Predator or prey? This 'switch' in the brain toggles when you're hunting or being hunted

Humans evolved to be both hunters and hunted; although Homo sapiens can take down large prey, our species is also vulnerable to big predators. Now, new research reveals how the human brain switches between these two modes of survival. 

The answer lies in the hypothalamus, a tiny structure nestled deep in the middle of the organ. This ancient brain region predates the evolution of vertebrates and thus appears in all vertebrate animals; similar brain regions also exist in invertebrates. The hypothalamus is known for performing very basic survival tasks, such as regulating body temperature, triggering the release of hormones, regulating circadian rhythms and sending out hunger cues. 

The new study, published Thursday (June 27) in the journal PLOS Biology, found that the hypothalamus also manages the survival behavior of switching between hunting and being hunted. 

The hypothalamus had previously been shown to take on this task in other mammals, such as mice. But the new research marks the first time the region has been shown to do so in humans, as well, the study authors wrote in their paper.

Related: Can animals really smell fear in humans?

The hypothalamus is small — about the size of a pea — and it's made up of even smaller nuclei that are too tiny for brain scanning techniques, such as functional magnetic resonance imaging (fMRI), to image. 

The researchers used several methods to overcome this problem. One involved determining the pulse of cerebrospinal fluid — a clear fluid that flows around and into gaps in the brain and spinal cord — and then correcting for this motion in their fMRI data. They also used a type of artificial intelligence called deep learning to detect and classify activity patterns that might otherwise be too subtle to catch. 

The team first had 277 volunteers play a video game in which they had to switch from hunting behavior to escaping behavior. The game consisted of a simple arena that the participants moved an avatar around. The color of the borders of the arena communicated whether the participants should be hunting or escaping from another computerized figure. 

These participants' brains weren't scanned, but the researchers studied the volunteers' actions to create a computer model that could differentiate when someone was in hunting or fleeing mode. 

Next, 22 other participants played the same game inside an fMRI scanner. This kind of brain imaging takes an indirect measure of brain activity that's based on the movement of blood and oxygen through different brain regions. When a given region of the brain is active, the flow of oxygenated blood to that area increases. 

For comparison purposes, the same 22 participants also did a task that involved just moving their avatar around the screen, without any particular drive to survive. 

The results revealed that the hypothalamus acted as a control center, facilitating the switch between predator and prey behaviors. It did this by communicating with a suite of other brain regions, including the amygdala, a region known for processing fear, and the ventromedial prefrontal cortex, which is known for being involved in decision-making tasks, including assessing risk in a given situation. This switch involved suppressing the behavior from the previous task. 

The hypothalamus continues to coordinate the new behavior after this switch occurs, staying active throughout the process. 

"These findings extend our understanding of the human hypothalamus from a region that regulates our internal body states to a region that switches survival behaviors and coordinates strategic survival behaviors," the authors wrote. 

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