Brain cells, switched by light

Neuroscientists could watch the brain at work long before they could test it. They saw which regions lit up during fear or sleep, but they could not switch a specific cell type on or off to see what changed.

The tools of the time were too blunt: electrodes stimulate every cell around their tip, while drugs act too slowly and spread too widely.

The answer from pond life

Green algae swim towards light, steered by light-sensitive proteins. In 2002 and 2003, Georg Nagel, Peter Hegemann and their colleagues showed that two of these proteins, channelrhodopsin-1 and channelrhodopsin-2, are channels that open when light falls on them. A single protein senses light and simultaneously allows an electric current through the cell membrane.

In 2005, Karl Deisseroth's group introduced channelrhodopsin-2 into mammalian nerve cells. A flash of blue light made the cells fire within milliseconds. By combining genetic targeting, which ensures only chosen cell types produce the protein, with fine optical fibres to deliver light deep into the brain, researchers could control defined cells in a living, moving animal. The method was named optogenetics. Later developments allowed scientists to switch cells off as well as on.

From correlation to cause

In mice, light revealed the neural circuits governing sleep, fear, anxiety, depression and movement. Stimulating one small group of neurons, for example, makes a sleeping mouse more likely to wake within thirty seconds.

Between 2006 and 2025, PubMed listed roughly 14,500 papers mentioning optogenetics. Stanford's training programme has since taught thousands of scientists, and these engineered genes have been shared with laboratories worldwide.

From algae to patients

Results in human trials are now beginning to arrive. Retinitis pigmentosa destroys the light-sensing cells of the eye. In a Phase I/II trial for patients with advanced disease, a gene therapy made surviving retinal cells sensitive to light, paired with goggles that projected light onto the treated eye. Six out of ten participants showed a clinically meaningful improvement in light sensitivity. Among those who completed visual tasks, four out of eight performed better at finding, locating or touching objects while wearing the goggles. The results were published in the New England Journal of Medicine in October 2026. Though this is a small, early trial and the treatment remains unapproved, it marks an important step.

The Nobel Prize in Physiology or Medicine 2026 was awarded jointly to Karl Deisseroth of the Howard Hughes Medical Institute and Stanford University, Peter Hegemann of Humboldt University of Berlin and Georg Nagel of the University of Würzburg, "for their discoveries concerning light-gated ion channels and optogenetics".