Karl Deisseroth has spent his career asking how a brain produces experience, and what happens when that experience becomes unbearable. He helped develop a way to investigate those questions by controlling nerve cells with light.
On October 5, 2026, he was named a recipient of the Nobel Prize in Physiology or Medicine, sharing the honor with Peter Hegemann and Georg Nagel for discoveries concerning ion channels activated by light and optogenetics. Their work changed the questions scientists could ask of a living brain.
For Karl, those questions have a human urgency. A psychiatrist and professor of bioengineering at Stanford, he has cared for people whose depression resisted treatment, whose experience of the world became difficult to communicate. In the consulting room, he could listen and offer treatment. In the laboratory, he confronted how much remained unknown about the biology behind their suffering.
He studied biochemistry at Harvard, then earned his doctorate and medical degree at Stanford. Literature mattered to him throughout his training. His writing pays close attention to how patients describe experiences that others struggle to understand. The words someone chooses, the effort required to speak, the difficulty of making another person understand: these belong to the clinical encounter, too.
The breakthrough behind optogenetics began with proteins found in microorganisms. Hegemann and Nagel studied channelrhodopsins, proteins that respond to light and allow electrically charged particles to cross a cell membrane. Deisseroth and his collaborators helped establish how these proteins could control the activity of neurons.
By giving selected nerve cells the genetic instructions to produce a light sensitive protein, researchers could activate them with brief pulses of light. Other proteins allowed activity to be suppressed. The timing could be measured in milliseconds, matching the speed at which neurons communicate.
Researchers could now change the activity of a particular population of cells and observe what happened to behavior. A circuit suspected of contributing to fear or motivation could be tested directly. The method spread through laboratories around the world, helping scientists investigate how particular cells contribute to memory, movement and emotion.
The distance between those experiments and relief for a patient remains substantial. Knowing how to alter a circuit in an experimental animal does not mean knowing how to treat a person whose depression has persisted despite repeated treatment. It does, however, offer a more exact place to begin.
His laboratory also helped develop CLARITY, introduced in 2013. The method removes light scattering fats from preserved tissue while anchoring many of its proteins and other molecules within a supporting gel. Researchers could examine connections through intact volumes of brain tissue, following structures that would otherwise have been divided among thin slices.
In his 2021 book, Projections: A Story of Human Emotions, Karl returned to the people behind the questions. He wrote its first chapter during his psychiatry residency, before opening his laboratory. The book draws on encounters with patients to explore the biological origins of feeling and the difficulty of describing an experience another person has never had.
That difficulty runs through his work. In psychiatry, an account of suffering must be heard and understood well enough to guide care. In the laboratory, a question must become specific enough to test. Karl has spent decades working at both, carrying the demands of patient care into a field capable of increasingly precise experiments.
The Nobel recognizes discoveries that gave neuroscience a new experimental reach. The clinical question remains unfinished: what has happened to this person, and what might help?
Portraits: https://www.christophermichel.com/New-Heroes/Karl-Deisseroth
