Humans have a tendency to authropormophise animals, even very simple ones. If a person swats at a fly and it moves, the assumption is that it was afraid but is it really? More specifically is it afraid in the same way that a human would be afraid in a life threatening situation.

A fruit fly is a very simple animal, with a very simple brain and limited capacity to store or process information. Additionally, if such a simple animal experiences ‘fear’ it implies that emotions developed fairly early on the evolutionary “tree of life”.

A new Caltech study, published in the journal Current Biology, attempted to understand what a fly “feels” in the hope of eventually understanding the the neural basis of human emotion. The researchers report that the fly’s response might be something analogous to what humans call “fear”.

Other animals, such as mice are biologically more similar to humans. However, according to the researchers, it is the simplicity of the fly that makes it more desirable as a research subject. The fly has a very simple neurological system that it is more difficult to misinterpret.

That does not necessarily mean that studying flies is easy.

“There are two difficulties with taking your own experiences and then saying that maybe these are happening in a fly. First, a fly’s brain is very different from yours, and second, a fly’s evolutionary history is so different from yours that even if you could prove beyond any doubt that flies have emotions, those emotions probably wouldn’t be the same ones that you have. For these reasons, in our study, we wanted to take an objective approach,” said William T. Gibson, the first author of the paper, in a statement.

Gibson and his colleague, biology professor David Anderson began by “deconstructing” the idea of emotions. As they explain it this involves breaking an emotional response down into its component parts.

“There has been ongoing debate for decades about what ’emotion’ means, and there is no generally accepted definition. In an article that Ralph Adolphs and I recently wrote, we put forth the view that emotions are a type of internal brain state with certain general properties that can exist independently of subjective, conscious feelings, which can only be studied in humans. That means we can study such brain states in animal models like flies or mice without worrying about whether they have ‘feelings’ or not. We use the behaviors that express those states as a readout,” said Anderson.

According to Gibson, emotions can be broken down into “emotion primativies” in much the same way that secondary colors can be broken down into their primary colors.

“And if we can show that fruit flies display all of these separate but necessary primitives, we then may be able to make the argument that they also have an emotion, like fear,” he said.

To understand ‘primitives’ the researchers give the example of human reactions to a gunshot. The initial sound triggers a primitive called ‘valance’, an immediate negative reaction. That is followed by ‘persistence’ which means that the individuals behaviour continues to be altered for some time after the initial stimulus. Next is ‘scalability’, the sound of 10 gunshots will provoke a stronger reaction than one gunshot.

According to Gibson, another primitive is the one that distracts you from your current activity. So, if you were otherwise occupied when you heard the shot the new stimulus would take over and prevent you from immediately returning to the activity. Finally there is ‘Trans-situationality, which means that you might later experience the same reaction to an unrelated stimulus such as a car backfiring.

These were the five primitives that the researchers were looking for in their experiment with the fruit flies. The team used an apparatus that repeatedly passed a dark paddle over the flies and a software program tracked their responses.

The researchers found that the flies displayed all of the emotional primitives they were testing for.

“These experiments provide objective evidence that visual stimuli designed to mimic an overhead predator can induce a persistent and scalable internal state of defensive arousal in flies, which can influence their subsequent behavior for minutes after the threat has passed. For us, that’s a big step beyond just casually intuiting that a fly fleeing a visual threat must be ‘afraid,’ based on our anthropomorphic assumptions. It suggests that the flies’ response to the threat is richer and more complicated than a robotic-like avoidance reflex,” said Anderson.

The researchers say that their next step will be to monitor the neural responses of animals while they are experiencing the emotional primitives.