The Brain. David Eagleman

the time we’re twenty-five years of age, the brain transformations of childhood and adolescence are finally over. The tectonic shifts in our identity and personality have ended, and our brain appears to now be fully developed. You might think that who we are as adults is now fixed in place, immoveable. But it’s not: in adulthood our brains continue to change. Something that can be shaped – and can hold that shape – is what we describe as plastic. And so it is with the brain, even in adulthood: experience changes it, and it retains the change.

      To get a sense of how impressive these physical changes can be, consider the brains of a particular group of men and women who work in London: the city’s cab drivers. They undergo four years of intensive training to pass the “Knowledge of London”, one of society’s most difficult feats of memory. The Knowledge requires aspiring cabbies to memorize London’s extensive roadways, in all their combinations and permutations. This is an exceedingly difficult task: The Knowledge covers 320 different routes through the city, 25,000 individual streets, and 20,000 landmarks and points of interest – hotels, theatres, restaurants, embassies, police stations, sports facilities, and anywhere a passenger is likely to want to go. Students of The Knowledge typically spend three to four hours a day reciting theoretical journeys.

       In an epic feat of memorization, London cab drivers learn the city’s geography by rote. After training, they can articulate the most direct (and legal!) route between any two points in the greater metropolitan area, without consulting a map. The end result of the challenge is a visible change in their brains.

      The unique mental challenges of The Knowledge sparked the interest of a group of neuroscientists from University College London, who scanned the brains of several cab drivers. The scientists were particularly interested in a small area of the brain called the hippocampus – vital for memory, and, in particular, spatial memory.

      The scientists discovered visible differences in the cabbies’ brains: in the drivers, the posterior part of the hippocampus had grown physically larger than those in the control group – presumably causing their increased spatial memory. The researchers also found that the longer a cabbie has been doing his job, the bigger the change in that brain region, suggesting that the result was not simply reflecting a pre-existing condition of people who go into the profession, but instead resulted from practice.

      The cab-driver study demonstrates that adult brains are not fixed in place, but instead can reconfigure so much that the change is visible to the trained eye.

       After learning The Knowledge, the hippocampuses of London cab drivers visibly changed shape – reflecting their improved skills of spatial navigation.

      It’s not just cab drivers whose brains reshape themselves. When one of the most famous brains of the twentieth century was examined, Albert Einstein’s brain did not reveal the secret of his genius. But it did show that the brain area devoted to his left fingers had expanded – forming a giant fold in his cortex called the Omega sign, shaped like the Greek symbol O – all thanks to his less commonly known passion for playing the violin. This fold becomes enlarged in experienced violin players, who intensively develop fine dexterity with the fingers of their left hand. Piano players, in contrast, develop an Omega sign in both hemispheres, as they use both hands in fine, detailed movements.

       Albert Einstein and his brain. The brain is viewed from above; the front of the brain is at the top of the picture. The orange shaded region is unusually enlarged – so much so that the extra tissue bunches up into what looks like an upside-down Greek letter omega.

      The shape of the hills and valleys in the brain is largely conserved across people – but the finer details give a personal and unique reflection of where you’ve been and who you are now. Although most of the changes are too small to detect with the naked eye, everything you’ve experienced has altered the physical structure of your brain – from the expression of genes to the positions of molecules to the architecture of neurons. Your family of origin, your culture, your friends, your work, every movie you’ve watched, every conversation you’ve had – these have all left their footprints in your nervous system. These indelible, microscopic impressions accumulate to make you who you are, and to constrain who you can become.

       Pathological changes

      Changes in our brain represent what we’ve done and who we are. But what happens if the brain changes because of a disease or injury? Does this also alter who we are, our personalities, our actions?

      On August 1st 1966, Charles Whitman took an elevator to the observation deck of the University of Texas Tower in Austin. Then the twenty-five-year-old started firing indiscriminately at people below. Thirteen people were killed and thirty-three wounded, until Whitman himself was finally shot dead by police. When they got to his house they discovered that he had killed his wife and mother the night before.

      There was only one thing more surprising than this random act of violence, and that was the lack of anything about Charles Whitman that would seem to have predicted it. He was an Eagle Scout, he was employed as a bank teller, and he was an engineering student.

       Police photograph of the body of Charles Whitman after he went on a murderous shooting spree at the University of Texas at Austin in 1966. In his suicide note, Whitman asked for an autopsy: he suspected that something was going awry in his brain.

      Shortly after killing his wife and his mother, he’d sat down and typed what amounted to a suicide note:

       I don’t really understand myself these days. I am supposed to be an average reasonable and intelligent young man. However, lately (I cannot recall when it started) I have been a victim of many unusual and irrational thoughts . . . After my death I wish that an autopsy would be performed on me to see if there is any visible physical disorder.

      Whitman’s request was granted. After an autopsy, the pathologist reported that Whitman had a small brain tumor. It was about the size of a nickel, and it was pressing against a part of his brain called the amygdala, which is involved in fear and aggression. This small amount of pressure on the amygdala led to a cascade of consequences in Whitman’s brain, resulting in him taking actions that would otherwise be completely out of character. His brain matter had been changing, and who he was changed with it.

      This is an extreme example, but less dramatic changes in your brain can alter the fabric of who you are. Consider the ingestion of drugs or alcohol. Particular kinds of epilepsy make people more religious. Parkinson’s disease often makes people lose their faith, while the medication for Parkinson’s can often turn people into compulsive gamblers. It’s not just illness or chemicals that change us: from the movies we watch to the jobs we work, everything contributes to a continual reshaping of the neural networks we summarize as us. So who exactly are you? Is there anyone down deep, at the core?

       Am I the sum of my memories?

      Our brains and bodies change so much during our life that – like a clock’s hour hand – it’s difficult to detect the changes. Every four months your red blood cells are entirely replaced, for instance, and your skin cells are replaced every few weeks. Within about seven years every atom in your body will be replaced by other atoms. Physically, you are constantly a new you. Fortunately, there may be one constant that links all these different versions of your self together: memory. Perhaps memory can serve as the thread that makes you who you are. It sits at the core of your identity, providing a single, continuous sense of self.

      But there might be a problem here. Could the continuity be an illusion? Imagine you could walk into a park and meet your self at different ages in your life. There you are aged six; as