2.2. THE HUMAN VISUAL SYSTEM

2.2.1. The Eye

The human visual system is one of the most complex in existence. Our visual system allows us to organize and understand the many complex elements of our environment. For nearly all animals, vision is just an instrument of survival. For humans, vision is not only an aid to survival, but an instrument of thought and a means to a richer life.

The visual system consists of an eye that transforms light to neural signals, and the related parts of the brain that process the neural signals and extract necessary information. The eye, the beginning of the visual system, is approximately spherical with a diameter of around 2 cm. From a functional point of view, the eye is a device that gathers light and focuses it on its rear surface.

A horizontal cross section of an eye is shown in Figure 2.10. At the very front of the eye, facing the outside world, is the cornea, a tough, transparent membrane. The main function of the cornea is to refract (bend) light. Because of its rounded shape, it acts like the convex lens of a camera. It accounts for nearly two-thirds of the total amount of light bending needed for proper focusing.

Behind the cornea is the aqueous humor, which is a clear, freely flowing liquid. Through the cornea and the aqueous humor, we can see the iris. By changing the size of the pupil, a small round hole in the center of the iris, the iris controls the amount of light entering the eye. Pupil diameter ranges between 1.5 mm ~ 8 mm, with smaller diameter corresponding to exposure to brighter light. The color of the iris determines the color of the eye. When we say that a person has blue eyes, we mean blue irises. Iris color, which has caught the attention of so many lovers and poets, is not functionally significant to the eye.

Behind the iris is the lens. The lens consists of many transparent fibers encased in a transparent elastic membrane about the size and shape of a small bean. The lens grows throughout a person’s lifetime. Thus, the lens of an eighty-year-old man is more than fifty percent larger than that of a twenty-year old. As with an onion, cells in the oldest layer remain in the center, and cells in newer layers grow further from the center. The lens has a hi-convex shape and a refractive index of 1.4, which is higher than any other part of the eye through which light passes. However, the lens is surrounded by media that have refractive indices close to its own. For this reason, much less light-bending takes place at the lens than at the cornea. The cornea has a refractive index of 1.38, but faces the air, which has a refractive index of 1. The main function of the lens is to accurately focus the incoming light on a screen at the back of the eye called the retina. For a system with a fixed lens and a fixed distance between the lens and the screen, it is possible to focus objects at only one particular distance. if faraway objects are in sharp focus, for example, close objects will be focused behind the screen. To be able to focus close objects at one time and distant objects at some other time, a camera changes the distance between the fixed lens and the screen. This is what the eyes of many fish do. in the case of the human eye, the shape of the lens, rather than the distance between the lens and screen, is changed. This process of changing shape to meet the needs of both near and far vision is called accommodation. This adjustability of the shape is the most important feature of the lens. Accommodation takes place almost instantly and is controlled by the ciliarv body, a group of muscles surrounding the lens.

Behind the lens is the vitreous humor, which is a transparent jelly-like substance. It is optically matched so that light which has been sharply focused by the lens keeps the same course. The vitreous humor fills the entire space between the lens and the retina and occupies about two-thirds of the eye’s volume. One of its functions is to support the shape of the eye.

Behind the vitreous humor is the retina, which covers about 65% of the inside of the eyeball. This is the screen on which the entering light is focused and light-receptive cells convert light to netural signals. All of the other eye parts we have discussed so far serve the function of placing a sharp image on this receptor surface. The fact that an image is formed on the retina, so the eye is simply an image catching device, was not known until the early seventeenth century. Even though the ancient Greeks knew the structure of an eye accurately and performed delicate surgery on it, they theorized that light-like rays emanate from the eye, touch an object, and make it visible. After all, things appeal “out there” In 1625. Schemer demonstrated that light enters the eye and vision stems from the light that enters the eye. By exposing the retina of an animal and looking through it from behind. he was able to see miniature reproductions of the objects in front of the eyeball.

There are two types of light-receptive cells in the retina. They are called cones and rods because of their shape. The cones, which number about 7 million, are less sensitive to light than rods, and are primarily for day (photopic) vision. They are also responsible for seeing color. The three types of cones are most sensitive to red, green, and blue light, respectively. This is the qualitative phvsiological basis for representing a color image with red, green. and blue monochrome images. The rods, which number about 120 million, are more sensitive to light than cones, and are primarily for night (scotopic) vision. Since the cones responsible for color vision do not respond to dim light, we do not see color in very dim light.

Rods and cones are distributed throughout the retina. However, their distribution is highly uneven. The distribution of the rods and cones in the retina is shown in Figure 2.11. Directly behind the middle point of the pupil is a small depressed dimple on the retina called the fovea. There are no rods in this small region, and most of the cones are concentrated here. Therefore, this is the regiot for the most accurate vision in bright light. When we look straight ahead at a,’ object, the object is focused on the fovea. Since the fovea is very small, we constantly move our attention from one region to another when studying a larger region in detail. The rods, which function best in night vision, are concentrated away from the fovea. Since there are no rods in the fovea, an object focused in the fovea is not visible in dim light. To see objects at night, therefore, we look at them slightly sideways.

There are many thin layers in the retina. Even though cones and rods arc light-receptive cells, so that it would be reasonable for them to be located closei to the vitreous humor, they are located farther away from it. Therefore, light has to pass through other layers of the retina, such as nerve fibers, to reach the cones and rods. This is shown in Figure 2.12. it is not clear why nature chose to do it this way, but the arrangement works. in the fovea, at least, the nerves are pushed aside so that the cones are directly exposed to light. Due to this particular arrangement, the optic nerve fibers have to pass through the light-receptive cell layers

on the way to the brain, instead of crossing the light-receptive cell layers throughout the retina, they bundle up at one small region the size of a pinhead in the retina, known as the blind spot. Since there are no light receptive cells in this region, we cannot see light focused on the blind spot.

When light hits cones and rods, a complex electrochemical reaction takes place, and light is converted to neural impulses, which are transmitted to the brain through the optic nerve fibers. There are about 130 million light-receptive cells (cones and rods), but only about I million nerve fibers. This means that one nerve fiber, on the average, serves more than 100 light-receptive cells. The nerve fibers are not shared equally. Some cones in the fovea are served by one nerve fiber each, increasing the visual acuity in this region. The rods, however, always share nerve fibers. This is one reason why visual acuity at night is not as good as it is during the day even though there are many more rods than cones.

After the optic nerve bundles leave the two eyes. the two bundles meet at an intersection called the optic chiasm. This is shown in Figure 2.13. Each of the two bundles is divided into two branches. Two branches, one from each of the two bundles, join together to form a new bundle. The remaining two branches form another bundle. This crossing of the nerve fibers from two eyes is partly responsible for our stereoscopic vision, which mixes the images from each eye to allow the visual field to be perceived as a 3-D space. These two new bundles go to the left and right lateral geniculate bodies, respectively. The original fibers end here and new fibers continue to the visual cortex, where the neural signals are processed and vision takes place. The visual cortex is a small part of the cortex. a mass of gray matter forming two hemispheres in the back of the brain. Little is known about how the visual neural signals are processed in the visual cortex.