The Structure of Eyes and Vision Ability

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Eyes are one of the five senses that are important for our everyday life, But sadly most people don’t understand how the eye works. This is just a basic overview of how the eye works.

To understand how the eye works we must first look at the light before it enters the eye. Light whether it be natural or artificial it has many wavelengths. We can only see the color spectrum of visible light which is about from 400 to 700 nanometers long. When light hits an orange most wavelengths get absorbed by the orange except for the part of the visible light spectrum that is orange, which is from 620 to 630 nanometers long. After the part of the light that does not get absorbed hits the orange it then reflects off the fruit and into our eyes.

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The sclera is the tissue that is the outside of the eye. The sclera is white in color and makes up 80 percent of the outside of the eyeball. It covers the whole outside of the eye expcet for the cornea and the optic nerve.

The sclera thickness is about 0.2 mm to 1 mm wide. The purpose of the sclera is to keep the shape of the eyeball. Its thick layers also protect it against serious damage like a tear or a deep scratch. The Cornea When the light hits our eye it first hits the cornea which is the transparent covering that is in the front of the eye. The cornea going left to right normally measures about 12 millimeters, and going from up to down is 11 millimeters, when viewed from the front. The cornea’s main purpose is to focus the light into the eye. The cornea also makes up for up to about 65 to 75 percent of the eye’s focusing power.

Next the light comes to the pupil and the iris which is right behind the cornea. The pupil’s purpose is to simply let the light into the eye. The normal pupil appears black and round. It is black due to light going into the pupil and getting absorbed without it being reflected back out. The pupil size mostly varies from person to person, but the pupil also changes with age, children and adults have a slightly larger pupil than seniors. The normal pupil size in grown-ups is from 2 to 4 millimeters wide in bright light, and they are from 4mm to 8mm in the dark. The iris is the round colored part of your eye that is wrapped around the iris. Its job is to constrict or enlarge the pupil. Its color ranges from brown, blue, and green to amber and grey. Together they control how much light is entering the eye. The pupil will enlarge if you are in a dark room or looking at something that is up close, But if you are in a brightly lit room or staring at something that is far away then the pupil will constrict.

The uvea in the middle layer of the eye, it has three parts the iris, ciliary body, and the choroid. The ciliary body surrounds the iris and cannot be seen because it is located behind the sclera. It purposes is to hold the lense in place and suspends it in place behind the pupil. Next is the choroid, it is located between the retina and the sclera. Its purpose in to keep the retina well-nourished because it contains many blood vessels.

After the iris and the pupil is the lense. After the light has went through the pupil it hits the lense. The lense is the transparent flexible tissue that is directly behind the pupil. Its job is to further focus the light on to the retina. The lense does a process called accommodation which lets the eye automatically focus on objects as you are moving. When you grow older the lense may lose its flexibility resulting in worse vision.

The retina is the membrane that lines the back of the eye, inside the retina are cells called photoreceptors. Photoreceptors can be categorized into two groups – rods and cones. The purpose for these cells is to turn light into electrochemical signals for the brain. Rods are used for nighttime vision while cones are responsible for daytime vision. Many of us have from 6 to 7 million cones. There are three types of cones, 64 percent of them respond best to red light, a third react strongly to green light, and the other 2 percent respond strongest to blue light. We with our three cone types, are better at Interpreting color than most animals, but many animals beat us in color vision. Many birds and fish have four types of cones, making them able to see ultraviolet light and light wavelengths shorter than what our eye can see. Some insects can also see in the ultraviolet light range. While rods are found in the outer edges of the retina the cones are located in the fovea and the macula. The fovea is a small area a bit above the optic nerve. Inside the fovea is a small depression in which cone cells are centralized, that is called the macula.

Once the signals from the photoreceptor cells reach the second and third layer of the retina. They meet a system of interneurons in the second layer and in the third are ganglion cells. These two layers exhibit complex receptive fields that can detect contrast changes in an image. Those changes appear sometimes as edges or shadows. After that the ganglion cells in the third layer gather this information along with other information about color. Then they send that information to the optic nerve.

The optic nerve primarily sends information from the thalamus to the occipital cerebral cortex. The thalamus is located just above the brainstem and the occipital cerebral cortex is found in the very back of the brain. The optical nerve also sends information from the retina to the lateral geniculate nucleus (LGN) which is located in the thalamus. The (LGN) divides the retina’s information into two streams, one having color and structure, and the other having contrast and motion. Cells that have color and structure make the top four layers of the (LGN), those four layers are the parvocellular layers. They are called parvocellular layers because the cells are small. Cells responsible for contrast and motion make the bottom two layers of the (LGN). They are called the magnocellular layers because the cells are large. Together the parvocellular and the magnocellular layers send information to the back of the brain through the optic nerve.

The primary visual cortex is located in the very back of the brain. Information from the retina and the lateral geniculate nucleus go through the optic nerve to get to the primary visual cortex. When cells reach the primary visual cortex or (V1) the cells arranged in a many ways so that the visual system can tell where objects are in space. First, the (V1) cells are organized with a point-to-point map. This lets the visual cortex position objects in two dimensions in the visual world, horizontal and vertical. The third step is depth, it is mapped in (V1) by comparing the information from both ot the eyes. Those signals are processed in stacks of cells called ocular dominance columns, it is a checkerboard pattern of connections between the left and right eye. During the first early years of life the cells in the visual cortex have a harder time arranging themselves. While (V1) helps us tell where objects are in space, (V2) helps refine our ability the interpret different colors. (V2) is largely responsible for interpretation of colors. Almost all higher features of vision are influenced greatly expectations based on experience. This information also goes to color and form perception in V3 and V4, to face and object recognition in the inferior temporal lobe, and to motion and spatial awareness in the parietal lobe. They give us with the ability to see and respond to the visual world very quickly.

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