Lasers make mice hallucinate

Aiming laser light into a mouse’s brain can make them “see” lines that aren’t there. This is the first time scientists have created a specific visual perception with lab trickery.

The experiment used optogenetics. This technique uses laser light to activate nerve cells in the brain called neurons. Scientists tweak the neurons to have a light-sensitive protein. This protein prompts the cells to send a signal in response to the light.

Optogenetics debuted about 15 years ago. Everyone hoped it would give scientists precise control over perception and the behaviors that follow, says neuroscientist Karl Deisseroth. He helped pioneer the technique. That ability could help unravel big questions, such as how certain groups of brain cells create experiences.

“It’s exciting to get to this point,” says Deisseroth, a Howard Hughes Medical Institute investigator at Stanford University in California.

Seeing things

Deisseroth’s team identified a group of about 20 neurons that activate when mice viewed either horizontal or vertical lines on a screen. Each mouse had been trained to lick water from a spout when it saw the lines it had been trained on.

The researchers then set out to make the mice hallucinate the lines using laser light. When the brain hallucinates, it sees something that isn’t really there.

Mice were first shown very faint real lines. Over time, the lines became so faint that the mice couldn’t see them. They failed to lick the water spouts. Hitting the group of neurons with laser light, though, improved the rodents’ performance. They licked their water spouts more often.

Then the researchers tested the mice in total darkness. The rodents could not see any lines. Shining laser light on the same group of 20 or so neurons caused mice to “see” lines and lick their spouts.

The light-stimulated neurons also prompted other neurons to fire off signals. This suggested that other vision cells acted as if the mouse had seen a real sight. The team reported the findings online July 18 in Science.

Beyond visions

Neuroscientist Conor Liston calls the work technically amazing. “I think every neuroscientist in this area will look at this with great interest,” says Liston, who works at Weill Cornell Medicine in New York City.

A few key advances led to the experiment’s success, Deisseroth says. Among them: Lasers controlled by liquid crystals, and the discovery of a protein called ChRmine. This protein responds to light — even dim light. That’s a helpful trait because too much light can damage the brain.

Similar approaches could let scientists create tastes, touches and smells, Deisseroth says. And this new method may let researchers control groups of neurons that are involved in more complex brain tasks. “You could easily imagine using similar tools to study memory,” Liston says.