4. ROLE
OF NERVE REGULATION FOR BEHAVIOUR
The Nervous System
The central nervous system’s neurons in the brain and spinal cord
communicate with the peripheral nervous system’s
sensory and motor neurons. The peripheral nervous system has two main divisions.
The somatic nervous system directs voluntary movements and reflexes. The
autonomic nervous system, through its sympathetic and parasympathetic
divisions, controls our involuntary muscles and glands. Like people clustering
into neighborhoods, neurons cluster into working networks.
The central
nervous system includes the brain and the spinal cord. The central nervous
system is so critical to your ability to function that it is entirely protected
by bone—the brain by your skull and the spinal cord by your spinal column. As
an added measure of protection, the brain and spinal cord are suspended in
cerebrospinal fluid to protect them from being jarred. The central nervous
system is aptly named. It is central to all your behaviors and mental
processes. And it is the central processing center—every action, thought,
feeling, and sensation you experience is processed through the central nervous
system. The most important element of the central nervous system is, of course,
the brain, which acts as the command center. We’ll take a tour of the human
brain in a later section. Think of the spinal cord as an old-fashioned but very
busy telephone switchboard, handling both incoming and outgoing messages.
Sensory receptors send messages along sensory nerves to the spinal cord, then
up to the brain. To activate muscles, the brain sends signals down the spinal
cord, which are relayed out along motor nerves to the muscles.
Most
behaviors are controlled by your brain. However, the spinal cord can produce
spinal reflexes—simple, automatic behaviors that occur without any brain
involvement. One of the simplest spinal reflexes involves a three-neuron loop
of rapid communication—a sensory neuron that communicates sensation to the
spinal cord, an interneuron that relays information within the spinal cord, and
a motor neuron leading from the spinal cord that signals muscles to react.
Spinal
reflexes are crucial to your survival. The additional few seconds that it would
take you to consciously process sensations and decide how to react could result
in serious injury. Spinal reflexes are also important as indicators that the
neural pathways in your spinal cord are working correctly. That’s why
physicians test spinal reflexes during neurological examinations by tapping
just below your kneecap for the knee-jerk spinal reflex or scratching the sole
of your foot for the toe-curl spinal reflex.
Reflex - a simple, automatic, inborn response to a
sensory stimulus.
The Peripheral Nervous System
The
peripheral nervous system is the other major division of your nervous system. The
word peripheral means “lying at the outer edges.” Thus, the peripheral nervous
system comprises all the nerves outside the central nervous system that extend
to the outermost borders of your body, including your skin. The communication
functions of the peripheral nervous system are handled by its two subdivisions:
the somatic nervous system and the autonomic nervous system. The somatic
nervous system takes its name from the Greek word soma, which means “body.” It
plays a key role in communication throughout the entire body. First, the
somatic nervous system communicates sensory information received by sensory
receptors along sensory nerves to the central nervous system. Second, it
carries messages from the central nervous system along motor nerves to perform
voluntary muscle movements. All the different sensations that you’re
experiencing right now are being communicated by your somatic nervous system to
your spinal cord and on to your brain. When you perform a voluntary action,such as turning a page of
this book, messages from the brain are communicated down the spinal cord, then
out to the muscles via the somatic nervous system. The other subdivision of the
peripheral nervous system is the autonomic nervous system. The word autonomic
means “self-governing.” Thus, the autonomic nervous system regulates
involuntary functions, such as heartbeat, blood pressure, breathing, and
digestion. These processes occur with little or no conscious involvement. This
is fortunate, because if you had to mentally command your heart to beat or your
stomach to digest the pizza you had for lunch, it would be difficult to focus
your attention on anything else.
However, the
autonomic nervous system is not completely self-regulating. By engaging in
physical activity or purposely tensing or relaxing your muscles, you can
increase or decrease autonomic activity. Emotions and mental imagery also
influence your autonomic nervous system. Vividly imagining a situation that
makes you feel angry, frightened, or even sexually aroused can dramatically
increase your heart rate and blood pressure. A peaceful mental image can lower
many autonomic functions. The involuntary functions regulated by the autonomic
nervous system are controlled by two different branches: the sympathetic and
parasympathetic nervous systems. These two systems control many of the same
organs in your body but cause them to respond in opposite ways. In general, the
sympathetic
nervous system arouses the body to expend energy, and the parasympathetic
nervous system helps the body conserve energy.
The
sympathetic nervous system is the body’s emergency system, rapidly activating
bodily systems to meet threats or emergencies. When you are frightened, your
breathing accelerates, your heart beats faster, digestion stops, and the
bronchial tubes in your lungs expand. All these physiological responses
increase the amount of oxygen available to your brain and muscles. Your pupils
dilate to increase your field of vision, and your mouth becomes dry, because
salivation stops. You begin to sweat in response to your body’s expenditure of
greater energy and heat. These bodily changes collectively represent the
fight-or-flight response—they physically prepare you to fight or flee from a
perceived danger. Whereas the sympathetic nervous system mobilizes your body’s
physical resources, the parasympathetic nervous system conserves and maintains
your physical resources. It calms you down after an emergency. Acting much more
slowly than the sympathetic nervous system, the parasympathetic nervous system
gradually returns your body’s systems to normal. Heart rate, breathing, and
blood pressure level out. Pupils constrict back to their normal size. Saliva
returns, and the digestive system begins operating again. Although the
sympathetic and parasympathetic nervous systems produce opposite effects, they
act together, keeping the nervous system in balance. Each division handles
different functions, yet the whole nervous system works in unison so that both
automatic and voluntary behaviors are carried out smoothly.
Sympathetic nervous system - branch of the autonomic nervous system
that produces rapid physical arousal in response to perceived emergencies or
threats.
Parasympathetic nervous system - branch of the autonomic nervous system
that maintains normal bodily functions and conserves the body’s physical
Sympathetic
“Fight or flight”
“E” division
Exercise, excitement, emergency, and embarrassment
Parasympathetic
“Rest and digest”
“D” division
Digestion, defecation, and diuresis
Endocrine glands communicate information from one part of the body to
another by secreting messenger chemicals called hormones into the bloodstream.
The hormones circulate throughout the bloodstream until they reach
specific hormone receptors on target organs or tissue. By interacting with the
nervous system and affecting internal organs and body tissues, hormones
regulate physical processes and influence behavior in
a variety of ways.
Metabolism, growth rate, digestion, blood pressure, and sexual
development and reproduction are just some of the processes that are regulated
by the endocrine hormones. Hormones are also involved in emotional response and
your response to stress.
Nervous & Endocrine System
Similarities:
They both monitor stimuli and react so as to maintain homeostasis.
Differences:
The NS is a rapid, fast-acting system whose effects do not always
persevere.
The ES acts slower and its actions are usually much longer lasting.
The central nervous system includes the brain and the spinal cord.
This is the primary internal communication network of the body; divided
into the central nervous system and the peripheral nervous system.
As the human fetus develops, brain cells
multiply, differentiate, and migrate to their final locations. By the fourth week of prenatal development,
new neurons are being generated at the rate of 500,000 per minute. By 24 weeks
of prenatal age, the brain has nearly its full complement of neurons. After
birth, the neurons grow in size and continue to develop new dendrites. Myelin
forms on neuron axons in key areas of the brain, such as those involved in
motor control. Axons also grow longer, and the branching at the ends of axons
becomes more dense. By adulthood, the fully mature
human brain weighs about three pounds.
The Endocrine System
The endocrine system, one of the body’s communication systems, is a
kindred system to the nervous system. Its glands release hormones at a slower
rate than neurotransmitters, resulting in a longer lasting effect. The feeling outlasts
the thought. However, the two systems are so closely interconnected that the
distinction between them is sometimes difficult to decipher.
Endocrine
glands communicate information from one part of the body to another by
secreting messenger chemicals called hormones into the bloodstream. The
hormones circulate throughout the bloodstream until they reach specific hormone
receptors on target organs or tissue. By interacting with the nervous system
and affecting internal organs and body tissues, hormones regulate physical
processes and influence behavior in a variety of ways. Metabolism, growth rate,
digestion, blood pressure, and sexual development and reproduction are just
some of the processes that are regulated by the endocrine hormones. Hormones are
also involved in emotional response and your response to stress. Endocrine
hormones are closely linked to the workings of the nervous system. For example,
the release of hormones may be stimulated or inhibited by certain parts of the
nervous system. In turn, hormones can promote or inhibit the generation of
nerve impulses. Finally, some hormones and neurotransmitters are chemically
identical. The same molecule can act as a hormone in the endocrine system and
as a neurotransmitter in the nervous system. In contrast to the rapid speed of
information transmission in the nervous system, communication in the endocrine
system takes place much more slowly. Hor mones rely on the circulation of the blood to deliver their
chemical messages to target organs, so it may take a few seconds or longer for
the hormone to reach its target organ after it has been secreted by the
originating gland. The signals that trigger the secretion of hormones are
regulated by the brain, primarily
by a
brain structure called the hypothalamus. The hypothalamus serves as the main
link between the endocrine system and the nervous system. The hypothalamus
directly regulates the release of hormones by the pituitary gland, a pea-sized
gland just under the brain. The pituitary’s hormones, in turn, regulate the
production of other hormones by many of the glands in the endocrine system.
This is why the pituitary gland is often referred to as the body’s master
gland. Under the direction of the hypothalamus, the pituitary gland controls
hormone production in other endocrine glands.
The
pituitary gland also produces some hormones that act directly. For example, the
pituitary produces growth hormone, which stimulates normal skeletal growth
during childhood. In nursing mothers, the pituitary produces both prolactin,
the hormone that stimulates milk production, and oxytocin, the hormone that
produces the let-down reflex, in which stored milk is “let down” into the
nipple. Interestingly, the pituitary gland can also secrete endorphins to
reduce the perception of pain. Another set of glands, called the adrenal
glands, is of particular interest to psychologists. The adrenal glands consist
of the adrenal cortex, which is the outer gland, and the adrenal medulla, which
is the inner gland. Both the adrenal cortex and the adrenal medulla produce
hormones that are involved in the human stress response. On stress, hormones
secreted by the adrenal cortex also interact with the immune system, the body’s
defense against invading viruses or bacteria. The adrenal medulla plays a key
role in the fight-or-flight response, described earlier. When aroused, the
sympathetic nervous system stimulates the adrenal medulla. In turn, the adrenal
medulla produces epinephrine and norepinephrine.
As they
circulate through the bloodstream to the heart and other target organs,
epinephrine and norepinephrine complement and enhance the effects of the
sympathetic nervous system. These hormones also act as neurotransmitters,
stimulating activity at the synapses in the sympathetic nervous system. The
action of epinephrine and norepinephrine is a good illustration of the
long-lasting effects of hormones. If you’ve noticed that it takes a while for
you to calm down after a particularly upsetting or stressful experience, it’s
because of the lingering effects of epinephrine and norepinephrine in your
body.
The Brain
Clinical observations have long revealed the general effects of damage
to various areas of the brain. But CT and MRI scans now reveal brain
structures, and EEG, PET, and functional MRI recordings reveal brain activity.
By surgically lesioning or electrically stimulating
specific brain areas, by recording the brain’s surface electrical activity, and
by displaying neural activity with computer-aided brain scans, neuroscientists
explore the connections among brain, mind, and behavior.
Older Brain Structures
The brainstem begins where the spinal cord swells to form the medulla,
which controls heartbeat and breathing. Within the brainstem, the reticular
formation controls arousal. Atop the brainstem is the thalamus, the brain’s
sensory switchboard. The cerebellum, attached to the rear of the brainstem,
coordinates muscle movement.
Between the brainstem and cerebral cortex is the limbic system, which is
linked to memory, emotions, and drives. One of its neural centers, the
amygdala, is involved in responses of aggression and fear. Another, the
hypothalamus, is involved in various bodily maintenance functions, pleasurable
rewards, and the control of the hormonal system.
The Hindbrain
The hindbrain
connects the spinal cord with the rest of the brain. Sensory and motor pathways
pass through the hindbrain to and from regions that are situated higher up in
the brain. Sensory information coming in from one side of the body crosses over
at the hindbrain level, projecting to the opposite side of the brain. And
outgoing motor messages from one side of the brain also cross over at the
hindbrain level, controlling movement and other motor functions on the opposite
side of the body.
This
crossover accounts for why people who suffer strokes on one side of their brain
experience muscle weakness or paralysis on the opposite side of their body. Our
friend Asha, for example, suffered only minor damage
to motor control areas in her brain. However, because the stroke occurred on
the left side of her brain, what muscle weakness she did experience was
localized on the right side of her body, primarily in her right hand.
Three
structures make up the hindbrain—the medulla, the pons, and the cerebellum. The
medulla lies directly above the spinal cord and contains centers active in the
control of such vital autonomic functions as breathing, heart rate, and
digestion. Because the medulla is involved in such critical life functions,
damage to it can result in death. The medulla also controls a number of vital
reflexes, such as swallowing, coughing, vomiting, and sneezing. Above the
medulla is a swelling of tissue called the pons, which represents the uppermost
level of the hindbrain. Bulging out behind the pons is the large cerebellum. On
each side of the pons, a large bundle of axons connects it to the cerebellum.
The word pons means “bridge,” and the pons is a bridge of sorts: Information
from various other brain regions located higher up in the brain is relayed to
the cerebellum via the pons. The cerebellum functions in the control of
balance, muscle tone, and coordinated muscle movements. It is also involved in
the learning of habitual or automatic movements and motor skills, such as
typing, writing, or gracefully backhanding a tennis ball.
Jerky,
uncoordinated movements can result from damage to the cerebellum. Simple
movements, such as walking or standing upright, may become difficult or
impossible. The cerebellum is also one of the brain areas affected by alcohol
consumption, which is why a person who is intoxicated may stagger and have
difficulty walking a straight line or standing on one foot. (This is also why a
police officer will ask a suspected drunk driver to execute these normally
effortless movements.) At the core of the medulla and the pons is a network of
neurons called the reticular formation, or the reticular activating system. The
reticular formation is composed of many groups of specialized neurons that
project up to higher brain regions and down to the spinal cord. The reticular
formation plays an important role in regulating attention and sleep.
The Midbrain
The midbrain
is an important relay station that contains centers important to the processing
of auditory and visual sensory information. Auditory sensations from the left
and right ears are processed through the midbrain, helping you orient toward
the direction of a sound. The midbrain is also involved in processing visual
information, including eye movements, helping you visually locate objects and
track their movements. After passing through the midbrain level, auditory and
visual information is relayed to sensory processing centers farther up in the
forebrain region, which will be discussed shortly. A midbrain area called the substantia nigra is involved in
motor control and contains a large concentration of dopamine-producing neurons.
Substantia nigra means
“dark substance,” and as the name suggests, this area is darkly pigmented. The substantia nigra is part of a
larger neural pathway that helps prepare other brain regions to initiate
organized movements or actions. In the section on neurotransmitters, we noted
that Parkinson’s disease involves symptoms of abnormal movement, including
difficulty initiating or starting a particular movement. Many of those
movement-related symptoms are associated with the degeneration of
dopamine-producing neurons in the substantia nigra.
The Forebrain
Situated
above the midbrain is the largest region of the brain: the forebrain. In
humans, the forebrain, also called the cerebrum, represents about 90 percent of
the brain. The size of the forebrain has increased during evolution, although
the general structure of the human brain is similar to that of other species
(Clark & others, 2001). Many important structures are found in the
forebrain region, but we’ll begin by describing the most prominent—the cerebral
cortex.
The Cerebral Cortex
The outer
portion of the forebrain, the cerebral cortex, is divided into two cerebral
hemispheres. The word cortex means “bark,” and much like the bark of a tree,
the cerebral cortex is the outer covering of the forebrain. A thick bundle of
axons, called the corpus callosum, connects the two cerebral hemispheres. The
corpus callosum serves as the primary communication link between the left and
right cerebral hemispheres. The cerebral cortex is only about a quarter of an
inch thick. It is mainly composed of glial cells and neuron cell bodies and
axons, giving it a grayish appearance—which is why the cerebral cortex is
sometimes described as being composed of gray matter. Extending inward from the
cerebral cortex are white myelinated axons that are
sometimes referred to as white matter. These myelinated
axons connect the cerebral cortex to other brain regions. Numerous folds,
grooves, and bulges characterize the human cerebral cortex. The purpose of
these ridges and valleys is easy to illustrate. Imagine a flat, three-foot by threefoot piece of paper. You can compact the surface area
of this piece of paper by scrunching it up into a wad. In much the same way,
the grooves and bulges of the cerebral cortex allow about three square feet of
surface area to be packed into the small space of the human skull.
Each hemisphere of the cerebral cortex—the neural fabric that covers the
hemispheres—has four geographic areas: the frontal, parietal, occipital, and
temporal lobes. Small, well-defined regions within these lobes control muscle
movement and receive information from the body senses. However, most of the
cortex—its association areas—is uncommitted to such functions and is therefore
free to process other information.
Some brain regions serve specific functions. The brain divides its labor
into specialized subtasks and then integrates the various outputs from its
neural networks. Thus, our emotions, thoughts, and behaviors result from the
intricate coordination of many brain areas. Language, for example, depends on a
chain of events in several brain regions. If one hemisphere is damaged early in
life, the other will pick up many of its functions, thus demonstrating the
brain’s plasticity. The brain becomes less plastic later in life. Frequently,
however, nearby neurons can partially compensate for damaged ones, as when a
person recovers from a stroke or brain injury.
Many
psychological processes, particularly complex ones, involve multiple brain
structures and regions. Even seemingly simple tasks—such as carrying on a
conversation or catching a ball—involve the smoothly coordinated synthesis of
information among many different areas of your brain. How is information
communicated and shared among these multiple brain regions? Many brain
functions involve the activation of neural pathways that link different brain
structures. Neural pathways are formed by groups of neuron cell bodies in one
area of the brain that project their axons to other brain areas. These neural
pathways form communication networks and circuits that link different brain
areas. As a result, damage to one area of the brain may disrupt many neural
pathways and affect many different functions. Thus, even though we’ll talk
about brain centers and structures that are involved in different aspects of
behavior, the best way to think of the brain is as an integrated system. As
part of our tour, let’s start with an overview of the methods that have been
used to study the human brain.
Each
cerebral hemisphere can be roughly divided into four regions, or lobes: the
temporal, occipital, parietal, and frontal lobes. Each lobe is associated with
distinct functions. Located near your temples, the temporal lobe contains the
primary auditory cortex, which receives auditory information. At the very back
of the brain is the occipital lobe. The occipital lobe includes the primary
visual cortex, where visual information is received. The parietal lobe is involved
in processing bodily, or somatosensory, information, including touch,
temperature, pressure, and information from receptors in the muscles and
joints. A band of tissue on the parietal lobe, called the somatosensory cortex,
receives information from touch receptors in different parts of the body. Each
part of the body is represented on the somatosensory cortex, but this
representation is not equally distributed. Instead, body parts are represented
in proportion to their sensitivity to somatic sensations. For example, your
hands and face, which are very responsive to touch, have much greater
representation on the somatosensory cortex than do the backs of your legs,
which are far less sensitive to touch. If body areas were actually proportional
to the amount of representation on the somatosensory cortex, humans would
resemble the misshapen. The largest lobe of the cerebral cortex, the frontal
lobe, is involved in planning, initiating, and executing voluntary movements.
The movements of different body parts are represented in a band of tissue on
the frontal lobe called the primary motor cortex. The degree of representation
on the primary motor cortex for a particular body part reflects the diversity
and precision of its potential movements.
Thus, it’s
not surprising that almost one-third of the primary motor cortex is devoted to
the hands and another third is devoted to facial muscles. The disproportionate
representation of these two body areas on the primary motor cortex is reflected
in the human capacity to produce an extremely wide range of hand movements and
facial expressions. The primary sensory and motor areas found on the different
lobes represent just a small portion of the cerebral cortex. The remaining bulk
of the cerebral cortex consists mostly of three large association areas. These
areas are generally thought to be involved in processing and integrating
sensory and motor information. For example, the prefrontal association cortex,
situated in front of the primary motor cortex, is involved in the planning of
voluntary movements. Another association area includes parts of the temporal,
parietal, and occipital lobes. This association area is involved in the
formation of perceptions and in the integration of perceptions and memories.
Studying the Brain. The Toughest Case to Crack
Imagine how
difficult it would be to try to figure out how something works without being
able to open it, take it apart, or watch it operate. Such has long been the
challenge faced by scientists investigating the workings of the human brain.
Because the brain is encased entirely by bone, it has been impossible to
directly observe a normal, living brain in action. One early approach to
mapping brain functions involved examining the bumps on a person’s skull. As
you can see in Science Versus Pseudoscience Box 2.1, this approach was not very
successful. Another obstacle is the complexity of the brain itself—complex not
only in its enormous number of interconnected neurons, but also in the
intricate structures, regions, and pathways formed by those neurons. But
scientists are not an easily discouraged lot.
Some of the
oldest methods of studying the brain are still commonly used. In the case
study, researchers systematically observe and record the behavior of people
whose brains have been damaged by illness or injury. Case studies of
individuals with brain damage have provided valuable insights into behavior in
such areas as memory, speech, emotion, movement, and personality. As you’ll see
later, the knowledge gained from such observations allowed scientists to begin
mapping the functions of the brain. However, generalizing results from a case
study must be done cautiously. By their very nature, case studies usually focus
on unusual situations or behaviors— in this case, brain disease or injury.
Because these behaviors or situations are out of the ordinary, they may not
reflect typical behavior. Another potential limitation to using case studies in
brain research is that injuries to the brain are rarely limited to specific,
localized areas or contained within well-defined anatomical boundaries. It’s
often difficult to be sure exactly which brain area is responsible for specific
behavioral problems. In addition, many brain areas are linked to other brain
areas, and damage in one area may disrupt functioning in another, otherwise
normal area.
A related
research method involves producing lesions—surgically altering, removing, or
destroying specific portions of the brain—and observing subsequent behavior. In
humans, lesions are sometimes produced for medical reasons, such as when part
of the brain is surgically altered or removed to relieve uncontrollable
seizures. Following such medical treatment, researchers can study the
behavioral effects of the lesions. Lesions are sometimes produced in animals to
systematically investigate the behavioral effects of damage in specific brain
areas.
Researchers
have also studied the behavioral effects of electrically stimulating specific
brain areas. This procedure usually involves implanting tiny electrified disks
or wires, called bipolar electrodes, into a specific brain area. Electrical
stimulation causes activation of the neurons in the area around the tip of the
electrode and usually produces the opposite behavioral effect of a lesion in
the same brain area.
The
invention of the electroencephalograph allowed scientists to record the brain’s
electrical activity through the use of large, disk-shaped electrodes placed
harmlessly on a person’s scalp. The graphic record of the brain’s electrical
activity that this instrument produces is called an electroencephalogram,
abbreviated as EEG. Modern electroencephalographs provide sophisticated
computerized analyses of the brain’s electrical activity, recording the
electrical activity of the brain from millisecond to millisecond. As technology
has become more advanced, so have the tools used to study the brain. In the
Focus on Neuroscience, we take a look at the new imaging techniques that allow
neuroscientists to see the human brain at work.
The Developing Brain
New Neurons
Throughout Life? Our guided tour will follow the same general sequence that the
brain follows in its development before birth. The human brain begins as a
fluid-filled neural tube that forms about two weeks after conception.
Gradually, the neural tube expands and develops into separate, fluid-filled
cavities, called ventricles, which are at the core of the fully developed
brain. Cerebrospinal fluid is manufactured in the ventricles by special glial
cells. We noted previously that cerebrospinal fluid acts as a shock absorber
for the central nervous system and cushions the brain.
As the human
fetus develops, brain cells multiply, differentiate, and migrate to their final
locations. By the fourth week of prenatal development, new neurons are being
generated at the rate of 500,000 per minute. By 24 weeks of prenatal age, the
brain has nearly its full complement of neurons. These neurons will continue to
function for decades throughout the person’s lifespan.
The fetal
brain is constantly changing, forming as many as 2 million synaptic connections
per second. Connections that are used are strengthened, while unused
connections are pruned (Rakic, 1995). Progressively,
the three major regions of the brain develop: the hindbrain, the midbrain, and
the forebrain. Over the course of fetal development, the forebrain structures
eventually come to surround and envelop the hindbrain and midbrain structures.
At birth,
the infant’s brain weighs less than a pound and is only about one-fourth the
size of an adult brain. After birth, the neurons grow in size and continue to
develop new dendrites. Myelin forms on neuron axons in key areas of the brain,
such as those involved in motor control. Axons also grow longer, and the
branching at the ends of axons becomes more dense. By adulthood, the fully
mature human brain weighs about three pounds.
For many
years, scientists believed that people and most animals did not experience
neurogenesis—the development of new neurons—after birth.With
the exception of birds, tree shrews, and some rodents, it was thought that the
mature brain could lose neurons but could not grow new ones. But new studies
offered compelling evidence that persuaded most neuroscientists to abandon that
dogma (Gross, 2000).
First,
research by psychologist Elizabeth Gould and her colleagues (1998) showed that
adult marmoset monkeys were generating a significant number of new neurons
every day in the hippocampus, a brain structure that plays a critical role in
the ability to form new memories. Gould’s groundbreaking research provided the
first demonstration that new neurons could develop in an adult primate brain.
Could it be that the human brain also has the capacity to generate new neurons
in adulthood? Researchers Peter Eriksson, Fred Gage, and their colleagues (1998)
provided evidence that it does. The subjects were five adult cancer patients,
whose agesranged from the late fifties to the early
seventies. These patients were all being treated with a drug to determine
whether tumor cells are multiplying. The drug is incorporated into newly
dividing cells and colors the cells. Under fluorescent light, this chemical
tracer can be detected in the newly created cells. The reasoning was that if
new neurons were being generated, the drug would be present in their genetic material.
Within hours
after each patient died, an autopsy was performed and the hippocampus was
removed and examined. The results were unequivocal. In each patient, hundreds
of new neurons had been generated since the drug had been administered, even
though all the patients were over 50 years old (see photo at left). The
conclusion? Contrary to the traditional scientific view, the hippocampus has
the capacity to generate new neurons throughout the lifespan (Eriksson &
others, 1998; Kempermann & Gage, 1999).
Is the
capacity to generate new neurons limited to just the hippocampus? Not according
to later research by Gould and her colleagues (1999b), which showed that adult
macaque monkeys continually develop new neurons that migrate to multiple brain
locations. These brain areas are involved in sophisticated cognitive abilities,
including memory, learning, and decision making. In the next section, we’ll
continue our guided tour of the brain. Following the general sequence of the
brain’s development, we’ll start with the structures at the base of the brain
and work our way up to the higher brain regions, which are responsible for
complex mental activity.
Commonly Used Brain-Imaging Techniques
Positron emission tomography, or a PET scan, generates images of the brain’s
activity by tracking the brain’s use of a radioactively tagged compound, such
as glucose, oxygen, or a particular drug. An invasive procedure, PET involves
injecting participants with a radioactive substance before the scan. The PET
scan then measures the amount of the radioactively tagged substance used in
thousands of brain areas while the person engages in some type of mental
activity. Over the course of several minutes, the information is collected,
analyzed, and averaged by computer. In the resulting color-coded images, the
areas of greatest brain activity are indicated by red and yellow colors.
Magnetic resonance imaging, or MRI, is a noninvasive procedure that provides
highly detailed images of the body’s internal structures, including the brain.
MRI is very versatile, producing thin “slice” images of body tissue from
virtually any angle. As the person lies motionless in a long magnetic tube,
powerful but harmless magnetic fields bombard the brain or other body area. In
response to these magnetic fields, the molecules of the body generate
electromagnetic signals, which are analyzed by computer to create the highly
detailed images. Tissues with high concentrations of water, such as fat, appear
lighter in color, while bone and other tissues with less water appear darker.
Functional magnetic resonance imaging, or fMRI, provides moment-by-moment
images of the brain’s changing activity. Using the same scanning hardware as an
MRI, fMRI also tracks changes in the brain’s blood flow and oxygen levels. Compared
to PET scans, fMRI produces a much sharper picture and can detail much smaller
brain structures. Another advantage of fMRI is that it provides a picture of
brain activity averaged over seconds rather than the several minutes required
by PET scans. Because fMRI is a noninvasive procedure, researchers can
repeatedly scan a single subject. In an fMRI image, the areas of greatest brain
activity are indicated by red and orange colors.
Brain scan
images have become so commonplace in news articles and popular magazines that
it’s easy to forget just how revolutionary brain imaging technology has been to
the field of neuroscience (Posner & DiGirolamo,
2000). Shown above are the three types of brain-imaging techniques most
commonly used in psychological research— PET scans, MRI, and functional MRI,
which is abbreviated fMRI. The descriptions explain how each brain-imaging
technique works and the kind of information it provides. How Psychologists Use
Brain-Imaging Technology Like other scientific data-gathering methods, brain
imaging is used for both descriptive and experimental research. A descriptive
study utilizing brain scans might compare the brain structure or functioning of
one carefully defined group of people with another. For example, MRI scans were
used to compare
How is brain imaging used in cognitive
neuroscience research?
In a typical experiment, a brain scan is taken during a control task or
condition, such as lying down with eyes closed. In the top row of PET scans in
the image on page 63, the control condition is shown in the middle PET scan. In
that particular study, the control condition consisted of resting while staring
at a fixed point. The control scan is compared to brain scans taken while the
participant is exposed to the experimental treatment or performing the
experimental task. In the top row of PET scans, the first PET scan is the
treatment task, which is labeled “Stimulation.” More specifically, the
treatment task in that study was looking at a flickering checkerboard pattern.
The difference between the PET scans is calculated to determine the brain
activity that can be attributed to the experimental condition (Gusnard & Raichle, 2001). In
the chapters to come, you’ll see several examples of experimental research that
use brain-imaging techniques.
Potential Limitations of Brain-Imaging Studies
As
technological advances occur, brain-imaging technology continues to improve,
offering increasingly detailed pictures of the intact living brain.
Nevertheless, brain-imaging research has some limitations. When you consider
the results of brain-imaging studies, including those presented in this
textbook, keep the following points in mind:
1. Most
brain-imaging studies involve small groups of subjects. Because of the limited
availability of sophisticated equipment and the high cost of brain-imaging
technology, brain-scan research tends to involve small groups of subjects,
often as few as few as a dozen or less. As is true with any research that
involves a small number of participants, caution must be exercised in
generalizing results to a wider population.
2. Most
brain-imaging studies involve simple aspects of behavior. Human behavior is
extraordinarily complex, and even seemingly simple tasks involve the smooth
coordination of multiple brain regions. Reading this paragraph, for example,
activates visual, language, memory, and auditory centers in your brain. As
psychologist William Uttal (2001) observes, “The more
complex the psychological process, the less likely it is that a narrowly
circumscribed [brain] region uniquely associated with that process will be
found.”
3. Knowing
what brain area is involved may tell us little about the psychological process
being investigated. Knowing the brain location of a psychological process does
not necessarily translate into an understanding of that process. For example,
identifying a particular brain structure as being involved in fear does little
to explain our psychological experience of fear (Miller & Keller, 2000).
Snapshots of brain activity can only be interpreted within the context of
psychological knowledge about the behavior being studied.
Looking at Brain-Scan Images
What should you notice when you look
at a brain scan image? First, read the text description carefully, so that you
understand the task or condition that is being measured. Second, when a controlcondition brain scan is shown, carefully compare the
control scan with the treatment scan, noting how the two scans differ. Third,
keep the limitations of brain-scan technology in mind, remembering that human
experience is much too complex to be captured by a single snapshot of brain
activity. Although brain-imaging research has its limitations, the advent of
sophisticated imaging technology has revolutionized our understanding of the
human brain. But brain-imaging technology has also revealed just how much
remains to be discovered about the most complex piece of matter known to exist
in the universe—the human brain.
Our Divided Brain
Clinical observations long ago revealed that the left cerebral
hemisphere is crucial for language. Experiments on people with a severed corpus
callosum have refined our knowledge of each hemisphere’s special functions.
Separately testing the two hemispheres, researchers have confirmed that in most
people the left hemisphere is the more verbal, and that the right hemisphere
excels in visual perception and the recognition of emotion. Studies of healthy
people with intact brains confirm that each hemisphere makes unique
contributions to the integrated functioning of the brain.
Biological psychology is the scientific study of the biological bases of
behavior and mental processes. This area of research is also called
biopsychology. Both terms emphasize the idea of a biological approach to the
study of psychological processes.
Biological psychology is one of the scientific disciplines that makes
important contributions to neuroscience—the scientific study of the nervous
system. As neuroscientists, biopsychologists bring
their expertise in behavior and behavioral research to this scientific
endeavor.
Some of the other scientific disciplines that contribute to neuroscience
include physiology, pharmacology, biology, and neurology.
Communication throughout the nervous system takes place via
neurons—cells that are highly specialized to receive and transmit information
from one part of the body to another.
By the end
of the 1700s it had already been well established that injury to one side of the brain could produce muscle
paralysis or loss of sensation on the opposite side of the body. By the early 1800s, animal experiments had shown that specific functions would be lost if particular
brain areas were destroyed. And, as discussed in
In the
1860s, more conclusive evidence for cortical localization was gathered by a
French surgeon and neuroanatomist named Pierre Paul Broca. Broca treated a series of patients who had great
difficulty speaking but could comprehend written or spoken language. Subsequent autopsies of these patients revealed a consistent finding—brain damage to
an area on the lower left frontal lobe. Today,
this area on the left hemisphere is referred to as Broca’s
area, and it is known to play a crucial role in
speech production.
About a
decade after Broca’s discovery, a young German
neurologist named Karl Wernicke discovered another
area in the left hemisphere that, when damaged, produced a different type of language disturbance. Unlike Broca’s patients, Wernicke’s
patients had great difficulty understanding spoken or written communications. They could speak quickly and easily, but their
speech sometimes made no sense. They sometimes used
meaningless words or even nonsense syllables, though their sentences seemed to be grammatical. In response to the
question “How are you feeling?” a patient
might say something like, “Don’t glow glover. Yes, uh, ummm,
bick, bo chipickers the dallydoe mick more
work mittle.” Autopsies of these patients’ brains revealed consistent damage to an
area on the left temporal lobe that today is
called Wernicke’s area.
The
discoveries of Broca and Wernicke provided the first
compelling clinical evidence that language and speech functions
are performed primarily by the left cerebral
hemisphere. If similar brain damage occurs in the exact same locations on the right hemisphere, these severe
disruptions in language and speech are usually not
seen.
The notion
that one hemisphere exerts more control over or is more involved in the processing of a particular psychological
function is termed lateralization of function.
Speech and language functions are lateralized on the left hemisphere. Generally, the left hemisphere exerts greater
control over speech and
language abilities in
virtually all right-handed and the majority of left-handed people.
The language
disruptions demonstrated by Broca’s and Wernicke’s
patients represent different types of
aphasia. Aphasia refers to the partial or complete inability to articulate ideas or understand spoken or written language
because of brain injury or damage. There are many different types of
aphasia. People with Broca’s aphasia
find it difficult or impossible to produce speech, but their comprehension of verbal or written words is
relatively unaffected. People with Wernicke’s
aphasia can speak, but they often have trouble finding the correct words and have great difficulty comprehending
written or spoken communication. In more severe
cases of Wernicke’s aphasia, speech can be characterized by nonsensical, meaningless, incoherent words, as
in the example given earlier. At the
beginning of this chapter, we described the symptoms experienced by our friend Asha in
the weeks before and the months following her stroke. Asha, who is right-handed, experienced the stroke in
her left hemisphere. About three days after her
stroke, an MRI brain scan showed where the damage had occurred: the left temporal lobe.
Asha
experienced many symptoms of Wernicke’s aphasia. Talking was difficult, not because Asha couldn’t speak, but because
she had to stop frequently to search for the
right words. Asha was unable to name even simple
objects, like the cup on her hospital dinner tray or
her doctor’s necktie. She recognized the objects but was unable to say what they were. She had great difficulty following
a normal conversation and understanding
speech, both in English and in her native language, Tulu. Asha also discovered that she had lost
the ability to read. She could see the words on the
page, but they seemed to have no meaning. Paul brought some of their Christmas cards to the hospital. Asha recalls, “When I realized I couldn’t read the Christmas cards, I thought my life was
over. I just lost it. I remember crying and telling the nurse, ‘I have a doctorate and I can’t read,
write, or talk!’ ” When we visited Asha
in the hospital, we brought her a Christmas present: a portable cassette tape player with headphones and some tapes of
relaxing instrumental music. Little did we realize how
helpful the music would be for her. One tape was a
recording of Native American flute music called Sky of Dreams. The music was beautiful and rather unusual,
with intricate melodies and unexpected, complex harmonies. Although it was very difficult for Asha to follow normal speech, listening to Sky of Dreams was an entirely different experience.
As Asha explained: I tried cranking up the music very high and it soothed me. I could sleep. At the time, the flute music seemed to be just perfectly timed with the way
my brain was working. It was tuning out
all the other noises so I could focus on just one thing and sleep. So I would play the music over and over again
at a very high level. I did that for a
long time because my mind was so active and jumbled that I couldn’t think.
Asha’s
language functions were severely disrupted, yet she was able to listen to and appreciate instrumental music—even very
complex music. Why? At the end of the next section, we’ll offer a possible explanation for what
seems to have been a disparity in Asha’s cognitive abilities following her stroke.
Cutting the Corpus Callosum. The
Since the discoveries by Broca and Wernicke, the most dramatic evidence illustrating the independent functions of the two cerebral
hemispheres has come from
a surgical procedure
called the split-brain operation. This operation is used to stop or reduce recurring seizures in severe
cases of epilepsy that can’t be treated in any other fashion. The procedure involves surgically cutting the corpus
callosum, the thick band of axons that
connects the two hemispheres. What was the
logic behind cutting the corpus callosum? An epileptic seizure typically occurs when neurons begin firing in a
disorganized fashion in one region of the brain.
The disorganized neuronal firing quickly spreads from one hemisphere to the other via the corpus callosum. If the
corpus callosum is cut, seizures should be
contained in just one hemisphere, reducing their severity or eliminating them altogether. This is exactly what happened
when the split-brain operation was first tried
in this country in the 1940s (Springer & Deutsch, 1998). Surprisingly, cutting the corpus callosum
initially seemed to produce no noticeable effect on the patients, other than reducing their epileptic seizures.
Their ability to engage in routine
conversations and tasks seemed to be unaffected. On the basis of these early observations, some brain researchers speculated
that the corpus callosum served no function
whatsoever (see Gazzaniga, 1995). One famous psychologist, Karl Lashley,
joked that its primary function seemed to be to keep the two hemispheres from sagging (Hoptman
& Davidson, 1994). In the 1960s, however, psychologist
and neuroscientist Roger Sperry and his colleagues
began unraveling the puzzle of the left and right
hemispheres. Sperry and his colleagues used the
apparatus shown in Figure below to test the abilities of split-brain patients.
They would direct a split-brain subject to focus on a point in the middle of a screen, while briefly flashing a word or
picture to the left or right of the midpoint. In
this procedure, visual information to the right of the midpoint is projected to
the person’s left hemisphere, and visual information to the left of the
midpoint is projected to the right hemisphere.
Behind the screen several objects were hidden from the split-brain subject. The subject could reach under a partition
below the screen to pick up the concealed
objects but could not see them (Sperry, 1982). In a typical experiment, Sperry
projected the image of an object concealed behind the screen, such as a hammer,
to the left of the midpoint. Thus, the image of the hammer was sent to the
right, nonverbal hemisphere. If a split-brain subject was asked to verbally
identify the image flashed on the screen, he could not do so and often denied
that anything had appeared on the screen. Why? Because his verbal left
hemisphere had no way of knowing the information that had been sent to his
right hemisphere.
However, if
a split-brain subject was asked to use his left hand
to reach under the partition for the object that had been displayed, he would correctly pick up the hammer. This was
because his left hand was controlled by the same
right hemisphere that saw the image of the hammer. Sperry’s experiments reconfirmed the specialized language abilities of
the left hemisphere that Broca and Wernicke had discovered more than a hundred years earlier. But notice, even though the
split-brain subject’s right hemisphere could not
express itself verbally, it still processed information and expressed itself nonverbally: The subject was able to pick up
the correct object. Over the last four decades,
researchers have gained numerous insights about the brain’s lateralization of functions by studying split-brain patients,
using brainimaging techniques with normal subjects, and other techniques (Gazzaniga, 1998).
On the basis
of this evidence, researchers have concluded that—in most people— the left hemisphere is superior in language
abilities, speech, reading, and writing. In contrast, the right hemisphere is more involved in nonverbal
emotional expression and visual-spatial tasks (Corballis & others, 2002). Deciphering complex visual cues, such as completing a puzzle or
manipulating blocks to match a particular design, also relies on right-hemisphere processing (Gazzaniga,
1995). And the right hemisphere excels in
recognizing faces and emotional facial cues, reading maps, copying designs, and drawing (Heller & others, 1998;
Reuter-Lorenz & Miller, 1998). Finally, the right
hemisphere shows a higher degree of specialization for musical appreciation or responsiveness—but not necessarily for
musical ability, which involves the use of the left
hemisphere as well (Springer & Deutsch, 1998). Figure summarizes the research findings for the different specialized
abilities of the two hemispheres for
right-handed people. As you look at the figure, it’s important to keep two points in mind. First, the
differences between the left and right hemispheres are
almost always relative differences, not absolute differences. In other words, both hemispheres of your brain are activated to some extent as you
perform virtually any task (Beeman
& Chiarello, 1998; Chabris
& Kosslyn, 1998). In the normal brain, the left and right hemispheres function in an integrated fashion, constantly
exchanging information (Banich,
1998). Thus, the hemisphere that typically displays
greater activation or exerts greater control
over a particular function. Second, many functions of the cerebral hemispheres, such as those involving the primary sensory and motor areas, are
symmetrical. They are located in the same place and
are performed in the same
way on both the left
and the right hemisphere.
Given the basic
findings on the laterality of different functions in the two hemispheres, can you speculate about why Asha was unable
to read or follow a simple conversation but could easily concentrate on a complex piece of music? Why were her language abilities so disrupted,
while her ability to focus on and appreciate music remained
intact after her stroke? A plausible explanation has to do with the location of the stroke’s damage on Asha’s
left temporal lobe. Because language functions are
usually localized on the left hemisphere, the stroke produced serious disruptions in Asha’s
language abilities. However, her right
cerebral hemisphere sustained no detectable
damage. Because one of the right hemisphere’s abilities is the appreciation of musical sounds, Asha
retained the ability to concentrate on and
appreciate music.
Plasticity. The Malleable Brain
In our
exploration of the biological foundations of behavior,
we’ve traveled from the activities of individual neurons to the complex interaction of the
billions of neurons that make up the human
nervous system, including the brain. Crucial to the development of a scientific understanding of brain functioning
were two areas—localization and
lateralization. Phrenology’s incorrect
interpretation of bumps on the head helped trigger scientific debate on the notion of localization—that different functions
are localized in different brain areas. The early
clinical evidence provided by Broca and Wernicke, and the later split-brain
evidence provided by Sperry and his colleagues, confirmed the idea of lateralization—that some functions are performed primarily by one cerebral hemisphere. The ideas of localization and lateralization
are complemented by another theme evident in this chapter—integration. Despite the high degree of
specialization
in the human
nervous system, the smooth functioning of the nervous system demands an equally high degree of integration
and harmony.Your ability to process new information and experiences, your memories
of previous experiences, your sense of who
you are and what you know, your actions and reactions—all are products of your brain working in harmony with the rest
of your nervous system.
The story of Asha’s stroke illustrated what can happen when this harmony is disrupted. Your physical survival and
conscious experience are mediated by the delicate
balance of chemicals and the complex, intricate connections in your nervous system. Asha
survived her stroke, but many people who suffer strokes do not. Of those who do survive a stroke, about one-third are left with
severe impairments in their ability to function.
What
happened to Asha? Fortunately, Asha’s
story has a happy ending. She was luckier than many stroke
victims—she was young, strong, and otherwise healthy. Asha’s recovery was also aided by her
high level of motivation, willingness to work hard,
and sheer will to recover. After being discharged from the hospital, Asha began
months of intensive speech therapy. Her speech therapist assigned a great deal of homework that
consisted of repeatedly pairing pictures with words, objects with words, and words with objects. Asha set a very high goal for herself: to return to
teaching at the university by the fall semester.With the help of her husband, Paul, and her
mother, Nalini, who traveled from