Brain scientists will tell you that the greatest problem facing human biology, and perhaps all of science, is cracking the code of consciousness. It means solving the long-intractable brain-mind conundrum: How does our material brain--the most complex physical system known--produce our immaterial but vital sense of awareness? Neuroscientists and philosophers argue fiercely about how to solve the riddle and whether it is even solvable. Some say consciousness is illusory. (Try to counter that one--a real headache.) Others say consciousness exists but at too complex a level for humans to fathom, like quantum mechanics is for monkeys. Still others believe consciousness will yield its secrets only when we discover new physical brain laws that could reveal its creation.

Christof Koch rejects all this skepticism. As one of the world's leaders in the field, the California Institute of Technology neuroscientist believes that consciousness is distinctly physical, that it can be described by existing neurological theories, and that he is on the way to figuring it out. He has some invaluable help in collaborators such as Tomaso A. Poggio, the neural-networks and artificial-intelligence guru at the Massachusetts Institute of Technology, and some lasting inspiration instilled by his close friend and longtime collaborator, the late Francis Crick, who with James D. Watson won the Nobel Prize for discovering the double-helix structure of DNA. The key to finding an answer, Koch says, is to trace the activity of neurons--"the neural correlates"--of the simplest type of consciousness, which is the awareness of something we see. "Some of my colleagues think Im naive," Koch remarks, "that this rather narrow focus wont reveal the workings. And they might be right. But as a scientist, I think this is the most likely way to solve this problem."

Koch draws this faith in part from history, where biology's Big Problems have so often been unraveled with a focus on simple systems. Austrian monk Gregor Mendel discovered the mechanics of heredity by splicing pea plants; British naturalist Charles Darwin saw the operation of natural selection in barnacles, birds and dog breeding. More recently, neuroscientist Eric Kandel won a Nobel Prize in 2000 for revealing the microbiology of memory by studying sea slugs. Koch believes that defining the mechanism behind the simplest kind of visual consciousness could similarly open a door to understanding higher levels of consciousness. He is buoyed by the highly supportive environment at Caltech, where he heads the Computation and Neural Systems program; by the efforts of the 20 people in his lab; and by the stimulation he gets from sharing ideas with many colleagues, from Kandel to France's eminent neuroscientist Stanislas Dehaene of INSERM, who think he is on the right track.

Another Koch admirer is neurologist and writer Oliver Sacks, who has known Koch since Crick introduced them in the late 1990s. Sacks, who admits a bias stemming from his own fascination with visual consciousness, thinks Koch's inquiry is "both the most fascinating and the most promising approach" to the consciousness problem. "There's a brilliant directness to it," Sacks says. "And with his energy and his mental quickness--well, you have to give the man an excellent chance."

Koch's energy is indeed striking. He dyes his hair orange and purple and wears clothes in the same hues, all of which seem natural extensions of an incandescent intellect and a physical restlessness so profound that in his 40s--he is 48 now--he took up running marathons and scaling huge cliffs. He rock-climbs as often as possible at the towering slabs in Joshua Tree National Park, two hours east of Caltech's Pasadena, Calif., campus, and he has scaled big walls in Yosemite National Park, including the 3,000-foot face of El Capitan. He talks fast and adds lots of hand gestures.

The son of German diplomats, Koch was born in Kansas City, Mo., raised in Germany, Amsterdam and Morocco, and sounds like a brainy Arnold Schwarzenegger. When referring to consciousness, he often rubs the top of his multicolored head with his fingertips, his long arm bent high above his six-foot four-inch frame. He always seems hurried but fully engaged. In studying consciousness, he seems to have found his ideal subject. He loves it intellectually, as the most absorbing and fascinating scientific problem imaginable. And he loves consciousness phenomenally, as something to experience.

This became evident to me when I first heard his cowboy yell, near the end of a long day in New York City. He had delivered two lectures to philosophers at New York University, made a round of visits to neuroscience labs there, then skipped dinner to give a riveting talk at the New York Academy of Sciences at its mansion off Fifth Avenue, after which he answered questions from the bedazzled audience for two hours. Finally, he escaped, and we took a cab ride downtown to Pastis, a fashionable West Village bistro. We pushed our way to the bar and ordered ales. The beer, along with the vibrant room full of chic people and the prospect of dinner, so pleased him that he unleashed a hair-raising hoot: "Yeee-Hooo!" I looked around, but no one in the boisterous crowd had minded. Smiling, Koch held out his beer to clink glasses. "Wonderful!" he shouted, gesturing at the happy scene around him. "I love it!"

When Koch started investigating awareness--in his late 20s, freshly arrived and then untenured at Caltech--he was told repeatedly that pursuing a theory of consciousness was professional suicide. Throwing down such a gauntlet may well have motivated Koch even more. Science has changed since then, too, as an explosion in neuroscience, particularly the ability to image the physical bases of various mental states, has made the problem more approachable. Every month researchers find new links in the obscure chain between sense and sensibility. Most investigators think someone will pull the whole thing together in the next 25 years, possibly in the next decade. Whoever does will very likely win a Nobel Prize.

Koch was permanently drawn into consciousness research through work with Crick. After co-discovering the structure of DNA in 1953, Crick worked another two decades in England on microbiology and embryology before accepting an offer in 1976 to join the Salk Institute for Biological Studies in La Jolla, Calif. Soon after his arrival he became interested in the consciousness problem, seeing it as the great remaining unexplored question in biology. He and Koch met three years later at the Max Planck Institute for Biological Cybernetics in Tübingen, Germany, where Koch was finishing his Ph.D. in neural processing. In 1986, when Koch joined the Caltech faculty (which put him two hours away from the Salk Institute), the two began a conversation about consciousness that quickly blossomed into an ongoing collaboration involving countless phone calls, visits, dinners and many joint publications.

It was a rich partnership in which likenesses transcended differences. Crick was a very distinguished 70 then, an ever urbane and distinctly British presence. Koch, 40 years younger, seemed exceedingly flamboyant. But they shared a quickness of mind and intellectual irreverence, an appreciation of each other's wit and warmth, and the conviction that disciplined, results-oriented science could solve this nebulous problem.

Crick died last summer. Koch misses him constantly. "I so often catch myself, after I hear about a new experiment or a new idea, thinking that this is what Francis thrived on," he says. "And that I would have called him to tell him about it and he would chide me gently or get intrigued and ask lots of questions. More than any scientist Ive ever known, Francis could focus on the most important points but be willing to abandon an idea if something suggested it was wrong. So few can do that. He always had a sure sense of where to go."

Koch and Crick agreed early on to focus on visual consciousness, and Koch retains that focus today. He is not after the higher-order consciousness that allows us to dream, imagine a chain of events or think our way through abstract problems. Rather he seeks the collections of neurons and physiological processes that produce the awareness of a particular sight. It is a simple agenda of enormous complexity--a clean entrance, Koch hopes, into the labyrinthine workings of consciousness. Being aware of seeing something requires the coordinated work of multiple brain regions. Say you are walking along the edge of a park, admiring a building across the street, when a bird enters your field of view. Your retina shoots an impulse down the optic nerve and through the thalamus (near the center of your head), which relays the sensory input to cortical processing areas. The impulses then move on to the primary visual cortex, called V1, in the back of your brain. But you are still not aware of the bird.

That happens, Koch asserts, only through a competition between a temporary coalition of neurons associated with the bird and other coalitions associated with other objects vying for your visual attention--the building you were admiring, the red car approaching up the street alongside the park. Each coalition engages, by Koch's estimate, not just the thalamus and V1 but other parts of the visual cortex at the brain's rear, as well as key cortical columns--bundles of nerves that run vertically through the cortex's six layers--in the medial, temporal and frontal lobes. It's a winner-take-all contest. If the bird is striking enough or you favor birds, its neuronal coalition wins out, and it enters and dominates your visual consciousness: you stop admiring the building and watch the bird. [To experience this neuronal competition, see box on opposite page.]

Why would we need such awareness? To serve as a sort of "executive summary" of reality that allows quick, effective action, Koch says. He speculates that such consciousness developed at some point in mammals because an explicit attentional awareness conferred an evolutionary advantage in finding food, spotting and evading predators, and (later) negotiating social interactions. Although we have subsequently evolved higher conscious functions for language, long-term planning and abstract thought, this simple visual awareness came first and is likely to yield most readily to investigation.

The Ultimate Climb
Koch and Crick's strategy arose from careful study and imaginative contemplation of modern neuroscience. In the past decade, the larger neuroscience and philosophy-of-mind community has come to consider this strategy one of the most promising empirical approaches to unveiling consciousness. Koch must now try to fulfill that promise. Because he is looking at the big picture (if in a concentrated way), he must track, evaluate and account for every major finding in neuroscience. Most of his own research focuses on identifying particular nodes and links that form the neural pathways that filter and create particular visual percepts (the mental result of perceiving, distinct from the act of perceiving). His work often involves cutting-edge tools such as brain scans, electroencephalography and neuronal probes to study monkeys or people as they view objects, faces or perception tricks.

As these investigations tie specific conscious perceptions to particular neuronal activity, Koch must find ways to test those correlations by turning on and off the suspected neurons in animals and, eventually, humans. Such "switching" methods currently involve electrical stimulation or nerve freezing; future techniques may include prodding genes to temporarily halt transmissions from targeted neurons. Koch and others have made remarkable progress in all these areas in just the past five years. But this is unpredictable work, and the possibility of following the wrong trail haunts them all.

Koch appears energized by the challenge. Actually he seems energized by everything. At Pastis he easily lasts till midnight, fueled by sea bass and a bottle of Beaujolais until, postdessert, with a glass of Armagnac at hand, he relates how crushed he was when his kids left home for college a couple of years before.

"Oh it was terrible!" he says laughing, talking as fast as ever. (Koch does not use commas.) It provoked the classic midlife crisis in which of course I am supposed to get a sports car and drive off with a student. But I didnt. My wife got a sports car. A Porsche. I got a new kitchen--new cabinets, range, black marble counters." He waves his hands along the appropriate planes to put cabinets and counters in place. "But it wasnt enough. The house is too quiet. Even with the three dogs. So I climb."

A few minutes later we exit into a snowstorm. The wind smacks wet flakes into our faces. We are getting ready to part when I remember I have brought him a gift, a collection of mountaineering accounts by the British climbers Peter Boardman and Joe Tasker. He wants to see it immediately, so we huddle in a doorway and look at photographs of Boardman and Tasker in the Himalayas, exposed on the planet's most austere heights: Changabang, Kanchenjunga, Gaurisankar.

"They're dead now," I tell him, "lost on Everest in 1982."

"But still," he says. He is entranced, shaking his head. "Look at this. It is Boardman and Tasker ascending the knife-edge ridge of Gaurisankar--an insane climb they somehow made and lived through. I mean, to do this--can you imagine? What a thing! I love it." Koch is so delighted he punches me in the arm and lets out another cowboy yell. Then he heads off through the snow.