The human brain weighs in at about 1.4 kg. You already know that its neurons and glial cells are busy humming away, giving you potentially brilliant ideas, consciousness, and feelings. But which neurons in which parts of the brain control which functions? To answer that question, neuroscientists had to find a way of describing the brain that allows researchers to communicate with one another. It can be helpful to talk about areas of the brain from “bottom to top,” noting how the different regions are specialized for different kinds of tasks. In general, simpler functions are performed at the “lower levels” of the brain, whereas more complex functions are performed at successively “higher” levels (see FIGURE 3.13). Or, as you will see shortly, the brain can also be approached in a “side-by-side” fashion: Although each side of the brain is roughly analogous, one half of the brain specializes in some tasks that the other half does not. Although these divisions make it easier to understand areas of the brain and their functions, keep in mind that none of these structures or areas in the brain can act alone: They are all part of one big, interacting, interdependent whole.

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Let us look first at the divisions of the brain, and the responsibilities of each part, moving from the bottom to the top. Using this view, we can divide the brain into three parts: the hindbrain, the midbrain, and the forebrain (see Figure 3.13).

If you follow the spinal cord from your tailbone to where it enters your skull, you will find it difficult to determine where your spinal cord ends and your brain begins. That is because the spinal cord is continuous with the hindbrain, an area of the brain that coordinates information coming into and out of the spinal cord. The hindbrain looks like a stalk on which the rest of the brain sits, and it controls the most basic functions of life: respiration, alertness, and motor skills. The structures that make up the hindbrain include: the medulla, the reticular formation, the cerebellum, and the pons (see FIGURE 3.14).

The medulla is an extension of the spinal cord into the skull that coordinates heart rate, circulation, and respiration. Beginning inside the medulla and extending upward is a small cluster of neurons called the reticular formation which regulates sleep, wakefulness, and levels of arousal. In one early experiment, researchers stimulated the reticular formation of a sleeping cat. This caused the animal to awaken almost instantaneously and remain alert. Conversely, severing the connections between the reticular formation and the rest of the brain caused the animal to lapse into an irreversible coma (Moruzzi & Magoun, 1949). The reticular formation maintains the same delicate balance between alertness and unconsciousness in humans. In fact, many general anesthetics work by reducing activity in the reticular formation, rendering the patient unconscious.

Behind the medulla is the cerebellum, a large structure of the hindbrain that controls fine motor skills. (Cerebellum is Latin for “little brain,” and the structure does look like a small replica of the brain.) The cerebellum orchestrates the proper sequence of movements when we ride a bike, play the piano, or maintain balance while walking and running. It contributes to the fine-tuning of behaviour: smoothing our actions to allow their graceful execution rather than initiating the actions (Smetacek, 2002). The initiation of behaviour involves other areas of the brain; as you will recall, different brain systems interact and are interdependent with one another.

The last major area of the hindbrain is the pons, a structure that relays information from the cerebellum to the rest of the brain. (Pons means “bridge” in Latin.) Although the detailed functions of the pons remain poorly understood, it essentially acts as a relay station or bridge between the cerebellum and other structures in the brain.

Sitting on top of the hindbrain is the midbrain, which is relatively small in humans. As you can see in FIGURE 3.15, the midbrain contains two main structures: the tectum and the tegmentum. The tectum orients an organism in the environment. The tectum receives stimulus input from the eyes, ears, and skin and moves the organism in a coordinated way toward the stimulus. For example, when you are studying in a quiet room and you hear a click behind and to the right of you, your body will swivel and orient to the direction of the sound; this is your tectum in action.

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The tegmentum is involved in movement and arousal; it also helps to orient an organism toward sensory stimuli. The midbrain may be relatively small, but it is a central location of neurotransmitters involved in arousal, mood, and motivation and the brain structures that rely on them (White, 1996). You could survive if you had only a hindbrain and a midbrain. The structures in the hindbrain would take care of all the bodily functions necessary to sustain life, and the structures in the midbrain would orient you toward or away from pleasurable or threatening stimuli in the environment. But this would not be much of a life. To understand where the abilities that make us fully human come from, we need to consider the last division of the brain.

When you appreciate the beauty of a poem, detect the sarcasm in a friend’s remark, plan to go skiing next winter, or notice the faint glimmer of sadness on a loved one’s face, you are enlisting the forebrain. The forebrain is the highest level of the brain—literally and figuratively—and controls complex cognitive, emotional, sensory, and motor functions. The forebrain itself is divided into two main sections: the cerebral cortex and the subcortical structures.

The cerebral cortex is the outermost layer of the brain, visible to the naked eye, and divided into two hemispheres. The subcortical structures are areas of the forebrain housed under the cerebral cortex near the centre of the brain (see FIGURE 3.16). We will have much more to say about the two hemispheres of the cerebral cortex and the functions they serve in the next section, fittingly saving the highest level of the brain for last. First, we will examine the subcortical structures.

The subcortical (beneath the cortex) structures are nestled deep inside the brain, where they are quite protected. If you imagine sticking an index finger in each of your ears and pushing inward until they touch, that is about where you would find the thalamus, hypothalamus, pituitary gland, limbic system, and basal ganglia (see Figure 3.16). Each of these subcortical structures plays an important role in relaying information throughout the brain, as well as performing specific tasks that allow us to think, feel, and behave as humans. Here we will give you a brief introduction to each, and you will read more about many of these structures in later chapters.

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The thalamus, hypothalamus, and pituitary gland, located in the centre of the brain, interact closely with several other brain structures. They relay signals to and from these structures and also help to regulate them.

The thalamus relays and filters information from the senses and transmits the information to the cerebral cortex. The thalamus receives inputs from all the major senses except smell, which has direct connections to the cerebral cortex. The thalamus acts as a kind of computer server in a networked system, taking in multiple inputs and relaying them to a variety of locations (Guillery & Sherman, 2002). However, unlike the mechanical operations of a computer (“send input A to location B”), the thalamus actively filters sensory information, giving more weight to some inputs and less weight to others. The thalamus also closes the pathways of incoming sensations during sleep, providing a valuable function in not allowing information to pass to the rest of the brain.

The hypothalamus, located below the thalamus (hypo- is Greek for “under”), regulates body temperature, hunger, thirst, and sexual behaviour. Although the hypothalamus is a tiny area of the brain, clusters of neurons in the hypothalamus oversee a wide range of basic behaviours, keeping body temperature, blood sugar levels, and metabolism within an optimal range for normal human functioning. Lesions to some areas of the hypothalamus result in overeating, whereas lesions to other areas leave an animal with no desire for food at all, highlighting that the hypothalamus plays a key role in regulating food intake (Berthoud & Morrison, 2008). Also, when you think about sex, messages from your cerebral cortex are sent to the hypothalamus to trigger the release of hormones. Finally, electric stimulation of some areas of the hypothalamus in cats can produce hissing and biting, whereas stimulation of other areas in the hypothalamus can produce what appears to be intense pleasure for an animal (Siegel et al., 1999). McGill University researchers James Olds and Peter Milner found that a small electric current delivered to a certain region of a rat’s hypothalamus was extremely rewarding for the animal (Olds & Milner, 1954). In fact, when allowed to press a bar attached to the electrode to initiate their own stimulation, rats would do so several thousand times an hour, often to the point of exhaustion!

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Located below the hypothalamus is the pituitary gland, the “master gland” of the body’s hormone-producing system, which releases hormones that direct the functions of many other glands in the body. The hypothalamus sends hormonal signals to the pituitary gland, which in turn sends hormonal signals to other glands to control stress, digestive activities, and reproductive processes. For example, when a baby suckles its mother’s breast, sensory neurons in her breast send signals to her hypothalamus, which then signals her pituitary gland to release a hormone called oxytocin into the bloodstream (McNeilly et al., 1983). Oxytocin, in turn, stimulates the release of milk from reservoirs in the breast. The pituitary gland is also involved in the response to stress. When we sense a threat, sensory neurons send signals to the hypothalamus, which stimulates the release of adrenocorticotropic hormone (ACTH) from the pituitary gland. ACTH, in turn, stimulates the adrenal glands (above the kidneys) to release hormones that activate the sympathetic nervous system (Selye & Fortier, 1950). As you read earlier in this chapter, the sympathetic nervous system prepares the body to either meet the threat head-on or flee from the situation.

The hypothalamus also is part of the limbic system, a group of forebrain structures including the hypothalamus, the hippocampus, and the amygdala, which are involved in motivation, emotion, learning, and memory (Maclean, 1970; Papez, 1937). The limbic system is where the subcortical structures meet the cerebral cortex.

The hippocampus (from Latin for “sea horse,” due to its shape) is critical for creating new memories and integrating them into a network of knowledge so that they can be stored indefinitely in other parts of the cerebral cortex. Individuals with damage to the hippocampus can acquire new information and keep it in awareness for a few seconds, but as soon as they are distracted, they forget the information and the experience that produced it (Scoville & Milner, 1957; Squire, 2009). This kind of disruption is limited to everyday memory for facts and events that we can bring to consciousness; memory of learned habitual routines or emotional reactions remains intact (Squire, Knowlton, & Musen, 1993). As an example, people with damage to the hippocampus can remember how to drive and talk, but they cannot recall where they have recently driven or a conversation they have just had. You will read more about the hippocampus and its role in creating, storing, and combining memories in the Memory chapter.

The amygdala (from Latin for “almond,” also due to its shape), located at the tip of each horn of the hippocampus, plays a central role in many emotional processes, particularly the formation of emotional memories (Aggleton, 1992). The amygdala attaches significance to previously neutral events that are associated with fear, punishment, or reward (LeDoux, 1992). As an example, think of the last time something scary or unpleasant happened to you: A car came barrelling toward you as you started walking into an intersection or a ferocious dog leaped out of an alley as you passed by. Ordinarily, these stimuli—a car or a dog—are fairly neutral; you do not have a panic attack every time you walk by a used car lot. But after a scary experience, the emotional significance attached to events involving those stimuli is the work of the amygdala (McGaugh, 2006). When we are in emotionally arousing situations, the amygdala stimulates the hippocampus to remember many details surrounding the situation (Kensinger & Schacter, 2005). For example, people who lived through the terrorist attacks of September 11, 2001 remember vivid details about where they were, what they were doing, and how they felt when they heard the news, even years later (Hirst et al., 2009). In particular, the amygdala seems to be especially involved in encoding events as fearful (Adolphs et al., 1995; Sigurdsson et al., 2007). We will have more to say about the amygdala in the Emotion and Motivation chapter. For now, keep in mind that a group of neurons the size of a lima bean buried deep in your brain help you to laugh, weep, or shriek in fright when the circumstances call for it.

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There are several other structures in the subcortical area, but we will consider just one more. The basal ganglia are a set of subcortical structures that direct intentional movements. The basal ganglia are located near the thalamus and hypothalamus; they receive input from the cerebral cortex and send outputs to the motor centres in the brain stem. One part of the basal ganglia, the striatum, is involved in the control of posture and movement. As we saw in the excerpt from Michael J. Fox’s book, people who suffer from Parkinson’s disease typically show symptoms of uncontrollable shaking and sudden jerks of the limbs and are unable to initiate a sequence of movements to achieve a specific goal. This happens because the dopamine-producing neurons in the substantia nigra (found in the tegmentum of the midbrain) have become damaged (Dauer & Przedborski, 2003). The undersupply of dopamine then affects the striatum in the basal ganglia, which in turn leads to the visible behavioural symptoms of Parkinson’s.

So, what is the problem in Parkinson’s: the jerky movements, the ineffectiveness of the striatum in directing behaviour, the botched interplay of the substantia nigra and the striatum, or the underproduction of dopamine at the neuronal level? The answer is all of the above. This unfortunate disease provides a nice illustration of two themes regarding the brain and behaviour. First, barely visible actions at the level of neurons in the brain can produce substantial effects at the level of behaviour. Second, the interaction of hindbrain, midbrain, and forebrain structures shows how the various regions are interdependent.

Our tour of the brain has taken us from the very small (neurons) to the somewhat bigger (major divisions of the brain) to the very large: the cerebral cortex. The cortex is the highest level of the brain, and it is responsible for the most complex aspects of perception, emotion, movement, and thought (Fuster, 2003). It sits over the rest of the brain, like a mushroom cap shielding the underside and stem, and it is the wrinkled surface you see when looking at the brain with the naked eye.

The smooth surfaces of the cortex—the raised part—are called gyri (gyrus if you are talking about just one), and the indentations or fissures are called sulci (sulcus when singular). Sulci and gyri represent a triumph of evolution. The cerebral cortex occupies roughly the area of a newspaper page. Fitting that much cortex into a human skull is a tough task. But if you crumple a sheet of newspaper, you will see that the same surface area now fits compactly into a much smaller space. The cortex, with its wrinkles and folds, holds a lot of brainpower in a relatively small package that fits comfortably inside the human skull (see FIGURE 3.17). The functions of the cerebral cortex can be understood at three levels: the separation of the cortex into two hemispheres, the functions of each hemisphere, and the role of specific cortical areas.

The first level of organization divides the cortex into the left and right hemispheres. The two hemispheres are more or less symmetrical in their appearance and, to some extent, in their functions. However, each hemisphere controls the functions of the opposite side of the body. This is called contralateral control, meaning that your right cerebral hemisphere perceives stimuli from and controls movements on the left side of your body, whereas your left cerebral hemisphere perceives stimuli from and controls movement on the right side of your body.

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The cerebral hemispheres are connected to each other by commissures, bundles of axons that make possible communication between parallel areas of the cortex in each half. The largest of these commissures is the corpus callosum, which connects large areas of the cerebral cortex on each side of the brain and supports communication of information across the hemispheres (see FIGURE 3.18). This means that information received in the right hemisphere, for example, can pass across the corpus callosum and be registered, virtually instantaneously, in the left hemisphere.

The second level of organization in the cerebral cortex distinguishes the functions of the different regions within each hemisphere of the brain. Each hemisphere of the cerebral cortex is divided into four areas, or lobes: From back to front, these are the occipital lobe, the parietal lobe, the temporal lobe, and the frontal lobe, as shown in Figure 3.17. We will examine the functions of these lobes in more detail in later chapters, noting how scientists have used a variety of techniques to understand the operations of the brain. For now, here is a brief overview of the main functions of each lobe.

The occipital lobe, located at the back of the cerebral cortex, processes visual information. Sensory receptors in the eyes send information to the thalamus, which in turn sends information to the primary areas of the occipital lobe, where simple features of the stimulus are extracted, such as the location and orientation of an object’s edges (see the Sensation and Perception chapter for more details). These features are then processed into a more complex “map” of the stimulus onto the occipital cortex, leading to comprehension of what is being seen. As you might imagine, damage to the primary visual areas of the occipital lobe can leave a person with partial or complete blindness. Information still enters the eyes, which work just fine. But without the ability to process and make sense of the information at the level of the cerebral cortex, the information is as good as lost (Zeki, 2001).

The parietal lobe, located in front of the occipital lobe, carries out functions that include processing information about touch. The parietal lobe contains the somatosensory cortex, a strip of brain tissue running from the top of the brain down to the sides (see FIGURE 3.19). Within each hemisphere, the somatosensory cortex represents the skin areas on the contralateral surface of the body. Each part of the somatosensory cortex maps onto a particular part of the body. If a body area is more sensitive, a larger part of the somatosensory cortex is devoted to it. For example, the part of the somatosensory cortex that corresponds to the lips and tongue is larger than the area corresponding to the feet. The somatosensory cortex can be illustrated as a distorted figure, called a homunculus (“little man”), in which the body parts are rendered according to how much of the somatosensory cortex is devoted to them (Penfield & Rasmussen, 1950). This map of the body in the brain was developed by Montreal neurosurgeon Wilder Penfield and his colleagues by stimulating the brains of their patients who were awake during surgery, and recording where the patient reported feeling a sensation (Penfield & Rasmussen, 1950). Penfield mapped both the somatosensory and motor cortex, and his maps are still used today. Directly in front of the somatosensory cortex, in the frontal lobe, is a parallel strip of brain tissue called the motor cortex. Like the somatosensory cortex, different parts of the motor cortex correspond to different body parts. The motor cortex initiates voluntary movements and sends messages to the basal ganglia, cerebellum, and spinal cord. The motor and somatosensory cortices, then, are like sending and receiving areas of the cerebral cortex, taking in information and sending out commands as the case might be.

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The temporal lobe, located on the lower side of each hemisphere, is responsible for hearing and language. The primary auditory cortex in the temporal lobe is analogous to the somatosensory cortex in the parietal lobe and the primary visual areas of the occipital lobe: It receives sensory information from the ears based on the frequencies of sounds (Recanzone & Sutter, 2008). Secondary areas of the temporal lobe then process the information into meaningful units, such as speech and words. The temporal lobe also houses the visual association areas that interpret the meaning of visual stimuli and help us recognize common objects in the environment (Martin, 2007).

The frontal lobe, which sits behind the forehead, has specialized areas for movement, abstract thinking, planning, memory, and judgment. As you just read, it contains the motor cortex, which coordinates movements of muscle groups throughout the body. Other areas in the frontal lobe coordinate thought processes that help us manipulate information and retrieve memories, which we can use to plan our behaviours and interact socially with others. In short, the frontal cortex allows us to do the kind of thinking, imagining, planning, and anticipating that sets humans apart from most other species (Schoenemann, Sheenan, & Glotzer, 2005; Stuss & Benson, 1986; Suddendorf & Corballis, 2007).

The third level of organization in the cerebral cortex involves the representation of information within specific lobes in the cortex. There is a hierarchy of processing stages from primary areas that handle fine details of information all the way up to association areas, which are composed of neurons that help provide sense and meaning to information registered in the cortex. For example, neurons in the primary visual cortex are highly specialized: Some detect features of the environment that are in a horizontal orientation, others detect movement, and still others process information about human versus nonhuman forms. Secondary areas interpret the information extracted by these primary areas (shape, motion, etc.) to make sense of what is being perceived; in this case, perhaps a large cat leaping toward your face. Similarly, neurons in the primary auditory cortex register sound frequencies, but it is the association areas of the temporal lobe that allow you to turn those noises into the meaning of your friend screaming, “Look out for the cat!” Association areas, then, help stitch together the threads of information in the various parts of the cortex to produce a meaningful understanding of what is being registered in the brain.

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A striking example of this property of association areas comes from the discovery of the mirror-neuron system. Mirror neurons are active when an animal performs a behaviour, such as reaching for or manipulating an object, and are also activated when the animal observes another animal performing the same behaviour. Mirror neurons are found in the frontal lobe (near the motor cortex) and in the parietal lobe (Rizzolatti & Craighero, 2004; Rizzolatti & Sinigaglia, 2010). They have been identified in birds, monkeys, and humans, and their name reflects the function they serve. Neuroimaging studies with humans have shown that mirror neurons are active when people watch someone perform a behaviour, such as grasping in midair. But they are more highly activated when that behaviour has some purpose or context, such as grasping a cup to take a drink (Iacoboni et al., 2005), and seem to be related to recognizing the goal someone has in carrying out an action and the outcome of the action, rather than to the particular movements a person makes while performing that action (Hamilton & Grafton, 2006, 2008; Iacoboni, 2009; Rizzolatti & Sinigaglia, 2010). In the Learning chapter we will find out more about the role of mirror neurons in learning.

Finally, neurons in the association areas are usually less specialized and more flexible than neurons in the primary areas. As such, they can be shaped by learning and experience to do their job more effectively. This kind of shaping of neurons by environmental forces allows the brain flexibility, or plasticity, our next topic.

The cerebral cortex may seem like a fixed structure, one big sheet of neurons designed to help us make sense of our external world. Remarkably, though, sensory cortices are not fixed. They can adapt to changes in sensory inputs, a quality researchers call plasticity (i.e., the ability to be molded). As an example, if you lose your middle finger in an accident, the part of the somatosensory area that represents that finger is initially unresponsive (Kaas, 1991). After all, there is no longer any sensory input going from that location to that part of the brain. You might expect the left middle-finger neurons of the somatosensory cortex to wither away. However, over time, that area in the somatosensory cortex becomes responsive to stimulation of the fingers adjacent to the missing finger. The brain is plastic: Functions that were assigned to certain areas of the brain may be capable of being reassigned to other areas of the brain to accommodate changing input from the environment (Feldman, 2009). This suggests that sensory inputs “compete” for representation in each cortical area. (See The Real World box below for a striking illustration of phantom limbs.)

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Plasticity does not only occur to compensate for missing digits or limbs, however. An extraordinary amount of stimulation of one finger can result in that finger “taking over” the representation of the part of the cortex that usually represents other, adjacent fingers (Merzenich et al., 1990). For example, concert pianists have highly developed cortical areas for finger control: The continued input from the fingers commands a larger area of representation in the somatosensory cortices in the brain. Consistent with this observation, recent research indicates greater plasticity within the motor cortex of professional musicians compared with nonmusicians, perhaps reflecting an increase in the number of motor synapses as a result of extended practice (Rosenkranz, Williamon, & Rothwell, 2007). Similar findings have been obtained with quilters (who have highly developed areas for the thumb and forefinger, which are critical to their profession) and taxi drivers (who have overdeveloped brain areas in the hippocampus that are used during spatial navigation) (Maguire, Woollett, & Spiers, 2006).

Plasticity is also related to a question you might not expect to find in a psychology text: How much exercise have you been getting lately? While we expect that you are spending countless happy hours reading this text, we also hope that you have been finding enough time for physical exercise. A large of number of studies in rats and other nonhuman animals indicate that physical exercise can increase the number of synapses and even promote the development of new neurons in the hippocampus (Hillman, Erickson, & Kramer, 2008; van Praag, 2009). Recent studies with people have begun to document beneficial effects of cardiovascular exercise on aspects of brain function and cognitive performance (Colcombe et al., 2004, 2006). Although these effects tend to be seen most clearly in older adults (okay, so it is time for your textbook authors to get on a treadmill), benefits have also been documented throughout the life span (Hertig & Nagel, 2012; Hillman et al., 2008; Roig et al., 2012). In fact, some researchers believe that this kind of activity-dependent brain plasticity is relevant to treating spinal cord injuries (which, as we saw, have a devastating impact on people’s lives), because understanding how to maximize plasticity through exercise and training may help to guide rehabilitation efforts (Dunlop, 2008). It should be clear by now that the plasticity of the brain is not just an interesting theoretical idea; it has potentially important applications to everyday life (Bryck & Fisher, 2012).

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