Hemispheres, Lobes, and Functions

At first glance, one notices that the outer portion of the brain is partitioned fore-and-aft into left and right halves, or cerebral hemispheres.Your two hemispheres are physically separate, but they’re joined by a thick band of nerve fibers called the corpus callosum (“callous body” in Latin),
as illustrated by the interior view in the figure. The corpus callosum carries signals back and forth between the hemispheres, enabling them to share information constantly. The outer part of your brain’s convoluted surface—the cortex—is demarcated by deep fissures, each referred to by scientists as a sulcus (collectively, sulci), separating various mounds or ridges, each referred
to as a gyrus (collectively, gyri). This sulcus-and-gyrus formation tends to maximize the surface area of the gray matter of the cortex, where the billions of neurons do the heavy work in our thinking processes. It’s also well known that the left hemisphere of the brain controls the right side of the body, and vice versa. Similarly, the sensory signals coming to the brain from the two sides of your body cross over to the opposite hemispheres, where they’re processed. Rather peculiarly, your visual neurons, which emerge from the retinas of your eyes, are segmented into left and right “fields.”That is, the nerves from the left half of your left retina and the left half of your right retina both go to your right hemisphere’s visual processing center, located in the occipital lobe at the rear of your brain. Similarly, the nerves from the right half of each retina go to the visual center in the occipital lobe of your left hemisphere. The optic nerves, which emerge from the back of each eyeball, fuse together into a junction called the optic chiasm, and immediately separate again, with each outgoing nerve branch switching over to the opposite hemisphere. This “crossover” effect, in which motor control and sensory processing are swapped between the two sides of the body and the two cerebral hemispheres, remains a mystery to scientists. The functional value of this design feature is open to speculation. Much of what we know about the functions of the brain comes from the study of brain-injured people. Scientists and physicians have long associated various cognitive, behavioral, and motor impairments with specific traumas to the brain and nervous system. Conversely, they can often diagnose specific brain injuries by testing for impairment of these specific functions. Incidentally, your brain cannot directly perceive the effects of trauma to itself. It has no sensory nerves of its own.
Amid the convolution of visible blobs and crevices on the surface of each hemisphere, one can discern four general subdivisions or lobes: the frontal lobe, which sits just behind your forehead; the temporal lobe, located on the side; the parietal lobe, which spans across the top of your brain; and the occipital lobe, located in the back of your skull. Each lobe is responsible for certain specific aspects of the thinking process.The left and right hemispheres each have the same four lobes, although the assignment of the functions differs somewhat between the two. Between any two people, these functional divisions of activity are very similar, although certain areas can vary somewhat from person to person.Two functional areas that seem to vary somewhat from person to person are the speech and language centers. For about 70 to 95 percent of us, these functions probably reside in the left hemisphere, as illustrated in the figure. Slightly above and behind your left ear, Wernicke’s area, named after German scientist Carl Wernicke (in science, you get to name a part of the body after yourself if you’re the first to discover it) handles the complex process of encoding ideas into language and interpreting the meaning of incoming verbal information. Just forward of your left ear, Broca’s area (named after French scientist Paul Broca)
controls your vocal apparatus. These two centers must work together closely for you to understand and use language. “Handedness”—the preference for using either the left or right
hand—is also not so simple as one might first think. Early researchers believed that handedness and speech were mostly contra-lateral—right-handers had their speech centers in their left hemispheres, so therefore left-handers must carry speech in their right hemispheres. More recent research indicates that left-handers are not simply the opposites— cranially speaking—of right-handers. Apparently, some of them are right-brained for language and some are not. Ambidextrous people complicate the question even further. It’s difficult for scientists to settle this question, because it would require opening up the skulls of large number of people and probing their brains—not a very humane approach to research. Your brain receives information from the various parts of your body and sends back instructions of various kinds by means of twelve pairs of cranial nerves, or nerve bundles (not shown in the figure), which emerge from the base of your skull and plug into your spinal cord. Each cranial nerve coordinates a particular collection of functions. Some of them only transmit information to the brain—the sensory nerves; some only transmit commands from the brain—the motor nerves; and some do both. The neurons you have over two hundred different kinds of them in your cortex, stacked in six layers—are specialized cells that seem to be designed to communicate with one another and with other cells in the body. A typical neuron has a blob-like central body with thousands of thread like receiving connections, or dendrites. Branching out from the cell body is a long tail, or axon, with a fatty myelin sheath, from which radiate many other outgoing connectors called axon terminals. These axons and their terminals make up the thick, fatty structure known as the white matter of the brain. Brain tissue, overall, is highly concentrated in fat, and people in certain cultures consider various animal brains a culinary delicacy. Each neuron receives information through its dendrites and passes it on through its axon terminals.These axons can vary in overall length from a small fraction of an inch to several feet. Unlike other body cells, neurons cannot replace themselves, with a few interesting exceptions. Neurons are continually flashing pulses to one another, at speeds of about two hundred miles per hour. The astronomical number of these potential neuron-to-neuron connections makes it possible for the brain to store vast amounts of information.The well-known brainwaves, measured by the electroencephalograph, portray a kind of electrical “music” created by the simultaneous, rhythmic firing of millions of neurons. Actually, neurons only account for about 10 percent of your brain’s cell count. There’s another type of cell, the much more abundant glial cell (from the Latin word meaning “glue”), which doesn’t carry nerve impulses,but supports the neurons in many ways. Scientists have previously thought of glial cells as a kind of passive “pudding” that surrounds and supports the neurons. New findings, however, suggest that the glial cells communicate chemically with one another, and may act in concert to help transmit information throughout the brain.They also transport nutrients, digest the bodies of dead neurons, guide the development of neurons in infancy, and manufacture the fatty myelin, which surrounds the axons of the neurons. The part of the brain we’ve been seeing from the outside—the cerebrum—is just one of three main divisions that reflect the evolutionary history of human development. This so-called cortex of the brain (from the Latin word meaning “tree bark”) is the most recent of the three primary structures, and it’s what makes us essentially human. The three primary brain structures are sometimes labeled the basal region, the mid-brain, and the cerebral cortex. (Note: scientists differ somewhat in the use of these labels and subdivisions, but these three seem to represent the most widely accepted architecture.)