The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

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.

These abilities (and the underlying mirror-neuron circuitry) are also seen in apes, but only in humans do they seem to have developed to the point of being able to model aspects of others’ minds rather than merely their actions. Inevitably this would have required the development of additional connections to allow a more sophisticated deployment of such circuits in complex social situations. Deciphering the nature of these connections—rather than just saying, “It’s done by mirror neurons”—is one of the major goals of current brain research.

It is difficult to overstate the importance of understanding mirror neurons and their function. They may well be central to social learning, imitation, and the cultural transmission of skills and attitudes—perhaps even of the pressed-together sound clusters we call “words.” By hyper-developing the mirror-neuron system, evolution in effect turned culture into the new genome. Armed with culture, humans could adapt to hostile new environments and figure out how to exploit formerly inaccessible or poisonous food sources in just one or two generations—instead of the hundreds or thousands of generations such adaptations would have taken to accomplish through genetic evolution.

Thus culture became a significant new source of evolutionary pressure, which helped select for brains that had even better mirror-neuron systems and the imitative learning associated with them. The result was one of the many self-amplifying snowball effects that culminated in Homo sapiens, the ape that looked into its own mind and saw the whole cosmos reflected inside.

CHAPTER 1

  Phantom Limbs and Plastic Brains

I love fools’ experiments. I am always making them.

—CHARLES DARWIN

AS A MEDICAL STUDENT I EXAMINED A PATIENT NAMED MIKHEY during my neurology rotation. Routine clinical testing required me to poke her neck with a sharp needle. It should have been mildly painful, but with each poke she laughed out loud, saying it was ticklish. This, I realized, was the ultimate paradox: laughter in the face of pain, a microcosm of the human condition itself. I was never able to investigate Mikhey’s case as I would have liked.

Soon after this episode, I decided to study human vision and perception, a decision largely influenced by Richard Gregory’s excellent book Eye and Brain. I spent several years doing research on neurophysiology and visual perception, first at the University of Cambridge’s Trinity College, and then in collaboration with Jack Pettigrew at Caltech.