How Your Eyes Work: Human Eyes Explained
Every day you see countless things. You see their shapes and colors and details, and your eyes and brain work so effectively together that vision is effortless. But do you ever wonder how your eyes work exactly?
Getting to know how your eyes work can help you appreciate them more and take better care of them.
In this post, we’ll look at how vision works and how we perceive objects. We’ll also answer some frequently asked questions about eyesight.
So, how do we see?
How Does the Human Eye Work?
Sight begins with light reflected by objects in your field of vision. The field of vision is the area of space you can see without moving.
Light passes through the tear film that keeps the surface of the eye lubricated. It enters the eye through the cornea, a transparent layer that helps focus the light. The cornea covers the pupil and the iris.
Next, light goes through the pupil, which controls how much light gets in, and passes through the crystalline lens. Much like the lens of a camera, the lens of the eye adjusts its shape to focus the light.
Fact: The human eyeball is about 1 inch in diameter. It grows until your twenties. After that, it maintains its size for the rest of your life.
Light also passes through the vitreous humor, a clear substance at the center of the eye, and then it reaches the retina. Lining the back of the eye, the retina is a layer of light sensors shaped like cones and rods.
These photoreceptor cells transform the light into electrical signals and send them through the optic nerve to the visual cortex at the back of the brain. The visual cortex is present in both hemispheres of the brain. Although it’s only 2 mm thin, it processes the raw visual information across multiple areas and combines it to form an image.
Fact: Different parts of the visual cortex perform different functions. Some respond to visual stimuli, while others respond to shapes, angles, symmetry, color, and movement.
The brain receives a slightly different image from each eye. But it combines them and draws on previous information to form 3D images. Because of the position of our eyes, we have a wider binocular field of vision than other animals. However, unlike say rabbits, which can see behind them, we have a much wider blind zone.
Did you know: The human eye also has a blind spot or a small area on the retina that lacks photoreceptors. This occurs at the point where the nerve fibers leave the retina to send nerve signals through the optic nerve. However, the brain draws on information from both eyes to fill in the gaps.
How Do We See Objects with Our Eyes?
Our eyes enable both conscious and unconscious vision. The main human eye function is to collect light and turn it into nerve impulses. It does this continuously, sending information to the visual cortex (except of course when we are asleep). The visual cortex splits this information into two brain pathways.
The Unconscious Pathway
One pathway, which passes through the parietal lobe, provides unconscious vision that guides movement. It’s what enables you to duck an object thrown at you. Conscious vision helps to protect us from potential threats without being as focused or as detailed as conscious vision.
The Conscious Pathway
The other pathway relies on the inferior temporal lobe and provides conscious vision. This part of the brain matches the color, shape, and other features of the perceived object to stored visual memories to help recognize it.
Key to seeing objects at night is the photopigment rhodopsin contained by the rods in the retina. While light decomposes these pigments, darkness regenerates them. It’s through this process of dark adaptation that our eyes can see in low light. However, while the human eyes can gradually adapt to low light conditions, they cannot see in complete darkness.
Fact: Our eyes keep moving while we sleep. However, despite their movements during the rapid eye movement stage, they don’t send any visual signals to the brain. Sleep gives the eyes, like the rest of the body, a break.
Face recognition also follows a specific pathway in the brain. The visual cortex sends clues about what we see to a specific face recognition area. From there, signals travel to the amygdala, which processes facial expressions, as well as to the frontal lobe, which enables the conscious recognition of faces.
It takes about half a second for the brain to interpret the visual information it receives from the eyes through the optic nerve.
Human Eye Terms Explained
The eye is our most complex sensory organ. Within it, many different structures come together to enable sight. Seeing may come to it easily, but it’s only because so many different structures work together effectively every day.
To better understand each part of the eye and its function, we’ve put together a small glossary of terms.
A clear layer made of cells and collagen, the cornea is shaped like a dome and acts as a window that controls how much light enters the eye. It bends the light that passes through it and handles most of the focusing power of the eye.
The black circle in the center of the iris, the pupil is an opening through which light enters the eye. Pupils are black because light doesn’t escape through them.
The colored circle surrounding the pupil, the iris controls the size of the pupil, making it dilate or contract. By doing so, it controls how much light enters the eye.
The iris consists of the dilator and sphincter muscles. It’s colored because it contains the pigment melanin, whose role is to block excessive light from entering the eye.
Fact: Different eye colors are the result of people having varying amounts of melanin in their irises. Brown-eyed people have plenty of melanin in their eyes while blue-eyed people have much less.
The crystalline lens is a clear structure behind the iris. Working together with the cornea, it allows the eye to focus on objects located at different distances. The lens of the eye works just like the lens of a camera or smartphone.
Did you know: It’s because the lens of the eye deteriorates with age that older people often need reading glasses.
Located at the back of the human eyeball, the retina is sensitive to light. It has millions of photoreceptors that receive visual information and turn it into electrical signals. The images that form on each retina are upside down, but the brain adjusts them as it processes them.
Rods and Cones
The photoreceptors in the eye consist of two main types of cells: rods and cones. Rods perceive brightness, shape, and size but not color. They work when the intensity of the light is low. Cones handle daylight vision. They perceive color and fine detail. Cones detect either green or blue or red. The visual cortex combines these colors to form all the others.
The focal point of the eye, the macula is a portion at the center of the retina that provides sharp vision. It’s the macula that provides us with the sharp and precise straight-ahead vision that enables us to read. The rest of the retina handles mostly peripheral vision.
Located at the center of the macula, the fovea is a small flat area that is densely packed with cones. It’s the part of the macula where the sharpest and most detailed images form.
The choroid is a layer of fine blood vessels that provide the retina with a steady supply of nutrients.
The optic nerve connects each retina to the brain. It’s about 30 mm long and consists of over 1 million nerve cells that carry the electrical signals from the retina to the brain.
The aqueous humor is a transparent watery fluid that circulates through the front part of the eyeball. It maintains a constant pressure inside the eye and bathes and nourishes the crystalline lens.
The rest of the human eyeball is filled with a clear, jelly-like substance. Known as the vitreous humor, this substance maintains the round shape of the eye and keeps its center clear for light to reach the retina.
The sclera is the white outer covering of the eye. Made of collagen fibers, it’s a tough covering that protects the more delicate structures of the eye. The six muscles that control the movement of the eye are attached to the sclera through tendons.
Located above each eye, the lacrimal glands supply the eyes with tear fluid continuously. Apart from water, tear fluid contains lipids, mucin, nutrients, and other substances that fight bacterial infections. All of these are crucial for the eyes to function normally.
When we blink, the eyelids help spread the tear film across the surface of the eye to wash away small particles of dust and bacteria and keep it lubricated. Imbalances in the tear film can lead to dry eyes.
The inside corners of your eyelids have tear ducts that drain excess tears into the nose. There are four tear ducts in total, one for each upper and lower eyelid.
In the end, here are a few commonly asked questions about vision answered.
How well can newborns see?
Newborns can only see the colors white, black, and red. They are nearsighted and can see best from 8 to 15 inches in front of them. Beyond this distance, they see blurry shapes.
Does vision deteriorate with age?
The crystalline lens of the eye loses its flexibility in time, which affects focus on objects up close. It’s why many people need eyeglasses for reading as they age. At the same time, our visual field decreases with age, reducing our peripheral vision.
Will using glasses or reading in the dark hurt your eyes?
Using corrective lenses won’t make your eyesight weaker. Nor does reading in the dark, though late reading may give you a headache and put more strain on your eyes.
Does using a screen daily hurt vision?
Using a computer or other screen daily increases the risk for dry eyes and eye strain. When staring at a screen for extended periods, your blink rate decreases. This can interfere with the tear film that covers the surface of the eyes.
Can dry eyes affect vision?
Dry eye symptoms can include blurred vision. Neglecting chronic dry eyes may lead in some cases to permanent eye damage and vision loss.
You can treat dry eyes in healthy ways, so it’s important to pay attention to your symptoms. Worried that you may have dry eyes? Our quick online dry eye test takes only 3 minutes.