The parietal lobes are primarily involved in processing touch, muscle, and joint information from the body and combining it with vision, hearing, and balance to give you a rich “multimedia” understanding of your corporeal self and the world around it. Damage to the right parietal lobe commonly results in a phenomenon called hemispatial neglect: The patient loses awareness of the left half of visual space. Even more remarkable is somatoparaphrenia, the patient’s vehement denial of ownership of her own left arm and insistence that it belongs to someone else. The parietal lobes have expanded greatly in human evolution, but no part of them has grown more than the inferior parietal lobules (IPL; see Figure Int.2). So great was this expansion that at some point in our past a large portion of it split into two new processing regions called the angular gyrus and the supramarginal gyrus. These uniquely human areas house some truly quintessential human abilities.
The right parietal lobe is involved in creating a mental model of the spatial layout of the outside world: your immediate environs, plus all the locations (but not identity) of objects, hazards, and people within it, along with your physical relationship to each of these things. Thus you can grab things, dodge missiles, and avoid obstacles. The right parietal, especially the right superior lobule (just above the IPL), is also responsible for constructing your body image—the vivid mental awareness you have of your body’s configuration and movement in space. Note that even though it is called an “image,” the body image is not a purely visual construct; it is also partly touch and muscle based. After all, a blind person has a body image too, and an extremely good one at that. In fact, if you zap the right angular gyrus with an electrode, you will have an out-of-body experience.
Now let’s consider the left parietal lobe. The left angular gyrus is involved in important functions unique to humans such as arithmetic, abstraction, and aspects of language such as word finding and metaphor. The left supramarginal gyrus, on the other hand, conjures up a vivid image of intended skilled actions—for example, sewing with a needle, hammering a nail, or waving goodbye—and executes them. Consequently, lesions in the left angular gyrus eliminate abstract skills like reading, writing, and arithmetic, while injury to the left supramarginal gyrus hinders you from orchestrating skilled movements. When I ask you to salute, you conjure up a visual image of the salute and, in a sense, use the image to guide your arm movements. But if your left supramarginal gyrus is damaged, you will simply stare at your hand perplexed or flail it around. Even though it isn’t paralyzed or weak and you clearly understand the command, you won’t be able to make your hand respond to your intention.
The frontal lobes also perform several distinct and vital functions. Part of this region the motor cortex—the vertical strip of cortex running just in front of the big furrow in the middle of the brain (Figure Int.2)—is involved in issuing simple motor commands. Other parts are involved in planning actions and keeping goals in mind long enough to follow through on them. There is another small part of the frontal lobe that is required for holding things in memory long enough to know what to attend to. This faculty is called working memory or short-term memory.
So far so good. But when you move to the more anterior part of the frontal lobes you enter the most inscrutable terra incognita of the brain: the prefrontal cortex (parts of which are identified in Figure Int.2). Oddly enough, a person can sustain massive damage to this area and come out of it showing no obvious signs of any neurological or cognitive deficits. The patient may seem perfectly normal if you casually interact with her for a few minutes. Yet if you talk to her relatives, they will tell you that her personality has changed beyond recognition. “She isn’t in there anymore. I don’t even recognize this new person” is the sort of heart-wrenching statement you frequently hear from bewildered spouses and lifelong friends. And if you continue to interact with the patient for a few hours or days, you too will see that there is something profoundly deranged.
If the left prefrontal lobe is damaged, the patient may withdraw from the social world and show a marked reluctance to do anything at all. This is euphemistically called pseudodepression—“pseudo” because none of the standard criteria for identifying depression, such as feelings of bleakness and chronic negative thought patterns, are revealed by psychological or neurological probing. Conversely, if the right prefrontal lobe is damaged, a patient will seem euphoric even though, once again he really won’t be. Cases of prefrontal damage are especially distressing to relatives. Such a patient seems to lose all interest in his own future and he shows no moral compunctions of any kind. He may laugh at a funeral or urinate in public. The great paradox is that he seems normal in most respects: his language, his memory, and even his IQ are unaffected. Yet he has lost many of the most quintessential attributes that define human nature: ambition, empathy, foresight, a complex personality, a sense of morality, and a sense of dignity as a human being. (Interestingly, a lack of empathy, moral standards, and self-restraint are also frequently seen in sociopaths, and the neurologist Antonio Damasio has pointed out they may have some clinically undetected frontal dysfunction.) For these reasons the prefrontal cortex has long been regarded as the “seat of humanity.” As for the question of how such a relatively small patch of the brain manages to orchestrate such a sophisticated and elusive suite of functions, we are still very much at a loss.
Is it possible to isolate a given part of the brain, as Owen attempted, that makes our species unique? Not quite. There is no region or structure that appears to have been grafted into the brain de novo by an intelligent designer; at the anatomical level, every part of our brain has a direct analog in the brains of the great apes. However, recent research has identified a handful of brain regions that have been so radically elaborated that at the functional (or cognitive) level they actually can be considered novel and unique. I mentioned three of these areas above: Wernicke’s area in the left temporal lobe, the prefrontal cortex, and the IPL in each parietal lobe. Indeed, the offshoots of the IPL—namely, the supramarginal and angular gyri, are anatomically nonexistent in apes. (Owen would have loved to have known about these.) The extraordinarily rapid development of these areas in humans suggests that something crucial must have been going on there, and clinical observations confirm this.
Within some of these regions, there is a special class of nerve cells called mirror neurons. These neurons fire not only when you perform an action, but also when you watch someone else perform the same action. This sounds so simple that its huge implications are easy to miss. What these cells do is effectively allow you to empathize with the other person and “read” her intentions—figure out what she is really up to. You do this by running a simulation of her actions using your own body image.
When you watch someone else reach for a glass of water, for example, your mirror neurons automatically simulate the same action in your (usually subconscious) imagination. Your mirror neurons will often go a step further and have you perform the action they anticipate the other person is about to take—say, to lift the water to her lips and take a drink. Thus you automatically form an assumption about her intentions and motivations—in this case, that she is thirsty and is taking steps to quench that thirst. Now, you could be wrong in this assumption—she might intend to use the water to douse a fire or to fling in the face of a boorish suitor—but usually your mirror neurons are reasonably accurate guessers of others’ intentions. As such, they are the closest thing to telepathy that nature was able to endow us with.