2026 Nobel Prize in Medicine: light, transformed into a switch for the brain

On Monday, the Karolinska Institute in Stockholm awarded the 2026 Nobel Prize in Medicine to the American Karl Deisseroth and the Germans Peter Hegemann and Georg Nagel for their discoveries concerning light-activated ion channels and optogenetics. According to the committee, this method has ushered in ‘a new era in neuroscience’: it allows us to see how nerve cells shape memories, emotions and behaviours in a living brain.

Neurons communicate via small electrical impulses. Optogenetics involves equipping a specific group of neurons with a light-sensitive protein, so that a flash of light can ‘switch them on’ or ‘switch them off’ at will. To achieve this, the instructions for producing the protein are introduced into these cells using genetic techniques; afterwards, a very fine optical fibre directs the light to the exact location in the brain.

The key is precision. Unlike drugs or electrodes, which affect everything in their path, light acts only on the selected cells, and does so in a fraction of a second.

The story begins far from the brain. In the early 2000s, Hegemann wanted to understand how the single-celled alga Chlamydomonas manages to swim towards the light. Together with Nagel, he identified the protein responsible: channelrhodopsin, a tiny pore that opens when exposed to light and allows electrical charges to pass through. They also found that it worked in any cell into which it was introduced.

In 2005, Deisseroth’s team at Stanford University made the decisive breakthrough: they inserted the channelrhodopsin gene into rat neurons and succeeded in activating them with pulses of blue light. Shortly afterwards, proteins were added that do the opposite – silencing the neurons – and the switch was complete.

For two decades, laboratories around the world have used this technique to move from simply observing the brain to actively interrogating it. It is no longer enough to see which area is activated or involved during a task: it is now possible to determine whether a specific group of neurons is necessary, or sufficient, to trigger that task.

It has thus been demonstrated, in laboratory animals, that activating just a few neurons can reactivate a specific memory, trigger or inhibit a movement, or modify behaviours such as fear, reward-seeking or anxiety. The technique has also been extended to other brain cells, such as astrocytes, and has revealed that they are not merely companions to neurons, but active participants.

This knowledge has laid the foundations for understanding many neurological disorders. In Parkinson’s disease, optogenetics has helped to identify which circuits are malfunctioning and why deep brain stimulation works. In epilepsy, animal studies have shown that it is possible to detect a seizure and stop it using light in real time. In Alzheimer’s disease, experiments in mice suggest that some memories are not erased, but rather become inaccessible, and that they can be recovered by stimulating the neurons that store them.

The technique has already reached patients: in 2021, the partial recovery of vision was reported in a person with retinitis pigmentosa. Applying it to the human brain still requires overcoming safety challenges, but the map it has drawn up is already indispensable.