Why does a tiny green alga called Chlamydomonas always swim toward the light? That question, asked decades ago by a curious German researcher, has now won its answer a Nobel prize — and reshaped how scientists study the brain itself. The 2026 Nobel prize in physiology or medicine has been awarded to Karl Deisseroth of Stanford University, Peter Hegemann of Humboldt University of Berlin, and Georg Nagel of the University of Würzburg. Hegemann first noticed that the alga's single "eye spot" held a light-sensitive protein, one that converted sunlight into an electrical signal almost instantly, steering the organism toward the glow.

Working together, Hegemann and Nagel identified the molecules responsible, called channelrhodopsins, and showed that shining blue light on them let charged particles flood into a cell, triggering an electrical pulse. When they inserted the protein's genetic code into mouse and human kidney cells in the lab, something startling happened: ordinary cells became sensitive to light. Deisseroth then carried the idea further, inserting the channelrhodopsin gene into the nerve cells of rats. The result was optogenetics, a technique that lets researchers switch individual neurons on or off with a beam of light, as precisely as flicking a light switch in a vast, dark house full of rooms.

Jonathan Levin, Stanford's president, said the discovery has deepened understanding of how neural circuits shape emotion and behaviour, with implications reaching into treatments for psychiatric and neurological disorders from Parkinson's to depression. The three laureates will share 12 million Swedish kronor, about £900,000. Their win also lands amid a quieter, less celebrated statistic: of 235 people who have received this prize since 1901, only 14 have been women, and not one has been Black.