In his quest to nail down the slippery stuff of consciousness, the neuroscientist David Eagleman studies the neural and psychological circuitry of the brain’s biological clocks.
Just how many clocks we contain still isn’t clear. The most recent neuroscience papers make the brain sound like a Victorian attic, full of odd, vaguely labelled objects ticking away in every corner. The circadian clock, which tracks the cycle of day and night, lurks in the suprachiasmatic nucleus, in the hypothalamus.
The cerebellum, which governs muscle movements, may control timing on the order of a few seconds or minutes. The basal ganglia and various parts of the cortex have all been nominated as timekeepers, though there’s some disagreement on the details. The standard model, proposed by the late Columbia psychologist John Gibbon in the nineteen-seventies, holds that the brain has “pacemaker” neurons that release steady pulses of neurotransmitters. More recently, at Duke, the neuroscientist Warren Meck has suggested that timing is governed by groups of neurons that oscillate at different frequencies. At U.C.L.A., Dean Buonomano believes that areas throughout the brain function as clocks, their tissue ticking with neural networks that change in predictable patterns. “Imagine a skyscraper at night,” he told me. “Some people on the top floor work till midnight, while some on the lower floors may go to bed early. If you studied the patterns long enough, you could tell the time just by looking at which lights are on.” Time isn’t like the other senses, Eagleman says. Sight, smell, touch, taste, and hearing are relatively easy to isolate in the brain. They have discrete functions that rarely overlap: it’s hard to describe the taste of a sound, the color of a smell, or the scent of a feeling. But a sense of time is threaded through everything we perceive. It’s there in the length of a song, the persistence of a scent, the flash of a light bulb. “There’s always an impulse toward phrenology in neuroscience—toward saying, ‘Here is the spot where it’s happening,’ ” Eagleman told me. “But the interesting thing about time is that there is no spot. It’s a distributed property. It’s metasensory; it rides on top of all the others.” The real mystery is how all this is coördinated. When you watch a ballgame or bite into a hot dog, your senses are in perfect synch: they see and hear, touch and taste the same thing at the same moment. Yet they operate at fundamentally different speeds, with different inputs. Sound travels more slowly than light, and aromas and tastes more slowly still. Even if the signals reached your brain at the same time, they would get processed at different rates. The reason that a hundred-metre dash starts with a pistol shot rather than a burst of light, Eagleman pointed out, is that the body reacts much more quickly to sound. Our ears and auditory cortex can process a signal forty milliseconds faster than our eyes and visual cortex—more than making up for the speed of light. It’s another vestige, perhaps, of our days in the jungle, when we’d hear the tiger long before we’d see it. In Eagleman’s essay “Brain Time,” published in the 2009 collection “What’s Next? Dispatches on the Future of Science,” he borrows a conceit from Italo Calvino’s “Invisible Cities.” The brain, he writes, is like Kublai Khan, the great Mongol emperor of the thirteenth century. It sits enthroned in its skull, “encased in darkness and silence,” at a lofty remove from brute reality. Messengers stream in from every corner of the sensory kingdom, bringing word of distant sights, sounds, and smells. Their reports arrive at different rates, often long out of date, yet the details are all stitched together into a seamless chronology. The difference is that Kublai Khan was piecing together the past. The brain is describing the present—processing reams of disjointed data on the fly, editing everything down to an instantaneous now. How does it manage it? The mind-body problem has been vexing Eagleman longer than most. Even as a boy, his mother told me, he had a tendency to “dissociate himself”—to assess his own inner workings from a cool, analytical distance. “My brain can do this,” he’d say. His mother was a biology teacher, his father a psychiatrist often called upon to evaluate insanity pleas, but their son was a creature outside their usual experience. “There were things about Dave that were a little bit funny,” his mother says. He wrote his first words at the age of two, on an Underwood typewriter. At twelve, he was explaining relativity to them. One of his favorite tricks was to ask for a list of random objects, then repeat it back from memory—in reverse order, if people wished. His record was four hundred items. As an undergraduate at Rice, Eagleman wanted to be a writer, but his parents persuaded him to major in electrical engineering instead. “It was like chewing on autumn leaves,” he says. An extended sabbatical ensued. After his sophomore year, Eagleman joined the Israeli Army as a volunteer, then spent a semester at Oxford studying political science and literature, and finally moved to Los Angeles to become a screenwriter and a standup comic. Nothing took. “I knew I had some intellectual horsepower,” he says. “But I didn’t know where my tires would catch purchase.” Back at Rice, he began to read books about the brain in his spare time and decided to take a course in neurolinguistics. “I was immediately enchanted just by the idea of it,” Eagleman says. “Here was this three-pound organ that was the seat of everything we are—our hopes and desires and our loves. They had me at page one.” Mathematicians, like rock musicians, tend to do their best work in their twenties and thirties. Not so neuroscientists. The Nobel Prizes in the field are usually earned in mid-career, after a few false starts and fruitless sidetracks. “Biology is special that way,” Eagleman says. “It takes years for people to get a feeling for the organism—for how nature actually works. Young people come in all the time knowing a bunch of fancy math. They say, ‘What if it’s like this computational model, this physical problem?’ They’re terrific ideas, but they’re wrong. Nothing works the way you think it should.” Eagleman was speaking from experience. As a grad student at Baylor, he leaned especially hard on his math skills at first, having had so little training in biology. For his doctoral work, he programmed a piece of virtual neural tissue so complex that it tied up the Texas Medical Center’s new supercomputer for days, prompting complaints from all over the university. “I remember, when he was writing it, he had a sack of raw potatoes under his desk,” his dissertation adviser, Read Montague, told me. “He would cook a potato in the microwave, put it in a cup, and lean over and bite it while he was typing. It kind of set the tone for my lab for the succeeding decade. It chased away the faint of heart.” Eagleman’s program was a theoretical as well as a technical feat: it showed that brain cells can exchange information not just through neurotransmitters but through the ebb and flow of calcium atoms. He went on to earn a postdoc at the prestigious Salk Institute, near San Diego. Once there, though, he fell under the spell of Francis Crick, a biologist interested in more than clever simulations. Crick was eighty-three when Eagleman met him, in 1999. He had won the Nobel Prize with James Watson almost forty years earlier, for deciphering the structure of DNA, but his research had taken a hard left turn since then, from genetics to the study of consciousness. “We’d have these seminars and he’d sit there and his head would nod, and I’d think, Oh, poor guy, the tolls of senescence,” Eagleman recalls. “Then he’d get this smile on his face and raise his hand—and just disembowel the speaker. I’d never seen anything like that.” For decades, brain researchers had taken their lead from behaviorists like B. F. Skinner. They treated their subject as a machine like any other, with inputs, outputs, and a shadowy mechanism in between. But Crick and a handful of other researchers believed that it was time to pry open Skinner’s black box—to at least begin to identify the mechanics of individual awareness. “When I started out, you basically weren’t allowed to talk about it,” Eagleman says. “Why does it feel like something to be alive? Why, when you put together millions of parts, does something suddenly have a sense of itself? All of this went out the window after B. F. Skinner. And it took a guy with Crick’s gravitas to come in and say, ‘You know what? This is a scientific problem—the most exciting of our time.’ ” Crick called it the scientific search for the soul.
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