ividual 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 also throws in a bit of neurochemistry, assuming no prior knowledge of biology, chemistry or psychology.
Except for the last two chapters, the book is not so much about connectomes as about how to use connectomes to frame neuroscience. We learn about the tools of the neuroscience trade—how they work and why they are important. From there we learn how patterns of neural connections give rise to our perceptions of the world, our reactions to perception and, ultimately, our uniqueness. Although obtaining connectomes for even the simpler brain of the mouse is beyond our technological abilities today, Mr. Seung lays out the technical hurdles and proposes some more attainable near-term goals. But he also makes a passionate case for a decade-long investment of time and energy, comparable to the Human Genome Project, to advance the cause.
Even in the current speculative realm, the connectome is a fascinating, occasionally frustrating, subject. One view is that each connectome or connectivity map will give rise to one and only one brain state and that different connectomes cannot give rise to the same state. Yet the widespread use of pharmaceutical agents such as Prozac and Ritalin suggests otherwise—that knowing the connectome is unlikely to tell us all we need to know about a person's thoughts, feelings, opinions and personality.
The levels of various chemicals in our brains can clearly be altered pharmaceutically. They are also influenced by diet, exercise, stress and normal biological cycles. Even if we know how the neurons are connected and the strength of their synapses, the amount of dopamine, for example, that is available in the brain at any given moment will influence firing patterns. This could cause the same neural network (a group of connected neurons) to give rise to different thoughts or different networks to give rise to similar thoughts.
Despite the fact that we have different brains and different neuronal configurations, when you and I smell a skunk it seems likely that we have equivalent mental states th