The 2025 Nobel Prize in Physiology or Medicine has been awarded to three scientists for the discovery and development of optogenetics, a technique that lets researchers switch individual nerve cells on and off using light. The winners are Karl Deisseroth of Stanford University in the United States and the German biophysicists Peter Hegemann of Humboldt University in Berlin and Georg Nagel of the University of Würzburg. The prize was announced on Monday at the Karolinska Institute in Stockholm, opening Nobel week. The three share 12 million Swedish kronor, a little over one million euros.

The story began far from neuroscience. Hegemann was curious about how Chlamydomonas, a green alga found in ponds and puddles, swims toward light. In the 1990s he found that the alga contains a light-sensitive protein that converts light into an electrical signal within about half a millisecond. The protein forms a channel in the cell membrane that opens when light hits it, letting charged particles through. Many colleagues were initially sceptical that such light-gated channels existed. Around 2002 to 2003, Hegemann and Nagel identified the responsible gene and showed that when it was introduced into frog egg cells, and later into human kidney cells and hamster cells, the cells became light-sensitive and produced electrical signals.

Deisseroth, a psychiatrist and neuroscientist, then took the idea into the brain. He introduced the algal gene into the nerve cells of rats and later living mice, showing that neurons could be activated with blue light and that the signal passed on to other nerve cells. He named the technique optogenetics. In one experiment, light-controlled whisker movements were triggered in mice. In another, his team identified neurons that seemed to store a fear memory: reactivating them made mice show fear without any reason for it.

Researchers say the importance lies in moving from observation to proof. Earlier methods, such as electrical stimulation, affected all neurons in an area at once, even though they can serve very different functions. Scientists could therefore mostly show correlations, not causes. Optogenetics lets them target specific cell types and test directly whether they drive a behaviour. According to the Nobel committee, it has helped reveal which brain regions regulate fear, thirst, hunger, attention and social behaviour, and it is used to study conditions such as depression, addiction, anxiety, Alzheimer's and Parkinson's disease.

Scientists quoted by European media also stressed what the prize says about basic research. Wim Vanduffel of KU Leuven in Belgium noted that nobody expected work on algae to produce such a breakthrough. Applications are now being explored beyond the laboratory: in 2021, optogenetics was used for the first time to restore partial, rudimentary vision to a blind person, and researchers are investigating uses for hearing loss, Parkinson's disease, schizophrenia and epilepsy. Treatments remain hopes rather than established therapies. The physics prize will be announced on Tuesday.