Fortunately, underlying all this lyrical complexity there is a basic plan of organization that’s easy to understand. Neurons are connected into networks that can process information. The brain’s many dozens of structures are ultimately all purpose-built networks of neurons, and often have elegant internal organization. Each of these structures performs some set of discrete (though not always easy to decipher) cognitive or physiological functions. Each structure makes patterned connections with other brain structures, thus forming circuits. Circuits pass information back and forth and in repeating loops, and allow brain structures to work together to create sophisticated perceptions, thoughts, and behaviors.
FIGURE INT.1 Drawing of a neuron showing the cell body, dendrites, and axon. The axon transmits information (in the form of nerve impulses) to the next neuron (or set of neurons) in the chain. The axon is quite long, and only part of it is shown here. The dendrites receive information from the axons of other neurons. The flow of information is thus always unidirectional.
The information processing that occurs both within and between brain structures can get quite complicated—this is, after all, the information-processing engine that generates the human mind—but there is plenty that can be understood and appreciated by nonspecialists. We will revisit many of these areas in greater depth in the chapters ahead, but a basic acquaintance now with each region will help you to appreciate how these specialized areas work together to determine mind, personality, and behavior.
The human brain looks like a walnut made of two mirror-image halves (Figure Int.2). These shell-like halves are the cerebral cortex. The cortex is split down the middle into two hemispheres: one on the left, one on the right. In humans the cortex has grown so large that it has been forced to become convoluted (folded), giving it its famous cauliflower-like appearance. (In contrast, the cortex of most other mammals is smooth and flat for the most part, with few if any folds in the surface.) The cortex is essentially the seat of higher thought, the tabula (far from) rasa where all of our highest mental functions are carried out. Not surprisingly, it is especially well developed in two groups of mammals: dolphins and primates. We’ll return to the cortex later in the chapter. For now let’s look at the other parts of the brain.
FIGURE INT.2 The human brain viewed from the top and from the left side. The top view shows the two mirror-symmetric cerebral hemispheres, each of which controls the movements of—and receives signals from—the opposite side of the body (though there are some exceptions to this rule). Abbreviations: DLF, dorsolateral prefrontal cortex; OFC, orbitofrontal cortex; IPL, inferior parietal lobule; I, insula, which is tucked away deep beneath the Sylvian fissure below the frontal lobe. The ventromedial prefrontal cortex (VMF, not labeled) is tucked away in the inner lower part of the frontal lobe, and the OFC is part of it.
FIGURE INT.3 A schematic drawing of the human brain showing internal structures such as the amygdala, hippocampus, basal ganglia, and hypothalamus.
Running up and down the core of the spinal column is a thick bundle of nerve fibers—the spinal cord—that conducts a steady stream of messages between brain and body. These messages include things like touch and pain flowing up from the skin, and motor commands rat-a-tat-tatting down to the muscles. At its uppermost extent the spinal cord pokes up out of its bony sheath of vertebrae, enters the skull, and grows thick and bulbous (Figure Int.3). This thickening is called the brainstem, and it is divided into three lobes: medulla, pons, and midbrain. The medulla and nuclei (neural clusters) on the floor of the pons control important vital functions like breathing, blood pressure, and body temperature. A hemorrhage from even a tiny artery supplying this region can spell instant death. (Paradoxically, the higher areas of the brain can sustain comparatively massive damage and leave the patient alive and even fit. For example, a large tumor in the frontal lobe might produce barely detectable neurological symptoms.)
Sitting on the roof of the pons is the cerebellum (Latin for “little brain”), which controls the fine coordination of movements and is also involved in balance, gait, and posture. When your motor cortex (a higher brain region that issues voluntary movement commands) sends a signal to the muscles via the spinal cord, a copy of that signal—sort of like a CC email—gets sent to the cerebellum. The cerebellum also receives sensory feedback from muscle and joint receptors throughout the body. Thus the cerebellum is able to detect any mismatches that may occur between the intended action and the actual action, and in response can insert appropriate corrections into the outgoing motor signal. This sort of real-time, feedback-driven mechanism is called a servo-control loop. Damage to the cerebellum causes the loop to go into oscillation. For example, a patient may attempt to touch her nose, feel her hand overshooting, and attempt to compensate with an opposing motion, which causes her hand to overshoot even more wildly in the opposite direction. This is called an intention tremor.
Surrounding the top portion of the brainstem are the thalamus and the basal ganglia. The thalamus receives its major inputs from the sense organs and relays them to the sensory cortex for more sophisticated processing. Why we need a relay station is far from clear. The basal ganglia are a strangely shaped cluster of structures that are concerned with the control of automatic movements associated with complex volitional actions—for example, adjusting your shoulder when throwing a dart, or coordinating the force and tension in dozens of muscles throughout your body while you walk. Damage to cells in the basal ganglia results in disorders like Parkinson’s disease, in which the patient’s torso is stiff, his face is an expressionless mask, and he walks with a characteristic shuffling gait. (Our neurology professor in medical school used to diagnose Parkinson’s by just listening to the patient’s footsteps next door; if we couldn’t do the same, he would fail us. Those were the days before high-tech medicine and magnetic resonance imaging, or MRI.) In contrast, excessive amounts of the brain chemical dopamine in the basal ganglia can lead to disorders known a choreas, which are characterized by uncontrollable movements that bear a superficial resemblance to dancing.
Finally we come to the cerebral cortex. Each cerebral hemisphere is subdivided into four lobes (see Figure Int.2): occipital, temporal, parietal, and frontal. These lobes have distinct domains of functioning, although in practice there is a great deal of interaction between them.
Broadly speaking, the occipital lobes are mainly concerned with visual processing. In fact, they are subdivided into as many as thirty distinct processing regions, each partially specialized for a different aspect of vision such as color, motion, and form.
The temporal lobes are specialized for higher perceptual functions, such as recognizing faces and other objects and linking them to appropriate emotions. They do this latter job in close cooperation with a structure called the amygdala (“almond”), which lies in the front ties (anterior poles) of the temporal lobes. Also tucked away beneath each temporal lobe is the hippocampus (“seahorse”), which lays down new memory traces. In addition to all this, the upper part of the left temporal lobe contains a patch of cortex known as Wernicke’s area. In humans this area has ballooned to seven times the size of the same area in chimpanzees; it is one of the few brain areas that can be safely declared unique to our species. Its job is nothing less than the comprehension of meaning and the semantic aspects of language—functions that are prime differentiators between human beings and mere apes.