Sensation & Reality

It is apparent that the world as we know it is created from sensory impressions. Less obvious is that what passes for "reality" is shaped by the senses. Our sensory organs can detect only a limited range of physical energies. Thus, events go unrecorded when the senses are not attuned to them. We have, for instance, no receptors for atomic radiation, X-rays, or microwaves. For this reason, it is possible to be injured by each of these energies without being aware of it.

Bob Edens had his sight restored at age 51 after being blind since birth: "I never would have dreamed that yellow is so...so yellow. I don't have the words, I'm amazed by yellow. But red is my favorite color. I just can't believe red. I can't wait to get up each day to see what I can see. And at night I look at the stars in the sky and the flashing lights. You could never know how wonderful everything is. I saw some bees the other day, and they were magnificent. I saw a truck drive by in the rain and throw a spray in the air. It was marvelous. And did I mention, I saw a falling leaf just drifting through the air?"

What would the world be like if new senses could be added --if we could "see" gamma rays, "hear" changes in barometric pressure, or "taste" light? We can only guess. It is far easier to imagine losing or regaining a sensory system. ...sensation is our window on the world. All our meaningful behavior, our awareness of physical reality, and our ideas about the universe ultimately spring from the senses.

I. GENERAL PROPERTIES OF SENSORY SYSTEMS:
We begin with a paradox. On one hand we have the magnificent power of the senses. In one instant you can view a star light-years away, and in the next, you can peer into the microscopic universe of a dewdrop. Yet vision, like the other senses, is also narrowly limited in sensitivity so that it acts as a data reduction system. That is, our senses routinely "boil down" floods of information into a select stream of useful data. Sensory selection can be seen in the fact that "light" is only a small slice of a broader range of energies. In addition to visible light, the electromagnetic spectrum includes infrared and ultraviolet light, radio waves, television broadcasts, gamma rays, and other energies. If our eyes were not limited to light sensitivity, "seeing" would be like getting hundreds of different "channels" at once. The confusion would be overwhelming. Obviously, selection of information is important.

Transducer. Some selection occurs simply because sensory receptors are biological transducers. A transducer is a device that converts one kind of energy into another. For example, a phonograph needle converts vibrations into electrical signals. Scrape the needle with your finger and the speakers will blast out sound. However, if you shine a light on the needle, or put it in cold water, the speakers will remain silent. Similarly, each sensory organ is most sensitive to a select range of energy, which it most easily translates into nerve impulses. As they transduce information, many sensory systems analyze the environment into important features before sending messages to the brain.

Perceptual features. They are basic elements of a stimulus pattern, such as lines, shapes, edges, spots, or colors.

Feature detectors . The neural circuits of many sensory systems act as feature detectors. In other words, they are highly attuned to specific stimulus patterns. Frog eyes, for example, are especially sensitive to small, dark, moving spots. Researcher Jerome Lettvin (1961) calls this sensitivity a "bug detector." It seems that the frog's eyes are "wired" to detect bugs flying nearby. But the insect (spot) must be moving. A frog may starve to death surrounded by dead flies. Code. In addition to selection and analysis, sensory systems code important features of the world into messages understood by the brain (Hubel & Wiesel, 1979). To see coding at work, try closing your eyes for a moment. Then take your fingertips and press firmly on your eyelids. Apply enough pressure to "squash" your eyes slightly. Do this for about 30 seconds and observe what happens. If you followed the instructions, you probably saw stars, checkerboards, and flashes of color called phosphenes. The reason for this is that the receptor cells in the eye, which normally respond to light, are also somewhat sensitive to pressure. Notice, though, that the eye is only prepared to code stimulation --including pressure --into visual features. As a result, you experience light sensations, not pressure. Also important in producing this effect is localization of function in the brain.

Localization of function. This means that the sensory receptors send messages to specific locations in the brain. Some brain areas receive visual information, others receive auditory information, still others receive taste, and so forth. Thus, the sensation you experience ultimately depends on which area of the brain is activated. One practical implication of such localization is that it may be possible to artificially stimulate specific brain areas to restore sight, hearing, or other senses. Researchers have already tested a system that uses a miniature television camera to create electrical signals that are applied to the visual cortex of the brain (Dobelle et al., 1974; Dobelle, 1977).Unfortunately artificial vision of this type still faces major hurdles. However, artificial hearing is proving more workable --as we will see later.

It is fascinating to realize that experiences such as "seeing" and "hearing" ultimately take place in the brain, not in the eye or ear. Each sense organ is merely the first link in a long chain that ends in the cell and fiber forest of the brain. Much as we may be tempted to think so, our sensory systems do not operate like cameras or tape recorders, sending back "pictures" of the world. Rather, they collect, transduce, analyze, code, and transmit an unending flow of data to an active, information-hungry brain.

Sensation. This incoming flow of information is what we refer to as sensation.

Perception. When the brain organizes sensations into meaningful patterns, we speak of perception.

II. PSYCHOPHYSICS
What is the quietest sound that can be heard? The weakest light that can be seen? The lightest touch that can be felt? The sense organs are our link to reality. What are their limits?

Psychophysics. Changes in physical stimuli are measured and related to psychological sensations, such as loudness, brightness, or taste. A basic question psychophysics asks is, What is the absolute minimum amount of energy necessary for a sensation to occur? The answer defines the absolute threshold for a sensory system.

Testing for absolute thresholds shows just how sensitive we are. It only takes 3 photons of light striking the retina to produce a sensation. A photon is the smallest possible "package" of light energy, and responding to 3 photons is the equivalent of seeing a candle flame 30 miles away.

SENSORY MODALITY - - - - - - - - - - - - - - - - - - - - - - - - - ABSOLUTE THRESHOLD

Vision - - - - - - - - - - - - - - - - - - - - - - - - - Candle flame seen at 30 miles on a clear, dark night.

Hearing - - - - - - - - - - - - - - - - - - - - - - - - - Tick of a watch under quiet conditions at 20 feet.

Taste - - - - - - - - - - - - - - - - - - - - - - - - - 1 Teaspoon of sugar in 2 gallons of water.

Smell - - - - - - - - - - - - - - - - - - - - - - - - - 1 Drop of perfume diffused into a three-room apartment.

Touch - - - - - - - - - - - - - - - - - - - - - - - - - A bee's wing falling on your cheek from 1 centimeter above.

Some sensory systems have upper limits as well as lower ones. For example, when the ears are tested for pitch (higher and lower tones), we find that humans can hear sounds down to 20 hertz (vibrations per second) and up to about 20,000 hertz. This is an impressive range --from the lowest rumble of a pipe organ to the highest squeak of a stereo "tweeter." On the lower end, the threshold is as low as practical. If the ears could respond to tones below 20 hertz, you would hear the movements of your own muscles (Oster, 1984). Imagine how disturbing it would be to hear your body creak and groan like an old ship each time you moved.

The 20,000 hertz upper threshold for human hearing, on the other hand, could easily be higher. Dogs, bats, cats, and other animals can hear sounds well above this limit. That's why a "silent" dog whistle (which may make sounds as high as 40,000 or 40,000 hertz) can be heard by dogs, but not by humans. For dogs, the sound exists. For humans, it is beyond awareness. It's easy to see how thresholds define the limits of the sensory world in which we live.

Perceptual defense. Not only do absolute thresholds vary from person to person, they also vary from time to time for a single person. The type of stimulus, the state of one's nervous system, and the costs of false "detections" all make a difference. Emotional factors are also important. Unpleasant stimuli, for example, may raise the threshold for recognition. This effect is called perceptual defense. "Dirty" words took longer to recognize when flashed on a screen that did "clean" words. Apparently it is possible to process information on more than one level and to resist information that causes anxiety, discomfort, or embarrassment (Dember & Warm, 1979).

Subliminal perception. Is this "subliminal" perception? Basically, yes. Anytime information is processed below the normal limen (threshold or limit) for awareness, it is subliminal. Subliminal perception was demonstrated by an experiment in which people saw a series of shapes flashed on a screen for 1/1000 second each. Later, they were allowed to see these shapes and other "new" shapes for as long as they wanted. At that time, they rated how much they liked each shape. Even tough they could not tell "old" shapes from "new," they gave "old" shapes higher ratings (Kunst-Wilson & Zajonc, 1980). It seems that the "old" shapes had become familiar and thus more "likable," but at a level below normal awareness. To summarize, there is evidence that subliminal perception occurs. However, well-controlled experiments have shown that subliminal stimuli are basically weak stimuli. Advertisers are better off using the loudest, clearest, more attention-demanding stimuli available --as most do.

Difference Thresholds. Also studied in psychophysics. Here we are asking, How much must a stimulus change (increase or decrease) before it becomes just noticeably different? The study of just noticeable differences (JNDs) led to one of psychology's first natural "laws." Called Weber's law, it can be roughly stated as follows: The amount of change needed to produce a JND is a constant proportion of the original stimulus intensity. It is really just an approximation, because it applies mainly to stimuli in the mid-range. For other than pure sensory judgments, it is even more approximate. Notice the big difference in auditory sensitivity (pitch and loudness) compared to taste. Very small changes in hearing are easy to detect. A voice or a musical instrument that is off pitch 1/3 of 1 percent will be noticeable. For taste, we find that a 20 percent change is necessary to produce a JND. If a cup of coffee has 5 teaspoons of sugar in it, one more (1/3 of 5) will have to be added before there is a noticeable increase in sweetness. It takes a lot of cooks to spoil the broth.

III. VISION.
1. Dimensions of Vision. The room in which you are sitting is filled with electromagnetic radiation, including light and other energies. The visible spectrum is made up of light of various wavelengths. The spectrum starts at wavelengths of 400 nanometers. A nanometer is one-billionth of a meter. Wavelengths at this end of the spectrum produce sensations of purple or violet. Increasingly longer wavelengths produce blue, green, yellow, and orange, until we reach red, with a wavelength of 700 nanometers.

This physical property of light --its wavelength --corresponds to the psychological experience of hue, or the specific color of a stimulus. White light is made up of a mixture of frequencies from the entire spectrum. Colors produced by a very narrow band of wavelengths are said to be very saturated, or "pure." A third dimension of vision, brightness, corresponds roughly to the amplitude (or "height") of light waves; light of greater amplitude carries more energy and appears brighter.

2. Structure of the Eye. By pushing the issue a bit, the eye could be used as a camera. When the light-sensitive back surface of the eye is bathed in alum solution, the last image to strike it will appear like a tiny photograph. This fact might make for a great murder mystery, but it's not much of a way to take a photograph. In any case, several of the basic elements of eyes and cameras are similar. Both have a lens that focuses images on a light-sensitive layer at the back of a closed space. In a camera, this layer is the film; in the eye, it is a layer of photoreceptors (light-sensitive cells) about the size and thickness of a postage stamp, called the retina.

3. Focusing. The front of the eye has a clear covering called the cornea. The curvature of this transparent "window" bends light rays inward. Next, the lens, which is elastic, is stretched or thickened by a series of muscles, so that more or less additional bending of light occurs. This bending, fattening, and stretching of the lens is called accommodation. In cameras, focusing is done more simply --by changing the distance between the lens and the film.

4. Visual Problems. The shape of the eye also affects focusing. If the eye is too short, nearby objects cannot be focused, but distant objects are clear. This is called farsightedness, or hyperopia. If the eyeball is too long, the image falls short of the retina, and distant objects cannot be focused. This condition results in nearsightedness, or myopia. When the cornea of the lens is misshapen, part of the visual field will be focused and part will be fuzzy. This problem is called astigmatism. All three visual defects can be corrected by placing glasses or contact lenses in front of the eye. These added lenses change the path of incoming light to restore crisp focusing.

Sometimes, with age, the lens becomes less flexible and less able to accommodate. Since the lens must do its greatest bending to focus nearby objects, the result is presbyopia (old vision), or farsightedness due to aging. If you need glasses for nearsightedness, you may need bifocals as you age. Bifocal lenses correct near vision and distance vision.

5. Light Control. There is one more major similarity between the eye and a camera. In front of the lens in both is a mechanism to control the aunt of light entering. This mechanism in a camera is the diaphragm; in the eye it is the iris. The iris is a colored circular muscle that expands and contracts to control the size of the pupil, the opening at the center of the eye. The iris is quite important for normal vision. The retina can adapt to changing light conditions, but only very slowly. By making rapid adjustments, the iris allows us to move quickly from darkness to bright sunlight, or reverse. In dim light the pupils dilate (enlarge), and in bright light they constrict (narrow). At the largest opening of the iris, the pupil is 17 times larger than at the smallest. Were it not for this, you would be blinded for quite some time upon walking into a darkened room.

6. Visual Receptors. At this point, our eye-camera comparison breaks down. From the retina on, vision becomes a complex system for analyzing patterns of light. Besides, the eye would make a very strange camera. First of all, the eye has two types of "film," consisting of receptor cells called rods and cones. Compared to the film in a camera, the visual receptors are backward. The rods and cones point toward the back of the eye, away from incoming light. In addition, the "film" has a hole in it. Each eye has a blind spot because there are no receptors where the optic nerve leaves the eye. And last, the eye is constantly in motion. This would be disastrous for a camera, but as we shall see later, it is essential for normal vision.

The cones and visual acuity. Cones, numbering about 6.5 million in each eye, work best in bright light. They also produce color sensations and pick up fine details. They lie mainly at the center of the eye. In fact, there is a small cup-shaped area in the middle of the retina called the fovea that contains only cones --about 50,000 of them. If you look at your thumbnail at arm's length, its image just about covers the fovea. Like a newspaper photograph made of many small dots, the tightly paced cones of the fovea produce the greatest visual acuity, or sharpness. In other words, vision is sharpest when an image falls on the fovea. Acuity steadily decreases as images are moved to the edge of the retina.

The rods and peripheral vision. Rods, numbering about 100 million, are unable to detect colors. Pure rod vision is black and white. However, the rods are much more sensitive to light than the cones are. The rods therefore allow us to see in very dim light. Peripheral Vision. Areas outside the fovea also get light, creating a large region of peripheral (side) vision. The rods are most numerous about 20 degrees from the center of the retina, so much peripheral vision is rod vision. Fortunately, the rods are quite sensitive to movement. Thus, while the eye gives its best acuity to the center of vision, it maintains a radarlike scan for movement in side vision. Seeing "out of the corner of the eye" is important for sorts, driving, and waling down dark alleys. Those who have lost peripheral vision suffer from tunnel vision, a condition much like wearing blinders.

Sailors, pilots, astronomers, and military spotters have long made use of another interesting fact about peripheral vision. Although the rods give poor acuity, they are many times more responsive to light than the cones are. Since most rods are 20 degrees to each side of the fovea, the best night vision is obtained by looking next to an object you wish to see.

7. Color Vision. What would you say is the brightest color? Red? Yellow? Blue? Actually, there are two answers to this question, one for the rods and one for the cones. The rods and cones differ in maximal color sensitivity, a difference that has practical importance. The cones are most sensitive to the yellowish green region of the spectrum. In other words, if all colors are tested in daylight (with each reflecting the same total amount of light) than yellowish green appears brightest. The increased use of yellow fire trucks and of Day-Glo yellow vests worn by roadside work crews reflects this fact. Remember that rods do not produce color sensations. If very dim colored lights are used, no color will be seen. Even so, one light will appear brighter than the others. When tested this way, the rods are most sensitive to blue-green lights. Thus, at night or in dim light, when rod vision prevails, the brightest-colored light will be one that is blue or blue-green. For this reason, police and highway patrol cars in many states now have blue emergency lights for night work. Also, you may have wondered why the taxiway lights at airports are blue. It seems like a poor choice, but blue is actually highly visible to pilots.

8. Color Theories. How do the cones record color sensations? No short answer can do justice to the complexities of color vision, but briefly, here is the best current explanation.

1. Trichromatic Theory of color vision holds that there are three types of cones, each most sensitive to a specific color: red, green, or blue. Other colors are assumed to result from combination of these three, whereas black and white sensations are produced by the rods. A basic problem with this theory is that four colors seem psychologically primary: red, green, blue, and yellow. This theory applies to the retina, where three types of light-sensitive visual pigments have been found. As predicted, each pigment is most sensitive to a different wavelength of light. The three peaks of sensitivity fall in roughly the red, green, and blue regions. As a result, the three types of cones fire nerve impulses at different rates when various colors are viewed. In further support of the three-color theory, researchers recently confirmed that each cone contains only one pigment and that each pigment is controlled by its own gene (Nathan et al., 1986).

2. Opponent-process Theory, a second view, attempts to explain why you can't have a reddish green or a yellowish blue. According to this theory, the visual system analyzes color into "either-or" messages. It is assumed that the visual system can produce messages for either red or green, yellow or blue, black or white. Coding one color in a pair (red, for instance) seems to block the opposite message (green), so a reddish green is impossible, but a yellowish red (orange) can occur. According to this theory, fatigue caused by making one response produces an afterimage of the opposite color as the system recovers. The opponent-process theory seems to explain events recorded in the optic pathways after information leaves the eye. So both theories appear to be correct at a particular level in the visual system.

9. Color blindness & Color weakness. A person who is completely color-blind sees the world as if it were a black and white movie. How do we know. In a few rare cases, people have been color-blind in only one eye and can compare. Two colors of equal brightness look exactly alike to the color-blind individual. The color-blind person either lacks cones or has cones that do not function normally. Total color blindness is rare. It is caused by changes in the genes that control red, green, and blue pigments in the cones (Nathans et al, 1986). Red-green color blindness is a recessive, sex-linked trait. This means that it is carried on the X, or female, chromosome. Women have two X chromosomes, so if they receive only one defective color gene, they still have normal vision. Color-blind men, however, have only one X chromosome, so they can inherit the defect from their mothers (who are usually not color-blind themselves). The red-green color-blind individual sees both reds and greens as the same color, usually a yellowish brown (Rushton, 1975). Red-green color-blind individuals have normal vision for yellow and blue, so their main problem is telling red lights from green. IN practice, this is not difficult. In the US, the red light is always on top, and the green light is brighter tan the red. Also, to help remedy this problem, most modern traffic signals have a "red" light that has a background of yellow light mixed with it, and a "green" light that is really blue-green.

Color weakness. Color weakness or partial color blindness, is more common. Approximately 8% of the male population (but less than 1% of women) are red-green color-blind. (Another form of color weakness, involving yellow and blue, is extremely rare.)

Ishihara test. A common test for color blindness and weakness. In the test, numbers and other designs made of dots are placed on a background also made of dots. The background and the numbers are of different colors (red and green, for example). A person who is color-blind sees only a collection of dots. The person with normal color vision can detect the presence of the numbers or designs.

10. Dark Adaptation. Dark adaptation is the dramatic increase in light sensitivity that occurs after entering the dark. Consider walking into a theater. If you enter from a brightly lighted lobby, you practically need to be led to your seat. After a short time, however, you can see the entire room in detail. Studies of dark adaptation show that it takes about 30 to 35 minutes of complete darkness to reach maximum visual sensitivity. When dark adaptation is complete, the eye can detect lights 10,000 times weaker than those to which it was originally sensitive.

What causes dark adaptation? Like the cones, the rods also contain a light-sensitive visual pigment. When struck by light, visual pigments bleach, or break down chemically. (The afterimages caused by flashbulbs are a direct result of this bleaching.) To restore light sensitivity, the visual pigments must recombine, which takes time. Night vision is due mainly to an increase of the rod pigment, rhodopsin. (One of the "ingredients" of rhodopsin is retinal, which the body makes from vitamin A. When too little vitamin A is available, less rhodopsin is produced. Thus, a person lacking vitamin A may develop night blindness --the person sees normally in bright light while using the cones, but becomes totally blind at night when the rods must function. Carrots are an excellent source of vitamin A, so they could improve night vision for someone suffering a deficiency, but not the vision of anyone with an adequate diet.) When completely dark-adapted, the human eye is almost as sensitive to light as the eye of an owl. Before artificial lighting, gradual adaptation at sunset posed few problems. Now we are often caught in temporary semiblindness. Usually this isn't dangerous, but it can be. Even though dark adaptation takes a long time, it can be wiped out by just a few seconds of viewing bright light. Under normal conditions, glare recovery takes about 20 seconds, plenty of time for an accident. After a few drinks, it may take 30 to 50 percent longer, because alcohol dilates the pupils, allowing more light to enter.

Is there any way to speed up dark adaptation? The rods are insensitive to extremely red light. To take advantage of this lack of sensitivity, submarines and airplane cockpits are illuminated with red light. So are the ready rooms for fighter pilots and ground crews. In each case, this allows people to move quickly into the dark without having to adapt. Because the red light doesn't stimulate the rods, it is as if they had already spent time in the dark.