The deepest significance of this year’s Nobel Prize in Physiology or Medicine lies not simply in what Karl Deisseroth, Peter Hegemann and Georg Nagel discovered, but in the new kind of question their work has made possible. For centuries, the brain has been the ultimate scientific black box: its billions of interconnected cells could be observed, but understanding precisely what individual circuits did was extraordinarily difficult. Optogenetics has begun to change that.

The breakthrough rests on an ingenious marriage of biology and light. Hegemann and Nagel helped establish the importance of channelrhodopsin, a light-sensitive protein found in algae. Deisseroth subsequently showed how such proteins could be used to control nerve cells. By making selected neurons responsive to light, researchers gained the ability to activate or silence particular circuits and then observe what followed. That distinction is crucial. It turns neuroscience from a discipline that largely asks what happens in the brain into one that can increasingly ask whether a particular circuit causes something to happen.

Researchers can investigate the neural mechanisms associated with memory, emotion and behaviour with a precision that earlier techniques could........

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