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Description
In this image of the middle ear, you can see the eardrum (left), hammer (top),
anvil (center), and stirrup (right). The stirrup is connected to the cochlea, which
houses the hair cells.
iStock.com/Didacta_produktionsbyra
Hearing Abilities
Hearing is not one sensory ability but many. People can detect sound, understand
spoken language, and appreciate the acoustical qualities of music. Those with normal
hearing can distinguish between sounds that are loud and soft or between those
produced by horns and those produced by stringed instruments. Although only 1 in
10,000 people have “absolute pitch “(an ability to identify a musical note as middle C,
F-sharp, or B-flat), we can all make judgments of “relative pitch”—enabling us to
know, for example, that a child’s voice is higher than a man’s (Oxenham, 2018;
Takeuchi & Hulse, 1993). In many ways, our auditory competence is impressive.
A particularly adaptive aspect of normal hearing that we take for granted is
auditory localization—the ability to tell the direction a sound is coming from.
Localization is needed to determine if the blaring siren you hear is coming from
behind you on the road, or if the approaching footsteps are coming from your left or
your right. This skill was vital to the survival of our primitive ancestors and is a matter
of life and death to all animals of prey.
auditory localization. The ability to judge the direction a sound is coming from.
It is usually easy to tell if a sound is coming from your left or your right, and those
who are not hard of hearing or deaf have auditory localization even as infants. In fact,
if you stand to the left or right of an infant and shake a rattle, the baby will often turn
its head in your direction, as if locating the source.
What makes auditory localization possible is that we hear in stereo, using two
ears spaced about 6 inches apart. If you are at a noisy gathering and someone on
your left calls your name, your left ear receives the signal before your right ear does
(it is closer to the source) and more intensely (your head is a barrier to the more
distant ear). The 6 inches of brain tissue and skull that separate your ears may seem
too little to matter, but the auditory system is sensitive. Unless a sound is directly
above, below, in front of, or behind you, the brain can detect small differences in
timing and intensity between the ears—and use these differences to locate the
source. This process is demonstrated in Figure 3.14 (Konishi, 1993; Middlebrooks &
Green, 1991). Researchers are now suggesting that vision also plays a role in
auditory localization, since congenitally blind participants demonstrate difficulty with
some auditory localization tasks (Cappagli, Cocchi, & Gori, 2017; Vercillo, Burr, &
Gori, 2016).
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Description
Figure 3.14 Auditory Localization
Source: Garrett, Brain and Behavior, 5e, 2018, SAGE Publications, Inc
Hearing Loss
It is natural to take your sensory competence for granted—until you do not have it.
Today, millions of people have hearing loss, ranging from a partial loss to profound
deafness. There are two kinds of hearing loss. The symptoms are the same, but the
causes and treatment are very different. One type is conductive hearing loss, in
which damage to the eardrum or to the bones in the middle ear diminishes their
ability to conduct sound waves. In this case, hearing can be partially restored through
surgery or by means of a hearing aid that amplifies sound waves—provided that the
inner-ear structures are intact (Ahmadi, Daramadi, Asadi-Samani, & Sani, 2017;
Woods et al., 2015). By contrast, sensorineural hearing loss results from inner-ear
damage to the cochlea, hair cells, or auditory nerve. Sensorineural hearing loss can
be caused by certain diseases or genetic mutations, by biological changes due to old
age, or by exposure to intensely loud noises (Tremblay, 2015). Some species can
regenerate hair cells, but humans are not one of them (Burns & Corwin, 2013).
However, genetic research on a mutation that causes hereditary deafness is tapping
into hair cell regeneration with the hopes of bringing sound to people who either lost it
or never had it (Mahmoodian-sani & Mehri-Ghahfarrokhi, 2017). Thanks to science,
we are getting closer to making hair cell regeneration a reality for humans (Lefèbvre,
Malgrange, & Moonen, 2008).
conductive hearing loss. Hearing loss caused by damage to the eardrum or
bones in the middle ear.
sensorineural hearing loss. Hearing loss caused by damage to the structures of
the inner ear.
Although it is not currently possible to regenerate working auditory hair cells in
humans and although conventional hearing aids do not fully restore hearing in
sensorineural hearing loss, cochlear implants have the ability to bypass hair cell
function (McCreery, Han, Pikov, Yadav, & Pannu, 2013). A cochlear implant uses a
tiny microphone to transmit sound to a processor worn behind the ear (similar to a
hearing aid). The processor then transmits the sound as an electrical signal to a
miniature electrode implanted in the cochlea (National Institute on Deafness and
Other Communication Disorders [NIDCD], 2018). The electrode stimulates the
auditory nerve—and an impulse is fired to the brain. Cochlear implants may enable
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many people who are profoundly deaf or severely hard-of-hearing to detect the
presence of sound. The NIDCD (2018) reports that, worldwide, more than 324,000
devices have been implanted in persons with hearing loss—adults and children—as
of December 2012. The sensations produced by these devices seem to vary from
one user to another. Some wearers derive little benefit from the implants, but
researchers believe benefit is correlated with age at implantation (Bruijnzeel, Ziylan,
Stegeman, Topsakal, & Grolman, 2016). A thorough literature review of improved
speech and language performance post–cochlear implantation revealed that earlier is
better; cochlear implantation before the age of 12 months resulted in better speech
production, auditory performance, and receptive language scores (Bruijnzeel et al.,
2016). In adults with hearing loss, quality of life for those with cochlear implants was
reported to be higher than for those without (Crowson, Semenov, Tucci, & Niparko,
2017). However, these reviewed studies have limitations, and researchers suggest
further testing with experimental rigor to determine the replicability and validity of
these findings.
Psychology Applied: When Is Loud Too Loud?
My Bloody Valentine is an American rock band known for its high decibel levels.
The band’s music has been described as “painfully loud—terrifyingly loud” (Richards,
2008). But when does loud become too loud, and is loud dangerous only when it
becomes painful? The amplitude of a sound, which determines its loudness to the
human ear, is measured in terms of decibels (dB). To understand this scale, it is
important to know that loudness increases in orders of magnitude (a 20 dB sound is
10 times as intense as a 10 dB sound, a 40 dB sound is 100 times as intense as a 20
dB sound, and so on). Constant daily exposure to sounds of over 85 dB (heavy street
traffic, subways, jackhammers, snowmobiles, lawnmowers, and vacuum cleaners)
can flatten the inner-ear hair cells over time and cause a gradual loss of hearing. It
makes you wonder how many people experience ringing in the ears, which often
occurs following exposure to loud sounds.
Ringing in the ears is common. The NIDCD (2016) estimates that 10 percent of
Americans have experienced ringing in the ear—tinnitus—and 13 percent of
Americans over 12 years of age have suffered from full or partial hearing loss, often
because of noise. Even a brief assault by an ear-shattering sound that exceeds 140
dB (a gunshot, an explosion, or a rocket launch) can tear the delicate inner-ear
tissues and cause permanent hearing loss. Review Figure 3.14, and you will see that
danger to the ear is all around us. Current research on sound exposure focuses on
the occupational hazards of noise in work settings and nonoccupational hazards such
as power tools or loud music (Hoffman, Dobie, Losonczy, Themann, & Flamme,
2017). Spend 2 hours at a rock concert or turn the stereo volume up to full blast, and
you may be putting your ears at risk (West & Evans, 1990). Many experts claim that if
your headphones can be heard by someone near you, then you are damaging your
ears. And the loss can be irreversible. Many famous musicians, such as Pete
Townshend, Sting, Ozzy Osbourne, and Eric Clapton, are now partially deaf. My
Bloody Valentine, whose founding member Kevin Shields suffers from tinnitus,
provides free earplugs at every concert. This is a wonderful service, especially since
hearing damage is a major concern for today’s adolescents and young adults (Jiang,
Zhao, Guderley, & Manchaiah, 2016). So, turn down the volume, and protect your
ears whenever you can, whether while mowing the lawn, vacuuming, or relishing your
next live music event.
Other Senses
Psychologists know more about seeing and hearing than about other sensory
systems, but these other systems are also essential to the adaptive human package.
You cannot see or hear the heat of a fire, the sting of a bee, the stench of a gas leak,
or the bitter taste of a poisonous plant. Nor can you see or hear the sensuous
pleasures of a scent-filled rose, creamy chocolate, or a soothing massage. Human
beings have developed the ability to detect, process, and integrate information from
many sources.
Smell
Dogs are known for their ability to sniff out faint scents and to track down animals,
criminals, and illegal drugs over time and long distances. In fact, dogs have such a
keen sense of smell that they can detect signs of sleep apnea in urine samples
(Koskinen et al., 2018) and harmful bacteria in cow’s milk (Fischer-Tenhagen, Theby,
Krömker, & Heuwieser, 2018)!
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We cannot smell it, but researchers, like Paula Jendrny and colleagues (2020),
are training dogs to detect the coronavirus before the most sophisticated technology
can.
JOEL SAGET / Contributor/Getty
In a contest of smelling ability, we humans would have little to brag about
compared to our canine buddies—who may well have the keenest noses in the
animal kingdom (Marshall & Moulton, 1981). But our own sense of smell, the product
of the body’s olfactory system, is more sensitive than you may realize—and
potentially more important. Since ancient times, medical doctors used the smell of
sweat, breath, urine, and other body odors to diagnose illness. Recently it was even
discovered that Joy Milne, a woman living in the United Kingdom, can detect
Parkinson’s disease by smell (Quigley, 2015). Milne is so accurate that, during one
controlled trial, she corrected researchers when they presented her with an olfactory
sample from a participant they thought was Parkinson’s free; they were wrong, and
Milne correctly diagnosed the disease with nothing but her sense of smell. The fact
that she is a woman may not be a coincidence. Research has found that women tend
to have greater olfactory acuity than men (Dalton, Doolittle, & Breslin, 2002;
Lundstrom & Hummel, 2006; Tubaldi, Ansuini, Tirindelli, & Castiello, 2008).
olfactory system. The structures responsible for the sense of smell.
When airborne odor molecules dissolve in the nose, they are trapped by the
hairlike olfactory receptors shown in this image.
Universal Images Group North America LLC / Alamy Stock Photo
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All smells have a chemical origin (Buck, 2000). Depending on their molecular
structure, substances emit odor-causing molecules into the air. Some objects, such
as glass and metal, have no smell (any scent these objects may emit comes from
impurities on the surface). Other objects, like the musk oil extracted from animal
sweat glands, produce overpowering odors. By breathing through the nose and
mouth, we inhale these airborne odorant molecules—which dissolve and become
trapped by olfactory receptors in the moist yellow lining of the upper nasal passages
just above the roof of the mouth. There are about 10 million of these hairlike
receptors in the human nose (the average dog has 200 million). Certain molecules
seem to fit certain types of receptors the way a key fits a lock. Once activated, they
trigger an action potential in the olfactory nerve. This nerve connects the nose to the
olfactory bulb, a bean-size organ that distributes information throughout the cortex
and to the nearby limbic-system structures that control memory and emotion. Smell is
the only sensation that is not routed to the cortex through the thalamus. As illustrated
in Figure 3.15, the olfactory bulb is its own private relay station (Doty, 2001).
Description
Figure 3.15 The Olfactory System
Source: Garrett, Brain and Behavior, 5e, 2018, SAGE Publications, Inc
Unlike other animals, humans do not need a sense of smell to mark territory, track
prey, signal danger, establish dominance hierarchies, or attract a mate. Also,
language does not provide an adequate supply of olfaction words, which makes it
hard for people to describe smells (Richardson & Zucco, 1989). Yet the millions of
olfactory receptors in the nose enable us to distinguish among 10,000 different odor
molecules—including the proposed “primary” odors of vinegar, roses, mint, rotten
egg, mothballs, dry-cleaning fluid, and musk (Amoore, Johnston, & Rubin, 1964).
Brain-imaging technology demonstrates that laboratory rats have a unique pattern of
receptor activity in the olfactory bulb and cortex in response to odors (Isaacson,
2010; Osmanski et al., 2014).
People from different cultures have the same olfactory capacity—and similar likes
and dislikes when it comes to the smell of plants, fruits, spices, and body odors.
Universally, people are drawn to flowers and other perfume-like fragrances and
disgusted by foul and sulfurous odors (Miller, 1997). At the same time, people from
different parts of the world are uniquely and adaptively attuned to the smells that
surround them. In Aroma: The Cultural History of Smell, Classen, Howes, and
Synnott (1994) illustrate the point in several cultures. In an Amazonian rain forest in
Colombia, the Desana separate the musky smell of “deep forest” animals (such as
the jaguar), and the sweet smell of “open field” animals (such as various rodents).
They also say they can detect by nose the presence of different neighboring tribes—
based on whether those tribes eat a steady diet of hunted game, fish, or roots and
vegetables.
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Smell is a primitive sense—yet so successful, suggests Diane Ackerman (1990),
“that in time the small lump of olfactory tissue atop the nerve cord grew into a brain”
(p. 20). Given this view, it is hardly surprising that researchers have tried to identify
pheromones in human beings. Pheromones are chemicals secreted by animals that
transmit signals to others, usually of the same species. Ants, bees, termites, and
other insects secrete chemicals that attract mates and “release” other behaviors.
Equipped with sensitive chemoreceptors on his antennae, for example, the male
emperor moth can detect the scent of a virgin female more than 6 miles away. Many
mammals are also sexually excited by scent, which is why female dogs in heat send
neighboring male dogs into a state of frenzy. Are humans similarly aroused? If so,
could the scent be bottled and manufactured? For perfumers, the potential is easy to
imagine. At this point, however, the research evidence is scant. Human sexuality is
far too complex to be chemically “controlled” by scent. However, some researchers
have found evidence that human pheromone can attract our attention and impact
emotional centers in the brain (Ferdenzi, Delplanque, Atanassova, & Sander, 2016;
Hummer, Phan, Kern, & Mcclintock, 2017).
pheromones. Chemicals secreted by animals that transmit signals—usually to
other animals of the same species.
Taste
Taste is a product of the body’s gustatory system (Doty, 1995; Santa-Cruz Calvo
& Egan, 2015). Like smell, taste is a chemical sensation. Put a morsel of food or a
drop of liquid on your tongue, and it will come into contact with clusters of hairlike
receptor cells called taste buds (illustrated in Figure 3.16). There are about 10,000
taste buds in the mouth. Some cling to the roof and back of the throat, but most line
the trenches and bumps on the surface of the tongue. Taste buds are packed most
densely on the tip of the tongue but are virtually absent from the center of the tongue.
Within each taste bud, between 50 and 150 receptor cells absorb the chemical
molecules of food and drink—and trigger neural impulses that are routed to the
thalamus and cortex. These cells are replaced every 10 days, so if you burn your
tongue on hot soup, the damage to your receptors will be repaired.
gustatory system. The structures responsible for the sense of taste.
taste buds. Nets of taste-receptor cells.
Description
Figure 3.16 Taste Buds
Gwen Shockey / Science Source; DE AGOSTINI PICTURE LIBRARY /
Contributor/Getty
There are five primary tastes: sweet, salty, sour, umami, and bitter (Calvo & Egan,
2015). Researchers have also found evidence that the tongue can detect fat
(Fukuwatari et al., 1997; Martin et al., 2011). All of your taste buds can react to these
primary tastes, regardless of their location on your tongue. However, the flavor of a
food is determined not only by taste but by other factors as well. You may have
noticed that after you have brushed your teeth in the morning, orange juice tastes
bitter. Chemical residues from the substance already eaten mix with what you are
currently eating to produce a new taste sensation. Temperature, texture, and
appearance are also important factors, which is why no one likes warm soda or
soggy potato chips and why the great chefs prepare dishes for the eye as well as for
the palate. By far the most important determinant of flavor is odor; it accounts for
about 80 percent of the taste we experience (Chartier, 2012). When you have an
illness that stuffs up your nose (Wolf, Renner, Tomazic, & Mueller, 2018), or COVID19 (Parma et al., 2020), food does not quite taste the same. Indeed, research shows
that people lose their ability to identify common flavors—such as chocolate, vanilla,
coffee, wine, and even onion and garlic—when they are prevented from smelling the
food (Mozell, Smith, Smith, Sullivan, & Swender, 1969). Furthermore, the flavors we
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become accustomed to are influenced by our culture (Spence, 2015). For example,
Spence (2015) mentions that people from Japanese cultures are frequently exposed
to almonds in savory dishes with pickled condiments, whereas those from Western
cultures more often experience almonds in sweet dishes, like cakes and toffee.
The more taste buds that dot your tongue, the more sensitive you are likely to be
to various tastes. Children have more taste buds than adults do, for example, which
may explain why they are often so picky about eating “grown-up” foods. Even among
adults, individuals differ in the number of taste buds they have—and in their
sensitivity to taste (Herbert, Platte, Wiemer, Macht, & Blumenthal, 2014). Indeed,
studies have shown that people can be divided into three groups: nontasters, medium
tasters, and supertasters (Tepper et al., 2009). At one extreme, nontasters (30
percent of the population) are unable to detect certain sweet and bitter compounds
and are in general less sensitive to taste. At the other extreme, supertasters (about
25 percent of the population) react strongly to certain sweet and bitter compounds.
Compared to most, supertasters use half as much sugar or saccharin in their coffee
or tea. They also are more likely to experience a burning sensation in response to the
active ingredient in chili peppers. These differences in taste sensitivity correspond
nicely to our physiological makeup. Using videomicroscopy to count the number of
taste buds on the tongue, researchers have found that nontasters have an average of
96 taste buds per square centimeter, medium tasters have 184, and supertasters
have 425. Put differently, the number of taste buds on the human tongue can range
from a low of 500 to a high of 10,000 (Bartoshuk & Beauchamp, 1994; Miller &
Reedy, 1990).
Touch
Every organism has a sense of touch. Sea snails withdraw their gills at the
slightest pressure. Sponges sense an intruder by feeling the water around them
quiver. As for us humans, often feeling is believing. Put up a “wet paint” sign, and you
will find, paradoxically, that it seems to invite touching rather than inhibit it. Tactile
sensations are unique in many ways. To begin with, touch is the only sensation with
receptors that are not localized in a single region of the body. We need eyes to see,
ears to hear, a nose to smell, and a tongue to taste. But the organ of touch, and the
site of its sensory receptors, is skin.
Skin is by far the largest organ of the body. It covers two square yards and weighs
6 to 10 pounds. It is multilayered, waterproof, and elastic, and is filled with hair
follicles, sweat-gland ducts, and nerve endings that connect to the central nervous
system. When you consider all the sensations that emanate from your skin—feeling
hot, cold, wet, dry, sore, itchy, scratchy, sticky, gooey, greasy, tingly, numb, and hurt—
you can see that touch involves not one sensory system but many (Craig & Rollman,
1999; Heller & Schiff, 1991; Zimmerman, Bai, & Ginty, 2014).
The sensations of touch are vital for survival and socialization. Without it, you
would not know that you are in danger of becoming frostbitten or burned; you would
not know if you have been stung by a bee; you would be unable to swallow food; and,
according to an interview with David Linden, it would be difficult to bond. David
Linden, an expert on touch at Johns Hopkins University, states that humans have an
emotional touch system that sends information to a part of the brain “crucial for
socially-bonding touch. This includes things like a hug from a friend, to the touch you
got as a child from your mother, to sexual touch” (Stromberg, 2015). Mammals
deprived of touch during development have demonstrated psychological issues,
higher than average levels of stress hormones, and immune deficiencies (Ardiel &
Rankin, 2010).
When other sensory systems fail, touch takes on even more importance. For
Virgil, the blind man whose eyesight was restored temporarily, shapes and textures
were particularly important for recognizing objects. When Virgil was handed a bowl of
fruit, he could easily distinguish among a slick plum, a soft fuzzy peach, a smooth
nectarine, and a rough, dimpled orange. He was even able to “see” through the
disguise of an artificial wax pear that had fooled everyone else. “It is a candle,” he
said, “shaped like a bell or a pear” (Sacks, 1995, p. 149).
It is important to distinguish between passive and active touch. In passive touch, a
person’s skin is contacted by another object, as when a cat rubs up against your leg.
In active touch, it is the person who initiates the contact, as when you pet your cat.
Psychologically, the effects are different. James Gibson (1962) tested subjects’ ability
to identify cookie cutters shaped like stars, triangles, circles, and so on. When the
objects were pressed lightly onto the hand (passive), they were identified correctly 29
percent of the time. When subjects actively explored the shapes with their fingers, the
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accuracy rate increased to 95 percent. Other research, too, shows that people can
make fine discriminations among common objects when they explore by grasping,
lifting, holding, squeezing, rubbing, and tracing the edges. For example, merely
touching an object reveals temperature, rubbing a finger across it reveals texture, and
molding the hand around it reveals its shape and volume (Klatsky & Lederman,
1992).
Active touch is what allows Virgil to feel the differences among plums, peaches,
oranges, and nectarines. It is also the key to Braille, the alphanumeric system that
allows many blind people to read. Braille letters and numbers consist of coded
patterns of raised dots on a page that readers scan with their fingertips. Some Braille
readers achieve a reading rate as high as 200 words per minute—which is
remarkable considering that the average rate among sighted readers is 250 words
per minute (Foulke, 1991). In one experiment, people who were blind that used their
Braille-reading fingers outperformed blindfolded sighted adults by 20 percent in their
ability to discriminate different shapes by touch (Stevens, Foulke, & Patterson, 1996).
The skin contains a wide range of sensory neurons such as nociceptors for pain,
pruriceptors for itch, thermoreceptors for temperature, and low-threshold
mechanoreceptors for touch (Zimmerman et al., 2014). One of the most striking
aspects of touch is that sensitivity to pressure or vibration is different from one part of
the body to another. To determine the thresholds for touch (how much force it takes
before a subject reports a feeling), researchers would apply a thin rod or wire to
different areas of skin and vary the pressure (Weinstein, 1968). In all cases, pressure
causes nerve endings to fire messages through the spinal cord, brainstem, and
thalamus en route to the somatosensory cortex (Burton & Sinclair, 1996; Greenspan
& Bolanowski, 1996).
Using Braille, persons who are blind can navigate a computer using their sense of
touch.
iStock.com/zlikovec
What’s Your Prediction?
People can distinguish objects by touch. Can we also identify live human faces in
this way? If so, do we rely on “geometric” cues (like nose size and cheekbone
structure) or “material” cues (like skin texture and temperature)? Andrea Kilgour and
Susan Lederman (2002) had college students manually explore an unfamiliar live
face up to the hairline. All the students wore blindfolds, headphones, and nasal
ointment to ensure that they relied only on a sense of touch. Afterward, they tried to
identify that face by hand from a group of three faces. Keeping in mind that they
could guess correctly 33 percent of the time, how accurate do you predict the
students were—33, 50, 75, 90, or 100 percent? What if they felt plaster masks of the
same faces, which preserved geometric information but not material cues? Would
that make them more accurate or less? The results were interesting. Students who
felt live faces were accurate 79 percent of the time; those who felt masks were
accurate in 59 percent of their identifications. Apparently, we can recognize faces by
hand, by using both geometric and material cues.
Temperature
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Normal human body temperature is 98.6 degrees Fahrenheit, or 37 degrees
Celsius (temperatures at the surface of the skin are slightly lower). There are two
striking facts about the sensation of temperature. First, temperature is, to a large
extent, relative to your current state. To demonstrate, fill three buckets with tap water
—one cold, one hot, and one at room temperature. Place your right hand into the cold
water and your left hand into the hot water, and leave them there for a minute. Then
place both hands together into the third bucket. You can probably predict the amusing
result: Both hands are now in the same water, yet your right hand feels warm and
your left hand feels cool. If you ever plunged into a cold pool or eased yourself into a
hot tub, you know that the sensations are triggered by temperatures that are well
above or below your own current “adaptation level” (Hensel, 1981).
The second fact about temperature is that there are two separate sensory
systems—one for signaling warmth, the other for signaling cold. Early studies showed
that some spots on the skin respond more to warming and others more to cooling
(Dallenbach, 1927). “Hot” is a particularly intriguing sensation in that it is triggered
when these warm and cold spots are simultaneously stimulated. Thus, when people
grasp two braided pipes—one with cold water running through it, the other with warm
water—they will pull away, complaining that the device is literally too hot to handle, as
illustrated in Figure 3.17. This effect of the “thermal grill,” first described by Torsten
Thunberg (Jutzeler, Warner, Wanek, Curt, & Kramer, 2017) is found not only among
humans but also in other animals (Bach, Becker, Kleinbohl, & Holzl, 2011;
Bouhassira, Kern, Rouaud, Pelle-Lancien, & Morain, 2005; Kammers, de Vignemont,
& Haggard, 2010). Apparently, the brain interprets the dual firing of both types of
temperature receptors as being caused by a burning hot stimulus. Is the sensation a
mere illusion? No. Neuroimaging studies show that gripping the entire grill (but not
the warm or cold bars alone) activates regions of the brain that process unpleasant
stimuli (Casey & Bushnell, 2000; Craig, Reiman, Evans, & Bushnell, 1996; Hofbauer,
Rainville, Duncan, & Bushnell, 2001).
Description
Figure 3.17 The Thermal Grill
Carolina Hrejsa/Body Scientific Intl.
Pain
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Pain is a dentist’s drill boring through a tooth. Pain is stepping on a thumbtack.
Pain is a pulled muscle, a splitting headache, a backache, sunburn blisters, and
stomach cramps. Ouch! Whatever the source, people are understandably motivated
to avoid and escape from pain. Yet pain is crucial to survival because it serves as a
red flag, a warning system that signals danger and the risk of tissue damage. Life
without pain may sound great, but it would be a life unlikely to last very long. The
case studies of children with a pain-specific genetic mutation illustrate the point. Born
with a congenital indifference to pain, these children often injure themselves without
realizing it. For example, these children are reported to repeatedly bang their heads
on the floor, bite into their tongues, and cut their hands—all without a pain response.
As a result, their bodies are often marked by burns, cuts, scrapes, and bruises
(Mansouri et al., 2014; Restak, 1988; Shorer, Wajsbrot, Liran, Levy, & Parvari, 2014).
For those of us who do feel pain, it is a subjective, emotionally charged sensation.
No single stimulus triggers pain the way that light does vision, no nerve endings in
the skin are specially dedicated to pain over other sensations, and people with similar
injuries often experience different degrees of pain. Clearly, the conditions that lead us
to report pain include not only the threat of bodily harm but also culture, religion,
personality, expectations, and other factors. Even gender plays a role, as male
research subjects tend to have a greater tolerance for painful stimulation than do
female subjects (Berkely, 1997; Keogh & Herdenfeldt, 2002). Why might this be?
Some argue that it is the role our society places on males to be tough. When a male
child cries, it is not abnormal to hear a caretaker say, “Boys don’t cry.” Others argue
that it might be a biological, or sex, difference. Regardless, theories of pain must take
into account all of these factors (Gatchel & Turk, 1999; Kruger, 1996) and, thus, lean
toward a biopsychosocial model (Bartley & Fillingim, 2013; Gatchel, Peng, Peters,
Fuchs, & Turk, 2007).
Gate-Control Theory
If you have a sore leg muscle or if you scrape your knee, nerve endings in the
skin send messages to the spinal cord through one of two types of nerve fibers. Dull,
chronic aches and pains—as in a sore muscle—are carried to the spinal cord by
“slow,” thin nerve fibers that also respond to nonpainful touch. Sharp, acute, piercing
sensations—as when you scrape your knee—are relayed through “fast” myelinated
fibers. In short, an express lane to the spinal cord is reserved for acute, emergencylike sensations. But how are these signals then sent from the spinal cord to the brain?
And can they be blocked when the pain is too intense to bear?
In an attempt to answer these questions, Ronald Melzack and Patrick Wall (1965)
proposed the gate-control theory of pain. According to this theory, the nervous
system can process only a limited number of sensory signals at once. When the
system is full, a neural “gate” in the spinal cord either blocks or allows the upward
passage of additional signals to the brain (the gate is not an actual structure but a
pattern of inhibitory neural activity). Research shows that although Melzack and Wall
were wrong about the physiological details of their theory, and the gate seems to be a
bit “leaky” instead of a perfect barrier (Sun et al., 2017), they were generally right
about the key point: Strong pain signals to the brain can be blocked (Melzack & Wall,
2001; Mendell, 2014; Pereira & Lerner, 2017).
gate-control theory. The theory that the spinal cord contains a neurological
“gate” that blocks pain signals from the brain when flooded by competing signals.
This theory has a valuable practical implication: that you can partially shut the
gate on pain by creating competing sensations. If you fall and hurt your knee, rubbing
it hard will send new impulses into the spinal cord—and inhibit other pain signals.
That is one reason it often helps to put ice on a bruise or to scratch the skin near a
mosquito bite. For chronic pain, such interventions as deep massage, electrical
stimulation, and acupuncture may provide temporary relief in the same way. It seems
paradoxical, but as the theory correctly suggests, you can ease the pain by causing
additional pain.
Psychological Control
One psychological approach people often use is to block the pain from
awareness. Just try not to think about it, okay? This advice sounds great, but beware:
The strategy can backfire. Research shows that the more we try to suppress a
particular thought, the more readily that thought pops to mind. Try not to think about
the itch you are not supposed to scratch, and the harder you try, the less likely you
are to succeed (Wegner, 1989). For adaptive reasons, pain sensations may be
particularly hard to suppress. Almost regardless of where we are, whom we are with,
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