Study reveals fruit flies exhibit the building blocks of emotion

May 14, 2015, California Institute of Technology
Drosophila sp fly. Credit: Muhammad Mahdi Karim / Wikipedia. GNU Free Documentation License, Version 1.2

A fruit fly starts buzzing around food at a picnic, so you wave your hand over the insect and shoo it away. But when the insect flees the scene, is it doing so because it is actually afraid? Using fruit flies to study the basic components of emotion, a new Caltech study reports that a fly's response to a shadowy overhead stimulus might be analogous to a negative emotional state such as fear—a finding that could one day help us understand the neural circuitry involved in human emotion.

The study, which was done in the laboratory of David Anderson, Seymour Benzer Professor of Biology and an investigator with the Howard Hughes Medical Institute, was published online May 14 in the journal Current Biology.

Insects are an important model for the study of emotion; although mice are closer to humans on the evolutionary family tree, the fruit fly has a much simpler neurological system that is easier to study. However, studying emotions in insects or any other animal can also be tricky. Because researchers know the experience of , they might anthropomorphize those of an insect—just as you might assume that the shooed-away fly left your plate because it was afraid of your hand. But there are several problems with such an assumption, says postdoctoral scholar William T. Gibson, first author of the paper.

"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 have emotions, those emotions probably wouldn't be the same ones that you have," he says. "For these reasons, in our study, we wanted to take an objective approach."

Anderson and Gibson and their colleagues did this by deconstructing the idea of an emotion into basic building blocks—so-called emotion primitives, a concept previously developed by Anderson and Ralph Adolphs, Bren Professor of Psychology and Neuroscience and professor of biology.

"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," Anderson says. "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."

A single fly walks back and forth, and briefly grooms, before a shadow moves overhead. The fly jumps and freezes in place, its wings and other appendages held motionless. After about 5 seconds, the fly suddenly breaks its freezing posture, and escapes. Credit: William T. Gibson/Caltech

Gibson explains by analogy that emotions can be broken down into these emotion primitives much as a secondary color, such as orange, can be separated into two primary colors, yellow and red. "And if we can show that 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."

The emotion primitives analyzed in the fly study can be understood in the context of a associated with human fear: the sound of a gunshot. If you hear a gun fire, the sound may trigger a negative feeling. This feeling, a primitive called valence, will probably cause you to behave differently for several minutes afterward. This is a primitive called persistence. Repeated exposure to the stimulus should also produce a greater response—a primitive called scalability; for example, the sound of 10 gunshots would make you more afraid than the sound of one shot.

Hungry flies feeding on a food patch show graded responses to a repetitive, overhead shadow stimulus. Moving from left to right, the three images show flies' positions after, respectively, 0, 3, and 5 passes of the shadow stimulus. Credit: William T. Gibson and David J. Anderson

Gibson says that another primitive of fear is that it is generalized to different contexts, meaning that if you were eating lunch or were otherwise occupied when the gun fired, the fear would take over, distracting you from your lunch. Trans-situationality is another primitive that could cause you to produce the same fearful reaction in response to an unrelated stimulus—such as the sound of a car backfiring.

The researchers chose to study these five primitives by observing the insects in the presence of a fear-inducing stimulus. Because defensive behavioral responses to overhead visual threats are common in many animals, the researchers created an apparatus that would pass a dark paddle over the flies' habitat. The flies' movements were then tracked using a software program created in collaboration with Pietro Perona, the Allen E. Puckett Professor of Electrical Engineering.

The researchers analyzed the flies' responses to the stimulus and found that the insects displayed all of these emotion primitives. For example, responses were scalable: when the paddle passed overhead, the flies would either freeze, or jump away from the stimulus, or enter a state of elevated arousal, and each response increased with the number of times the stimulus was delivered. And when hungry flies were gathered around food, the stimulus would cause them to leave the food for several seconds and run around the arena until their state of elevated arousal decayed and they returned to the food—exhibiting the primitives of context generalization and persistence.

The experimental apparatus, including the walking arena for the flies, and the shadow paddle. Credit: William T. Gibson
"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," Anderson says. "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."

In the future, the researchers say that they plan to combine the new technique with genetically based techniques and imaging of brain activity to identify the that underlies these defensive behaviors. Their end goal is to identify specific populations of neurons in the fruit fly brain that are necessary for emotion primitives—and whether these functions are conserved in higher organisms, such as mice or even humans.

Although the presence of these primitives suggests that the flies might be reacting to the stimulus based on some kind of emotion, the researchers are quick to point out that this new information does not prove—nor did it set out to establish—that flies can experience fear, or happiness, or anger, or any other feelings.

"Our work can get at questions about mechanism and questions about the functional properties of emotion states, but we cannot get at the question of whether or not flies have feelings," Gibson says.

Explore further: Brain compass keeps flies on course, even in the dark

More information: Behavioral Responses to a Repetitive Stimulus Express a Persistent State of Defensive Arousal in Drosophila, Current Biology, 2015. dx.doi.org/10.1016/j.cub.2015.03.058

Related Stories

Brain compass keeps flies on course, even in the dark

May 13, 2015

If you walk into a dark room, you can still find your way to the light switch. That's because your brain keeps track of landmarks and the direction in which you are moving. Fruit flies also boast an internal compass that ...

What model organisms can teach us about emotion

February 21, 2010

Scientists know little about how the brain creates and controls emotions - an uncertainty that presents a major obstacle in the effort to develop treatments for emotional disorders. "The study of the brain science of emotion ...

Emotions in the brain

September 17, 2014

This year has been a busy one for biologist David Anderson, Caltech's Seymour Benzer Professor of Biology. In 2014 alone, Anderson's lab has reported finding neurons in the male fly brain that promote fighting and, in the ...

The neural processes of evaluating the emotions of others

July 1, 2014

When evaluating another person's emotions – happy, sad, angry, afraid – humans take cues from facial expressions. Neurons in a part of the brain called the amygdala "fire" in response to the visual stimulation as information ...

Using fruit flies to understand how we sense hot and cold

March 4, 2015

Innately, we pull our hand away when we touch a hot pan on the stove, but little is known about how our brain processes temperature information. Northwestern University scientists now have discovered how a fruit fly's brain ...

Recommended for you

Paleontologists report world's biggest Tyrannosaurus rex

March 22, 2019

University of Alberta paleontologists have just reported the world's biggest Tyrannosaurus rex and the largest dinosaur skeleton ever found in Canada. The 13-metre-long T. rex, nicknamed "Scotty," lived in prehistoric Saskatchewan ...

NASA instruments image fireball over Bering Sea

March 22, 2019

On Dec. 18, 2018, a large "fireball—the term used for exceptionally bright meteors that are visible over a wide area—exploded about 16 miles (26 kilometers) above the Bering Sea. The explosion unleashed an estimated 173 ...

Coffee-based colloids for direct solar absorption

March 22, 2019

Solar energy is one of the most promising resources to help reduce fossil fuel consumption and mitigate greenhouse gas emissions to power a sustainable future. Devices presently in use to convert solar energy into thermal ...

9 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

fabian_brodersen
3 / 5 (2) May 14, 2015
I think the concept we call emotion is just the types of weighting of different associations within all stimulus one recieves.

Everything that can learn those associations can have emotions, therefore a fly can have emotions aswell. They're just different in terms of their Umwelt.

gkam
3.7 / 5 (3) May 14, 2015
I thought we already found that tomato worms know what leaves to eat because of positive emotional feedback from the same neurotransmitters we get when we perform life-affirming acts.
RobertKarlStonjek
2 / 5 (3) May 15, 2015
As was first detailed by William James back in the 19th century, a fear response does not necessarily involve a fear emotion and a fear emotion does not necessarily involve a response.

These 'scientists' have confounded a fear response with a fear emotion. The two are barely even related. If these 'scientists' had bothered to study *human* psychology or psychiatry they would know that fear can and does occur entirely independently of triggering stimuli, as in the cases of phobias, paranoia, delusions and psychosis.

This is conclusive proof that the mechanisms are separate. Subjective emotion is experienced in the neocortex (flies don't even have one of these) whereas the fear response is mediated by the brain stem (flies do have one of these).

Which model of emotion says that the basic form of emotion is a physical response? None do.

These 'scientists' are drawing upon a folk psychology model of emotions ~ why not use a current Scientific model???
RobertKarlStonjek
2 / 5 (3) May 15, 2015
Consider this: A computer is programmed to respond to a dangerous situation like a virus attack by closing its ports. Does the computer have a fear emotion or a fear response?

Clearly no emotion is involved, just a pre-programmed response to a particular condition. The difference between these responses and emotion is that emotion exists independently of, but in sympathy with, the stimulus and response. Emotion requires a cognitive process to experience it, as in the case of all human emotion but not to all emotion-like responses that can occur without any accompanying emotion.
gkam
3 / 5 (2) May 15, 2015
Emotions are reactions to neurotransmitters. The brains of insects react like ours do when we perform life-affirming behavior.
JVK
1 / 5 (1) May 16, 2015
These 'scientists' are drawing upon a folk psychology model of emotions ~ why not use a current Scientific model???


The current scientific model links microbes to humans via the conserved molecular mechanisms of the biologically-based nutrient-dependent biophysically constrained chemistry of RNA-mediated cell type differentiation and ecological adaptations.

For example, see: Affective Neuronal Selection: The Nature of the Primordial Emotion Systems http://www.ncbi.n...3540967/

Evolutionary theorists can't seem to grasp the fact that selection of nutrients must lead from self vs other/ immune system and olfactory system recognition to the nutrient-dependent physiology of reproduction in all genera.

They want mutations to lead to evolution and won't accept the fact that ecological variation leads to ecological adaptations without the pseudoscientific nonsense of their ridiculous theories.

They don't like the current scientific model.
Vietvet
3.7 / 5 (3) May 16, 2015
@JVK

Your model isn't the current scientific model.

Why not list the biologists who explicitly endorse your model?
RobertKarlStonjek
1 / 5 (1) May 23, 2015
gkam, neurotransmitter abundances reflect the current condition and need of the organism.

Contrast a fax machine. We can say that the people in the office respond to the state of the fax machine. This is true. But we know that the fax machine is merely a way station, a communication device and is not the source of behaviour. We know that somebody sends the faxes instructing these office workers.

The same is true of the brain in general along with the state of neurons, glia and neurotransmitters.

We also note that if a virus enters the fax machine, or a hacker or a fool in the office fakes a fax, that behaviour can be initiated at the fax machine. Same for neurotransmitter abundances. If they change for reasons other than a response to conditions then behavioural change can be initiated at the neurotransmitter level and corrected with a medication that controls those abundances (in principle).

But as long as the system is working, neurotransmitters are not the source.
gkam
3 / 5 (2) May 23, 2015
They are the source of emotion. Yes, they also have sources themselves,

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.