Deric Bownds

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.



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.


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. 1

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. 2 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. 3 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.)



Our brain is constructed of layers that reflect stages in our evolutionary history.


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.



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. 4 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. 5 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. 6



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.



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.


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). 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. 8

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.



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").



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. 9 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. 10

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.) 11 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 12 . 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. 13



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. 14 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. 15

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), 16 and electromagnetoencephalography (MEG). 17 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. 18 Brain regions that become more active during memory, perception, attention, orientation, emotions, and overall vigilance are being mapped. 19 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. 20 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. 21

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. 22 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. 23 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. 24 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. 25 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. 26 Depression can be correlated with increased activity of the amygdala (see Chapter 10). 27 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). 28



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. 29 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.


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.

1. This material is taken from Bear et. al., 1996, Ch. 7.

2. See the popular account given by Ornstein and Thompson, 1984

3. MacLean, 1990.

4. Most of this section is abstracted from Kandel et al., 1991 and Calvin, 1990.

5. Churchland, 1995, offers the following metaphor: It would take about 500,000 television screens, each having about 200,000 pixels, to cover a tower of the World Trade Center. Transform the surface of that skyscraper containing 100 billion pixels (neurons) into a sheet of aluminum foil so thin it can be crumpled up and stuffed into a grapefruit. That's your cerebral and cerebellar cortex.

6. Scannell and Young, 1993

7. Much of this history section is taken from Kandel et al, 1991.

8. See Brothers, 1997, for an extended discussion of this important point.

9. Kosslyn and Koenig, 1992, pg. 109 ff.

10. This section is drawn largely from Calvin, 1990.

11. For material on hemispheric specialization see: Kandel et al, 1991, Ch 53,54; Gardner, 1987, pg. 275; Gazzaniga and LeDoux, 1978; Springer and Deutsch, 1993; Efron, 1990,Hellige, 1993;

12. The lateralization of the brain is reflected in how we used our bodies. Most people favor their right hands and feet because the left hemisphere is more suited for the control of fine movements. Many people favor one eye. Try, with both eyes open, holding up your right thumb at arm's length under an object directly across the room from you. Now alternately close your left and right eyes, and see if your thumb appears to jump to the right or left with respect to the distant object. If you are right-eyed like half the population, your thumb will jump to the right when you close your right eye but stay put when hyou close your left eye, because your right eye contributes more to your perception of the visual world than does your left. If you see little or no jumping, your are neither strongly left-eyed or right-eyed. About 5% of the population is left-eyed, the thumb appears to jump to the left.

13. Bradshaw and Rogers, 1993.

14. Morton et al., 1993.

15. Springer and Deutsch,1993.

16. See Menon and Kim, 1999, for a brief review of spatial and temporal limits in cognitive neuroimaging with fMRI. For a review of efforts to correlate brain imaging, cognition, and neural modeling, see Horwitz et al, 1999. Grimson et al., 1999 describe the use of imaging in brain surgery.

17. Crease, 1993; Posner, 1993

18. Humphreys, 1966.

19. Posner and Raichle, 1994.

20. Roland, 1993.

21. Sergent, 1994.

22. Peterson et al, 1988

23. Paulesu et al 1993

24. Corbetta, 1993

25. Posner, 1993: Bihan et al, 1993; Kosslyn et al., 1995

26. Blakeslee, S. 1992a, Clausiusz, 1996.

27. Posner and Raichle, 1994.

28. Goleman, 1993b.

29. Gibbons, 1992

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