Why don’t we notice when we blink? We don’t notice when we blink because our brain suppresses its visual center and makes sure our eyes are pointing in the same direction.
Everybody is different, and we all blink a different number of times depending on the time of day, the condition of our eyes, the weather, and an enormous range of factors, but generally, we blink about 17 times per minute. This drops if we are focusing on something, such as a book or a computer screen. That is one of the reasons why our eyes feel so tired if we have spent a long time using a computer or looking at our phones.
A blink takes roughly 150 milliseconds. That means, if we blink 17 times a minute, our eyes are closed for 2.5 seconds of every minute. We blink to lubricate our eyes and to remove any dust or debris that might be there. In those 150 milliseconds, the eyelids close and the eyeball rolls upward and slightly inwards towards the nose. The eyeball also withdraws about 1 mm into the eye socket. This movement is so that the eyelids can effectively cover the entire eyeball, and the rolling motion helps the eye to be completely lubricated. The lubrication comes from the lacrimal glands, which are located above and to the far side of each eye. They produce tears that drain down through tiny holes in the eyelid. These tears are called basal tears, and they are spread all over the eyeball during a blink, before running away through the tear duct and down the nasolacrimal duct, which runs into your nose. These are different from emotional tears because only a small amount of liquid is released. When you cry emotional tears, there is too much liquid for the tear duct to cope with, and they run down your face. The lubricant released to clean and protect the eye contains oils and nutrients.
After the blink is finished, the eyelids open and the eye refocuses on the same place it was focused on before. All of this is an unconscious action, just like breathing. You are only aware of it if you actually stop and think about blinking. It is controlled by the globus pallidus, which is an area in the basal ganglia. When you consciously think about blinking, the control shifts to a different part of the brain, the orbitofrontal cortex and the primary motor cortex, which is why you are aware of it. As soon as you stop thinking about it, the control shifts back.
So, if we blink 17 times a minute and we have our eyes shut for almost three seconds, why don’t we see darkness? The main reason for this is that the brain suppresses the areas it uses for visual input it uses during a blink. This was tested by scientists shining very powerful fiber-optic lights into subjects’ mouths. The light was so intense that it penetrated the roofs of their mouths and hit their retinas. They wore goggles to cut out the external light, and they were attached to functional magnetic resonance imagers. The amount of light hitting their retinas didn’t change even when their eyes were closed, but when they blinked, the fMRI showed that the parts of their brain used for vision stopped for those few milliseconds.
The second thing the brain does is to make sure the eye is pointed at and focused on the same thing it was looking at before the blink started. To do this, it uses the oculomotor system, which controls the movement of the eye and the muscles that focus the lens. When the eye opens after the blink, the brain takes in the new image, compares it to the old image, and then repositions that eye so that the images match. It does this, literally, in the blink of an eye. This was shown by experimenters who had subjects sit in a dark room and stare at a dot on a screen. Infrared cameras were used to observe the motion of their eyes. Every time they blinked, the dot was moved one centimeter to the side. The subjects didn’t notice the movement, but their brains did. When they opened their eyes after the blink, their eyes automatically adjusted so they were pointing at the new location of the dot. After a while, their eyes adjusted even before they fully opened because the brain was predicting where it would need to be looking. And this is why we don’t see black when we blink and this is what I learned today.
Sources
https://en.wikipedia.org/wiki/Blinking
https://news.berkeley.edu/2017/01/19/blink
https://www.livescience.com/335-blink-noticing.html
https://my.clevelandclinic.org/health/body/tears
https://my.clevelandclinic.org/health/body/24415-lacrimal-apparatus
Photo by Nothing Ahead: https://www.pexels.com/photo/selective-focus-close-up-photo-of-woman-s-face-with-her-eyes-closed-3660639/