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Thursday, August 9, 2007Unsolved Mystery Of Brain
Unsolved Mysteries Of The BrainWhat we know—and don’t know—about how we think.by David Eagleman
7. How is time represented in the brain?
Hundred-yard dashes begin with a gunshot rather than a strobe light because your brain can react more quickly to a bang than to a flash. Yet as soon as we get outside the realm of motor reactions and into the realm of perception (what you report that you saw and heard), the story changes. When it comes to awareness, the brain goes through a good deal of trouble to synchronize incoming signals that are processed at very different speeds.
For example, snap your fingers in front of you. Although your auditory system processes information about the snap about 30 milliseconds faster than your visual system, the sight of your fingers and the sound of the snap seem simultaneous. Your brain is employing fancy editing tricks to make simultaneous events in the world feel simultaneous to you, even when the different senses processing the information would individually swear otherwise.
For a simple example of how your brain plays tricks with time, look in the mirror at your left eye. Now shift your gaze to your right eye. Your eye movements take time, of course, but you do not see your eyes move. It is as if the world instantly made the transition from one view to the next. What happened to that little gap in time? For that matter, what happens to the 80 milliseconds of darkness you should see every time you blink your eyes? Bottom line: Your notion of the smooth passage of time is a construction of the brain. Clarifying the picture of how the brain normally solves timing problems should give insight into what happens when temporal calibration goes wrong, as may happen in the brains of people with dyslexia. Sensory inputs that are out of sync also contribute to the risk of falls in elderly patients.
We grew up with the near-future promise of smart robots, but today we have little better than the Roomba robotic vacuum cleaner. What went wrong?8. Why do brains sleep and dream?
One of the most astonishing aspects of our lives is that we spend a third of our time in the strange world of sleep. Newborn babies spend about twice that. It is inordinately difficult to remain awake for more than a full day-night cycle. In humans, continuous wakefulness of the nervous system results in mental derangement; rats deprived of sleep for 10 days die. All mammals sleep, reptiles and birds sleep, and voluntary breathers like dolphins sleep with one brain hemisphere dormant at a time. The evolutionary trend is clear, but the function of sleep is not.
The universality of sleep, even though it comes at the cost of time and leaves the sleeper relatively defenseless, suggests a deep importance. There is no universally agreed-upon answer, but there are at least three popular (and nonexclusive) guesses. The first is that sleep is restorative, saving and replenishing the body’s energy stores. However, the high neural activity during sleep suggests there is more to the story. A second theory proposes that sleep allows the brain to run simulations of fighting, problem solving, and other key actions before testing them out in the real world. A third theory—the one that enjoys the most evidence—is that sleep plays a critical role in learning and consolidating memories and in forgetting inconsequential details. In other words, sleep allows the brain to store away the important stuff and take out the neural trash.
Recently, the spotlight has focused on REM sleep as the most important phase for locking memories into long-term encoding. In one study, rats were trained to scurry around a track for a food reward. The researchers recorded activity in the neurons known as place cells, which showed distinct patterns of activity depending upon the rats’ location on the track. Later, while the rats dropped off into REM sleep, the recordings continued. During this sleep, the rats’ place cells often repeated the exact same pattern of activity that was seen when the animals ran. The correlation was so close, the researchers claimed, that as the animal “dreamed,” they could reconstruct where it would be on the track if it had been awake—and whether the animal was dreaming of running or standing still. The emerging idea is that information replayed during sleep might determine which events we remember later. Sleep, in this view, is akin to an off-line practice session. In several recent experiments, human subjects performing difficult tasks improved their scores between sessions on consecutive days, but not between sessions on the same day, implicating sleep in the learning process.
Understanding how sleeping and dreaming are changed by trauma, drugs, and disease—and how we might modulate our need for sleep—is a rich field to harvest for future clues.
9. How do the specialized systems of the brain integrate with one another?
To the naked eye, no part of the brain’s surface looks terribly different from any other part. But when we measure activity, we find that different types of information lurk in each region of the neural territory. Within vision, for example, separate areas process motion, edges, faces, and colors. The territory of the adult brain is as fractured as a map of the countries of the world.
Now that neuroscientists have a reasonable idea of how that territory is divided, we find ourselves looking at a strange assortment of brain networks involved with smell, hunger, pain, goal setting, temperature, prediction, and hundreds of other tasks. Despite their disparate functions, these systems seem to work together seamlessly. There are almost no good ideas about how this occurs.
Nor is it understood how the brain coordinates its systems so rapidly. The slow speed of spikes (they travel about one foot per second in axons that lack the insulating sheathing called myelin) is one hundred-millionth the speed of signal transmission in digital computers. Yet a human can recognize a friend almost instantaneously, while digital computers are slow—and usually unsuccessful—at face recognition. How can an organ with such slow parts operate so quickly? The usual answer is that the brain is a parallel processor, running many operations at the same time. This is almost certainly true, but what slows down parallel-processing digital computers is the next stage of operations, where results need to be compared and decided upon. Brains are amazingly fast at this. So while the brain’s ability to do parallel processing is impressive, its ability to rapidly synthesize those parallel processes into a single, behavior-guiding output is at least as significant. An animal running must go left or right around a tree; it cannot do both.
There is no special anatomical location in the brain where information from all the different systems converges; rather, the specialized areas all interconnect with one another, forming a network of parallel and recurring links. Somehow, our integrated image of the world emerges from this complex labyrinthine network of brain structures. Surprisingly little study has been done on large, loopy networks like the ones in the brain—probably in part because it is easier to think about brains as tidy assembly lines than as dynamic networks.
10. What is consciousness?
Think back to your first kiss. The experience of it may pop into your head instantly. Where was that memory before you became conscious of it? How was it stored in your brain before and after it came into consciousness? What is the difference between those states
An explanation of consciousness is one of the major unsolved problems of modern science. It may not turn out to be a single phenomenon; nonetheless, by way of a preliminary target, let’s think of it as the thing that flickers on when you wake up in the morning that was not there, in the exact same brain hardware, moments before.
Neuroscientists believe that consciousness emerges from the material stuff of the brain primarily because even very small changes to your brain (say, by drugs or disease) can powerfully alter your subjective experiences. The heart of the problem is that we do not yet know how to engineer pieces and parts such that the resulting machine has the kind of private subjective experience that you and I take for granted. If I give you all the Tinkertoys in the world and tell you to hook them up so that they form a conscious machine, good luck. We don’t have a theory yet of how to do this; we don’t even know what the theory will look like.
One of the traditional challenges to consciousness research is studying it experimentally. It is probable that at any moment some active neuronal processes correlate with consciousness, while others do not. The first challenge is to determine the difference between them. Some clever experiments are making at least a little headway. In one of these, subjects see an image of a house in one eye and, simultaneously, an image of a cow in the other. Instead of perceiving a house-cow mixture, people perceive only one of them. Then, after some random amount of time, they will believe they’re seeing the other, and they will continue to switch slowly back and forth. Yet nothing about the visual stimulus changes; only the conscious experience changes. This test allows investigators to probe which properties of neuronal activity correlate with the changes in subjective experience.
The mechanisms underlying consciousness could reside at any of a variety of physical levels: molecular, cellular, circuit, pathway, or some organizational level not yet described. The mechanisms might also be a product of interactions between these levels. One compelling but still speculative notion is that the massive feedback circuitry of the brain is essential to the production of consciousness.
In the near term, scientists are working to identify the areas of the brain that correlate with consciousness. Then comes the next step: understanding why they correlate. This is the so-called hard problem of neuroscience, and it lies at the outer limit of what material explanations will say about the experience of being human.