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20/20 VISION Part I: Eyesight, Perception and Vision

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20/20 VISION Part I: Eyesight, Perception and Vision 20/20 VISION Part I: Eyesight, perception and vision In this first part of our three-part exploration of 20/20 vision, we are going to look at our physical eyes and how they work. Then we will explore perception and how the brain actually converts all that sensory information into images. Finally, we will look at some of the techniques for improving our vision naturally and ask if these really work. 20/20 vision is a term used to express normal visual acuity (the clarity or sharpness of vision) measured at a distance of 20 feet. ... If you have 20/100 vision, it means that you must be as close as 20 feet to see what a person with normal vision can see at 100 feet. Having 20/20 vision does not necessarily mean you have perfect vision. 20/20 vision only indicates the sharpness or clarity of vision at a distance. Other important vision skills, including peripheral awareness or side vision, eye coordination, depth perception, focusing ability and colour vision, contribute to your overall visual ability. Some people can see well at a distance but are unable to bring nearer objects into focus. This condition can be caused by hyperopia (far-sightedness) or presbyopia (loss of focusing ability). Others can see items that are close but cannot see those far away. This condition may be caused by myopia (near-sightedness). Is it possible to see better than 20/20? Yes, it's indeed possible to have sharper than 20/20 vision. In fact, most people with young, healthy eyes are capable of identifying at least some of the letters on the 20/15 line or even smaller letters on the Snellen chart. Most government agencies and health care institutions agree that legal blindness is defined as a visual acuity (central vision) of 20/200 or worse in the best seeing eye or a visual field (peripheral vision) that is limited to only 20 degrees. There is no maximum prescription. Lenses can be made to compensate for just about any degree of myopia, astigmatism, or hyperopia so that your eye can see 20/20 with them on (provided they are healthy and you are too). NWR 2020 Vision/A Tisdall Reg Charity: 295198 Page 1 Vision Vision is a broader term than visual acuity or eyesight. In addition to clarity of sight or simply a description of the ability to see, the term "vision" includes all interactions between the eyes and the brain, and all neurological processes that take place in the brain to make the sense of vision possible. Also, unlike simple eyesight or Snellen (high contrast) visual acuity, measures of vision include contrast sensitivity, the ability to track moving objects with smooth and accurate eye movements, colour vision, depth perception, focusing speed and accuracy, and more. Because of the broader nature of the word “vision,” what is commonly called “20/20 vision" should really be called "20/20 visual acuity" or “20/20 eyesight.” What is 20/20? The term "20/20" and similar fractions (such as 20/60, 20/40 etc.) are visual acuity measurements. They also are called Snellen fractions, named after Herman Snellen, the Dutch ophthalmologist who developed this eyesight measurement system in 1862. In the Snellen visual acuity system, the top number of the Snellen fraction is the viewing distance between the patient and the eye chart. In the United States, this distance typically is 20 feet; in the UK it is 6 metres (therefore 20/20 is equal to 6/6). At this testing distance, the size of the letters on one of the smaller lines near the bottom of the eye chart has been standardised to correspond to "normal" visual acuity — this is the "20/20" line. If you can identify the letters on this line but none smaller, you have normal 20/20 visual acuity. The increasingly larger letter sizes on the lines on the Snellen chart above the 20/20 line correspond to worse visual acuity measurements (20/25; 20/32; etc.); the lines with smaller letters below the 20/20 line on the chart correspond to visual acuity measurements that are even better than 20/20 vision (e.g. 20/16; 20/10). The single big "E" at the top of most Snellen eye charts corresponds to 20/200 visual acuity. If this is the smallest letter size you can discern with your best corrective lenses in front of your eyes, you are considered legally blind. NWR Theme: 2020 Vision/ A Tisdall Reg Charity: 295198 Page 2 There are over two million people in the UK living with sight loss. This includes around 350,000 people registered as blind or partially sighted who have severe and irreversible sight loss. It also includes those that have uncorrected refractive error or cataracts that may be reversed. INTERESTING FACTS ABOUT EYES • Your eyes are about 1 inch across and weigh about 0.25 of an ounce. • The human eye can differentiate approximately 10 million different colours. • Our eyes remain the same size throughout life, whereas our nose and ears never stop growing. • The human eye blinks an average of 4,200,000 times a year. The purpose of blinking is to lubricate the eyes. Adults blink around 15 - 20 times a minute, which researchers say is more than the required amount to keep the eyes moist. • Eyes are made up of over 2 million working parts. • Each individual eye contains 107 million cells and all are light sensitive. • Your eye is the fastest muscle in your body. Hence, the phrase: “In the blink of an eye.” • The world’s most common eye colour is brown. However, brown eyes are blue eyes underneath. Consequently, a person can receive surgery in order to make their brown eyes blue. People with blue eyes share the same ancestor. Originally, all human beings had brown eyes, until a genetic mutation occurred between 6,000 and 10,000 years ago. Every single blue-eyed person shares this very distant relative. • “Don’t It Make My Brown Eyes Blue” is a song recorded by American country music artist Crystal Gayle. The story goes that Richard Leigh wrote it while his dog, Amanda, sat at his feet, staring up at him with her big brown eyes. Strangely, a few years later, a rubbish collector threw rocks at the dog, hitting her in one of her eyes. Amanda developed cataracts, and one of her brown eyes literally turned blue. • Some people are born with mismatched eye colours. This condition is known as heterochromia and is usually the result of a relative lack or excess of pigment in one eye. It is most often inherited but may also occur due to disease or injury. • The cornea is the transparent covering of the iris and pupil. It protects your eyes from dirt and germs, as well as some of the sun’s UV rays. If your cornea becomes damaged you will experience distorted vision, because the light that enters your eye is interfered with. NWR Theme: 2020 Vision/ A Tisdall Reg Charity: 295198 Page 3 • It’s a myth that liars make less eye contact. In fact, a well-practised liar will try to overcompensate as an attempt to “prove” they are telling the truth, by making too much eye contact and holding a gaze. • Human corneas are very similar to a shark’s cornea. This similarity means that sharks’ eyes can be used as replacements in human eye surgeries. • Anisocoria is a condition where a person’s pupils are not the same size. It can be present at birth or can be developed over time, however it is very rare. Sometimes, people with this condition will notice that the difference in size is only temporary, and they return to their normal sizes again. David Bowie actually had this condition, making his eyes appear different colours when in fact they were the same. It happened after a school yard scrap when he was a teenager – although he never minded saying that it gave him added mystique. • The “floaters” in your vision are permanent. They are mainly made up of protein strands floating inside the eye’s vitreous, casting shadows on the retina. Because the vitreous is completely stagnant, they will remain there indefinitely unless surgically removed. • It’s impossible to sneeze with your eyes open. Your eyes and nose are connected by cranial nerves, so the stimulation from a sneeze travels up one nerve to the brain, then down another nerve to the eyelids, typically prompting a blink. • Tears help protect our eyes from infection. Any dirt and dust that has managed to pass the defence of our eyelashes and brows is washed away by tears. They keep our eyes clean and moist and are filled with antibodies that fight infection. • Our eyes close automatically to protect us from perceived dangers. The superb reflex control of our eyelids allows them close automatically when they detect that an object is too close to the eye or there is sudden bright light. • We have two eyeballs for depth perception. Our eyes work together to help us judge the size and distance of objects, so that we can safely navigate around them. (The Stereoscope was a Victorian device that made use of this fact to create some of the first 3D images. Two photos were taken 3 inches apart and then set next to each other on a piece of card that was inserted into viewing binoculars perched on your nose – this meant that your eyes could suddenly see the image in 3D) NWR Theme: 2020 Vision/ A Tisdall Reg Charity: 295198 Page 4 Viewing Rome through a Stereoscope • Your eyes contain 7 million cones which help you see colour and detail, as well as 100 million cells called rods which help you see better in the dark.
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