Medical writing about the mind and brain is in rude health. Sigmund Freud was its first successful modern exponent, though his writing sometimes strayed too far towards imaginative literature. Like Freud, Karl Deisseroth tries to locate human behaviour and feeling within neurons and the energy flowing through them. Unlike Freud, when Deisseroth talks about such things he is not being metaphorical – at least not always. A professor of bioengineering and psychiatry at Stanford University, he brings a unique perspective to bear on fundamental questions about our capacity for love, for enduring pain and for sometimes causing it.

Deisseroth is no dabbler in the laboratory; nor is he a mere translator of neuroscience into accessible language. He’s a genuine pioneer whose prize-winning research is redefining our understanding of the brain. He is also a practising clinician who emphasises that his clinical work is a powerful guide to his scientific thinking.

There are many tools of the neuroscientist’s trade, though no one technique can provide a comprehensive picture of how the brain operates. We can look to ‘experiments of nature’, where brain damage reveals what we have through what has been lost. With modern imaging technology, we can observe shifting levels