Sensation & Reality [Cont.]

1. What is the stimulus for hearing? If you throw a stone into a quiet pond, a circle of waves will spread in all directions. In much the same way, sound travels as a series of invisible waves of compression (peaks) and rarefaction (valleys) in the air. Any vibrating object --a tuning fork, the string of a musical instrument, or the vocal cords --will produce sound waves by setting air molecules in motion. Other materials, such as fluids or solids, will also carry sound. But sound does not travel in a vacuum. Movies that show characters reacting to the "roar" of alien starships or to titanic battles in deep space are in error.

The frequency of sound waves (the number of waves per second) corresponds to the perceived pitch of a sound. The amplitude, or physical "height," of a sound wave tells how much energy it contains. Psychologically, amplitude corresponds to sensed loudness.

2. How are sounds converted to nerve impulses? What we call the "ear" is only the pinna, or visible, external part of the ear. In addition to being a good place to hang earrings or balance pencils, the pinna acts like a funnel to concentrate sounds. After they are guided into the ear, sound waves collide with the eardrum (tympanic membrane), which is like a tight drumhead within the ear canal. The sound waves set the eardrum in motion. This, in turn, causes three small bones called the auditory ossicles to vibrate. The third ossicle is attached to a second membrane, or drumhead, called the oval window. As the oval window moves back and forth, it makes waves in a fluid within the cochlea. The cochlea is really the organ of hearing, since it is here that waves in the fluid are detected by tiny hair cells, which generate nerve impulses to be sent to the brain.

3. How are higher and lower sounds detected? The frequency theoryof hearing states that as pitch rises, nerve impulses of the same frequency are fed into the auditory nerve. This explains how sounds up to about 4000 hertz reach the brain. But higher tones require a different explanation. The place theory of hearing states that high tones register most strongly at the base of the cochlea (near the oval window). Lower tones, on the other hand, mostly move hair cells near the outer tip of the cochlea. Pitch is therefore signaled by the area of the cochlea most strongly activated. Place theory also explains why hunters sometimes lose hearing in a narrow pitch range. "Hunter's notch," as this is called, occurs when hair cells are damaged in the area activated by the pitch of gunfire.

4. Deafness. There are three principle types.

l. Conduction Deafness occurs when the eardrums or ossicles are damaged or immobilized by disease or injury. Such damage reduces the transfer of sounds to the inner ear. In many cases, conduction deafness can be overcome by a hearing aid, which makes sounds louder and clearer.

2. Nerve Deafness is a hearing loss resulting from damage to the hair cells or auditory nerve. Hearing aids are of no help in this case, because auditory messages are blocked from reaching the brain. However, a new artificial hearing system is making it possible for some persons with nerve deafness to break through the wall of silence.

3. Stimulation deafness is of special interest, because many jobs, hobbies, and pastimes can cause it. Stimulation deafness occurs when very loud sounds damage hair cells in the cochlea (as in "hunter's notch"). If you work in a noisy environment or enjoy loud music, motorcycling, snowmobiling, hunting, or similar pursuits, you may be risking stimulation deafness. The hair cells, which are about as thick as a cobweb, are very fragile and easily damaged. If a ringing sensation known as tinnitus follows exposure to loud sounds, chances are that hair cells have been damaged.

V. SMELL & TASTE.
Unless you are a wine taster, a perfume blender, a chef, or a gourmet, you may think of olfaction (smell) and gustation (taste) as least important among the senses. Certainly, a person could survive without these two chemical senses. Just the same, the chemical senses occasionally prevent poisonings, and they add pleasure to our lives every day.

l. The sense of smell. The receptors for smell respond primarily to gaseous molecules carried in the air. As air enters the nose, it passes over roughly 20 million nerve fibers embedded in the lining of the upper nasal passages. Airborne molecules passing over the exposed fibers trigger nerve signals that are sent to the brain. How are different odors produced? This is still something of a mystery. One hint comes from the fact that it is possible to develop a sort of "smell blindness" for a single odor. This loss, called anosmia, suggests that there are specific receptors for different odors. Indeed, scientists have noticed that molecules having a particular odor are quite similar in shape. Specific shapes have been identified for the following odors: Floral (flower-like), camphoric (Camphor-like), musky (Have you ever smelled a sweaty musk ox?), minty (mint-like) , and etherish (like either or cleaning fluid). This does not mean, however, that there are different olfactory receptors comparable to the three types of cones in vision. Each receptor in the nose is probably sensitive to many molecules or combinations of molecules.

It is currently believed that there are different shaped "holes," or depressions, on the odor receptors. Like a piece fit in a puzzle, a molecule produces an odor when it matches up with a hole of the same shape. This is called the lock and key theory. Although there are some exceptions, the theory seems to explain many odors.

2. Taste. There are at least four basic taste sensations: sweet, salt, sour, and bitter. We are most sensitive to bitter, less sensitive to sour, even less sensitive to salt, and least sensitive to sweet. This order may have helped prevent poisonings when most humans foraged for food, because bitter and sour foods are more likely to be inedible. Flavors seem more varied than suggested by the four taste qualities because we tend to include sensations of texture, temperature, smell, and even pain ("hot" chili peppers) along with taste. Smell is particularly important in determining flavor. The four primary tastes are detected by taste buds located mainly on the top of the tongue, but also at other points inside the mouth. As food is chewed, it dissolves and enters the taste buds, where it sets off nerve impulses to the brain. Like the skin senses, taste receptors are not equally distributed. Some areas of the tongue are more sensitive to certain tastes than others. Some differences in tastes are generic. The chemical phenylthiocarbamine, or PTC, tastes bitter to about 70 percent of those tested and has no taste for the other 30 percent. The sense of taste also varies with age. Taste cells have a life of only several days. With aging, cell replacement slows down, so the sense of taste diminishes. Aside from this fact, most taste preferences are acquired.

VI. THE SOMESTHETIC SENSES
Even the most routine activities, such as walking, running, or passing a sobriety test, would be impossible without somesthetic information from the body. The somesthetic senses (soma means "body," esthetic means "feel") include the skin senses (touch), the kinesthetic senses (receptors in the muscles and joints that detect body position and movement), and the vestibular senses (receptors in the inner ear used to maintain balance). (The vestibular senses also contribute to motion sickness).

1. Skin Receptors. They produce at least five different sensations: light touch, pressure, pain, cold, and warmth. Receptors with particular shapes appear to specialize somewhat in various sensations. However, the surface of the eye, which only has free nerve endings, can produce all five sensations. Altogether, the skin has aut 200,000 nerve endings for temperature, 500,000 for touch and pressure, and 3 million for pain.

A. Does the number of receptors in an area of skin relate to its sensitivity? Yes. Your skin could be "mapped" by applying heat, cold, touch, pressure, or pain to points all over your body. Such testing would show that the sin receptors are fund in varying numbers, and that sensitivity generally matches the number of receptors in a given area. Generally speaking, important areas such as the lips, tongue, face, hands, and genitals, have a higher density of receptors.

B. There are many more pain receptors than other kinds. Why is pain so heavily represented, and does the concentration of pain receptors also vary? Pain receptors vary in their distribution like the other [ ?] senses. There are an average of about 232 pain points per square centimeter behind the knee, 184 per centimeter on the buttocks (an area preferred by many parents for spankings), 60 on the pad of the thumb, and 44 on the tip of the nose (Geldard, 1972).

C. There are two kinds of pain. Pain carried by large nerve fibers is sharp, bright, fast, and seems to come from specific body areas. This is the body's warning system. Quickly disappears. Much as we dislike warning pain, it is usually a signal that the body has been, or is about to be, damaged. Without warning pain, we would be unable to detect or prevent injury. A second type of pain is carried by small nerve fibers. This type is slower, nagging, aching, widespread, and very unpleasant. It gets worse if the pain stimulus is repeated. This is the body's reminding system. A sad thing about the reminding system is that it often causes agony even when the reminder is useless, as in terminal cancer, or when pain continues after an injury has healed.

VII. ADAPTATION, ATTENTION, AND GATING
Each of the senses we have described is continuously active. Even so, many sensory events never reach awareness. One reason for this is sensory adaptation, a second is selective attention, and a third is sensory gating. Let's see how information is filtered by these processes.

1. Sensory adaptation. A decline in the number of nerve impulses generated by sensory receptors exposed to an unchanging stimulus. Refers to a decrease in sensory response to a constant or unchanging stimulus. The olfactory (smell) receptors are among the most quickly adapting. When exposed to a constant odor, they send fewer and fewer nerve impulses to the brain until the odor is no longer noticed. Adaptation to sensations of pressure from a wristwatch, waistband, ring, or glasses is based on the same principle. Sensory receptors generally respond best to changes in stimulation. As David Hubel says, "We need above all to know about changes; no one wants or needs to be reminded 16 hours a day that his shoes are on." The rods and cones, like other receptor cells, would respond less to a constant stimulus were it nor for the fact that the eye normally makes thousands of tiny movements every minute. These movements are caused by tremors in the eye muscle known as physiological nystagmus. Although they are too small to be seen, these movements shift visual images from one receptor cell to another. Constant shifting of the eyes ensures that images always fall on fresh, unfatigued receptors. Evidence for this comes from experiments in which subjects are fitted with a special contact lens that has a miniature slide projector attached to it. Since the projector follows the exact movements of the eye, an image can be stabilized on the retina. When this is done, projected geometric designs fade from view within a few seconds (Pritchard, 1961).

2. Selective Attention. Voluntarily focusing on a selected portion of sensory input, most likely by rerouting messages within the brain. The so-called "seat-of-your-pants phenomenon" also relates to the functioning of sensory systems. As you sit reading this chapter, receptors for touch and pressure in the seat of your pants are sending nerve impulses to your brain. Even though these sensations have been present all along, you were probably not aware of them until just now. The seat-of-your-pants phenomenon is an example of selective attention. We are able to "tune in on" any of the many sensory messages bombarding us while excluding others. Another example of this is the "cocktail party effect." When you are in a group of people, surrounded by voices, you can still select and attend to the voice of the person with whom you are conversing. Or if that person gets dull, you can eavesdrop on conversations all over the room. Selective attention appears to be based on the ability of various brain structures to select and divert incoming sensory messages. But what about messages that haven't reached the brain? Is it possible that some are blocked while others are allowed to pass? Recent evidence suggests there are sensory gates that control the flow of incoming nerve impulses in just this way.

3. Sensory Gating of Pain. Alteration of incoming sensory messages in the spinal cord, before they reach the brain. A fascinating example of sensory gating is provided by Ronald Melzack and Patrick Wall, who are studying "pain gates" in the spinal cord (Melzack & Wall, 1983). Melzack and Wall noticed, as you may have, that one type of pain will sometimes cancel another. This suggests that pain messages from different nerve fibers pass through the same neural "gate" in the spinal cord. If the gate is "closed" by one pain message, other messages may not be able to pass through. Messages carried by large, fast nerve fibers seem to close the spinal pain gate directly. Doing so can prevent slower, "reminding system" pain from reaching the brain. Pain clinics use this effect by applying a mild electrical current to the skin. Such stimulation, felt only as a mild tingling, can greatly reduce more agonizing pain. Messages from small, slow fibers seem to take a different route. After going through the pain gate, they pass on to a "central biasing system" in the brain. Under some circumstances, the brain then sends a message back down the spinal cord, closing the pain gates. Melzack and Wall believe that this type of gating explains the painkilling effects of acupuncture. As the acupuncturist's needles are twirled, heated, or electrified, they activate small pain fibers. These relay through the biasing system to close the gates to intense or chronic pain. Acupuncture has an interesting side effect not predicted by sensory gating. People given acupuncture often report feelings of light-headedness, relaxation, or euphoria. How are these feelings explained? The answer seems to lie in the body's newly discovered ability to produce opiate-like chemicals. To combat pain, the brain causes the pituitary gland to release a chemical called beta-endorphin (from endo, "within," and orphin, "opiate") which is similar to morphine. Endorphins are related to a class of brain chemicals known as enkephalins. Receptor sites for endorphins are found in large numbers in the limbic system and other brain areas associated with pleasure, pain, and emotion. Both acupuncture and electrical stimulation cause a buildup of endorphins in the brain. In other words, the nervous system makes its own "drugs" to block pain. The central biasing system, which closes pain gates in the spinal cord, is highly sensitive to morphine and other opiate painkillers. The discovery of endorphins and their painkilling effect has caused quite a stir in psychology. At long last it appears possible to explain a number of puzzling phenomena, including runner's "high," masochism, acupuncture, and the euphoria sometimes associated with childbirth and painful initiation rites in primitive cultures. In each instance, there is reason to believe that pain and stress cause the release of endorphins. These, in turn, induce feelings of pleasure or euphoria similar to morphine intoxication.