196
CHAPTER 6
Sensation and Perception
papillae [pa-PILL-ee]
Knoblike elevations on
the tongue, containing
the taste buds. (Singular :
papilla . )
taste buds
Nests of taste-receptor
cells.
• Why do saccharin and caffeine taste bitter to
some people but not to others?
• Why do you have trouble tasting your food
when you have a cold?
• Why do people often continue to "feel" limbs
that have been amputated?
Other Senses
Psychologists have been particularly interested in vi­
sion and audition because of the importance of
these senses to human survival. However, research
on the other senses is growing dramatically, as
awareness of how they contribute to our lives in­
creases and new ways are found to study th em.
Taste: Savory Sensations
Taste, or gustation, occurs because chemicals stim­
ulate thousands of receptors in the mouth. These re­
ceptors are located primarily on the tongue, but
some are also found in the throat, inside the cheeks,
and on the roof of the mouth. If you look at your
tongue in a mirror, you will notice many tiny
bumps; they are called papillae (from the Latin
for "pimpl es"), and they come in several forms. In
all but one of these forms, the sides of each papil­
la are lined with taste buds, which up close look
a little like segmented oranges (see Figure 6.9) . Be­
cause of genetic differences, human tongues can
have as few as 500 or as many as 10,000 taste buds
(Miller & Reedy, 1990).
The taste buds are commonly referred to, mis­
takenly, as the receptors for taste. The actual re­
ceptor cells, however, are inside the buds, 15 to 50
to a bud. These cells send tiny fibers out through an
opening in the bud; the receptor sites are on these
fibers. The receptor cells are replaced by new cells
about every ten days. However, after age 40 or so,
the total number of taste buds (and therefore re­
ceptors) declines.
Traditionally, researchers have considered four
tastes to be basic: salty, sour, bitter, and sweet, each
produced by a different type of chemical. Many re­
searchers now also include a fifth taste, umami (from
the Japanese for "delicious"), which is the taste of
monosodium glutamate (MSG). Umami is found
in many protein-rich foods, but its inclusion re­
mains somewhat controversial. (We know when
something is bitter or salty, but no one ever says,
"Yum, this steak sure has a great umami taste. ")
The basic tastes are part of our evolutionary her­
itage: Bitterness and sourness help us identify foods
that are rancid or poisonous; sweetness helps us
identify foods that are healthful or rich in calories;
salt is necessary for all boclily functions; and umami
(if it is basic) could have helped us identify protein­
rich foods.
The basic tastes can be perceived at any spot
on the tongue that has receptors, and differences
among the areas are small. Interestingly, the cen­
ter of the tongue contains no taste buds, and so it
cannot produce any sort of taste sensation. But, as
in the case of the eye's blind spot, you will not usu­
ally notice the lack of sensation because the brain
fills in the gap.
When you bite into an egg or a piece of bread
or an orange, its unique flavor is composed of some
combination of the four or five basic tastes, but thf
physiological details are still hazy. It has even bee
difficult to find the receptors for the basic tastes, (
though recently researchers have proposed can'
dates for the receptors that process bitter, SW(
and umami (Chaudhari, Landin, & Roper, 2(
Huang et al., 1999; Max et a!., 2001; Montma
et a!., 2001; Zhang et at., 2003).
Taste receptor cell
Papilla
FIGURE 6 .9
Taste Receptors
The illustration on the
left shows taste buds
lining the sides of a
papilla on the tongue 's
surface . The illustration
on the right shows an
enlarged view of a single
taste bud.
Taste buds
Sensation and Perception
Everyone knows that people live in somewhat
different "taste worlds" (Bartoshuk, 1998). Some
people love broccoli and others hate it. Some peo­
ple can eat chili peppers that are burning hot, and
others cannot tolerate the mildest jalapeno. One
reason for these differences is genetic. In the Unit­
ed States, about 25 percent of people are by na ture
supertasters who find saccharin, caffeine, broccoli,
and many other substances to be unpleasantly bit­
ter. (Women are overrepresented in this group.)
"Tasters," in contrast, detect less bitterness, and
"nontasters" detect none at all. Supertasters also
perceive sweet tastes as sweeter and salty tastes as
saltier than other people do, and they feel more
"burn" from foods such as ginger, pepper, and hot
chilies (Bartoshuk et al., 1998; Lucchina et al.,
1998). Supertasters, it seems, have more taste buds,
and certain papillae on their tongues are smaller, are
more densely packed, and look different from those
of non tasters (Reedy et aI., 1993).
Other taste preferences are a matter of culture
and learning. Many North Americans who enjoy
raw oysters, raw smoked salmon, and raw herring
are nevertheless put off by other forms of raw
seafood that are popular in Japan, such as sea
urchin and octopus. And within a given culture,
some people will greedily gobble up a dish that
makes others turn green. Some of these learned
taste preferences probably begin in the womb or
during breast feeding. A baby whose mother drank
carrot juice while pregnant or nursing is likely to be
more enthusiastic about eating porridge mixed with
carrot juice than porridge rnixed with water, where­
as babies without this exposure show no such pref­
erence (Mennella, Jagnow, & Beauchamp, 2001).
The attractiveness of a food can also be affect­
ed by its color, temperature, and texture. As
Goldilocks found out, a bowl of cold porridge is
not nearly as delicious as one that is properly heat­
ed. And any peanut butter fan will tell you that
chunky and smooth peanut butters just don't taste
the same. Even more important for taste is a food's
odor. Subtle flavors such as chocolate and vanilla
would have little taste if we could not smell them
(see Figure 6.10). Smell's influence on flavor ex­
plains why you have trouble tasting your food
when you have a stuffy nose. Most people who
chronically have trouble tasting things have a prob­
lem with smell, not taste.
Smell: The Sense of Scents
The great author and educator Helen Keller, who
became blind and deaf as a toddler, once called
Apricot
CHAPTER 6
197
h-- - ; ' - - -:.--"
Chocolate
Coffee
Dill pickle juice
Garlic
Lemon
Onion
Root beer
Water
Wine
o
20
40
60
80
100
Percent correct
FIGURE 6.10
Taste Test
The orange bars show the percentages of people who
could identify a substance dropped on the tongue when
they were able to smell it The blue bars show the per­
centages who could identify a SUbstance when they were
prevented from smelling it (Mozell et ai, 1969)
smell "the fallen angel of the senses." Yet our sense
of smell, or olfaction, although seemingly crude
when compared to a bloodhound's, is actually quite
good, and is far more useful than most people
realize.
The receptors for smell are specialized neu­
rons embedded in a tiny patch of mucous mem­
brane in the upper part of the nasal passage, just
beneath the eyes (see Figure 6. 11 on page 198).
Millions of receptors in each nasal cavity respond
to chemical molecules in the air. When you in­
hale, you pull these molecules into the nasal cav­
ity, but they can also enter from the mouth,
wafting up the throat like smoke up a chimney.
These molecules trigger responses in the recep­
tors, and these responses combine to yield the
yeasty smell of freshly baked bread or the spicy
fragrance of a eucalyptus tree. Signals from the re­
ceptors are carried to the brain's olfactory bulb
by the olfactory nerve, which is made up of the re­
ceptors' axons. From the olfactory bulb, they trav­
el to a higher region of the brain.
Figuring out the neural code for smell has been
a real challenge. Of the 10,000 or so smells we
198
CHAPTER 6
Sensation and Perception
Olfactory tract
Olfactory tract
To cerebral cortex
.0(
I'o:-=
==-- Olfactory
nerve fiber
+-.......,.,..- Olfactory
cell
Olfactory hairs
(receptors)
FIGURE 6.11
Rec:eptors for Smell
Airborne chemical molecules (vapors) enter the nose and circulate through the nasal cavity, where the smell receptors are
located. The receptors' axons make up the olfactory nerve, which carries signals to the brain . When you sniff, you draw more
vapors into the nose and speed their circulation. Vapors can also reach the nasal cavity through the mouth by way of a pas­
sageway from the throat.
detect (rotten, burned, musky, fruity, spicy, flowery,
resinous, putrid .. ,), none seems to be more basic
than any other. Moreover, as many as a thousand
kinds of receptors exist, each kind responding to a
Smell has not only evolutionary but also cultural significance. These pil­
grims in Japan are purifying themselves with holy incense for good luck
and health .
part of an odor molecule's structure (Axel, 1995;
Buck & Axel, 1991 ). But researchers are making
progress; they have discovered that distinct odors
activate unique combinations of receptor types, and
they have succeeded in identifying some of those
combinations (Malnic et aI., 1999) .
Although smell is less vital for human sur­
vival than for the survival of other animals, it is
still important. We sniff out danger by smelling
smoke, food spoilage, or poison gases. Thus, a
deficit in the sense of smell is nothing to turn up
your nose at. Such a loss can come about because
of infection, disease, injury, or smoking. A person
who has smoked two packs a day for ten years
must abstain from cigarettes for ten more years
before the sense of smell returns to normal (Frye,
Schwartz, & Doty, 1990).
Human odor preferences, like taste preferences,
vary. In some societies, people use rancid fat as a
hair pomade, but anyone in North America who
did so would quickly have a social problem. With­
in a particular culture, context and experience are
all-important. The very same chemicals that con­
tribute to unpleasant body odors and bad breath
also contribute to the pleasant bouquet and flavor
of cheese.
Sensation and Perception
INV
,
CHAPTER 6
199
THE SMELL OF TASTE
Demonstrate for yourself that smell enhances the sense of taste . Take a bite of a slice of apple, hOldi~g
your nose, and then do the same with a slice of raw potato. You may find that you can't taste much dif­ ference! If you think you do taste a difference, maybe your expectations are influencing your response. Try the same thing, but close your eyes and have someone else feed you the slices. Can you still tell them apart? Senses of the Skin
The skin's usefulness is more than just skin deep.
Besides protecting our innards, our 2 square yards
of skin help us identify objects and establish inti­
macy with others. By providing a boundary be­
tween ourselves and everything else, the skin also
gives us a sense of ourselves as distinct from the
en vironmen t.
The basic skin senses include touch (or pres­
sure), warmth , cold, and pain. Within these four
types are variations such as itch, tickle, and painful
burning. Although certain spots on the skin are
esp ecially sensitive to the four basic skin sensa­
tions, for many years scientists had difficulty find­
ing distinct receptors for these sensations, except
in the case of pressure. A few years ago, however,
Swedish researchers found a new kind of nerve
fiber that seems to be responsible for at least some
types of itching (Schmelz et al., 1997) . And more
recently, scientists identified a possible cold recep­
tor (McKemy, Neuhausser, & Julius, 2002; Peier
et al., 2002).
Perhaps specialized fibers will also be discov­
ered for other skin sensations . In the meantime,
many aspects of touch continue to baffle science­
for example, why gently touching adjacent pres­
sure spots in rapid succession produces tickle; and
why the simultaneous stimulation of warm and
cold spots produces not a lukewarm sensation but
the sensation of heat. Decoding the messages of
the skin senses will eventually tell us how we are
able to distinguish sandpaper from velvet and glue
from grease.
The Mystery of Pa in
Pain, which is not only a skin sense but also an in­
ternal sense, has come under special scrutiny. Pain
differs from other senses in an important way:
When the stimulus producing it is removed, the
sensation may continue-sometimes for years.
Chronic pain disrupts lives, puts stress on the
body, and causes depression and despair. (For
ways of coping with pain, see "Taking Psycholo­ gy With You. ") The Gate-Control Theory of Pain For many years, a leading explanation of pain has been the gate-control theory, which was first proposed by Canadian psychologist Ronald Melzack and British physiologist Patrick Wall (1965). Ac_~
cording to this theory, pain impulses must
:v:~,
get past a "gate" in the spinal cord . The gate
~~;.
J'~
is not an actual structure, but rather a pattern
of neural activity that either blocks pain mes­ "Gate" closed by
incoming
sages coming from the skin, muscles, and internal
impulses from
organs or lets those signals through. Normally, the
large fibers or
gate is kept shut, either by impulses com­
from the brain;
opened by
ing into the spinal cord from large fibers If "gate" is open,
impulses from
then pain
that respond to pressure and other kinds
smaller fibers
impulses reach
of stimulation or by signals coming down
the brain
from the brain itself. But when body tis­
sue is injured, the large fibers are damaged and
smaller fibers open the gate, allowing pain messages
to reach the brain unchecked .
Because the gate-control theory emphasizes
the role of the brain in controlling the gate, it cor­
rectly predicts that thoughts and feelings can in­
fluence our reactions to pain. When we dwell on
our pain, focusing on it and talking about it con­
stantly instead of acting in spite of it, we often in­
tenSify our experience of it (Sullivan, Tripp, &
Santor, 1998) . Conversely, when we are distract­
ed from our pain, we may not feel it as we usual­
ly would-which is why we hear, from time to
time, of athletes who are able to finish a perfor­
mance despite sprained ankles or even broken
bones. The gate-control theory also correctly pre­
dicts that mild pressure, or other kinds of stimula­
tion, can interfere with severe or protracted pain
by closing the spinal gate. When we vigorously rub
gate-control theory
a banged elbow or apply ice packs, h eat, or stim­
The theory that the expe­
ulating ointments to injuries, we are applying this
rience of pai n depends in
principle.
part on whether pain im­
r('/j)..
Updating the Gate-Control Theory The
gate-control theory has been extremely useful, but
it does not completely explain the many instances
pulses get past a ne uro­
logical "gate" In the spinal
cord and thus reach the
brain .
- -- - - - - -- - - -------------- -- -- -- -- -- - - - - -- -- - - -
200
CHAPTER 6
Sensation and Perception
of severe, chronic pain that occur without any sign
of injury or disease. In the strange phenomenon of
phantom pain, for instance, a erson continues to
feelJ?aiI1JhaLs_e.eminglY.~Qmesj.rQIJi~~at­
ecfTimb or from an organ that has been surgically
amputee m~.y._ie.eUlie Sj:!!!le acliirig,
removed.
b ~i.ug.ro.L.Sba,IrLRain· Irom so!es,_calf cramps,
throbbing toes, or even ingrown toenail s that he
or she endured before the surgery. Even when the
spinal cord has been completely severed, amputees
often continue to report phantom pamrITiITfEfreas
below the break.
ere are mTn~Tve Impurses lor
tlrespmal cora gate to block or let through. So why
is there pain?
These puzzles have led Ronald Melzack (1992,
1993) to revise the gate-control theory. The brain,
he says, not only responds to incoming signals from
sensory nexyes but also is ~le
of generating p'aTn(anaot:fier sensa­
-ttons) enTirelyOn its own . An ex­
tensive matrbnnetworl()Of neurons
in the brain gives us a sense of our
own bodies and body parts. When
this matrix produces abnormal pat­
terns of activity, the result is pain.
Such abnormal patterns can occur
not only because of input from pe­
ripheral nerves, but al so as a result
of memories, emotions, expecta­
tions, or signals from various brain centers. In
the case of phantom pain, the abnormal patterns
may arise because of a lack of sensory stimula­
tion, or because of the person's efforts to move
a nonexistent limb. Evid ence that brain areas as­
sociated with a mis si ng limb continue to func­
tion in its absence is consistent with this view
(Davis et al., 1998).
At present, however, no general theory com­
pletely explains pain, which has turned out to be
extremely complicated, both physiologically and
psychologically. Different kinds of pain (from a pin­
prick, or a bruise, or a stomach ulcer) involve dif­
ferent chemical changes and changes in the activity
of neuron s at the site of injury or disease, and also
in the spinal cord and brain . Genetic differences in
the production of painkilling endorphins affect peo­
ple's perception of pain; a blow experienced as
crushing to one person may seem much milder to
another (Zubieta et aI., 2003). Pain is also affected
by cultural beliefs about whether it is appropriate
. to notice symptoms and express distress, and by
psychological factors, such as stress and a focus on
oneself. It can rise and fall in epidemics, as nation-
An
kinesthesis
[KIN-es-THEE-sis]
The sense of body posi­
tion and movement of
body parts; also cal led
kinesthesia.
equilibrium
The sense of balance ,
al outbreaks of back pain, whipla sh, and repetitive
motion injuries illu strate (Gawande, 1998). The
people who suffer during such epidemics are not
faking it, and their pain is not "j ust in their heads."
But it may be in their brains.
The Environment Within
We usually think of our senses as pipelines to the
"outside" world, but two senses keep us informed
about the movements of our own bodies.
Kinesthesis tells us where our body parts are lo­
cated and lets us know when they move. This in­
formation is provided by pain and pressure
receptors located in the muscle s, joints, and ten­
dons (tissues that connect muscles to bones). With­
out kinesth esis, you could not touch your finger to
your nose with your eyes shut. In fact, you would
have trouble with any voluntary movement. Think
of how hard walking is when your leg has "fallen
asleep" or how clumsy chewing is when a dentist
has numbed your jaw.
Equilibrium, or the sense of balance, gives us
information about our bodies as a whole. Along
with vision and touch, it lets us know whether we
are standing upright or on our heads and tells us
when we are falling or rotating. Equilibrium relies
Dancers, divers, and gymnasts turn their kinesthetic
talents into artistry.
Sensation and Perception
primarily on three semicircular canals in the
inner ear (see Figure 6.8 on page 194). These thin
tubes are filled with fluid that moves and presses on
hairlike receptors whenever the head rotates. The
receptors initiate messages that travel through a
part of the auditory nerve not involved in hearing.
Normally, kinesthesis and equilibrium work
together to give us a sense of our own physical
reality, something we take utterly for granted but
should not. Oliver Sacks (1985) told the heart­
breaking story of Christina, a young British
woman who suffered irreversible damage to her
kinesthetic nerve fibers because of a mysterious
inflammation. At first. Christina was as floppy as
a rag doll; she could not sit up, walk, or stand.
Then, slowly, she learned to do these things, re­
CHAPTER 6
201
lying on visual cues and sheer willpower. But her
movements remained unnatural; she had to grasp
a fork with painful force or she would drop it.
More important, despite her remaining sensitivi­
ty to light touch on the skin, she could no longer
experience herself as physically embodied: "It's
like something's been scooped right out of me,
right at the centre .. .. "
With equilibrium, we come, as it were, to the
end of our senses. Every second, millions of sen­
sory signals reach the brain, which combines and
integrates them to produce a model of reality from
moment to moment. How does it know how to do
this? Are our perceptual abilities inborn, or must we
learn them? We turn next to this issue.
Can you make sense of the following sensory problems?
1. April always has trouble tasting foods, especially those with subtle flavors. What is the most likely
explanation of her difficulty?
2. May has chronic shoulder pain. How might the gate-control theory and its revision explain her pain?
3. June, a rock musician, does not hear as well as she used to. What is a likely explanation?
Answers:
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• Do babies see the world the way adults do?
• What psychological motives could cause people
to "see" the face of a religious figure on a cin­
namon bun?
Perceptual Powers:
Origins
and Influences
What happens when babies first open their eyes?
Do they see the same sights, hear the same sounds,
smell the same smells, taste the same tastes as an
adult does? Are their strategies for organizing the
world wired into their brains from the beginning?
Or is an infant's world, as William James once sug­
gested, only a "blooming, buzzing confusion," wait­
ing to be organized by experience and learning?
The truth lies somewhere between these two
extremes.
Inborn Abilities
In human beings, most basic sensory abilities, and
many perceptual skills, are inborn or develop quite
early. Infants can distinguish salty from sweet and
can discriminate among odors. They can distinguish
a human voice from other sounds. They will star­
tle to a loud noise and turn their heads toward its
source, showing that they perceive sound as being
semicircular canals
Sense organs in the inner
ear, which contribute to
equilibrium by responding
to rotation of the head,