The Ultimate Brain Quest
Deciphering how human thought works is mind-bendingly difficult, but researchers now know where to start.
Thomas Deerinck/NCMIR/Photo Researchers
Nerve cells in the brain. Neurons are supported and protected by smaller, and even more numerous, glial cells.
"Every day we recall the past, perceive the present and imagine the future. How do our brains accomplish these feats? It's safe to say that nobody really knows," Sebastian Seung writes early in Connectome, his exploration of how researchers have at least made a start toward understanding how those feats are accomplished. Mr. Seung, a professor of brain science at the Massachusetts Institute of Technology, is an amiable guide, witty and exceptionally clear in describing complex matters for the general reader.
He begins with the observation that each of us is unique, differing from one another in uncountable ways. These differences arise in part because we have different genes that influence brain development and, accordingly, behavior. Also important are gene-by-environment interactions and gene expression: You might have a genetic predisposition toward alcoholism, but without the right environmental triggers, that gene may never become active. The sequencing of the human genome has provided a map of our 20,000 genes, and we are gradually gaining insights into what these genes do, individually and in combination.
Neuroscientists posit that all of our hopes, desires, beliefs and experiences are encoded in the brain as patterns of neural firings. Just how this happens is not precisely understood, as the author attests, but we have made great strides in understanding how neurons communicate with one another. Progress has also been made in mapping which brain systems control which kinds of operations (my own field of research): One system is responsible for lifting your foot, another senses the pain when you stub your toe; one system helps you to solve arithmetic problems, another enjoys "La Bohème." A new approach to studying brains and individual differences involves making maps of how neurons connect to one another. Following the term genome, these are called connectomes.
"Why study connectomes if genomics is already so powerful?" Mr. Seung asks. "The answer is simple: Genes alone cannot explain how your brain got to be the way it is. As you lay nestled in your mother's womb, you already possessed your genome but not yet the memory of your first kiss."
The human brain contains 100 million neurons, and each neuron makes thousands of connections on average. If we assume that each distinct connection pattern gives rise to a distinct brain state—like the effervescent sensation after that first kiss—the number of brain states exceeds the number of known particles in the universe. Your experiences, memories, personality and thoughts are thus encoded in the ways your neurons connect to one another. The next big frontier is mapping those trillions of neural connection patterns to their brain states. By observing a particular network of neurons firing, researchers should know (in theory) whether you are thinking about love or money, beer or burgers.
The associational networks of your brain determine to a large degree how you understand the world and your place in it. When prompted with "red" you might respond "apple," while another person may respond "flame." Your fears and phobias, likes and attractions, were influenced by a lifetime of outcomes and associations. The information thus contained in your connectome is what sets you apart from everyone else, what describes your unique personality. "Information," according to Mr. Seung, "is the new soul."
But before readers can profitably learn about the connections, they need to understand something about neural physiology, neurotransmitters, synapses and neuroanatomy. Mr. Seung is equal to the task: Connectome offers the equivalent of a college course on neuroscience, covering such technical matters as spike trains, cortical layering, ion channels, and the function and structure of axons and dendrites. He a