Science Brain and Behavior contiuned 7

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Visit the Brain and Behavior Web site
(www.worthpublishers.com/kolb)
and go to the Web links for additional
information about the evolution of the
brain.
The juxtaposition of excitation and inhibition is central to how the nervous system
produces behavior. The same principle that governs behavior governs the activity
of individual neurons. Neurons can pass on information to other neurons either by
being active or by being silent. That is, they can be “on” or “off.” Some neurons in the
brain function primarily to excite other neurons, whereas others function to inhibit
other neurons. These excitatory and inhibitory effects are produced by various neurochemicals.
Just as individual neurons can act to excite or inhibit other neurons, brain nuclei
(or layers) can do the same to other nuclei (or layers). These actions are especially obvious
in the motor systems. Inhibiting reading and initiating walking to the telephone,
for example, result from the on and off actions of specific motor-system nuclei.
Now imagine that one of the nuclei that normally inhibits some movement is injured.
The injury creates an inability to inhibit that particular response. This symptom
can be seen in people with frontal-lobe injury who are often unable to inhibit talking
at inappropriate times or cursing.Recall the story of Phineas Gage in Chapter 1. In contrast,
people with injury to excitatory areas cannot initiate movement. Injury in an area
that normally initiates speech can leave a person unable to talk at all.
Thus, brain injury can produce either a loss of behavior or a release of behavior.
Behavior is lost when the damage prevents excitatory instructions; behavior is released
when the damage prevents inhibitory instructions.
Principle 6: The Central Nervous System
Functions on Multiple Levels
Similar sensory and motor functions are carried out in various
parts of the CNS—that is, the spinal cord, brainstem, and forebrain.
But why are multiple areas with overlapping functions
needed? It seems simpler to put all the controls for a certain function
in a single place. Why bother with duplication? It turns out
that, as the brain evolved, new areas were added but old ones were
retained, as described in “Optimizing Connections in the Brain” at
the beginning of this chapter. The simplest solution has been to
add new structures on top of existing ones.
We see this “descent with modification” solution in the evolution from primitive
vertebrates to amphibians to mammals. Primitive vertebrates, such as lampreys,make
only whole-body movements to swim—movements controlled by the spinal cord and
hindbrain. Amphibians developed legs and corresponding neural control areas in the
brainstem. Land mammals later developed new capacities with their limbs, such as independent
limb movements and fine digit movements. These movements, too, required
new control areas, which were selected for in the forebrain.We therefore find
three distinct areas of motor control in mammals: the spinal cord, the brainstem, and
the forebrain.
A century ago, John Hughlings-Jackson suggested that the addition of new brain
structures in the course of evolution could be viewed as adding new levels of nervous
system control. The lowest level is the spinal cord, the next level is the brainstem,
and the highest level is the forebrain. These levels are not autonomous, however. To
move the arms, the brainstem must use circuits in the spinal cord. Similarly, to make
independent movements of the arms and fingers, such as in tying a shoelace, the cortex
must use circuits in both the brainstem and the spinal cord. Each new level offers
a refinement and elaboration of the motor control provided by one or more lower
levels.
John Hughlings Jackson
(1835–1911)
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68 ! CHAPTER 2
We can observe the operation of functional levels in the behavior of people with
brain injuries. Someone whose spinal cord is disconnected from the brain cannot voluntarily
move a limb, because the brain has no way to control the movement. But the
limb can still move automatically to withdraw reflexively from a noxious stimulus, because
the circuits for moving the muscles are still intact in the spinal cord. Similarly, if
the forebrain is not functioning but the brainstem is still connected to the spinal cord,
a person can still move, but the movements are relatively simple: there is limited limb
use and no digit control.
The principle of multiple levels of function can also be applied to the cortex in
mammals, which evolved by adding new areas, mostly sensory-processing ones. The
newer areas essentially added new levels of control that provide more and more abstract
analysis of inputs. Consider the recognition of an object, such as a car. The simplest
level of analysis recognizes the features of this object such as its size, shape, and
color. A higher level of analysis recognizes this object as a car. And an even higher level
of analysis recognizes it as Susan’s car with a dent in the fender. Probably the highest
functional levels are cortical regions that substitute one or more words for the object
(SUV for an automobile, for instance) and can think about the car in its absence.
When we consider the brain as a structure composed of multiple levels of function,
these levels clearly must be extensively interconnected to integrate their processing
and create unified perceptions or movements. The nature of this connectivity in
the brain leads to the next principle of brain function: the brain has both parallel and
hierarchical circuitry.
Principle 7: Brain Systems Are Organized Both
Hierarchically and in Parallel
The brain and spinal cord are semiautonomous areas organized into functional levels,
and, even within a single level, more than one area may take part in a given function.
How then,with these different systems and levels, do we eventually obtain a unified conscious
experience? Why, when we look at Susan’s car, do we not have the sense that one
part of the brain is processing features such as shape while another part is processing
color? Or why,when we tie our shoelaces, are we not aware that different levels of motor
control are at work to move our arms and fingers and coordinate their actions?
These questions are part of the binding problem, which focuses on how the brain
ties together its various activities into a whole perception or behavior. The solution to
the binding problem must somehow be related to the ways in which the parts of the
nervous system are connected. The two alternative possibilities for “wiring” the nervous
system are serial or parallel circuits.
A serial circuit hooks up in a linear series all the regions concerned with a given
function, as shown in Figure 2-33A. Consider seeing Susan’s car again. In a serial system,
the information from the eyes would go first to a region (or regions) that performs
the simplest analysis—for example, the detection of specific properties, such as color
and shape. This information would then be passed on to another region that sums up
the information and identifies a car. The information would next proceed to yet another
region that compares this car with stored images and identifies it as Susan’s car.
Notice how the perceptual process entails the hierarchical flow of information sequentially
through the serial circuit from simple to complex.
One difficulty with hierarchical models, however, is that functionally related structures
in the brain are not always linked in a linear series. Although the brain has many
serial connections, many expected connections are missing. For example, within the
visual system, not all cortical regions are connected to one another. The simplest
Binding problem. A theoretical
problem with the integration of sensory
information. Because a single sensory
event is analyzed by multiple parallel
channels that do not converge on a single
region, there is said to be a problem in
binding together the segregated analyses
into a single sensory experience.
Figure 2-33
Models of Neural Information
Processing (A) Simple serial
hierarchical model of cortical processing
similar to that first proposed by
Alexandre Luria, who conceptualized
information as being organized by the
brain into three levels: primary,
secondary, and tertiary. (B) In Daniel
Felleman and David van Essen’s
distributed hierarchical model, multiple
levels exist in each of several processing
streams. Areas at each level interconnect.
(A) Primary
Secondary
Tertiary
(B)
Level 4
Level 3
Level 2
Primary
Level 3 Level 3
Level 4
Level 2
Level 4 Level 4
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HOW DOES THE NERVOUS SYSTEM FUNCTION? ! 69
explanation is that the unconnected regions must have very different functions, as we
shall see in Chapter 8.
Another solution to the binding problem is to imagine multiple hierarchical systems
that operate in parallel but are also interconnected. Figure 2-33B illustrates the
flow of information in such a distributed hierarchy. If you trace the information flow
from the primary area to levels 2, 3, and 4, you can see the parallel pathways. These
multiple parallel pathways are also connected to each other. However, the connections
are more selective than those that exist in a purely serial circuit.
The visual system provides a good example of such parallel hierarchical pathways.
Let’s return once again to Susan’s car. As we look at the car door, one set of visual pathways
processes information about its nature, such as its color and shape, whereas another
set of pathways processes information about door-related movements, such as
those required to open it.
These two visual systems are independent of each other, with no connections between
them. Yet your perception when you pull the door open is not one of two different
representations—the door’s size, shape, and color on the one hand, and the
opening movements on the other.When you open the door, you have the impression
of unity in your conscious experience.
Interestingly, the brain is organized into multiple parallel pathways in all its subsystems.
Yet our conscious experiences are always unified.We will return to this conundrum,
as well as to the binding problem, at the end of this book. For now, keep in mind
that your commonsense impressions of how the brain works may not always be correct.
Principle 8: Functions in the Brain Are Both
Localized and Distributed
In our consideration of brain organization, we have so far assumed that functions can
be localized in specific parts of the brain. This assumption makes intuitive sense, but
it turns out to be controversial.One of the great debates in the history of brain research
has been about what aspects of different functions are actually localized in specific
brain regions.
Perhaps the fundamental problem is that of defining a function. Consider language,
for example. Language includes the comprehension of spoken words, written
words, signed words (as in American Sign Language), and even touched words (as in
Braille). Language also includes processes of producing words orally, in writing, and by
signing, as well as constructing whole linguistic compositions, such as stories, poems,
songs, and essays.
Because the function that we call language has many aspects, it is not surprising
that these aspects reside in widely separated areas of the brain.We see evidence of this
widespread distribution in language-related brain injuries. People with injuries in different
locations may selectively lose the abilities to produce words, understand words,
read words, write words, and so forth. Specific language-related abilities, therefore, are
found in specific locations, but language itself is distributed throughout a wide region
of the brain.
Memory provides another example of this same distributed pattern.Memories can
be extremely rich in detail and can include sensual material, feelings, words, and much
more. Like language, then, aspects of memory are located in many brain regions distributed
throughout a vast area of the brain.
Because many functions are both localized and distributed in the brain, damage to
a small brain region produces only focal symptoms. Massive brain damage is required to
completely remove some function. For instance, a relatively small injury could impair
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some aspect of language functioning, but it would take a very widespread injury to
completely remove all language abilities. In fact, one of the characteristics of dementing
diseases, such as Alzheimer’s, is that people can endure widespread deterioration of
the cortex yet maintain remarkably normal language functions until late stages of the
disease.
SUMMARY
How can we view the nervous system for functional analysis? In contrast with a twopart
anatomical organization, the human nervous system can be viewed as composed
of three semiautonomous functional divisions. The central nervous system includes
the brain and the spinal cord. The somatic nervous system consists of the spinal nerves
that enter and leave the spinal column, going to and from muscles, skin, and joints in
the body and of the cranial nerves that link the CNS to the head, neck, and internal organs.
The autonomic nervous system controls the body’s internal organs. Its sympathetic
(arousing) and parasympathetic (calming) divisions, work in opposition to each
other. The law of Bell and Magendie states that tracts and nerves entering a dorsal
structure carry sensory information to and from receptors in the body, whereas nerves
entering and leaving on the ventral side carry motor information to the periphery.
What gross external and internal features constitute the brain? Under the tough,
protective meninges that covers the brain lie its two major structures—the larger cerebrum,
the most recently evolved structure of the nervous system, and the smaller cerebellum.
Both structures are divided into symmetrical hemispheres covered with gyri
(bumps) and sulci (cracks). At the base of the brain, where it joins the spinal cord, the
brainstem is visible, as are the cranial nerves. A frontal section through the brain reveals
the fluid-filled ventricles inside it, as well as the white matter, gray matter, and
reticular matter that make up its tissue. Apparent in sagittal section are the corpus callosum,
which joins the two hemispheres, and beneath it the subcortical structures that
control more basic neural functions in concert with the SNS and ANS.
What are the basic structures and functions of the brainstem? The brainstem consists
of the hindbrain, midbrain, diencephalon, and cerebellum.Within the hindbrain, the
reticular formation activates the forebrain, the pons serves as a bridge from the cerebellum
to the rest of the brain, and the medulla controls such vital functions as breathing.
In the midbrain, the tectum, a dorsal structure, processes information from the
eyes and ears and produces orienting movements related to these sensory inputs. Ventral
to the tectum, the nuclei of the tegmentum orchestrate movement-related functions.
The diencephalon, or between brain, consists mainly of the thalamus and the
In Review .
Knowing the parts of a brain and some general notions of what they might do is only the
beginning. Learning how the parts work together allows us to proceed to a closer look, in
the chapters that follow, at how the brain produces behavior. We have identified eight
principles about nervous system functioning for review on a regular basis, with an eye toward
understanding the general concept rather than simply memorizing the principle. The
balance created by the whole nervous system through excitation and inhibition, balance
within the functioning brain, and balance within individual cells all work together to produce
behavior.
70 ! CHAPTER 2
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hypothalamus. The thalamus collects sensory inputs from the cranial nerves and brainstem
and sends this information to the cortex, whereas the hypothalamus integrates
the autonomic nervous system with the central control of species-typical behaviors
such as sexual activity and feeding.
What are the important structures and functions of the forebrain? The forebrain is the
largest, most anterior region of the brain. Its wrinkled outer surface is the neocortex.
Sulci, especially deep ones called fissures, form the anatomical boundaries of the four
lobes on each cerebral hemisphere: frontal, parietal, temporal, and occipital. The cells
of the cortex form six distinctive layers based on their specialized sensory,motor, or integrative
functions. Extensive interconnections to other brain regions are essential to
the directing role of the cortex in top-down neural processing. The basal ganglia, lying
just below the white matter of the cortex, interact with the brainstem, primarily in directing
movement. Also important in the forebrain are the structures of the limbic system.
The amygdala regulates emotional behavior, whereas the hippocampus and the
cingulate cortex both have roles in memory, in motivation, and in orienting and navigating
the body in space.
How does the somatic nervous system work? The cranial nerves of the somatic nervous
system link the muscles of the face and some internal organs to the brain. Some
cranial nerves are sensory, some are motor, and some combine both functions. The
spinal nerves transmit afferent sensory input from the skin, muscles, and joints of the
body to the CNS. Efferent connections to the skeletal muscles give the SNS control over
the body’s movements on the side where the nerves are located. The spinal cord functions
as a kind of minibrain for the peripheral (spinal) nerves that enter and leave its
five spinal regions. Each region works relatively independently, although CNS fibers interconnect
them and coordinate their activities.
How does the autonomic nervous system work? The ANS acts either to activate (sympathetic
division) or to inhiibit (parasympathetic division) the body’s internal organs
through two parallel divisions that balance out internal acitivity. The parasympathetic
division directs the organs to “rest and digest,” whereas the sympathetic division prepares
for “fight or flight.”
What basic principles govern nervous system functioning? Knowing how the parts of
the nervous system work together helps us to understand how the brain produces behavior.
Here are eight guiding principles:
1. The sequence of processing within the brain is “in ! integrate ! out.” The term
integrate refers to the creation of new information as cells, nuclei, and brain layers
sum the inputs that they receive from different sources.
2. Sensory and motor functions are separated throughout the nervous system, not
just in the periphery but in the brain as well.
3. Most brain circuits are crossed, meaning that the right cerebral hemisphere is connected
to the left side of the body, whereas the left hemisphere is connected to the
body’s right side.
4. The brain, though largely symmetrical, also has asymmetrical organization appropriate
for controlling tasks such as language and spatial navigation.
5. The nervous system works through a combination of excitatory and inhibitory
signals.
6. The nervous system operates on multiple levels of function, which range from
older or more primitive to higher functional levels that evolved more recently.
Tasks are often duplicated among these multiple levels.
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72 ! CHAPTER 2
REVIEW QUESTIONS
1. What are the three primary functions of the brain?
2. What features of the brain are visible from the outside?
3. Contrast the anatomical and functional divisions of the nervous system.
4. Expand the Bell and Magendie law to include the entire nervous system.
5. In what sense is the nervous system crossed?
6. In what sense is the activity of the nervous system a summation of excitatory
and inhibitory processes?
7. What does it mean to say that the nervous system is organized into levels of
function?
FOR FURTHER THOUGHT
In the course of studying the effects of the removal of the entire cerebral cortex on the
behavior of dogs, Franz Goltz noticed that the dogs were still able to walk, smell, bark,
sleep, withdraw from pain, and eat. He concluded that functions must not be localized
in the cerebrum, reasoning that only widely distributed functions could explain how
the dogs still performed all these behaviors despite so much lost brain tissue. On the
basis of the principles introduced in this chapter, how would you explain why the dogs
behaved so normally in spite of having lost about one-quarter of the brain?
RECOMMENDED READING
Diamond, M. C., Scheibel, A. B., & Elson, L. M. (1985). The human brain coloring book.
New York: Barnes & Noble. Although a coloring book might seem an odd way to learn
7. Brain circuits are organized to process information both hierarchically and in
parallel.
8. Functions exemplified by memory and language are both localized and distributed
in the brain.
KEY TERMS
neuroscience interact ive
There are many resources available for
expanding your learning online:
www.worthpublishers.com/kolb
Try the Chapter 2 quizzes and
flashcards to test your mastery of the
chapter material. You’ll also be able to
link to some other sites that will
reinforce what you’ve learned.
www.brainmuseum.org/evolution/
index.html
Link to brain atlases that include
photographs, stained sections, and
movie clips from humans, monkeys,
and even a dolphin. Note the
differences among the brains (the
increasing complexity as you move
from mouse to human) as well as the
similarities (try to find brain nuclei
that are the same across species).
www.williamcalvin.com/1990s/
1998SciAmer.htm
Read an article about the theory of
brain evolution from Scientific
American.
On your Foundations CD-ROM, you’ll
be able to view an entire module on the
Central Nervous System. This module
includes a rotatable, three-dimensional
brain and many sections of the brain
through which you can move with the
click of a mouse. In addition, the
Research Methods module includes
various computer tomographic and
magnetic resonance images of the
brain, including a video clip of a
coronal MRI scan.
afferent, p. 40
autonomic nervous system
(ANS), p. 38
basal ganglia, p. 53
binding problem, p. 68
brainstem, p. 42
cerebrospinal fluid (CSF),
p. 40
cerebrum, p. 40
corpus callosum, p. 45
cranial nerve, p. 57
cytoarchitectonic map,
p. 54
dermatome, p. 58
diencephalon, p. 49
efferent, p. 40
excitation, p. 66
forebrain, p. 53
gray matter, p. 42
gyrus (pl. gyri), p. 41
hindbrain, p. 49
hypothalamus, p. 51
inhibition, p. 66
law of Bell and Magendie,
p. 58
limbic system, p. 53
meninges, p. 40
midbrain, p. 49
neocortex (cerebral cortex),
p. 53
nerve, p. 47
nucleus (pl. nuclei), p. 47
orienting movement, p. 51
parasympathetic system,
p. 60
reticular formation, p. 51
somatic nervous system
(SNS), p. 38
stroke, p. 42
sulcus (pl. sulci), p. 41
sympathetic system, p. 60
tectum, p. 51
tegmentum, p. 51
thalamus, p. 53
tract, p. 47
ventricle, p. 42
vertebra, p. 54
white matter, p. 42
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neuroanatomy, many students find this book to be a painless way to study the relations
between brain structures.
Jerison, H. J. (1991). Brain size and the evolution of mind. New York: American Museum of
Natural History.What is the mind and why do we have language? These questions and
many more are discussed by the leading expert in brain evolution. This monograph is a
fascinating introduction to the issues surrounding why the brain grew larger in the
primate evolutionary branch and what advantage a large brain might confer in creating
a richer sensory world.
Heimer, L. (1995). The human brain and spinal cord: Functional neuroanatomy and
dissection guide (2nd ed.). New York: Springer Verlag. If coloring books aren’t your
thing, then Heimer’s dissection guide to the human brain will provide a more
traditional, as well as sophisticated, guide to human brain anatomy.
Luria, A. R. (1973). The working brain. Harmondsworth, England: Penguin. Luria was a
Russian neurologist who studied thousands of patients over a long career. He wrote
a series of books outlining how the human brain functions, of which The Working
Brain is the most accessible. In fact, The Working Brain is really the first human
neuropsychology book. Although many of the details of Luria’s ideas are now outdated,
his general framework for how the brain is organized is substantially correct.
Zeki, S. (1993). A vision of the brain. London: Blackwell Scientific. Humans are visual
creatures. Zeki’s book uses the visual system as a way of introducing the reader to
how the brain is organized. It is entertaining and introduces the reader to Zeki’s ideas
about how the brain functions.
HOW DOES THE NERVOUS SYSTEM FUNCTION? ! 73
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Focus on New Research: Programming Behavior
Cells of the Nervous System
Neurons
Glial Cells
Focus on Disorders: Brain Tumors
Focus on Disorders: Multiple Sclerosis
Internal Structure of a Cell
Elements and Atoms
Molecules
Parts of a Cell
Genes, Cells, and Behavior
Focus on New Research: Knocking out Genes
Chromosomes and Genes
Genotype and Phenotype
Dominant and Recessive Alleles
Genetic Mutations
Mendel’s Principles Apply to Genetic Disorders
Focus on Disorders: Huntington’s Chorea
Chromosome Abnormalities
Genetic Engineering
74 !
C H A P T E R3
What Are the Units of Nervous
System Function?
Left: Dr. Dennis Kunkel/Phototake. Middle: Tom Sanders/The Stock Market.
Right: Phototake.