Researchers have transformed normally timid lab mice into snapping, super-efficient killers by manipulating circuits in the brain's "fear center" — the amygdala.
Their findings show just where the predatory mechanism comes from in the brain, and show that, in mice, anyway, it links the muscles of the jaw, shoulder and forelimb. They work together to create a fast and efficient pounce.
It creates a somewhat horrifying scenario but sheds light on precisely where in the brain hunting skills are centered. It's a mechanism common to all higher animals, including humans.
The team used a technique called optogenetics to control the mice. It involves genetically modifying specific brain cells using a virus, and then employing a laser to activate the neurons.
Once they'd homed in on the correct circuit, the transformation was instant, the team reports in the journal Cell.
"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, who also works at the nearby John B. Pierce Laboratory.
They've got movies showing how it works. When they're in their normal, natural state, the mice ignore sticks and other objects in their cages, and actively try to avoid a moving robotic toy.
When the laser pulses on, they instantly attack the objects, biting hard. They also pounced quickly on live crickets.
"Behavior was interrupted immediately upon laser deactivation. Such attacks were never observed when laser source was off," the team wrote.
It's almost irresistible to compare the mice to zombie killers, but de Araujo says the analogy is not quite fair. The laser activation does not prompt the mice to attack one another and it simply heightens natural hunting behavior.
But they do show that vertebrates — animals with a spine — have evolved a coordinated mechanism linking the brain to the jaws and to limbs needed to seize food.
"Our findings imply the central amygdala as a modular command system," de Araujo's team wrote. "It is a major evolutionarily player in shaping the brain," he added. "There must be some primordial subcortical pathway that connects sensory input to the movement of the jaw and the biting."
Not only that, but the brain cells control the jaw strength. When they damaged certain neurons, it weakened the bite of the mice. "They fail to deliver the killing bite," de Araujo said in a statement.
It's counterintuitive that this system would start in the amygdala. "It's an area more traditionally linked to avoidance and fear and fight or flight behavior," de Araujo told NBC News.
That bears more study.
So could they make real zombies? It would be hard. To activate the brain cells, the laser has to shine right on them. That requires a fiber optic cable to be surgically implanted in the skull. The mice are tethered to their cages.
Chemicals can be used to activate the brain cells, but they are much harder to control, de Araujo said.
And the behavior controlled by this brain system has a purpose. "The system is not just generalized aggression," de Araujo said. "It seems to be related to the animal's interest in obtaining food."