A blue light affixed to a mouse's skull flicks on. In less than a second, the rodent charges toward a scuttling plastic bug, grabs the toy and ferociously nibbles at it.
Researchers at Yale University used this blue light to hijack the brains of mice. With the flip of a switch, the indifferent critters are transformed into determined hunters. This response, the researchers found, originates deep in the amygdala — commonly known as the brain's emotion and motivation center.
Writing in the journal Cell, neurobiologist Ivan de Araujo and colleagues describe this almond-shaped brain region as a "command center" for predatory behavior. From the amygdala, two independent sets of neurons extend to sections of the brain dedicated to the pursuit and killing of prey.
"When we stimulated these neurons, [the mice] became more efficient hunters," De Araujo said. "They captured more and larger numbers of prey in shorter amounts of time."
The scientists were able to stimulate the appropriate neurons using a technique called optogenetics. They re-engineered specific cells in the center of the amygdala so that they would fire when the laser was turned on.
The laser-activated mice would chase down just about anything, including the bug-like toy, chips of wood, bottle caps and live crickets.
It's tempting to compare these mice to raging zombies, attacking whatever unfortunate victim happens to get too close. But De Araujo said there was a key difference: the laser-activated mice did not attack other mice, nor did they eat more. Instead, the laser seemed to enhance behaviors related to finding potential food.
"Generally, upon laser activation, mice readily seize, bite and often ingest non-edible objects," the study authors wrote. "Laser activation also abolished natural preferences for edible over non-edible items."
Within 100 milliseconds of stimulating a mouse amygdala, the animal's neck and jaw muscles tensed up, the researchers observed. When faced with a non-edible object or a live cricket, the mice would stretch out their necks, clutch their prey with two forepaws and bite down.
When the laser turned off, so did the hunting behavior.
The researchers found similar results when they activated the key mouse neurons using designer drugs instead of lasers.
In addition, they were able to shut down hunting behaviors by killing certain neurons. For example, if the neurons controlling the jaw and neck muscles were destroyed, the mouse could chase and grab its prey, but it could not bite hard enough to kill.
De Araujo said it's likely that most vertebrates, from rodents to big cats and primates, are wired to hunt in the same way. When the brain receives sensory information — such as the sight, smell or sound of prey — it passes through the amygdala, which activates the appropriate motor responses.
But tracking the path from sensory input to motor output has proved tricky, De Araujo said. There's no obvious input from the eyes, nose or ears into the amygdala. Yet somehow, the brain solves the task with remarkable efficiency.
"We have to disentangle that network," De Araujo said.
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