Forest, jungle, labyrinth, maze – the human brain attracts dizzying analogies, and with good reason: our brains contain around one hundred thousand million nerve cells (neurons), linked by ‘millions of miles of gossamer neurites’ permitting around one thousand million million interconnections (synapses), the points at which cells exchange information with one another. It has long been thought that these interconnections provide a key to human nature: while the broad pattern of connections between brain regions is similar in every healthy human brain, their details – their number, size and strength – are thought to underpin our individuality, as synapses are ‘plastic’, shaped by experience. This shaping process obeys the ‘Hebbian rule’, named after the influential Canadian psychologist Donald Hebb, which dictates that ‘cells that fire together wire together’. It is the consistent coactivation of brain cells representing your kitchen or your beloved, for example, and the resulting synaptic changes in your brain, that allow you to summon them up in your mind’s eye without, I would imagine, too much trouble. The sum total of the connections within your brain, your ‘wiring diagram’, constitutes your ‘connectome’. In Sebastian Seung’s view, argued provocatively here, ‘you are your connectome’.
His book is an engaging introduction to the study of the human wiring diagram. This project has been given fresh impetus by some recent techniques facilitating the tracing of neuronal interconnections. These range from methods working at very fine scales, examining individual synapses in slices of brain under huge magnifications,