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Chapter 3
Structures of Mind
In Chapter 2 we traced the story
of nervous system evolution as far as the appearance of vertebrates and invertebrates
and also covered some ideas in evolutionary biology. This chapter moves on
to deal with the evolution and structures of vertebrate nervous systems and
brains, and we begin to discuss how to study the functions of the brain. Some
of the material on brain structures and functions may strike you as being a
digression from our emphasis on evolution, but it is only a temporary one,
necessary for continuing to tell our evolutionary story in subsequent chapters.
Origins and Structures of the Vertebrate
Nervous System
Vertebrates and their nervous systems
appeared 400--500 million years ago. The human nervous system, like that of
fish, amphibians, and reptiles, has two major subdivisions. The central nervous
system (CNS) includes the brain and spinal cord; the peripheral nervous system
(PNS) is everything else (see Figure 3-1). Input occurs both at the periphery,
as with touch receptors in our fingers, and at the level of the brain. The
retinas in our eyes are parts of the brain. Output can involve moving a peripheral
limb, but it also might consist of regulating the diameter of a blood vessel
inside the brain. The sensory branch of the PNS deals with stimuli that impinge
on our skin, as well as internal visceral stimuli such as those associated
with digestion, excretion, and other regulatory functions. The motor portion
of the PNS is conventionally divided into a voluntary or somatic nervous system,
which regulates movements of striated muscles of torso and limbs, and an involuntary
or autonomic nervous system, which is concerned with involuntary functions
such as digestion and blood pressure regulation.
Figure 3-1
Divisions of the vertebrate nervous
system. This figure offers an oversimplified description of the major divisions
of the vertebrate nervous system. The first distinction is between brain and
spinal cord (the central nervous system, or CNS) and any other parts of the
nervous system that are peripheral to these structures (the peripheral nervous
system, or PNS). The PNS then consists of a sensory branch (information coming
into the CNS) and a motor branch (information leaving the CNS to regulate muscle
movements). The further divisions of the motor portion of the PNS that are
shown are described in the text.
The autonomic nervous system becomes
increasingly complex in the transition from lower to higher vertebrates, mirroring
the increased sophistication of behaviors associated with the four basics F's
(fighting, fleeing, feeding, and fornicating). It comprises two sets of motor
systems that act on our body organs with opposing effects. Activation of the
sympathetic nervous system correlates with arousal and energy generation: The
heart beats faster, the liver converts glycogen to glucose, bronchi of the
lung are dilated for greater oxygen transfer capacity, digestion is inhibited,
and secretion of adrenaline from the adrenal medulla is stimulated. Activity
of the parasympathetic system is approximately the mirror image of this: a
calming and a return to emphasis on vegetative, self-maintenance functions.
Heart rate decreases and energy storage and digestion are enhanced.
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DESIGN NOTE: IMPORTANT POINT
The activities of our sympathetic
and parasympathetic nervous systems correlate with our two major areas of activity:
engagement and arousal versus rest and rebuilding of resources.
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We also have a secondary brain
in our bodies: the gut brain or enteric nervous system. It has an ancient origin,
for the first nervous systems occurred in tubular animals that sat on rocks
and waited for food to pass by. As more complex brains and central nervous
systems evolved for finding food and sex, the gut's nervous system stayed where
it acted, rather than moving to the newer central structures. This is the "brain" that
gives us butterflies in our stomachs when we are about to go on stage. When
the central nervous system gets anxious, the enteric brain mirrors this agitation.
The enteric brain forms independently of our central nervous system during
our early embryonic development and only later becomes connected to the brain
proper via a cable, the vagus nerve.
Layers of the Brain
The deepest structure of the vertebrate
brain, which evolved over 500 million years ago, is the enlargement of the
top of the spinal cord that consists of the hindbrain, which generates the
medulla, pons, and cerebellum, and the midbrain, which forms the tectum (see
Figure 3-2). This part of the brain handles breathing, heart rate, and other
basic bodily functions necessary for survival. It also regulates general levels
of alertness and monitors important environmental information such as the presence
of food, predators, or a potential mate. We share with primitive vertebrates
a so-called "little brain," or cerebellum, which attaches to the
rear of the hindbrain and is important in coordinating movement and memories
for learned responses. In our brains it also participates in some of the higher
cognitive functions. Like our cerebral cortex and many other structures in
the brain, it is bilateral, having a left and a right lobe. Finally, the forward
portion of the brain, the forebrain, consisting of the thalamus, hypothalamus,
and cortex, is much smaller in fish and reptiles than it is in higher vertebrates.
Figure 3-2
Major divisions of the
vertebrate brain. The hindbrain comprises the medulla (which contains centers
that regulate several autonomic visceral functions such as breathing, heart
and blood vessel activity, swallowing, vomiting, and digestion), the pons (which
also participates in some of these activities), and the cerebellum. All of
the sensory and motor information that is communicated between brain and body
passes through the medulla and pons. The cerebellum plays a central role in
sensing and regulating movement and posture. The midbrain has centers for receiving
and processing sensory information, particularly auditory and visual information,
and is important in regulating states of arousal. The forebrain consists of
the diencephalon (which contains the thalamus and hypothalamus) and the telencephalon.
The thalamus controls access to the cerebral cortex, and the hypothalamus performs
regulatory functions described in the text. The bottom portion of the telencephalon
forms the olfactory cortex, the hippocampus, the amygdala, and other components
of the limbic system. In mammals the top of the telencephalon forms the "new
cortex," or neocortex.
The tiny hypothalamus serves as
the Health Maintenance Organization of the body, regulating its homeostasis,
or stable state of equilibrium. The hypothalamus also generates behaviors involved
in eating, drinking, general arousal, rage, aggression, embarrassment, escape
from danger, pleasure and copulation. It does an amazing number of housekeeping
chores for such a small piece of tissue. Its lateral and anterior parts seem
to support activation of the parasympathetic nervous system: drop in blood
pressure; slowing of pulse; and regulation of digestion, defecation, assimilation,
and reproduction in such a way as to contribute on the whole to rest and recovery.
The medial and posterior hypothalamus regulate activation: acceleration of
pulse and breathing rates, high blood pressure, arousal, fear and anger. Stimulation
of specific groups of cells in these areas can elicit pure behaviors. For example,
rats placed in an experimental situation where they can press a lever to stimulate
a pleasure center will do so to the exclusion of eating and drinking. Stimulation
of another area can produce rage.
We can be described, to use a model
proposed by the neuroanatomist Paul MacLean, as having a three-part brain,
or triune brain, consisting of a reptilian brainstem core, an old mammalian
brain called the limbic system, and the neocortex. Figure
3-3 shows MacLean's symbolic representation of the model, and Figure 3-6 (see
next section) indicates how the inner limbic portions of the cortex are overlaid
by the external neocortex. The limbic system appeared between 200 and 300 million
years ago and was initially dominated by smell input. Corresponding structures
are found in reptiles and birds. One component of the limbic system, the hippocampus,
plays a key role in storing memories. Another, the amygdala, mediates emotional
reactions relevant to survival.
Figure 3-3
The model of
the triune brain proposed by Paul McLean, indicating brain areas added during
vertebrate evolution. The reptilian brain is the main seat of innate or instinctive
behaviors regulating primitive survival issues. The old mammalian brain,
or the limbic system, expresses innate motivational value systems that interact
with the newer cortex, or neocortex, which manages propositional information
and declarative knowledge about the world.
The triune brain model suggests
that we think of ourselves as having a hierarchy of three brains in one.
These can act semi-independently but still are interconnected and functionally
dependent on each other. During evolution, the newer structures have both
encapsulated the older ones and led to their alterations. At the core is
a reptilian brainstem, the reticular formation, and striate cortex. It is
the seat of basic survival behavior patterns of the species that have strong
genetic specification. This is the brain that is crucial in basic hunting,
feeding, and reproductive behaviors. This deep structure in our brains is
analogous to the entire brain of reptiles, and it roughly resembles that
of a 45-day-old human embryo, before the development of higher structures.
A second system, the old mammalian brain, or the limbic system, is the seat
of motives and emotions and is capable of responding to present information
in the light of memories of past information. It is sometimes called the
rhinencephalon to indicate that its regions evolved from structures previously
associated with the sense of smell. (To be sure, many anatomists consider
this model an oversimplification, perhaps even a misleading one, but its
underlying evolutionary perspective is appropriate.)
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DESIGN NOTE: IMPORTANT POINT
Our brain is constructed of layers
that reflect stages in our evolutionary history.
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In trying to understand a structure
as complicated as the brain, scientists have attached names to portions that
can be distinguished by position or shape and then have suggested functions
for these named structures. Although it is hard to imagine proceeding in any
other manner, this approach has been criticized as leading people to accept
the structure-function assignments as established, when in fact they usually
have been informed speculations. Emotions, for example, involve much more than
the limbic system, and parts of the limbic system serve functions not related
to affect, such as declarative memory. Thus it is important to be aware that
terms like "limbic system" are in fact oversimplifications---broad
anatomical names used to substitute for detailed physiological explanations.
It is most useful to make reference to smaller functional neuronal groups whenever
possible, further subdividing currently defined subregions of structures such
as the amygdala, hippocampus, and other functional groups of cortical and visceral
neurons.
The older core structures of the
brain, such as the brainstem and limbic system, are somewhat analogous to a
liver or a kidney in that they are organized less for thought than for automatic
action and for analysis that determines body temperature, blood flow, digestion,
heartbeat, and every blink and swallow, as well as rapid emotional reactions.
The hominid cortex sends cortical projections back down to these lower centers
to generate more "top-down" regulation of pathways that in monkeys
and lower mammals are relatively autonomous. Thus voluntary movements, especially
of the hands, come under more cortical control.
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DESIGN NOTE: IMPORTANT POINT
Newer structures of the brain encapsulate
older ones. Their feedback to lower levels of the brain can modulate the way
in which more ancient structures regulate homeostasis, emotions, and movement.
*********
The Cerebral Cortex
In lower mammals the cerebral cortex
that overlays the limbic system is a simple, smooth structure, but in the sequence
rat -> cat -> monkey -> human,
it increasingly arches into a complex horseshoe shape as a temporal lobe emerges.
This process is shown in Figure 3-4. (The newer cortex, or neocortex, in mammals
is frequently referred to as the cerebral cortex, even though, properly speaking,
the cerebral cortex also contains older, deep-lying structures such as the
limbic system.) The rat neocortex is largely devoted to primary visual, auditory,
and sensorimotor activities. More complicated brains, as in the cat, introduce
more crenulations, or infoldings of the cortex, and association areas appear
that deal with several sensory modalities rather than just one. Crenulations
increase the brain's surface area enormously; the effect is rather like that
of crumpling a piece of paper into a ball so that it can fit in a small space.
Our human cortex, which is deeply furrowed, carries this to an extreme. Increasing
complexity of the cortex seems to correlate with the development of social
interactions within mammalian species, where a need arises to assess the probable
reactions of others to close approach, to compete for food or mates, to distinguish
among individuals, and perhaps remember who is reliable and who is not. Even
though our brains are vastly more complex than rat brains, the myriad anatomical
subdivisions in brainstem, midbrain, and forebrain are similar.
Figure 3-4
Evolution of the mammalian neocortex,
illustrated by drawings of the brains of the rat, cat, monkey, and human. The
most striking changes are the emergence of a temporal lobe, progressive infolding
of the cortex that increases its surface area, and an increase in the size
of the frontal lobe. The drawing of the human cortex shows the four major lobes
of the brain, which are discussed in Figures 3-5 and 3-6.
Figures 3-5 and 3-6 illustrate
the four major divisions, or lobes, found in each hemisphere of our cerebral
cortex: the frontal, temporal, occipital, and parietal lobes. The frontal lobe
is concerned with our working memory, planning for future action, and control
of movement. The temporal lobe, the horseshoe bend that points forward, deals
with hearing as well as aspects of learning, memory, and emotion. The rear
portion of the cortex, which consists of the occipital lobe, deals with vision,
and the parietal lobe in the middle plays a role in somatic sensation, body
image, and analysis of spatial relationships.
Figure 3-5
Regions of the cerebral cortex.
The frontal, parietal, occipital, and temporal lobes are shown. Primary areas
involved with just one modality (such as vision, hearing, sensing the body,
or moving the body) are shaded. The rest of the cortex, the association cortex,
integrates different functions. Also shown are two important functional areas
that we use for understanding and generating speech. Broca's area, near the
primary motor cortex, which controls the mouth and lips, is required for speaking;
and Wernicke's area, near the primary auditory cortex, is involved in comprehending
spoken words.
The cortex is an extensively folded
sheet of nerve cell bodies and their connections that is only about 1/8 inch
thick. The crests of the foldings are called gyri (gyrus, singular); the grooves
are called sulci (sulcus, singular) or fissures. In
the human brain, primary motor, somatic sensory, visual, and auditory cortexes
take up only a relatively small area, as shown in Figure 3-5; the remainder
is association cortex (see the "Association of Functions with Different
Brain Regions" section below). The parietal-temporal-occipital association
cortex located at the junction of these lobes integrates somatosensory, visual,
and auditory input and is involved in language. The limbic association cortex,
comprising adjacent parts of the frontal and temporal lobes, is important in
emotion and memory. The prefrontal association cortex is required for cognitive
behavior and motor planning. It is apparently a locus of working memory---the
temporary storing of information used to guide a future action. In all mammalian
brains, the cortex contains four major connectional clusters formed by visual,
auditory, somato-motor, and frontal-limbic areas. Each of the sensory systems
is hierarchically organized, with a primary sensory area sending information
on to higher stations more closely associated with the frontal-limbic complex.
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DESIGN NOTE: IMPORTANT POINT
Different areas of the brain may
specialize in one kind of activity, but they never work alone. Generating behaviors
requires the collaboration of many different centers; function is spread throughout
the brain.
*********
Figure 3-6
A cutaway view of the cerebral
cortex shown in Figure 3-5, with the extent of the frontal, parietal, and occipital
lobes indicated by brackets. A portion of the left hemisphere in front of the
central sulcus has been cut away to show what the interior looks like. The
drawing of the little homunculus, or person, along this cut indicates how regions
of the body are represented along the surface of the cortex. The neocortex
lies on top of the old mammalian brain (the limbic system, shown by stippling)
and brainstem. The internal part of the cutaway reveals the white matter. Its
color derives from the fatty insulating myelin sheaths that cover masses of
axons going to and from the cortex and connecting different parts of the cortex.
These axons connect to the grey-appearing neurons of the cerebral cortex, or
grey matter.
The cerebral cortex is divided
into two hemispheres, each of which is responsible for the opposite half of
the body. The left hemisphere receives information from, and controls the movement
of, the right side of the body, and vice versa. The two hemispheres communicate
with each other via a band of nerve fibers called the corpus callosum. In the
central top third of each of our cerebral hemispheres, the central sulcus separates
a precentral gyrus concerned with motor function---the primary motor cortex---from
a postcentral somatosensory gyrus that deals with sensing our body surfaces---the
primary somatic sensory cortex. The central sulcus divides our "past" from
our "future" in that posterior sensory areas are registering events
that have occurred, whereas activity in anterior motor areas corresponds to
responses that are about to occur.
Several structures shown in Figure
3-6 contain maps of our body surfaces or of our visual world. (These structures
are central to the discussion of brain plasticity in Chapter 6.) A map of the
opposite side of the body is represented along the cortical folds of both the
precentral and postcentral gyri in each hemisphere. This map of the body surface,
which is usually referred to as a somatotopic map, spans from head to foot
and is shown in Figure 3-6. We know exactly where to insert an electrode in
the left precentral gyrus to command a particular finger of the right hand
to move, and we know where in the right postcentral gyrus to record the sensory
response to scratching a finger of the left hand. Moving toward the rear of
the cortex, we find that another kind of map, a topographical map, or representation
of the external visual world, is plotted along the folds of the primary visual
cortex occupying the rearmost portion of the occipital lobe. In this case we
know, for example, exactly where to place an electrode to record the response
to a small object in any portion of our visual field. This visual map is duplicated
many times across the brain, and over 30 distinct areas deal with aspects of
visual processing. We are very visual animals, and more than half the cells
in our brains process visual information.
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DESIGN NOTE: SELF-EXPERIMENT
In our brains, as in those of all
vertebrates, the two sides of the body are sensed and controlled separately.
The right side of the cortex moves and senses the left side of the body, and
vice versa. You can gain some appreciation of your own bilateral nature by
performing a very simple exercise on yourself. Close your eyes and for several
minutes pay attention just to relaxing the left side of your face, especially
the muscles around your left eye. Please do this before reading further! Now,
note the rest of your left side. Does it feel more relaxed also? If so, it
is probably because changes in the face region of the right precentral and
postcentral gyri spread to include the brain regions that represent the rest
of the left side of the body. Now compare your left and right sides. I suspect
that you will notice a clear difference, the left being more relaxed than the
right. Changes in muscle tone tend to spread along one side of the body, but
not necessarily to the other side. Later, you might like to see whether this
exercise works the same way when you start with the right side of your face.
*************
As you can see by now, brain anatomy
is fiendishly complicated. It used to be said that it takes about 10 years
of study for a bright and eager young person to learn what is known about the
development and position of the various brain nuclei (here a nucleus is a clearly
distinguishable mass of nerve cells). This rule of thumb no longer applies,
because new information is accumulating at such a rate that any normal human
who tries to keep up with it will merely fall farther and farther behind. Furthermore,
it is easy to put considerable effort into learning some finer details of brain
anatomy---and then discover that you have forgotten them within a few months
unless you use them constantly. Rather than trying to keep it all in our heads,
we now go look it up in brain anatomy databases on the Internet. In this book,
we will mostly avoid the problem of knowing and naming structures. Apart from
naming major landmarks, we will not delve into anatomical detail.
MODERN STUDIES OF BRAIN FUNCTION
The brief introduction to brain
anatomy that we have just offered permits us now to begin discussing how brain
function is studied, a topic that will engage us in subsequent chapters of
this book. Modern studies on brain function began in the 19th century against
the backdrop of the phrenology fad, which purported to map different areas
on the head and brain the phrenologists supposed were responsible for complex
human traits such as spirituality, hope, firmness, destructiveness, and the
like (see Figure 3-7). Phrenology
was discredited toward the end of the century, when observations on animals
and humans with brain damage revealed the association of more elemental functions
with specific areas of the cerebral cortex. The legacy of phrenology, however,
still permeates efforts to understand the brain, for even though we no longer
imagine that we will find brain areas corresponding to sprirituality, hope,
or sublimity we still in our common language make reference to assumed "mental" entites
such as willpower, emotion, superego, boredom, reasoning, and so on, and often
take the further dubious step of assuming that these correspond to empirically
established entities in the brain.
In 1861, Broca described patients
who could understand language but could not speak, even though they sometimes
could utter isolated words or sing a melody. They were found, on postmortem
examination, to have a lesion in posterior regions of the frontal lobes of
their left hemispheres, now called Broca's area (see Figure 3-5).
Figure 3-7The 19th-century phrenologists
thought that different contours on a person's head, such as bumps and dips,
were related to different personality traits. Although phrenology was shown
to warrant little credence, it spawned the view that different parts of the
brain may be responsible for different functions and actions. This view is
central to our efforts today, to discover how the different parts of the brain
control different aspects of our lives.
In 1876, Wernicke reported a new
kind of aphasia, or loss of the power to use or comprehend words, that involved
language comprehension rather than execution. Patients could speak, but they
could not understand. The cortical lesion responsible was located on a posterior
part of the temporal lobe now called Wernicke's area (see Figure 3-5). Wernicke
proposed that Broca's area controls the motor programs for coordinating speech,
for it is located just in front of motor areas that control the mouth, tongue,
and vocal cords. Wernicke's area is related to word perception, the sensory
component of language. It is located near the auditory cortex and near the
association cortex that integrates auditory, visual, and somatic sensation
into complex perceptions. Wernicke predicted that a further type of aphasia
would arise if the communication between the two areas was compromised. This
is indeed found in conduction aphasia, which is characterized by an incorrect
use of words, termed paraphasia. Patients with conduction aphasia have lesions
in the fiber pathways that connect Broca's and Wernicke's areas. They can
understand words that are heard and seen but cannot repeat simple phrases.
They omit parts of words, substitute incorrect words, jumble grammar and syntax,
and are painfully aware of their own errors.
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DESIGN NOTE: IMPORTANT POINT
Studies in the 19th century on
patients with brain lesions led to the identification of areas that are important
in the comprehension and generation of speech.
*********
In Germany in 1870, Fritsch and
Hitzig showed that characteristic movements of the limbs can be produced in
dogs by electrically stimulating certain regions of the brain. This sort of
work was refined in this country, in the 1930s, by Woolsey and Rose. They surveyed
a large number of different mammalian and marsupial brains to show that tactile
stimuli elicit responses from discrete regions of the cerebral cortex---fields
of cells that could be defined unambiguously. In the 1950s, Wilder Penfield
used small electrodes to stimulate the cortex of awake patients during brain
surgery, carried out under local anesthesia, for epilepsy. He tested specifically
for areas that produce disorders of language to ensure that the surgery would
not damage the patient's communication skills. He confirmed from patients'
own reports the localizations assigned by Broca's and Wernicke's studies. He
also mapped areas associated with specific sensory modalities and areas the
stimulation of which elicited specific strong memories.
In studying human brains, we have
the considerable advantage that humans can verbally report on what they are
doing, whereas monkeys, cats, and rats cannot. Animal studies are not useless,
however, for if we compare the detailed anatomy of a rat's brain with ours,
we find that over 90% of the same basic groups or clusters of nerve cells are
present in both, even though their shapes and sizes can be quite different.
We can use indirect tricks to show that these clusters appear to serve roughly
corresponding functions---for example, that working memory is located in the
frontal lobes, and that spatial visual representations require the right parietal
lobe. Although areas that represent functions such as these can always be found
in our brains, the variation among different humans is much larger than is
usually realized. The area occupied by primary visual cortex in some individuals
can be double or half the average. The elements of the motor strip aren't always
in the same order, and patches of sensory cortex are sometimes found in front
of the motor strip. This variation can blur our sense of the overall average
organization, but the point is that if we could map one individual brain in
some detail, we might get the impression of considerable specialization, each
little patch of cortex doing a somewhat different job. Such specialization
has been found during neurosurgery and by imaging the activity of living brains.
Association of Functions with Different
Brain Regions
Early mapping studies were most
effective in identifying areas that are the early recipients of sensory information
or the final direct generators of action. They didn't reveal much about the
sophisticated thinking and processing carried out by the various association
areas we have mentioned. Clues to what each of the areas might be doing have
come from studies on people in whom physical trauma, strokes, or cancer have
caused damage confined to a specific region. Such clues have to be interpreted
with some caution, because the effects of brain lesions often are not simple.
For example, we cannot conclude from a brain lesion that alters a particular
behavior that the region the lesion affects is the sole generator of that behavior.
The region might be an essential processing subsystem or might regulate connections
among subsystems. It might reduce overall activation or lead to compensatory
changes. Given these caveats, it is gratifying that many conclusions about
the functions of different brain areas based on lesion studies have now been
confirmed by direct imaging of our living brains (this is described in the
section "Imaging the Activity of the Brain").
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DESIGN NOTE: IMPORTANT POINT
When we remove a resistor from
a radio, a howling sound may result, but this does not prove that the resistor
served the function of being a howl suppressor. Similarly,
loss of a behavior by lesion to a brain region does not prove that this region
is the sole generator of the behavior.
*********
Let's begin with the frontal lobe,
where injury can cause a variety of both obvious and subtle defects. Damage
to its rear portion along the central sulcus, the primary motor cortex, can
cause weakness or paralysis of specific body parts (see Figures 3-5 and 3-6).
The premotor cortex in front of the motor strip has several maps of the body
surface. The lateral portion just in front of the motor strip's area for mouth
and face is Broca's area, where lesions impair speech generation. Premotor
areas, sometimes called supplementary motor areas, interact with numerous other
brain regions to plan sequences of actions, as in linking movements together.
Musicians and athletes couldn't do without them. If you imagine making a finger
movement but don't actually move, the premotor cortex works a lot harder than
the rest of the brain. If you actually move, the motor strip also becomes active.
Patients who have had strokes that injured supplementary motor cortex usually
have trouble with skilled movements such as speech and gestures. They might
be able to perform each action of a sequence separately, but they still have
trouble chaining the actions together into a fluent motion. A common neurological
test is to ask patients to tap their fingers rapidly and then change rhythm,
or to draw a pattern and then switch to drawing a different pattern. Sometimes
patients with premotor problems cannot easily change from one rhythm to another
or switch from drawing one pattern to the next.
The prefrontal cortex is almost
everything in front of the premotor cortex and motor strip. Distractibility
and confabulation are the foremost sign of injury here. For example, a patient
might set out to say something or do something and get easily distracted into
saying or doing something else. Confabulations, a term used in this context
to mean spurious comments or explanations, occur mainly with injuries to the
frontal lobe's inside surfaces, not with injuries to the side or top. Prefrontal
functions include strategy and evaluation, abstract and creative thinking,
fluency of thought and language, volition and drive, selective attention, capacity
for emotional attachments, and social judgment. Patients with prefrontal lesions
sometimes get "stuck" in an ongoing strategy or sequence. Damage
to the bottom surface of the frontal lobe causes storytelling that meanders
or gets obsessive. These localizations of function are vague---not at all comparable,
for example, to the predictable location of hand function in the middle of
the motor strip.
The limbic association cortex is
involved in emotion and memory. Lesions in the frontal lobe portion of the
limbic association cortex, such as prefrontal lobotomy, have been used with
psychotic patients to reduce affect and anxiety. This procedure, along with
other forms of psychosurgery, has now been largely discontinued. Lesions in
the temporal lobe portion of the association cortex impair the formation of
long-term memory. Chapter 10 describes how the limbic association cortex plays
a central role in integrating our rational and emotional minds.
The association areas of the parietal
lobes are involved in attention to the spatial aspects of sensation and in
the manipulation of objects in space. Different lesions cause many different
striking deficits, including abnormalities in body image and perception of
spatial relations. A partial list includes agnosia, the inability to perceive
objects through otherwise normally functioning sensory channels; optic ataxia,
a deficit in using visual guidance to grasp an object; dysgraphia, writing
disability; and dyscalculia, the inability to carry out mathematical calculations,
sometimes along with left-right confusion. Lesions of the nondominant (usually
the right) parietal lobe can cause anosognosia, denial of a part of the body.
Patients with this deficit fail to dress, undress, or wash the affected side,
and they deny that the affected arm or leg belongs to them when the limb is
passively brought into their field of vision. Such patients have lost a sense
of self with respect to these parts of their bodies.
Specializations of the Cerebral
Hemispheres
One of the most fascinating stories
about association of separate functions with different brain areas comes from
observations demonstrating that the two cerebral hemispheres have distinctive
specializations. (Experiments showing that the two hemispheres can in some
circumstances act as separate selves are described in Chapter 7.) The
hemispheres are asymmetrical, the left slightly larger than the right in most
humans. One means of revealing differences between the hemispheres has been
to inject sodium amytal, a fast-acting barbiturate, into the left or right
internal carotid artery. The drug is preferentially carried to the hemisphere
on the same side as the injection and produces a brief period of dysfunction
of that hemisphere. The effects of this procedure on language have revealed
some relationships between handedness and speech functions. Nearly all right-handed,
and most left-handed, people use the left hemisphere for speech, but 15% of
the lefties have right-hemisphere speech, and some left-handed people (about
5%) have control of speech in both the right and left hemispheres. Sodium amytal
tests have an interesting effect on mood. Left injection tends to produce brief
depression, and right injection, euphoria. These effects occur at doses smaller
than those needed to block speech. This observation suggests that functions
related to mood may be lateralized, and further experiments have confirmed
that this is so (see Chapter 10).
Many studies have now demonstrated
functional specializations of the two hemispheres .
The left hemisphere is most adept at language, math, logic operations, and
the processing of serial sequences of information. It has a bias for the detailed,
speed-optimized activities required for skeletal motor control and processing
of fine visual details. The right hemisphere is stronger at pattern recognition,
face recognition, spatial relations, nonverbal ideation, the stress and intonation
component of language, and parallel processing of many kinds of information.
It has a visceral motor bias and deals with large time domains. It appears
to specialize in perception of the relationship between figures and the whole
context in which they occur, whereas the left hemisphere is better at focused
perception. While working with their hands, most right-handed people use the
left hand (right hemisphere) for context or holding and use the right hand
(left hemisphere) for fine detailed movement. Understanding and generating
the stress and intonation patterns of speech that convey its emotional content
is a right-hemisphere function, as is music. Several famous composers have
had strokes in the left hemisphere that rendered them unable to speak, but
they were still able to compose music. It is worth pointing out that cerebral
asymmetry of cognitive abilities is not confined to humans but is observed
in other mammals and also in birds.
*********
DESIGN NOTE: IMPORTANT POINT
A number of our faculties appear
to be concentrated more in one hemisphere than in the other. The left hemisphere
has a bias for focused and detailed, high-speed serial processing, whereas
the right specializes in pattern recognition, nonverbal ideation, and parallel
processing.
*********
Pop psychology in the 1970s carried
the idea of cerebral specialization to such an extreme that one heard people
referring to themselves as "left brain" or "right brain" thinkers,
depending on whether they were more intellectual, rational, verbal, and analytical,
or more emotional, artistic, nonverbal, and intuitive. It is curious that the
latter group of people tend to sit on the right-hand side of classrooms, whereas
the former sit on the left. The
situation is not really so black and white, because it now is clear that in
normal brains, interactions between the hemispheres are required for all major
cognitive functions. There is much evidence that the capacity of one hemisphere
to perform a particular task deteriorates when the corpus callosum connecting
the two hemispheres is cut. Thus, even though lateral specializations are observed,
full competence and expression of these specializations require crosstalk between
the hemispheres and other parts of the brain. The fact that a particular brain
region is required for an activity or competence does not mean that this region
is the sole locus of that activity. It is more likely to be a necessary part
of a larger chain of loci throughout the brain, all of which must cooperate.
Imaging the Activity of the Brain
Until recently, almost everything
we knew about localization of language and other functions came from clinical
studies of patients with brain lesions. Now, however, metabolically or electrically
active regions of the normal living brain can be monitored with noninvasive
procedures such as positron emission tomography (PET scanning), magnetic resonance
imaging (MRI), and electromagnetoencephalography
(MEG). These techniques all
monitor some aspect of the metabolic changes that accompany nerve cell activity,
and their variety and sophistication are increasing each year. As shown in
Figure 3-8, activation of the visual cortex can be observed during the reading
of words. Listening activates the temporal cortex. Broca's area and supplementary
motor areas become more active during speaking. Thinking tasks recruit the
inferior frontal cortex. A region of lateral inferior frontal cortex is activated
during recognition of nonliving objects, whereas medial extra-striate cortex
responds to living things. Brain
regions that become more active during memory, perception, attention, orientation,
emotions, and overall vigilance are being mapped. Imaging
studies suggest that the cortex consists of many small "fields," cortical
processing units between 600 and 3000 cubic millimeters in volume that contain
approximately 4 x 107 neurons and 1012 synapses. There
is great enthusiasm for the prospects of imaging studies, and popular magazines
routinely print beautiful pictures of brain areas that light up during the
performance of different activities. However, experts are questioning a number
of assumptions and raising technical issues that make these studies less definitive
than they may appear at first glance.
Later chapters in this book raise
numerous questions for which imaging experiments provide an answer. For example,
does a word that is read also have to have an auditory representation before
it can be associated with a meaning, or can visual information be transferred
directly to Broca's area? The latter is the case. Wernicke's area becomes active
only when words are heard. The conclusion is that words presented visually
or orally use different brain pathways to reach Broca's area, which is associated
with language output. The PET technique has generated interesting studies on
language localization (indicated in Figure 3-8). A word recognition task, involving
visual input alone, lights up the occipital lobe. Instruction to speak---that
is, to output a word---lights up the temporal-parietal-occipital association
area, and instruction to associate a word with some use lights up frontal lobe
association areas. Active cognitive
processes, such as detection, identification, and use association, can be highlighted
by subtracting from them the activity related to passive recognition of the
stimulus being used.
Figure 3-8
Imaging of cerebral blood flow
during different mental activities, shown by positron emission tomography (PET).
Stippling indicates areas that become more active when the subject is viewing
words, listening to words, speaking words, and generating a verb that indicates
a use for a noun that is presented.
During the use of short-term memory
to retain a list of letters, the left supramarginal gyrus (near the junction
of the parietal and occipital lobes) become more active, and during subvocal
rehearsal (making rhyming judgments for letters), Broca's area is stimulated. During
the viewing of an object, the processing of the color, motion, and form components
of the visual image can be associated with separate areas. Imaging
techniques reveal a convergence of what we see and what we think. The psychologist
Stephen Kosslyn has shown that imagining what a telephone looks like can activate
the primary visual cortex, just as though the same telephone were actually
observed. Imagining a movement
can activate premotor cortex that is active when the movement is actually carried
out.
Imaging studies are being used
to understand mental disease and to follow the effects of clinical intervention.
Drug or behavioral therapy for obsessive-compulsive disorder, which involves
dysfunctional repetitive behaviors such as constant hand washing, can decrease
activity in forebrain structures associated with habit learning and complex
movements. Depression can be
correlated with increased activity of the amygdala (see Chapter 10). In
schizophrenic patients who report auditory hallucinations (hearing voices),
Broca's area becomes active. This observation suggests that auditory hallucinations
have more to do with the generation of words in the brain than with listening
to them (which involves Wernicke's area).
*********
DESIGN NOTE: IMPORTANT POINT
Imaging studies demonstrate that
the hyperactivity of frontal brain regions observed in patients with obsessive-compulsive
disorder can be diminished by drug therapy or by behavioral therapy that trains
the patient to regard symptoms as "a part of my brain that is not working,
that is giving out false brain messages." When the patient's attitude
in response is "I'm not going to let this bothersome part of my brain
run the show," a systematic change in the metabolic activity of the frontal
cortical regions involved occurs.
*********
Imaging experiments performed on
humans as well as other animals suggest that all behaviors, including higher
cognitive and affective mental functions, can be localized to specific regions
or constellations of regions within the brain. We experience mental processes
such as perceiving, thinking, learning, and remembering as continuous and indivisible.
Each is in fact composed of several independent information-processing components
and requires the coordination of several distinct brain areas. A central idea
is that functions localized to discrete regions in the brain are not complex
faculties of mind (the model of the old phrenologists), but rather are elementary
operations. The more elaborate faculties are constructed from the serial and
parallel (distributed) interconnections of several brain regions. This is why
damage to a single area need not lead to the disappearance of a specific mental
function. And even if the function does disappear, it may partially return
because the undamaged parts of the brain reorganize to some extent to perform
the lost function.
Our information on the association
of different brain regions with different kinds of mental activities is becoming
so overwhelming that no one individual could recall it all or keep all of it
in mind at one time. For this reason, several efforts are under way to make
databases on the brain accessible over the Internet, just as databases of DNA
and protein sequences are now available. Thus
an investigator who has conducted some PET scan experiments correlating increased
blood flow in a particular area with a distinct mental activity will be able
to find out readily whether lesions in that area are known to influence the
same activity.
Summary
Present-day humans function with
peripheral and central nervous systems whose basic design was laid down 400--500
million years ago. A system for voluntary action works alongside a more autonomic
system whose two branches regulate arousal (sympathetic system) and restorative
(parasympathetic system) functions. Hindbrain and mid-cortical regions regulate
housekeeping, memory storage, and emotional behaviors. In higher mammals, the
cortex becomes much larger and infolded as its regions specialize and collaborate
in generating more complicated social communication and, finally, forethought
and language. Frontal, parietal, occipital, and temporal lobes specialize in
carrying out different sensory, motor, and integrative functions. In general,
the posterior portions of the cortex register events that have occurred, and
the anterior portions process what we are about to do in reaction to these
events. The two cerebral hemispheres contain corresponding sensory, motor,
and integration areas that control the opposite side of the body, but at the
same time, they exhibit distinctive global specializations. Damage to specific
regions of the brain caused by tumors, strokes, or mechanical trauma has yielded
important insights into the functions of different cortical areas. A large
number of behavioral deficits in vision, hearing, movement, decision making,
and socially appropriate behavior, which are described in more detail in Part
III of this book, have now been correlated with damage to different brain areas.
The recent development of sophisticated technology for imaging the activity
of living brains is bringing about a revolution in our understanding of brain
function. It is now possible to demonstrate directly the activation of functional
areas suggested by brain lesion studies. Brain changes associated with depression,
drug use, schizophrenia, and other disorders are being documented at increasing
levels of detail.
The toolkit of basic facts that
we have now assembled about brain evolution and brain function can serve as
a foundation for our consideration of the minds of animals. We begin this examination
in Chapter 4, where we consider the kinds of intelligence that might be attributable
to monkeys and apes. In this way, we will seek an understanding of the platform
on which subsequent stages of hominid intelligence are built.
Questions for Thought
1. Structures such as the brainstem
and limbic system have been present throughout mammalian evolution, whereas
the extensive prefrontal cortex characteristic of hominids has appeared only
over the past several million years. What functions seem to be served by the
newer structures, as contrasted with the older? Do these differences justify
a generalization that the newer structures are more "advanced" than
the ones that have been present longer?
2. The "triune brain" model
depicts our lower brain structures as being reptilian in origin, the middle
limbic structures as early mammalian, and the extensive neocortex of primates
as later mammalian. Several commentators have suggested that this means we
are run by three brains in parallel, any one of which might be in ascendance
at a given time. What arguments supporting this idea and what arguments opposing
it can you think of?
3. What are the major functional
specializations of the four different lobes of our brains? Suppose it is demonstrated
that a lesion of a discrete brain area causes a deficit (for example, say a
lesion in the occipital cortex abolishes the ability to see colors). Does this
tell us that the region in question is responsible for our color vision? When
you read about the attribution of a function to a particular brain area, what
cautions should you keep in mind?
4. A patient with a small lesion
in the right parietal lobe loses the ability to comprehend or speak language.
Given the known specializations of the left and right hemispheres, is this
expected or is it surprising? What functions appear to be distinctive to the
two different hemispheres?
Suggestions for Further General
Reading
Calvin, W.H. 1990. The Cerebral
Symphony. New York: Bantam Books. This is an engaging account of how the brain
works, and it is the source of some of the material in this chapter on localization
of different brain functions.
Greenfield, S.A., ed. 1996. The
Human Mind Explained. New York: Holt. An elementary, accessible, encyclopedic,
and well-illustrated presentation of brain structures and functions.
Posner, M.I., and Raichle, M.E.
1994. Images of Mind. New York: Freeman. This book provides a fascinating introduction
to the new brain-imaging technologies and what can be learned from them.
Reading on More Advanced or Specialized
Topics
Deacon, T.W. 1990. Rethinking mammalian
brain evolution. American Zoologist 30:629--705. This is a detailed and thoughtful
consideration of changes that occurred during the evolution of the mammalian
brain.
Gazzaniga, M.S., Ivry, R.B., & Mangun,
G.R. 1998. Cognitive Neuroscience---The Biology of the Mind. New York: Norton.
This recent book provides an introduction to some of the areas of cognitive
neuroscience mentioned in this chapter.
Kandel, E.R., Schwartz, J.H., & Jessell,
T, eds. 1991. Principles of Neural Science. New York: Elsevier/North-Holland.
If you want to invest in one heavy medical neuroscience textbook, the most
recent edition of this work would be a reasonable choice. It is the source
of some of the historical material mentioned in this chapter.
Kosslyn, S.M., & Koenig, O.
1992. Wet Mind---The New Cognitive Neuroscience. New York: Free Press. This
is a graduate-level introduction to how the functions of different brain areas
are elucidated by a variety of approaches, such as imaging and studying the
consequences of brain lesions.
Kosslyn, S.M., Thompson, W.L.,
Kim, I.J., & Alpert, N.M. 1995. Topographical representations of mental
images in primary visual cortex. Nature 378:496--498. A demonstration that
areas of the visual cortex that become active during viewing of an object are
also activated by imagining that object.
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