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Scientists figure out what turns mice into killers

Killer mice

Killer mice Killer mice.

Tucked deep within the temporal lobe of the human brain are a pair of almond-shaped bundles of neurons which control emotional and motivational responses called the amygdala.

It’s a well-studied neural region, functioning as a processor of information and initiator of a whole host of reactions, such as fear and appetite conditioning but also compassionate responses and memory association, all part of a set of Pavlovian, instant reactions to environmental conditions that are made possible by the amygdala.

Now, through experiments with mice in which two subgroups of neurons within the amygdala were stimulated with lasers, researchers from Yale University in Connecticut believe they have uncovered a hunting impulse in specific areas within the mice’s amygdala.

Before the study “it wasn’t clear how the brain was possessing and putting together a bunch of information in order to properly perform hunting behaviuor,” says study co-author Luis Tellez of the Yale University School of Medicine, who says that the research was made possible by the use of a light-based technique called optogenetics which allows for precise activation of specific neuron clusters via laser light. In a statement, Tellez calls optogenetics, “the new standard for system neuroscience and also for circuitry distinction.”

To produce the reactions in mice, the researchers found that when one particular set of neurons were illuminated, the mice responded by chasing moving objects around their cages, while activation of a second set caused the mice to bite nearby objects.

The researchers released startling video footage of the experiment, which shows mice acting in seemingly normal fashion (as normal as one might expect with electrodes stuck into to one’s brain, we assume) when the lasers are off, but when turned on, the mice instantly became agitated, jumping and chasing after objects.

“This area, the central amygdala, seems to allow the animal precise control over the muscles involved in pursuing and capturing prey,’’ said Ivan de Araujo, associate professor of psychiatry at Yale School of Medicine and senior author of the paper, recently published in the journal Cell.

The researchers are quick to point out that their results should not be misinterpreted as instigating a zombie-like “kill reaction” in mice, saying that the mice appear very much aware of their behaviour even with the specified neurons activated. The mice did not attack other mice while the lasers were turned on, for instance. “This is a behaviour that is triggered by the need for food and not as a killer mechanism,” says de Araujo. “That means that there is a recognition mechanism in the brain that detects the presence of a member of the same species and aborts predatory behaviour,”

Whether comparable stimulation of the amygdala in humans would produce a similar “hunting instinct” is unclear, although research has shown that the human amygdala is also associated with the desire to eat. A 2012 study from researchers at McGill University in Montreal, for instance, found that food cues and cravings were linked to increased blood oxygen level responses in the left amygdala.

Below: Tiny lasers turn lab mice into brutal killers

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About The Author /

Jayson is a writer, researcher and educator with a PhD in political philosophy from the University of Ottawa. His interests range from bioethics and innovations in the health sciences to governance, social justice and the history of ideas.

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