| Vision Tutorial | |
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| How we see color | ||
| Iris | ||
| Depth Perception |
§ Rods |
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| Vitreous Humor |
§ Cones |
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| Vision problems |
§ Fovea |
Briefly, the eye is analogous to a camera. It consists of an outer lens, called the cornea, which provides most of the refractory properties of the eye. Behind the cornea is the colored part of the eye that is called the iris. In the middle of the iris is a whole that can be varied in diameter. This hole is called the pupil and is analogous to the aperture of a camera; it is the part that regulates how much light reaches the back of the eye. Directly behind the pupil is the lens, which can be varied in thickness in order to complete the focusing on the image. A large fluid filled chamber behind the lens before we reach the back of the eye, called the retina, where the image is transferred to the brain. It is on the retina that the image must be finely focused for correct vision. Defects in any of the above structures can render the eye unable to focus the image by itself . If so, corrective measures, such as laser surgery, glasses, or contacts are in order.
The cornea is the major refractory, or focusing, part of the eye. The lens, held in place by ligaments and controlled by the ciliary muscles, brings the image into fine focus on the retina. As you can see in the picture above, the Cornea and lens turned image upside down on the retina! Fortunately, the brain turns it right side up again as we perceive it. The iris is a circular muscle that regulates the amount of light that enters the eye. In dim light, it relaxes to let in more light. In quick changes of light intensity (such as waking up abruptly when someone turns on the light) it contracts so quickly that it is sometimes associated with pain. The eye is able to keep its shape because it is “inflated” by a jelly-like substance called the vitreous humor. Sometimes, debris floats in this chamber and is responsible for the “ floaters” we sometimes see in our vision. The eye is moved by muscles that are attached to it, and the nerves that stimulate the muscles that move both eyes are “connected” in such a way as to make both eyes move at once. This allows us to have what is called “binocular vision.” Because of the different positions of each eye on our head, each eye sees the same image from slightly different angles. Some theories claim this phenomenon is what allows us to have depth perception. Try this phenomenon out by putting your finger together at arms length in front of your eyes, while focusing on something further away, such as the wall. As you put your fingers together, it will look like they are touching before they actually are, and when they finally do touch, it will look like there is a sausage between your fingers.
The Retina is the part the eye that transforms the light (electromagnetic radiation) signal into a chemical signal that can be transmitted by the brain. All of the cells that make up the retina send signals to the brain along Axons. Axons are the paths, like wires, that transmit the information from one cell to another. These axons all leave the eye in one place, the optic nerve. This nerve transmits the signal from the retina to the brain. However, where the optic nerve leaves the eye, there are no light receptors, creating a blind spot-- like a hole in our visual field. Based on the picture surrounding the blind spot, our brain fills it in, so that we do not notice it. It does not do a perfect job, however. For example, in the picture below, If the letters were to land exactly in the blind spot, you would not see them, only a straight bar. This is because your brain calculates a bar going into the blind spot, and one coming out of it, and so fills in the blind spot with bar.
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The retina is made up of many different kinds of cells. The light actually travels through the entire retina, (including blood vessels, which also contributes to the floaters we sometimes see) before it reaches the photo receptors. This may seem ineffective, but it allows the photo receptive discs to be close to the back of the eye and to so regenerate. The photo receptors on the back of the eye change the light into a chemical signal that they then transfer to the brain.
As the message is being sent to the brain, it is filtered and processed to enhance the image. The rest of the cells in the retina play a role, in this along with other cells in the brain, particularly the lateral geniculate nucleus. Only 40% of the message that leaves the retina actually gets to the visual part of the cortex (outer part of the brain). The miracle of vision is that our visual system does much more than produce an image and send it to the brain like a video camera to a TV. Not only does it enhance the image, but it also incorporates emotion, memories, intelligence, potential danger, rates of moving objects, object recognition, and much, much more.
The fovea is the main focal point of the eye and is the part of the eye that is primarily responsible for visual acuity and color vision. This is because it contains only cones, or the retinal cell that responds to bright light and color. Outside the fovea, the color photo receptors are mixed with rods, the photo receptors that respond to dim light, and they do not give information about color. Hence, in dim light, the eye does not see color well. We are able to see many colors because there are three types of color receptors that absorb different frequencies, or colors, of light: red, yellow, and blue. The brain integrates the signals it gets from each color receptor to enable us to see an enormous amount of colors.
Irregularities in the eye cause problems with our vision. The following is a summary of the major vision problems:
Astigmatism -- The irregular curvature of the cornea causes light to focus on more than one point on the retina. For example, the cornea could be shaped like an egg instead of a ball.
Cataract -- The cornea or lens, that are normally clear, are cloudy.
Farsightedness (Hyperopia) -- The eye can focus on far objects only. This is caused when the lens focuses the image behind the retina ("Far" from the center of the eye).
Irregular curvature -- The cornea of the eye is wavy, so nothing is focused well. The only way to correct this is with something hard to shape the cornea--like a hard, gas permeable contact lens.
Nearsightedness (Myopia) -- The eye can focus on near objects. This is caused when the lens focuses the image in front of the retina ("Near" to the center of the eye).
Presbyopia -- Inability to focus on near or far objects. This is usually associated with people of old age whose lens has become rigid and won't focus as well. People with this need bifocals.
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