The Eye: How We See
The animal world displays a tremendous variety in the anatomy of eyes. The eyes of insects and shrimp are, for the most part, simple, light-sensitive structures. The eyes of predatory birds like hawks and eagles are designed to see very small objects at long distances, and are in some ways more complex than the human eye. Yet in spite of this diversity in structure, visual performance is basically similar. Each animal eye is eminently suitable for the task it performs, and the task is basically to transform light energy into information relevant to the organism.
We will consider the basic structure and neural connections of the human eye. We will see how the eye is constructed, and where the actual receptors are. We shall also learn what stimulates the eye and causes experiences of color.
THE ANATOMY OF THE EYE
The eye, the sense organ of vision, is a compact optical apparatus that is stimulated by light to give a sensation of vision. Before we can discuss how the light becomes vision, we have to know how the eye is structured and what the light goes through before finally reaching the receptors in the very back of the eye.
Figure 10. A human eye (frontal) -- missing, but not an important figure
Figure 11. Cross section of human eye (showing retina, lens, pupil opening, iris, msucle, etc.) .
Figure 10 shows the eye as you see it. The white of the eye is the sclera, the colored portion is the iris, and the black circle is the pupil. But these visible portions are only a small part of the eye, as shown in Figure 11. The transparent dome at the front is the cornea; it is continuous with the sclera, or white of the eye. The colored iris is a circular muscle that acts like the shutter on a camera; it regulates the amount of light that enters the eye. When the surroundings are very bright the iris has a tiny opening in the center. When the surroundings are dim the iris has a very large opening. This opening in the iris is the pupil. Directly behind the pupil is the lens. The lens also changes shape to focus on objects at various distances. The lens is relatively flat when you gaze into the distance and quite rounded when you look at something up close, like a book. The space in the eyeball in front of the lens is filled with a watery fluid. The space behind the lens is filled with a gelatinous substance.
Covering the inside of the eye, behind the muscles that shape, or accommodate, the lens, is the retina. The purpose of the lens is to focus images on the retina. If it does not, eyeglasses can redirect the path of light so that the image is focused on the retina. The actual receptors that produce the sensation of sight are located in the retina.
Figure 12. Schematic cross section of the retina (shows Rods, and cones and their connection.)
We understand the function of each part of the optical system
Figure 12 shows a schematic cross section of the retina. At the very back are the receptors, the rods and cones. Actually, the receptors are behind at least nine layers of retina. When light finally stimulates these receptors, a nerve impulse is generated and carried by the small fibers to the optic nerve at the back of the eye. Thus, before light can contribute to a sensation of sight, it passes through the cornea, the watery fluid, the lens, the gelatinous substance, and many layers of retina, finally reaching the rods and cones.
Where does the nerve impulse that begins at a rod or cone end up? As you learned in the last unit, the visual projection area is in the occipital lobe, at the back of the brain. There are two occipital lobes and two eyes. But all the impulses from one eye do not go to the same part of the brain.
Figure 13 shows the general path of the optic nerves. Impulses from the right side of each eye travel to the right occipital lobe. Impulses from the left side of each eye travel to the left occipital lobe. The point where the two optic nerves cross is called the optic chiasma. At the optic chiasma, half the nerve fibers from each eye cross to the opposite side of the brain. Damage to the optic nerve in front of the optic chiasma could cause blindness in one eye. Damage behind the optic chiasma or in the occipital cortex could cause partial blindness in each eye.
Figure 13 Visual pathways
We have traced the path of light from outside the body to the receptors in the retina. We have traced the path of the nerve impulse from the receptors in the retina to the visual projection areas in the occipital cortex. But we have not yet made the jump from light to sight. What really makes us see? How is it that you can read a book or rightly say, "I see a blue and silver Corvette"? To answer these questions, we must look first at the stimuli -- the light, and secondly, at the receptors -- the rods and cones.
We know a good deal about the properties of light
Light, like sound, is best represented as a wave. Light waves visible to the human eye range from about 400 to 700 millimicrons (millionths of a millimeter) in length. The different wavelengths of light are seen as different colors, ranging from violet at the short end to red at the long end of the visual spectrum. Mixtures of wavelengths cause us to see colors not on the spectrum. A mixture of all wavelengths is seen as white, while a lack of any wavelength is seen as black. The wavelength determines the hue, or color, while the mixture of wavelengths determines the purity, or saturation. The amplitude (height) of the light wave determines the brightness of the object.
Click here for an image of the light spectrum
The perceived color of an object is determined by which light waves it reflects. Yellow paint, for example, reflects the wavelengths of light that we see as yellow; it absorbs wavelengths that give us red, blue, and all other hues. Blue paint reflects only the wavelengths that give us blue. When yellow and blue paints are mixed, the result reflects the wavelengths we see as green. The wavelengths of red, blue, and yellow are all absorbed by the green paint.
Mixing lights is quite different. Energy is added at each wavelength. Yellow and blue lights mix to produce a grayish white, for the eye is then stimulated by all the wavelengths in each light. Similarly, the mixture of red and green lights will give yellow, while the mixture of red and green pigments will give brown. Mixing paints is subtractive, because combining colors reduces the number of wavelengths reflected to the eye. This is the type of color mixture with which we are all familiar. Mixing lights is additive, because wavelengths reflected to the eye are increased by the mixture. The results of light mixtures are often surprising to those of us whose experience with paints leads them to expect subtractive results.
And color exists only in the brain, not out there somewhere. A book entitled "Color: A multidisciplinary approach" by Heinrich Zollinger (1999) covers the physics, chemistry, biology, and culture of color. Thomas Lazar (2000) in reviewing the book, makes the point that color has served as a versatile way of communicating -- flowers, insects, squids, birds, and of course human means through art and literature.
A SFSU course on color (CFS 240) Color and Design) has some interesting information on it's web page: http://online.sfsu.edu/~janets/color.htm
The rods and cones are not evenly distributed in the retina. One area at the center of the back of the eyeball, the fovea, is densely packed with cones. Around the fovea are both rods and cones, but there are fewer and fewer cones as the distance from the fovea increases. The area of the retina closest to the lens muscles contains mostly rods. This is called the periphery, or edge, of the retina, and the fovea is considered the center. We still are not certain how light waves physically stimulate the receptors in the eye
These two types of receptors seem to have different functions. The rods function especially in dim light, and they do not seem to produce sensations of color. White, black, and intermediate shades of grey can be seen by a person who has only rod vision. The cones function best in bright light. They give us sensations of color, as well as of saturation (degree of color). Damage to the retina, which is fairly common among victims of diabetes mellitas, can result in different visual sensations. Damage to the fovea can interfere with color sensations.
It is not really known at this point just how different length light waves
produce different sensations. One theory, the Young-Helmholtz theory,
assumes that there are three different types of cones, each especially
responsive to a particular wavelength -- that of red, green, or blue. Another
theory, proposed first by Hering, also postulates three types of cones, but
assumes that each responds to a pair of wavelengths, red-green, yellow-
blue, or black-white (Hurvich and Jameson, 1957). The Hering theory
appears to be supported by the phenomenon of color blindness.
The human eye, basically, discriminates on three dimensions:
b) blue-yellow, and
A person who is totally blind discriminates none of these. A person who has normal vision discriminates all of them; he is, therefore, a trichromat (three color discriminator). On rare occasions, a person is totally color-blind; he sees only shades of grey. Such a person monochromat. The partially color- blind person is a dichromat. There are two types of dichromats: those who cannot discriminate between red and green, and those who cannot discriminate between blue and yellow. By far the most common type of color blindness is the red-green type, and there seem to be several variations of this. In most cases, color blindness is hereditary. It was discussed in Unit Two, Human Development, as a sex-linked genetic defect.
Now test yourself without looking back.
1. The lens in the eye focuses images on the:
a. optic chiasma.
c. sclera. d.
2. The receptors that are stimulated by light are:
a. Iocated in the retina. b.
just behind the cornea. c.
called rods and cones. d.
called organs of Corti.
3. Which of the following does light pass through before it stimulates the rods and cones?
a. The cornea
b. The optic chiasma
c. The lens
d. The fluid behind the lens
1 ) Rods__________
a. Especially sensitive to colors
b. Primarily sensitive to black, white, and gray
c. In greatest concentration in the fovea
d. In greatest concentration in the periphery
e. Evenly distributed in retina
5. The wavelength of visible light:
a. is longer than a sound wave. b.
determines the hue that is seen. c.
determines the saturation of a color. d. is
seen as a specific color.
6. A person with the most common form of color blindness:
a. cannot distinguish between red and green. b.
cannot distinguish between yellow and blue. c.
can distinguish only black, white, and gray. d. is
called a trichromat.
Refer to this drawing as you answer the following.
The clear dome that covers the front of the eye is the_________________________________________________________ 4
The circular muscle that regulates the amount of light that enters
the eye is the_____________________________________________________________ 1
The structure that changes shape, or accommodates, to focus images properly is the______________________________________8
The actual receptors for vision are located in the _________________________________________________________ 10
Which of the following does light pass through on its way to the
d. Some layers of the retina
e. The optic chiasrna
|Most effective in . . .||dim light||bright light|
|Hues seen||black, white, gray||all colors|
|Highest concentration||edge of retina||fovea|
Refer to this drawing as you answer the following.
The receptors that cause us to see the color green are___________________________________________ 5
The receptors that are very highly concentrated in the center of the retina are the________________________________________________7
The receptors that are most active in night vision are the_____________________________________________2
The receptors that generate nerve impulses in response to light-wave stimulation are the
3 b, c, d
6 rods and cones
1 0 retina
Drawing showing view from above with left and right visual fields and the optic chiasma
Nerve impulses that begin in the left side of the right eye travel to
which hemisphere of the brain?______________________________4
The visual projection area is in which lobe of the brain?__________________________________1
The point which, if cut through, would cause total blindness is the
|cannot distinguish between red and green||dichromat|
|cannot distinguish between blue and yellow||dichromat|
|can only distinguish light and dark||monochromat|
The most common form of color blindness is red-green. Such a person would be a_________________________________________2
4 NOW TAKE PROGRESS CHECK 2
1 occipital cortex
5 optic chiasma
1. The amount of light that enters the eye is regulated by the:
b. optic chiasma.
d. size of the pupil.
2. The receptors in the eye mat are highly concentrated in the fovea are called __________________
3. The receptors of visual perception are located on the________________
4. Nerve impulses from the left side of the right eye end up in the:
a. right side of the brain.
b. left side of the brain.
c. right visual projection area.
d. left occipital lobe.
5. Which of the following would be produced by one specific wavelength of light?
1 ) Trichromat_______
a. Normal vision
b. Totally color-blind
c. Red-green color-blind
d. Blue-yellow color-blind
e. Totally blind
UNIT 6 Table of Contents
February 14, 2005