Under the Hood

Response To Danger: How The Brain Decides Whether To Flee Or Freeze

While 2018 is far away from the age when we were under constant threat from predatory wild animals, the fundamental defensive behaviors are still embedded in our system. But how does the brain decide on what strategy to implement when threatened?

The paper titled "Speed dependent descending control of freezing behavior in Drosophila melanogaster" was published in the journal Nature Communications on Sept. 12.

"Just like any other animal in nature, our reaction to a threat is invariably one of the following three: escape, fight or freeze in place with the hope of remaining unnoticed," explained Marta Moita who led the new study with Maria Luisa Vasconcelos at the Champalimaud Centre for the Unknown in Lisbon, Portugal.

They conducted an experiment with fruit flies, placing them inside a covered dish and exposing them to a "threat," i.e. a dark, expanding circle. (This is unfamiliar and rather scary from the perspective of a fly.)

When observing their reactions, many of the flies froze on the spot. Sometimes, they remained motionless for minutes, even if they were in an awkward position. But some of the flies ended up fleeing from the dark circle. This was a very exciting thing to see, according to Vasconcelos, as it meant the flies were trying to decide between alternative strategies just like human beings would.

Next, the researchers used machine vision software which allowed them to take a closer look at the behavior of the flies when they were responding to the danger. This led them to an unexpected discovery.

It turned out, if the fly was moving slowly, it would freeze. But if the fly was walking quickly, it would flee from the threat.

"This result is very important: it is the first report showing how the behavioral state of the animal can influence its choice of defensive strategy," Vasconcelos stated.

The team went on to use genetic tools to observe the neurons responsible for the behavior of the flies. Among the thousands of neurons in the brains of the flies, the freezing was controlled by two identical neurons, one on each side of the brain.

When the neurons were turned off, flies did not freeze anymore, but still managed to escape from the threat. However, when they turned the neurons on without the presence of a threat, the flies only froze when walking slowly, not when walking quickly. 

"This is exactly what we were looking for: how the brain decides between competing strategies. And moreover, these neurons are of the type that sends motor commands from the brain to the 'spinal cord' of the fly. This means that they may be involved not only in the choice but also in the execution," Moita pointed out. "And because defensive behaviors are common to all animals, our discoveries provide a good starting point towards identifying the 'rules of the game' that define how all animals choose to defend themselves."