Making up our changing minds

NEUROSCIENCE:Sebastian Seung plans to map the billions of neural connections in our brains in order to discover what makes each of us unique

Connectome: How the Brain’s Wiring Makes Us Who We Are By Sebastian Seung Allen Lane, 359pp. £20

‘NO ROAD, NO TRAIL can penetrate this forest. The long and delicate branches of its trees lie everywhere, choking space with their exuberant growth . . . All the trees of this dark forest grew from 100 billion seeds planted together. And, all in one day, every tree is destined to die.”

This is the arresting opening of Sebastian Seung’s Connectome: How the Brain’s Wiring Makes Us Who We Are, one of the most eagerly awaited scientific books of the year. He is describing the human brain, “an organ so vast in its complexity that it might as well be infinite”. Only the most determined and fearless kind of explorer could hope to penetrate such terrain. But Seung has already set out to do just that: to map all the connections in the brain, neuron by neuron, synapse by synapse.

Consider the scale of his ambition: the human brain contains 100 billion neurons, with each neuron making thousands of connections, or synapses. Seung uses the analogy of the complex flight maps found in the back of airline magazines, with each neuron an airport and each synapse a route between cities. But in the case of the human brain this flight map includes 100 billion cities with 10,000 flights per day from each. That unimaginably complex map is what he calls the “connectome”.

Described by Steven Pinker as a rising star in neuroscience, the young and charismatic Seung is professor of computational neuroscience at Massachusetts Institute of Technology, having originally trained as a physicist at Harvard. What makes him such a star is not just his dazzling intelligence, on full view in this book, but also his intellectual boldness.

Why map the connectome? Because therein lies the explanation of human uniqueness, why you think, feel and react the way you do and not the way your sibling, spouse or best friend does. Everything about us – personality, emotions, thoughts, memories – is encoded in these dense, tangled patterns of neural connections. Seung states his proposition simply. People are different – or, more precisely, minds are different – because connectomes differ. You are your connectome.

His choice of the word connectome rather than the more conventional term “wiring diagram” is revealing, evoking the word genome, meaning an individual’s entire DNA sequence. Seung thus lays claim to a field of discovery as potentially revolutionary in its scientific impact as genomics. But he goes further: the connectome theory of individual differences is not just compatible with genetic theory but richer, more complex, less deterministic. It includes the effect of experiences on the brain and thereby opens up the possibility of reshaping our connectome by changing the way we think and react.

At the moment of conception you already possess your genome, your unique DNA sequence, and that does not change. But at every moment of your life thereafter you are laying down memories of your experiences and what you have learned from them. So, unlike your fixed genome, your connectome keeps changing. Our minds are influenced by genes, especially when the brain is wiring itself up during infancy and childhood, but we play an important role in wiring our own brains.

To explain the complex interplay between our fixed and changing nature, Seung uses the analogy of a stream flowing through a stream bed. The stream bed is your connectome, made from your unique patterns of neural synapses, influenced by both your genome and your previous experiences. The stream is the flow of neural activity created by your day-to-day experiences. Just as the contours of the stream bed define and guide the flow of the stream, the constant flow of experiences slowly moulds and changes the stream bed over time.

The connectome or stream bed changes in four basic ways, what Seung calls the four Rs. Neurons reweight their connections by making them stronger or weaker. They reconnect them by creating or destroying synapses. They rewire them by growing or pulling back branches. Or they regenerate them by creating entirely new neurons and eliminating existing ones.

The most important goal of neuroscience, Seung argues, is to harness the power of the four Rs. If that is successful, then neuroscience can play a profound role in understanding and treating baffling conditions such as autism and schizophrenia, or find ways to heal Parkinson’s disease and brain injuries. It could give us the knowledge to affect real personal change. “In the end,” he says, “every important goal in life boils down to changing our brains.”

To give us a sense of the scale of the challenge, he notes that it took more than 12 years to map the 7,000 connections in the nervous system of the 1mm-long lowly roundworm. The connectome in the human brain is 100 billion times bigger. Yet, undaunted, he predicts that the human connectome will be mapped by the end of this century.

His confidence is based on the powerful technologies now available, the capacity of computers to collect and store immense databases of brain images generated by sophisticated machines such as electron microscopes, new “staining” methods, or “diamond knives” that can cut brain tissue as thin as 50 nanometres, 1,000 times thinner than a human hair. He describes these tools of the trade with precision, clarity, even love.

He argues persuasively for more sustained investment in these new technologies, and for the kind of bold, long-term ambition that made the Human Genome Project possible, the boldness to take on a challenge that would require a decade of intense scientific effort, such as mapping the entire connectome of a mouse or just one region of the human brain. He reminds the sceptics that before the Human Genome Project began, sequencing an entire human genome also looked impossible.

This is an intellectually exhilarating and scientifically daring book, beautifully written, exquisitely precise yet still managing to be inspirational. But be warned: it is not a book for the fainthearted. Be prepared to grapple with terms such as dendrites, axons, neurotransmitters, pyramidal tracts, multiple converging synapses and postsynaptic density. It provides a (very good) crash course in neuroscience.

If you stick the course there is a little light relief in the last few chapters, where Seung launches energetically into a discussion of immortality. There is an interesting, if slightly bizarre, detour into cryonics and the Alcor Life Extension Foundation, which, in return for $200,000, will freeze your body indefinitely in the hope that one day science will be capable of resurrecting you. He explores the possibility of “uploading” your connectome, effectively transferring your mind and identity to a computer, and thereby living immortally as a computer simulation.

He predicts that a prerequisite, a computer powerful enough to simulate the electrical signals circulating in your brain, could conceivably exist by the end of this century. He is, of course, not promising immortality, but for those who dream on he offers the assurance that their only hope is the mapping of the connectome.

My dream is more practical. You know one of the questions beloved of profile writers: who are the guests you would choose to have at your dream dinner party? For mine, Sebastian Seung is already sitting on one of the chairs.

Dr Maureen Gaffney is adjunct professor of psychology and society at University College Dublin. Her book Flourishing is published by Penguin Ireland