Introduction
Have you ever wondered why we blink our eyes? An ELI5 explanation reveals that a blink is a momentary car wash for your eyes; it occurs when a thin layer of tears containing mucus and oil spreads over the surface of your eyes, freeing them from dust and keeping them hydrated. Your eyes need to be kept moist for several reasons: dry eyes feel uncomfortable, and the cornea, the transparent front part of the eye, can be harmed by prolonged dryness. Apart from hydrating your eyes, your blinks can give your brain incredible and perfectly timed breaks from visual stimuli, helping you to stay attentive. For example, if you are concentrating hard on reading or looking at a screen, you may blink less. So, blinking is the body’s way of gifting us mini-breaks in between bouts of seeing the world!
The Mechanics of Blinking
Blinking is mostly an involuntary reflex – your body blinks about 15-20 times a minute, even when you aren’t paying attention to it. It is also a voluntary action: we can voluntarily blink our eyelids to keep our eyes lubricated, in situations such as suddenly being blindsided by a Jerry Springer episode on the TV in front of us.
A series of muscles in the eyes, eyelids and nearby structures contract when we blink. Principal among them is the orbicularis oculi, which forms a ring around the eyeball to create an upper and lower eyelid by pressing the upper and lower eyelashes together. Located in the upper eyelid, the levator palpebrae superioris is another muscle which raises the upper eyelid. Another muscle, the superior tarsal muscle, wraps around the eyeball and aids in maintaining the position of the eyelid.
Keeping Our Eyes Moist
Blinking is probably very important to lubricate our eye. It spreads a thin film of tears across your eyes. These are produced in a thin channel that runs along the upper part of each eye and is called the, yes I’m really naming this for you, the upper palpebral fissure. Tears come from the two lacrimal glands (you knew which two) located up and outside your eyes, a little above the inner corner of your eyes. The tear film is extremely important. There’s a top oily layer that’s crucial for slowing evaporation of the film. Beneath that is a water layer, and lastly, there’s a watery-ish mucous layer that helps get the right mix of oxygen and nutrients to your eyes.
Greasy layer: coming from the meibomian glands, this layer slows down the evaporation of the tears.
The watery layer: This thick middle layer is produced by the lacrimal glands and washes away particles and delivers nutrients.
The mucous layer: This inner layer helps the aqueous layer spread out evenly over the surface of the eye, and it also holds the tear film in place on the eye.
Protecting Our Eyes from Harm
Blinking is protective. We will have a blinking shutting or closing of the eyelids to get rid of debris like small particles of dust and dirt that are in the eyes. There is also a rapid sort of blink reflex that might take place in on the order of 1/10 of a second if something is going to come into the eye very fast so as to protect it from something that would hurt the eye.
Providing Moments of Rest
Our visual cortex thus processes staggering amounts of visual information – blinking allows the brain to take a break, refocusing the brain’s neurotransmitters and recharging our eyes. Given the amount of screen time we spend today, eye-closed periods offer valuable respite to the eyes that work non-stop. Scientific studies have demonstrated that even after focusing on reading or computer tasks, people continue to blink less than normal – a fact that underlies the headaches and eye-discomfort we often experience after long spells of staring at a screen.
Blinking and Communication
There’s yet another type of nonverbal communication performed by the eyelids: as a way to signal emotions and social cues. A cascade of quick blinking, for example, can mean surprise or disbelief, while turning a blind eye in rapid bursts can signal irritation. Prolonged eye contact, with fewer blinks, can be a sign that you’re holding your gaze somewhere (perhaps a romantic interest), but it can also be seen as a threat under other social circumstances. Excessive blinking conveys nervousness or a feeling of unease. Our eyelids can thus subtly signal our emotions to others.
Variations in Blinking Rates
Being in different emotional states and performing different types of activity can increase or decrease our blink rate. For example, when we are deep in thought or daydreaming, we might blink more than when we are reading, driving, or working at a computer. People who work at computers in dry, high-airflow environments often experience dry eyes.
The Role of Blinking in Eye Health
The most commonly cited reason we blink is perhaps not the most correct: to keep our eyes wet and clear of dryness or foreign particles. As important as these perceptual benefits might be, blinking plays in equal measure a broader role to keep all the eye’s tissues in good health. In particular, the distribution of nutrients and oxygen throughout the eye is crucial. The cornea, the clear front window of the eye, has no blood vessels of its own. All of its nourishment has to be transported in by other means. That is exactly what blinking is meant for. The layer of tears that spreads across the eye during each blink contains a wealth of beneficial proteins, enzymes and antibodies, which prevent infection and inflammation and sweep away accumulated metabolic waste products from the surface of the eye.
The Neurological Aspect of Blinking
These are joined together by a set of neurological pathways, which trigger blinking through a combination of the brainstem and lower levels of the nervous system, utilizing reflex arcs involving the trigeminal nerve (touch and pressure sense around the eye) as well as the facial nerve (eye muscles). When we blink voluntarily, the cerebral cortex plays a role in regulating the blink and also by coordinating it with other events such as speech and facial expressions.
Conditions Affecting Blinking
Even though people can override nearly all of the autonomic nervous system’s effects, dwindling blinking can be such a humiliation that I resort to alpha-blockers (drugs for hypertension, now used to prevent blinking disorders; yes, I’m toting prescription drugs for a blinking disorder around in Beatle gloves). Some medical conditions can also mess with your blink rate. Dry eye syndrome can make you blink more, as you try to refresh your increasingly gritty eyes. Parkinson’s disease can cut your blinking rates, which will leave you with dry, irritated eyes. Other conditions, such as blepharitis (inflammation of the eyelids), can put a damper on your blinks.
Blinking in Different Species
It’s not as if humans were the first to think up blink reflexes – other animals do it all the time. A whole range of species blink away to wipe away grit, to restore clarity and to preserve their eyeballs. But the frequency and mechanisms that facilitate blinking vary immensely, with some animals, such as reptiles, sporting a clear third eyelid – a blinking mechanism that slides sideways right over the lens to act as a protective nictitating membrane, and also acts as a means of distributing moisture to keep the eyes clear. Birds, similarly, have a third eyelid, a third, inner and smallest upper eyelid called the palpebra tertia that slides sideways, hidden within blinking action, to act as a reserve of moisture and also as a protective shield, without encumbering depth perception.
Conclusion
Blink. It involves a few thousand muscles moving in unison, and lasts roughly a sixth of a second. We do it every few seconds, hundreds of times a day, and trillions of times throughout a lifetime. And yet, we rarely think twice about it. Your eye isn’t actually bobbing when you blink, it’s swimming. Your blink in this continuous web isn’t just random movement, it’s a brief but essential action that can keep you seeing clearly, keep your eyes safe, and give your brain a break. Too fast and your eyes dry out; too slow, and your vision ghosts. Blinking allows your eyelashes to sweep the sensitive surface of the eye clear of debris, and occludes your field of vision so that your brain can momentarily stop processing every little thing. It’s a complex biological system allowing two very different parts of an animal’s nervous system to exist in harmony, one that has played an under-appreciated role in the modern development of vision. The survival of eyes in the wild suggests that resolving when an organism might turn their brain’s focus from outward to inward was essential. We take blinking for granted now – but it’s not a habit we’ve always had.
External Links:
- Anatomy of the Eye: American Academy of Ophthalmology
- Tear Film Layers: National Eye Institute
- Eye Strain from Screens: Mayo Clinic on Digital Eye Strain