When we see danger, we react. Whether we choose to run, hide or face the threat, our "instant" decision is the result of a complex brain mechanism that integrates visual data and triggers an appropriate response. How does this happen? A new study published in the journal Nature explains it.
In the animal kingdom, vision is vital for survival. This important sense informs the brain about predators and other threats, and, in turn, the brain generates an appropriate reaction: courage or fear, fight or flight.
But how is this process carried out? How do animals - humans - integrate visual information with the appropriate brain circuits that control our emotional states and then our behavior and our actions?
A team of scientists led by Andrew Huberman of the Faculty of Medicine at Stanford University in California (USA) has found that certain brain circuits are "responsible" for the decision to fight or flee in the face of danger.
Although the study was conducted in mice, the findings are equally relevant for humans. In fact, the results have important implications for understanding and controlling post-traumatic stress disorder (PTSD), addiction and phobias.
To examine the response of rodents to a threat, the researchers simulated the approach of a bird of prey and used the neuronal marker c-Fos to track the activity of mice's neurons. They found greater activity in the neurons that clustered in a structure called the ventral midline of the thalamus. Through brain mapping they were able to see what sensory information was entering and what information came out of this area.
They discovered that the median line of the thalamus receives information from a wide range of brain areas that process internal states, such as fear, but that sends information very selectively to only two main areas: the basolateral amygdala and the medial prefrontal cortex.
The amygdala processes fear, aggression and other emotions, while the medial prefrontal cortex uses its executive function to modulate emotional responses (as well as anxiety).
'Turn on' the courage switch
By looking at the trajectory in the brain, the researchers wondered if the selective inhibition of certain neurons along these pathways produced specific fight or flight reactions. To find out, they stimulated only the activity of the xiphoid nucleus while confronting the rodents with the image of the bird of prey. This caused the mice to freeze in front of the predator.
After that, they stimulated the activity of the tract that goes from the reuniens nucleus to the medial prefrontal cortex. This caused a surprising reaction: the mice became aggressive, and prepared to defend themselves. An answer of courage or undeniable value.
"You could hear their tails banging against the side of the camera, it's the equivalent in the mouse to slap and hit your chest and say, 'OK, let's fight!'", Explains Andrew Huberman, project leader.
A second experiment confirmed the results: exclusively stimulating the nucleus reuniens for half a minute before putting them in front of the predator, producing the same behavioral response: instead of hiding, the mice shook their tails and exposed themselves in unprotected areas, ready to fight.
Huberman says the findings are very relevant to humans, given that human brains have a structure similar to the ventral midline of the thalamus.
It suggests that people living with phobias, anxiety or PTSD may soon benefit from the findings, since reducing activity in this area or adjacent neural groups can help these people overcome their fears.
"This opens the door to future work on how to change from paralysis and fear to being able to face challenges in a way that improves our lives," Huberman says.
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