It seems like the stuff of nightmares – scientists changing the minds of timid mice and turning them into killers. But that’s exactly what has happened in a new study involving laboratory mice.

Ivan de Araujo, a neurobiologist at Yale University in New Haven, Connecticut, and his colleagues set out to find what role the almond-shaped amygdala in the brain has to play in predatory behavior. In doing so, they found a switch that turns on the killer instinct of a mouse using a laser stimulator.

The technique called optogenetics involved giving mice a particular virus that made the neurons in their brains sensitive to light. They then used optic fibre to shine blue light directly on to the amygdala to see if any reactions took place in the behavior of the mice.

To their astonishment, the mice started to react by tensing their jaw and neck muscles and portrayed hunter-like behavior both when there was ‘prey’ in the cage and when there wasn’t. The mice even pounced quickly on crickets.

“We’d turn the laser on and they’d jump on an object, hold it with their paws and intensively bite it as if they were trying to capture and kill it,” said Ivan de Araujo, an associate professor of psychiatry at the Yale University School of Medicine.

Despite the horror-zombie story that seems to have been created, the researchers say it’s not quite a fair comparison as the mice aren’t ever attacking each other but merely heightening their hunting instincts.

“The system is not just generalized aggression,” de Araujo said. “It seems to be related to the animal’s interest in obtaining food.”

It seems the amygdala has a very big evolutionary part to play in shaping the brain and “there must be some primordial subcortical pathway that connects sensory input to the movement of the jaw and the biting” as the team wrote in the study.

Details of the research was published in the journal Cell.