening and closing are the orbicularis oculi and levator palpebrae superioris muscle. The orbicularis oculi closes the eye, while the contraction of the levator palpebrae muscle opens the eye. The MllerŐs muscle, or the superior tarsal muscle, in the upper eyelid and the inferior palpebral muscle in the lower 2 eyelid are responsible for widening the eyes. These muscles are not only imperative in blinking, but they are also important in many other functions such as squinting and winking. The inferior palpebral muscle is coordinated with the inferior rectus to pull down the lower lid when one looks down. Also, when the eyes move, there is often a blink; the blink is thought to help the eye shift its target point.
Though one may think that the stimulus triggering blinking is dry or irritated eyes, it is most likely that it is controlled by a "blinking center" of the globus pallidus of the lenticular nucleus - a body of nerve cells between the base and outer surface of the brain. Nevertheless, external stimuli can contribute. The average length of a blink is 100-400 milliseconds. Closures in excess of 1000ms were defined as microsleeps.
Greater activation of dopaminergic pathways dopamine production in the striatum is associated with a higher rate of spontaneous eye blinking. Conditions in which there is reduced dopamine availability such as Parkinson's disease have reduced eye blink rate, while conditions in which it is raised such as schizophrenia have an increased rate.
Infants do not blink at the same rate of adults; in fact infants only blink at an average rate of one or two times in a minute. The reason for this difference is unknown, but it is suggested that infants do not require the same amount of eye lubrication that adults do because their eyelid opening is smaller in relation to adults. Additionally, infants do not produce tears during their first month of life. Infants also get a significant amount more sleep than adults do and, as discussed earlier, fatigued eyes blink more. However, throughout childhood the blink rate increases, and by adolescence, it is usually equivalent to adults.
It is a myth that women blink nearly twice as much as men. Average male and female blink rates are almost identical, although women using Oral contraceptives blink 32% more often for unknown reasons. Generally, between each blink is an interval of 2-10 seconds; actual rates vary by individual averaging around 10 blinks per minute in a laboratory setting. However, when the eyes are focused on an object for an extended period of time, such as when reading, the rate of blinking decreases to about 3 to 4 times per minute. This is the major reason that eyes dry out and become fatigued when reading.
Eye blinking can be a criterion for diagnosing medical conditions. For example, excessive blinking may help to indicate the onset of Tourette syndrome, strokes or disorders of the nervous system. A reduced rate of blinking is associated with Parkinson's disease. Parkinson's patients have a distinct serpentine stare that is very recognizable.
Blink if your brain needs a rest PhysOrg - December 28, 2012
Why do we spend roughly 10 percent of our waking hours with our eyes closed - blinking far more often than is actually necessary to keep our eyeballs lubricated? Scientists have pried open the answer to this mystery, finding that the human brain uses that tiny moment of shut-eye to power down. The mental break can last anywhere from a split second to a few seconds before attention is fully restored. During that time, scans that track the ebb and flow of blood within the brain revealed that regions associated with paying close attention momentarily go offline. And in the brief break in attention, brain regions collectively identified as the "Default Mode Network" power up.
Discovered less than a decade ago, the default mode network is the brain's "idle" setting. In times when our attention is not required by a cognitive task such as reading or speaking, this far-flung cluster of brain regions comes alive, and our thoughts wander freely. In idle mode, however, our thoughts seldom stray far from home: We contemplate our feelings; we wonder what a friend meant by a recent comment; we consider something we did last week, or imagine what we'll do tomorrow.
Most of us take between 15 and 20 such moments of downtime per minute, and scientists have observed that most blinking takes place near or at the point of an "implicit stop": While reading or listening to another person, that generally comes at the end of a sentence; while watching a movie, for instance, we're most likely to blink when an actor turns to leave the scene or when the camera shifts to follow the dialogue.
The study, published this week in the journal Proceedings of the National Academies of Science, studied 20 healthy young subjects in a brain scanner as they watched "best bits" snippets from the British comedy "Mr. Bean." An earlier study had shown the researchers which implicit breakpoints in the "Mr. Bean" video most commonly elicited a spontaneous blink, so researchers knew when to look for changes in the brain's activation patterns.
Sure enough, when subjects blinked, the researchers detected a momentary stand-down within the brain's visual cortex and somatosensory cortex - both involved with processing visual stimuli - and in areas that govern attention. The circuitry of the Default Mode Network stepped up to fill the momentary lapse in attention, and then yielded again as order - and attention - was restored.
In a separate experiment, the researchers established that the momentary rest that blinking appears to represent is a deliberate act, and not just a response to an absence of stimuli. When researchers inserted roughly nine 165 milliseconds of blank screentime per minute into the "Mr. Bean" video, subjects' Default Mode Network did not activate in response. Although the video gave them a blink's worth of time to rest, subjects did not recognize a breakpoint in the action, and therefore remained attentive.
Though the current study didn't examine the relationship between blinking and deception, others have: While telling a lie, liars have been found to blink less - possibly because the act of deception requires rapt and uninterrupted attention to pull it off. In the seconds after telling a lie, however, the liar will blink far more frequently than a truth-teller. Perhaps the resulting downtime is necessary for the liar to consider whether the deceived person was buying the fib - or whether it was worth telling in the first place.
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