Switching the Brain on with Light: The 2026 Nobel Prize in Physiology or Medicine

On 5 October, the Nobel Assembly at Karolinska Institutet announced that the 2026 Nobel Prize in Physiology or Medicine goes to Karl Deisseroth, Peter Hegemann and Georg Nagel for their discoveries on light-gated ion channels and optogenetics. Optogenetics is a method that allows researchers to switch individual nerve cells on or off inside a living brain using light, and it has been used to study how neurons give rise to memories, emotions and behaviour. The story starts with a green alga that swims towards the sun. How did a pond organism end up in a Nobel citation, and what has it changed for people so far?

A wish written down in 1979

The human brain contains roughly 86 billion neurons of many different types, packed side by side and wired to one another in ways that we are still trying to comprehend. For most of the twentieth century, researchers could listen to these cells with electrodes or stimulate them with electric current. The trouble with an electrode is that it excites whatever sits within reach, whether that is the cell of interest, its neighbours or the fibres that happen to pass through. In 1979, Francis Crick wrote that a great obstacle for neuroscience was the lack of a way to control one type of neuron while leaving the others alone. He later suggested that light might do the job, since a beam can be aimed at a target and switched on and off within milliseconds. Neurons, however, do not respond to light. Someone would have to give them that ability.

An alga with a sense of light

Peter Hegemann spent much of his early career asking how Chlamydomonas reinhardtii, a single-celled green alga, finds light. The cell has a tiny eyespot and swims towards moderate light, which feeds its photosynthesis, and away from light that is too strong. Hegemann and colleagues observed that the alga reacted within milliseconds, far faster than a chain of chemical reactions would allow. That speed suggested that the light sensor was itself an electrical gate in the cell membrane.

Around the year 2000, sequences from the alga’s genome began to appear in public databases. Hegemann and Georg Nagel, a biophysicist who worked on ion channels in Frankfurt with Ernst Bamberg, found two genes that resembled the light-sensitive pigments of microbes. Nagel placed them in frog eggs and in cultured human cells, switched on a lamp and measured an electric current. The first protein, described in 2002, let protons through. The second, published in 2003 and named channelrhodopsin-2, opened under blue light and let positively charged ions such as sodium flow into the cell.

In plain terms, neurons talk to each other through electrical impulses, produced when ions flow through channels in their membrane. Channelrhodopsin is a channel with a light-sensitive lock. Blue light opens it, as ions enter and if enough of them do, the neuron fires.

From a curiosity to a tool

Karl Deisseroth, a psychiatrist and bioengineer at Stanford, treated patients with depression and other disorders, and knew how little the field could say about the neuronal circuits involved. In 2005, his laboratory, with Edward Boyden and Feng Zhang among the authors, delivered the channelrhodopsin-2 gene into mammalian neurons using a virus. When pulses of blue light reached the cells, they fired with millisecond precision and fell silent when the light stopped. The result appeared in Nature Neuroscience, one of the most renowned journals in the field.

What turned the finding into a tool was genetic targeting. By attaching the gene to control sequences that are active in only one cell type, researchers made sure that only those cells produced the protein. Light can then be delivered to a mouse through an optical fibre implanted in the skull, and only the labelled cells respond. Later work added proteins that silence neurons instead of exciting them, along with variants that react to other colours and open faster or slower. With both an on and an off switch available, a circuit could be tested for whether it is necessary for a behaviour as well as whether it is sufficient to trigger it.

What the switch has shown

What do you learn when you can turn a handful of cells on? In 2007, shining light on orexin-producing neurons in the hypothalamus woke sleeping mice. In 2012, Susumu Tonegawa’s group labelled the neurons that were active while mice learned to fear a chamber. Reactivating those cells with light made the animals freeze in a different, harmless chamber, which showed that a memory could be switched on artificially. In models of Parkinson’s disease, Deisseroth’s laboratory reported in 2009 that stimulating the connections that arrive from the cortex relieved symptoms, a clue about how deep brain stimulation, a treatment already used in patients, may achieve its effect.

The same logic has been applied to anxiety, feeding, pain, addiction and social behaviour. Each experiment connects a behaviour to a defined group of cells, and that connection is what a drug or a surgical target needs.

From mice to patients

Clinical use is much newer. In 2021, a team led by José-Alain Sahel and Botond Roska reported in Nature Medicine a man blind from retinitis pigmentosa who regained some vision. A virus had carried a channelrhodopsin variant into cells of his retina, and he wore goggles that translated camera images into pulses of amber light. After months of training, he could locate and count objects on a table and reportedly make out the stripes of a pedestrian crossing. Researchers such as Tobias Moser in Göttingen are working on optical cochlear implants, which could separate sound frequencies more finely than the electrical devices in use today.

The caveats are considerable. The retinal result concerns one patient and a partial recovery. Reaching neurons deep inside a human brain requires gene therapy and a light source placed in the tissue, and both carry risks that a mouse study does not have to answer for. Optogenetics is, for now, far more a laboratory instrument than a treatment.

A prize with three names

A Nobel Prize can hold at most three laureates, and this story has more people in it. Ernst Bamberg collaborated with Nagel on the experiments that characterised channelrhodopsin. Edward Boyden was part of the 2005 paper that brought the protein into neurons. Gero Miesenböck had shown in 2002 that fly neurons carrying light-sensitive proteins could be controlled with light, using a different set of genes. Which discoveries deserve a place on the stage is a question the Nobel Committee answers every year, and every year the answer leaves colleagues outside the room.