Have you ever noticed your mouth watering when you see a picture of lemons or melting chocolate, even though you are nowhere near real food? Did you ever get a mental image of children playing from the smell of leaves in the autumn? Do you ever feel a little itchy just looking at rough wool? And would you eat lettuce if it was limp and brown? All of these connections are examples of how we rely on information collected by all of our senses to develop an indispensable mental catalogue of sight that helps us recognize objects and their contexts.
But of the five senses we use every day, we rely most on our sense of sight. Some 80% of the information we take in is provided by our eyesight. And its no wonder. Our eyes can gather the most detailed information over the greatest distances of any of our measurable senses. Take taste and touch for example. When we taste or touch things, we actually need to be in contact with an object. Or consider our sense of smell. While smells do travel, we still have to be fairly close to the source to take them in. Sound? It may be detectable from a little further away, but we would need a radio or telephone to hear as far as the eye can see on earth — and beyond. For as we gaze at the stars in the night sky, our eyes can see the flickerings of light that has traveled through space from thousands of years away!
Central vision in humans
Sight has evolved differently in different species. Horses have outstanding lateral vision so they can scan the horizon quickly for predators. Cats have excellent vertical vision for climbing up trees and looking downwards. Some birds can see ultraviolet light that may help them navigate better. But we humans have developed strong, full-color central vision to recognize the details of our immediate environment.
In the human eye, most of our color-sensing cone cells are clustered in the middle of the retina (where we “see” inside the eye). This means that once we have focused on an object, we can instantly identify the fine details of its shape, texture and color using our central vision. But the image of an object on the retina will fade away after a few moments. So to gather new input, our eyes instinctively drift around still objects to keep them in focus. Our life-long fascination with detail is developed in early infancy, as is the ability to move our eyes, track moving objects and focus.
Like many animals, we humans use our eyes as our main means of monitoring our environment. Although we cannot scan the horizon with the skill and ease of a deer, for example, the amount of information we receive out of the corner of our eyes is truly impressive. What we see out of the corner of our eyes is called peripheral vision.
Rod cells clustered around the outside of the retina give us the extra sensitivity we need to detect the most minute changes in light and movement for excellent peripheral vision. Thanks to our rod cells, we effortlessly take in the essential features of our surroundings within a panoramic range, while our central vision is at work. If something in the corner of the eye demands our attention, we can move our eyes to use our central vision in no time at all. That’s where our eye muscles and the functions of binocular vision come in!
Binocular vision
In binocular vision, the images received in both eyes are combined into a single, sharp image in the blink of an eye. In order to enjoy crisp binocular focus without any double vision, the image of an object should fall on corresponding points in each retina. Six muscles attached to our eyeballs move to achieve this crystal clear binocular focus. These muscles can turn the eye in any direction thousands of times a day — and they are still at work when we are asleep!
To focus on objects that are within close range, both eyes converge (come closer together). Our eyes diverge (push further apart) to focus on objects that are far away. The slight differences in the angles of the images we receive from both eyes produce full three-dimensional sight. But our ability to see in three dimensions — stereoscopic vision — is usually only fully developed around age 6 or 7. Why does it take so long? Because our eyes do not tell us what they see, we tell our eyes what to look for!
People with full vision in one eye only are a fascinating example of our mind’s ability to create optical illusions. Even though they do not have the sensing ability of two eyes to rely upon, they do not necessarily see the world as entirely two-dimensional (flat). Based on light, shade, colors, relative sizes of familiar objects and a vast catalogue of sensory experiences, one eye is often enough to perceive depth with great accuracy!
Half of the human brain is dedicated to visual processing, allowing us to take in a maximum of 100 letters per second when we scan a text quickly with our eyes. That’s about 100 bits per second in computer terms. Not bad! But did you know we can absorb the equivalent of 1 billion bits per second just glancing at a three-dimensional object?
Our brains think and learn best in three dimensions and when our other senses are somehow engaged. Unfortunately, most of the information we are asked to understand is presented in two dimensions, on flat pages and computer screens. For this reason, interactive virtual reality is emerging as the learning and training tool of the future. By understanding all of Nature’s gifts, science may be able to help use them all to their full capacity!
Can blind people see? In the future, the answer to that question may be “yes” for some. Experiments have revealed that by stimulating the retina with preprogrammed configurations of ultrasound, some blind people are able to see black and white outlines and images.
A healthy retina has a natural ability to retain images that is called persistence of vision. It is persistence of vision that allows us to see continuous, smooth movement in film. A movie projector throws about 24 still images on a screen in one second. But each picture on the screen is presented to the eye before the previous image in the brain fades on our retina, so that one image seems to merge into the next.
So if cameras can successfully digitize images and translate them into recognizable ultrasound impulses for the retina and the brain, we may well be witness to the birth of yet another astounding human sense — virtual sight!
For more information
On taste and sight, see
http://www.mayohealth.org/mayo/9707/htm/taste.htm
On perception and sensing, see
http://members.aol.com/osleye/eyeworks.htm
http://members.aol.com/osleye/eyeworks.htm
On learning and sight, see
http://www.aoanet.org/
Sources
Adler, R, Adler, I. Your eyes. New York (NY): The John Day Company; 1992.
Begbie, GH. Seeing and the eye; an introduction to vision. Garden City (NY): National History Press; 1996.
Cohen, NS. Out of sight into vision: there is more to good vision than reading the fine print. Toronto: Collier Macmillan Canada; 1997.
Kwiko, ML. Eyes. Toronto: Key Porter Books; 1994.
Rainwater, J. Vision: How, Why and What We See. New York (NY): Golden Press; 1992.
