The Brain and the Nervous System
All too often, you don’t know what you’ve got untilit’s gone. And so it goes with brains: Much of what we know about this magnificent organ comes from studying what happens when it is damaged. Because the resulting problems depend on where the image is located, we have evidence that the brain is made up of specialized regions. The achievements that we can easily observe—thinking, feeling, or acting—require exquisite coordination among these areas. To begin describing these diverse regions, let’s briefly tour your cerebral cortex, the brain’s outermost layer.
Place your fingertips on your forehead. A few centimeters beneath your skull, the frontal lobes serve as the brain’s executive center—essentially the CEO of your mind. These lobes are crucial for many human feats: our capacities to plan for the future, think abstractly, and control impulses. Right up front in your skull, they are especially vulnerable to impact. What can damage here do?
Phillipa, a 35-year-old teacher living near Auckland, New Zealand, suffered a terrible transformation after a burglar broke into her home and, while attacking her, bludgeoned her head. Once a calm professional and devoted mother, Phillipa became wild and unpredictable—undressing in front of strangers, swearing at passersby, and sobbing at the slightest provocation. Heartbroken, Phillipa’s husband eventually placed her in an institution. These days, she is usually happy to see her family but never seems to miss them. In fact, she curses and yells at them when they stay too long.
Now walk your fingers to the crown of your head. Directly below, your parietal lobes weave together sensory information to create your sense of spatial layout. They represent this information via a succession of maps. One map describes the space around you; another corresponds to your skin, so you can distinguish a touch on your leg from one on your arm; yet another tracks your body position.
If any of the maps are disrupted, the effects can be bewildering. Consider Arthur (a pseudonym), a physician in the Midwest. Arthur’s body map lacks the brain cells that should represent his right leg, below the mid-thigh. As a result, his brain can’t integrate sensations coming from this portion of his leg—and he has always struggled with a disconcerting desire to have his lower leg amputated. If asked, he calmly traces a line with his finger exactly where he’d like the surgeon to cut.
Now drop your fingers to your temples. Just beneath them, the temporal lobes interpret sounds, including speech. The effects of damage here are evident in the case of J. S., whose temporal lobe damage, following severe viral encephalitis, drastically altered his language and memory abilities. When shown pictures of objects, for instance, J. S. labels them with numerals. He can form sentences of no more than three words and often misunderstands what others say. Still, many of his other capacities remain intact—so he can easily do math and abstract reasoning problems.
To complete this tour, walk your fingers to the back of your head. Here your occipital lobes interpret visual information arriving from your eyes. Different sections of these lobes are each responsible for a certain type of analysis. One region determines the colors in the input; another interprets motion, another assesses shapes, and so on. Damage to any of these areas causes a corresponding visual deficit. Some people are unable to see color: To them, the world looks covered in ash. Others can’t perceive movement: They see the bus is here, and know it used to be over there—but never sense its motion.
We have mentioned only a few functions of the four lobes; many more brain structures regulate everything from appetite to anger, from learning to liking. We’ll consider many of these structures—where they are and how they contribute to our overall functioning. Throughout, we’ll view the brain essentially as a piece of biological machinery—a very modern perspective. The idea that thoughts and feelings had a physi-cal origin was regarded for millennia as deeply heretical, and we’ll start by setting some historical context for modern inquiries into the biology of the mind. Our inquiry then turns to the neurons—the cells that make up the nervous system—and the way they process and transmit information. Of course, neurons function within a large system, so we turn next to the brain. We’ll explore the architecture of the nervous system, and then zoom in to examine the cerebral cortex, including the functions mentioned above.
As you master the brain’s intricate parts, try not to lose sight of the miraculous whole: Your brain weighs about three pounds, maybe 2% of your body weight, but it generates your beliefs, personality, habits, skills, emotions, memories, and hopes. How this is possible is one of modern science’s great mysteries. We’ll return often in this text to this remarkable intersection of body and mind, cells and self.