The Brain. David Eagleman
epileptic seizure on his fifteenth birthday. From there, the seizures grew more frequent. Faced with a future of violent convulsions, Henry underwent an experimental surgery – one which removed the middle part of the temporal lobe (including the hippocampus) on both sides of his brain. Henry was cured of the seizures, but with a dire side effect: for the rest of his life, he was unable to establish any new memories.
But the story doesn’t end there. Beyond his inability to form new memories, he was also unable to imagine the future.
Picture what it would be like to go to the beach tomorrow. What do you conjure up? Surfers and sandcastles? Crashing waves? Rays of sun breaking through clouds? If you were to ask Henry what he might imagine, a typical response might be, “all I can come up with is the color blue”. His misfortune reveals something about the brain mechanisms that underlie memory: their purpose is not simply to record what has gone before but to allow us to project forward into the future. To imagine tomorrow’s experience at the beach, the hippocampus, in particular, plays a key role in assembling an imagined future by recombining information from our past.
Across the US, more than 1,100 nuns, priests, and brothers have been taking part in a unique research project – The Religious Orders Study – to explore the effects of aging on the brain. In particular the study is interested in teasing out the risk factors for Alzheimer’s disease, and it includes subjects, aged sixty-five and over, who are symptom-free and don’t exhibit any measurable signs of disease.
Keeping a busy lifestyle into old age benefits the brain.
In addition to being a stable group that can be easily tracked down each year for regular tests, the religious orders share a similar lifestyle, including nutrition and living standards. This allows for fewer so-called “confounding factors”, or differences, that might arise in the wider population, like diet or socioeconomic status or education – all of which could interfere with the study results.
Data collection began in 1994. So far, Dr. David Bennett and his team at Rush University in Chicago have collected over 350 brains. Each one is carefully preserved, and examined for microscopic evidence of age-related brain diseases. And that’s only half the study: the other half involves the collection of in-depth data on each participant while they’re alive. Every year, everyone in the study undergoes a battery of tests, ranging from psychological and cognitive appraisals to medical, physical, and genetic tests.
Hundreds of nuns have donated their brains for examination after their death. Researchers were caught off guard by the results.
When the team began their research, they expected to find a clear-cut link between cognitive decline and the three diseases that are the most common causes of dementia: Alzheimer’s, stroke and Parkinson’s. Instead, here’s what they found: having brain tissue that was being riddled with the ravages of Alzheimer’s disease didn’t necessarily mean a person would experience cognitive problems. Some people were dying with a full-blown Alzheimer’s pathology without having cognitive loss. What was going on?
The team went back to their substantial data sets for clues. Bennett found that psychological and experiential factors determined whether there was loss of cognition. Specifically, cognitive exercise – that is, activity that keeps the brain active, like crosswords, reading, driving, learning new skills, and having responsibilities – was protective. So were social activity, social networks and interactions, and physical activity.
On the flip side, they found that negative psychological factors like loneliness, anxiety, depression, and proneness to psychological distress were related to more rapid cognitive decline. Positive traits like conscientiousness, purpose in life, and keeping busy were protective.
The participants with diseased neural tissue – but no cognitive symptoms – have built up what is known as “cognitive reserve”. As areas of brain tissue have degenerated, other areas have been well exercised, and therefore have compensated or taken over those functions. The more we keep our brains cognitively fit – typically by challenging them with difficult and novel tasks, including social interaction – the more the neural networks build new roadways to get from A to B.
Think of the brain like a toolbox. If it’s a good toolbox, it will contain all the tools you need to get a job done. If you need to disengage a bolt, you might fish out a ratchet; if you don’t have access to the ratchet, you’ll pull out a wrench; if the wrench is missing you might try a pair of pliers. It’s the same concept in a cognitively fit brain: even if many pathways degenerate because of disease, the brain can retrieve other solutions.
The nuns’ brains demonstrate that it’s possible to protect our brains, and to help hold on to who we are for as long as possible. We can’t stop the process of aging, but by practicing all the skills in our cognitive toolbox, we may be able to slow it down.
I am sentient
When I think about who I am, there’s one aspect above all that can’t be ignored: I am a sentient being. I experience my existence. I feel like I’m here, looking out on the world through these eyes, perceiving this Technicolor show from my own center stage. Let’s call this feeling consciousness or awareness.
Scientists often debate the detailed definition of consciousness, but it’s easy enough to pin down what we’re talking about with the help of a simple comparison: when you’re awake you have consciousness, and when you’re in deep sleep you don’t. That distinction gives us an inroad for a simple question: what is the difference in brain activity between those two states?
One way to measure that is with electroencephalography (EEG), which captures a summary of billions of neurons firing by picking up weak electrical signals on the outside of the skull. It’s a bit of a crude technique; sometimes it’s compared to trying to understand the rules of baseball by holding a microphone against the outside of a baseball stadium. Nonetheless, EEG can offer immediate insights into the differences between the waking and sleeping states.
When you’re awake, your brain waves reveal that your billions of neurons are engaged in complex exchanges with one another: think of it like thousands of individual conversations in the ballgame crowd.
When you go to sleep, your body seems to shut down. So it’s a natural assumption that the neuronal stadium quiets down. But in 1953 it was discovered that such an assumption is incorrect: the brain is just as active at night as during the day. During sleep, neurons simply coordinate with one another differently, entering a more synchronized, rhythmic state. Imagine the crowd at the stadium doing an incessant Mexican wave, around and around.
Consciousness emerges when neurons are coordinating with one another in complex, subtle, mostly independent rhythms. In slow-wave sleep, neurons are more synchronized with one another, and consciousness is absent.
As you can imagine, the complexity of the discussion in a stadium is much richer when thousands of discrete conversations are playing out. In contrast, when the crowd is entrained in a bellowing wave, it’s a less intellectual time.
So who you are at any given moment depends on the detailed rhythms of your neuronal firing. During the day, the conscious you emerges from that integrated neural complexity. At night, when the interaction of your neurons changes just a bit, you disappear. Your loved ones have to wait until the next morning, when your neurons let the wave die and work themselves back into their complex rhythm. Only then do you return.
THE MIND–BODY PROBLEM
Conscious awareness is one of the most baffling puzzles of modern neuroscience. What is the relationship between our mental experience and our physical brains?