New research sheds light on how our brains react when faced with danger.

Image credits spcbrass / Flickr.

Hear that? If you listen really hard, you can actually make out the sound of nothing hunting you right now. Safely ensconced in our society, we tend to take this for granted. Make no mistake, however: it’s anything but.

That’s exactly why we (and basically every other animal) evolved from the ground up with self-preservation in mind. Despite our sheltered existence, the brain circuits that generate our responses to perceived threats are still very much alive to this day. In a bid to better understand how these networks operate, and why they work the way they do, researchers at the Champalimaud Centre for the Unknown (CCU) in Lisbon, Portugal, set about to terrify the pants off some very tiny flies.

Fly, fruit fly!

“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,” says Marta Moita, co-lead author of the study.

“These behaviours are fundamental, but we still don’t know what the rules of the game are,” adds the study’s first author Ricardo Zacarias. “In each situation, how does the brain decide which of the three strategies to implement and how does it ensure that the body carries it through?”

Fruit flies (Drosophila melanogaster) might not seem like the coolest or smartest organism out there — in all honesty, they’re not — but they do have a few saving graces: they’re easy and cheap to care for in large numbers and they’re low maintenance. They also procreate fast and with a fury, so there’s always plenty of them to experiment on.

Given their simpler natures (and wings), Moita admits, many people “believed that flies only escape”, but the research showed that’s not the case. They devised an experiment in which the flies didn’t have the option of flying away and then spooked them to see their reaction.

The flies were placed in covered dishes and were then shown an expanding dark circle, which ” is how a threat looks like to a fly,” Moita explains. With flying away out of the question, the flies froze, the team reports. In a perfect mirror of the same behavior in mammals, birds, and several other species, the flies remained completely motionless for minutes on end. There’s no doubt as to why the flies froze since they would maintain positions that were obviously awkward and uncomfortable for them, such as half crouches, or holding a leg or two “suspended in the air,” Moita explains.

Some flies, however, decided to make a dash for it.

“This was very exciting,” says Vasconcelos, “because it meant that similarly to humans, the flies were choosing between alternative strategies.”

The next step was to take a closer look at what triggered each response. For this goal, the team used machine vision software to produce highly-detailed accounts of each fly’s behavior. Analyzing this data revealed that the flies’ response was determined by their walking speed at the moment the threat appeared. If the fly was walking slowly, it would freeze. By contrast, if it was traveling at speed, it would attempt to run away instead.

“This result is very important: it is the first report showing how the behavioural state of the animal can influence its choice of defensive strategy,” Vasconcelos points out.

The team later identified a single pair of neurons that underpin these defensive behaviors. The pair — with one neuron on each side of the flies’ brain — decided whether the flies would freeze or not. When the team inactivated these neurons, the flies stopped attempting to freeze and just ran away from threats all the time.

When the team artificially forced the neurons to stay active all the time, even without a threat being present, the flies would freeze depending on their walking speed — the fly would freeze if it was walking slowly, but not if it was walking quickly.

“This result places these neurons directly at the gateway of the circuit of choice,” says Zacarias.

“This is exactly what we were looking for: how the brain decides between competing strategies,” Moita adds. “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”.

The findings should help provide a starting point for identifying how the brains of other species handle defense, the team explains, as “defensive behaviors are common to all animals”.

The paper “Speed dependent descending control of freezing behavior in Drosophila melanogaster” has been published in the journal Nature.