EYE FUNCTIONHuman eye function is more sophisticated than any man-made optical device. Dr. Carolina Valdivia explains how the eye works to produce the images that we see. One way to understand how the eye functions is to compare it to a camera. In order to capture an image, I focus my camera on an object. Specific amounts of light are permitted to pass through a small hole in the camera called an aperture and are focused by a lens to create a visual impression on film or computer chip. The human eye is much more sophisticated than this, but its basic function is similar. Let's first identify structures in the eye that are analogous to those in a camera.
EYE FUNCTION AND REFRACTION The fundamental task of the eye is to focus light originating from sources outside of the eye onto the retina, which is inside and at the back of the eye. The physical process at work to make this happen is called refraction. You can see refraction by looking at the photo to the right of a pencil in a glass of water. You see the pencil through two different media: the atmosphere and the water. The pencil appears to bend when it enters the water. However, if you were to extract the pencil from the water you would see that it is not bent at all. It is straight. This tells us that it is the light itself that is bent when the pencil is placed in the water. Refraction is a critical process in eye function because it helps to aim light rays on the retina.
We do not encounter the same intensity of light in all contexts throughout the day and night. To address this challenge, the iris (colored part of the eye) adjusts the size of the pupil. This aspect of eye function regulates the amount of light entering the eye to more or less within a constant range. In situations where the intensity of light great, the pupillary opening is small. If the light is too great or sudden to make iris adjustment possible, we either close ours eyes or look away from the source. Conversely, when light is scarce, the iris dilates (expands) the pupillary opening to allow as much light as possible to enter.
A disadvantage of making one-to-one comparisons between eye function and a camera is that misconceptions can arise. One of these is that the lens of the eye is where most of the focusing takes place. This is not true. Instead, about 70% of the eye's focusing power occurs at the cornea – the boundary between the atmosphere and aqueous humor fluid. Why? The reason for this is a property of refraction. It turns out that the cornea and lens are comprised of similar materials, so this alone cannot account for the 70/30 difference in focusing power between the two structures. Instead, the difference between the two media at the corneal boundary (atmosphere and aqueous humor) is great, so the corresponding amount of refraction is great. However, the difference between the two media separated by the lens inside the eye (aqueous humor and vitreous humor) is not nearly as great. In fact, the vitreous humor in the back part of the eye (vitreous chamber) has only a slightly greater density than the aqueous humor in the front part (anterior chamber), so not as much refraction occurs as light enters and exits the lens. The job of the lens is to make sure that the rays refracted by the cornea come to a sharp focus on the retina. It accomplishes this by adjusting its shape, an action that is analogous to the auto-focus function on cameras, but markedly more sophisticated. This elegant seamless auto-focus function is made possible by the ability of the lens to change its refractive power by altering its form and thickness. This eye function is called accommodation. The optical power of a lens is measured in dioptres, with increasing numbers representing stronger optical power. In a camera, optical power is achieved by combining the optical power of individual lenses. This is efficient, because two lenses of different dioptre ratings can be used individually or combined to yield three different levels of optical power (3 lenses can produce 6 unique levels). Because the eye has just one lens, it uses a different strategy to adjust its optical power. Through accommodation, the lens of the eye can change its shape (either thinner or thicker) by tensing or relaxing the muscles of the eye. This unique property is continuous and makes eye function unsurpassed by any other optical device in its focusing ability.
 Does the eye have a zoom function like my camera?No. In its most basic form, a zoom lens consists of different small optical glass lenses inside a camera that are moved either closer to, or further away from, each other depending on whether you want to zoom in or to zoom out. Since the eye only has a single lens, a zoom function is not possible. However, accommodation enables the eye to move from a focus on a distant to a close object and back to the distant object more or less instantaneously, something that no camera is capable of doing. EYE FUNCTION AND IMAGE PROCESSING Once light rays are bent at the lens, they pass through the vitreous humor, a jelly-like substance filling the back part of the eye (vitreous chamber), and arrive at the retina. The retina, which makes up the inner lining of the eyeball functions like a camera film, or a charge-coupled device (CCD) of a digital camera. The retina is a complex multi-layered structure consisting of three types of light-sensitive photoreceptors: rods, cones, and photosensitive ganglion cells. These photoreceptors are responsible for converting light rays (called photons) into electrical impulses through a process called signal transduction. These impulses are processed and transmitted through the
optic nerve
to the visual cortex in the brain, where the image is processed and perceived.
How do the eyes work together?Humans have binocular vision, which means that our eyes work together rather than separately like those of many birds and reptiles. To accomplish this, both eyes normally aim at the same spot. The brain then combines the two images into a single three-dimensional representation. This three-dimensional image gives us depth perception, a very important aspect of eye function. In some people, a lack of coordination between the extraocular muscles disrupts proper alignment of the eyes and impedes their ability to focus on the same spot. This condition is known as strabismus and it tends to have negative affects on depth perception. When the eyes are not properly aligned, two different pictures are sent to the brain. In a young child, the brain learns to ignore the image of the misaligned eye and sees only the image from the straight or better-seeing eye. The child then loses depth perception. Adults who develop strabismus often have double vision because the brain cannot ignore the image from the misaligned eye. Below are some other common conditions in which eye function is not optimal.
COLOR AND NIGHT VISION Turn your head from side to side and gaze upon your surroundings. As you re-direct your attention from point to point, the images appear to be seamless. This seamless quality is possible because your eye functions to update images, including the details of motion and color, on a time scale so rapid that a break in the action is almost never perceived. The intricate details of images, ranges of color, contrast and quality, and the perception of seamless motion that your eyes and brain achieve is more sophisticated than any apparatus or instrumentation invented. Humans are able to detect only a small portion of the electromagnetic spectrum. Light or visible radiation occupies only a small portion of this spectrum and ranges from about 400 nm to 700 nm in terms of wavelength. Other wavelengths are not visible because they neither are transmitted by the ocular media nor are they absorbed by the photopigment of the eye.
In a photo studio, red or blue light often is used to illuminate objects. This is because rods reflect red and blue light and do not become bleached, allowing for preservation of night vision. However, rods absorb green light and therefore, green light cannot be used.
What causes color blindness?.Color blindness occurs when there is a problem with the color-sensing materials (pigments) in the cones of the retina. There are two major types: people who have difficulty distinguishing between red and green, and people who have difficulty distinguishing between blue and yellow. Most color blindness is inherited as an X-linked recessive trait. This is just a fancy way of saying that the genes responsible for it are located on the X chromosome (also known as a "sex chromosome"). Since males have a single X chromosome that they always inherit from their mothers, color blindness in males is inherited through their mothers. They also are 16 times more likely to express the trait than females. The reason for this is that color blindness is a recessive trait. This means that females must have a copy of the genetic variation on each of their two X chromosomes, one that they have inherited from their mother (who may or may not be color blind) and the other from their father (who will be color blind). If both parents are color blind, all of their children will be color blind. EYE FUNCTION AND EMOTIONS The eyes often are called the windows to the soul. We communicate and express emotion with our eyes in ways that defy words. When we are shocked or surprised, our eyes open wide. If we are confused, our eyes squint; angry, they appear to narrow; excited, they brighten. Couples value eye contact much more in their relationship than do mere acquaintances. The look of love, which is expressed predominantly through the eyes, has been an elusive subject explored by poets, artists, and writers for centuries. Interestingly, the perception of eyes narrowing during anger is not only one experienced by the observer, but also is encountered by the angry individual. This is because our peripheral vision is reduced during anger, producing a narrowing effect. During anger, the fight or flight response is invoked (norepinephrine-epinephrine pathway). Our bodies respond to stress, either real or imagined, by either switching into battle mode or escape mode. Heart and respiratory rates increase, blood is rushed to muscles, and reflexes are heightened, among other things. Reduced peripheral vision is believed to better enable us to focus on the threat and not be distracted by other things around us. When we think about eyes and emotion, crying usually comes to mind. People shed tears when they are upset, happy, angry, depressed, or otherwise moved emotionally. But is the function of crying the same for all contexts? Many of us believe that having a good cry can make you feel emotionally and physically better. Some recent scientific research supports this. Dr. William Frey and colleagues analyzed the chemical composition of emotional tears and found that they contain three chemicals released by the body during stress: 1) Leucine-enkephalin — a mood-elevating and pain reducing endorphin 2) ACTH — a hormone considered to be the most reliable indicator of stress and 3) Prolactin — the hormone that regulates milk production in mammals. Frey believes that crying is the body's attempt of ridding itself of built-up toxins, thus reducing stress and avoiding depression. However, additional research is needed to elucidate the role of emotional tears in eye function.
It is possible to manipulate eye function so that images perceived do not match objective reality. This commonly is called an optical illusion. There are numerous examples of these but they basically fall into three categories.
Although we typically do not think of it this way, nearly all painted art is in fact an optical illusion, as it tricks the eye into seeing three dimensions on the canvas when in fact there are only two. Another way of tricking eye function is through magic. Magicians rely on the fact that peripheral vision lacks the clarity and sharpness of central vision. You probably have read a book, played a video game, or watched a television program and experienced being lost to the world. The reason for this is that our peripheral vision significantly is reduced during times of intense concentration. Thus, the goal of magicians is to bring observers into a state of intense concentration and to focus their central vision on meaningless distractions, while simultaneously performing the actual trick in the area of peripheral vision.
Return from Eye Function to Glaucoma Information
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When light crosses a boundary between two different things, in this case the atmosphere and the cornea (along with a fluid called the aqueous humor that lies behind it), a portion of its energy is reflected back into the atmosphere, but the remaining part is transmitted through the cornea into the interior of the eye. As the transmitted portion of light passes through the corneal boundary it is refracted or bent. 
It turns out that science does not know the answer to this. Although we know what induces emotional crying, we do not know why we do it. In other words, what unique physiological function does crying serve that cannot be dealt with, or at least dealt with as efficiently, by other body processes?
