Fast and Furious: Flight Decisions in Fruit Flies
By Marissa Sumathipala
Imagine you are a tiny fruit fly darting through the air. The ridges of a wooden table are endless mountain ranges, the carpet a monochromatic forest. Suddenly, you see the shadow of a fly swatter looming over you. In a fraction of a second, you reorient your body and leap off the table. This type of lightning-quick instinct and evasive maneuvering is an astonishing feat of nature. How does such a tiny insect go from perceiving a shadow to executing a complex escape maneuver?
The neural basis of behavior—how cells in our brain signal each other and produce actions—continues to puzzle neuroscientists. How do brains make behavioral choices? How did the fly make the perfect sequence of decisions to avoid the swatter so quickly? It seems far removed, but understanding decision-making in flies could one day shed light on how humans make more complex decisions, from what beverage to drink in the morning to what college to attend.
Fight or flight responses, like the kind elicited when a fly sees an incoming fly swatter, are a promising behavioral paradigm: fast, consistent, and reproducible. When a fly escapes, it’s much more than a simple knee jerk response. Fruit flies sift through visual cues in their environment, such as the approaching shadow from the fly swatter. This information undergoes complex processing through several sets of neurons in the brain.1 Then, the brain makes a series of decisions based on the processed information, such as determining how to escape if threatened: Is the object food or foe? If foe, how much time do I have to escape? What kind of escape maneuver am I going to use? Lastly, the fly executes an escape sequence: lift wings, shift weight, extend legs rapidly, beat wings, bend legs.
Dr. Gwyneth Card at the Janelia Research Campus is systematically tracing neurons in the fly brain to answer one of the most fundamental questions in neuroscience: how we make decisions. When faced with a rapidly looming stimulus, essentially a large circle getting closer and closer, flies will execute a rapid escape maneuver 2. Getting off the ground as fast as possible is their priority. A fly won’t bother lifting its wings or precisely positioning itself, all of which taking up precious time. Dr. Card has traced back the decision for this rapid escape to a single signal fired by a crucial neuron called the giant fiber neuron. A tiny electrical impulse, called an action potential, in this giant fiber neuron can override the fly’s slower, precise escape response and replace it with the quick, imprecise one 3. For the fly, the decision between the fast and slow response makes the difference between life and death.
The giant fiber neuron bridges the junction between the brain and the ventral nerve cord in flies, making it a perfect candidate to understand how the brain sends messages to the muscles through the spinal cord and elicits a behavior in humans. Neurons connecting the brain and spinal cord are called descending neurons and are the focus of Dr. Card’s research. Descending neurons are analogous to a bottleneck of critical information in decision making. Specialized neurons carefully analyze features in the environment—how fast an object is approaching, how big it is, and from what direction it’s coming—and send it to one of these descending neurons. The descending neuron encodes this information and carries a message in the form of an electrical impulse, called an action potential. This action potential then tells the fly’s muscles what to do. Card studies these ‘secret messages’ descending neurons send by measuring the electrical impulses different cells send and receive during a behavior.
To unravel the circuits of neurons that underlie decision making in a dynamic environment, Card has created a novel apparatus that captures high speed videos of flies as they’re exposed to looming circles that mimic an approaching predator. Card and her colleagues analyze different escape sequences, breaking them down into individual components that come together to produce a concerted, complex behavior. Then, she toggles individual neurons one at a time to map out the circuits responsible for each part of the behavior.4
Though Card has made incredible progress in understanding the neural basis of escape responses, she’s only just begun to scratch the surface. Her work stands testament to how complex and fascinating our brains are. Understanding how fruit flies analyze and respond to their surroundings is one small part of a massive, global project to understand how the brain functions. Key to our sense of existence, this complex organ lies at the core of our being. Everything we see, think, and feel is orchestrated by this maze of neuronal circuitry, and untangling its inner workings is key to understanding how it defines us.
References
1. Card, G. M. (2012). Escape behaviors in insects. Current Opinion in Neurobiology, 22(2), 180–186.
2. Card, G., & Dickinson, M. (2008). Performance trade-offs in the flight initiation of Drosophila. Journal of Experimental Biology, 211(3), 341–353.
3. Reyn, C. R., Breads, P., Peek, M. Y., Zheng, G. Z., Williamson, W. R., Yee, A. L., … Card, G. M. (2014). A spike-timing mechanism for action selection. Nature Neuroscience, 17(7), 962–970.
4. Reyn, C. R. von, Nern, A., Williamson, W. R., Breads, P., Wu, M., Namiki, S., & Card, G. M. (2017). Feature Integration Drives Probabilistic Behavior in the Drosophila Escape Response. Neuron, 94(6), 1190–1204.e6.
Imagine you are a tiny fruit fly darting through the air. The ridges of a wooden table are endless mountain ranges, the carpet a monochromatic forest. Suddenly, you see the shadow of a fly swatter looming over you. In a fraction of a second, you reorient your body and leap off the table. This type of lightning-quick instinct and evasive maneuvering is an astonishing feat of nature. How does such a tiny insect go from perceiving a shadow to executing a complex escape maneuver?
The neural basis of behavior—how cells in our brain signal each other and produce actions—continues to puzzle neuroscientists. How do brains make behavioral choices? How did the fly make the perfect sequence of decisions to avoid the swatter so quickly? It seems far removed, but understanding decision-making in flies could one day shed light on how humans make more complex decisions, from what beverage to drink in the morning to what college to attend.
Fight or flight responses, like the kind elicited when a fly sees an incoming fly swatter, are a promising behavioral paradigm: fast, consistent, and reproducible. When a fly escapes, it’s much more than a simple knee jerk response. Fruit flies sift through visual cues in their environment, such as the approaching shadow from the fly swatter. This information undergoes complex processing through several sets of neurons in the brain.1 Then, the brain makes a series of decisions based on the processed information, such as determining how to escape if threatened: Is the object food or foe? If foe, how much time do I have to escape? What kind of escape maneuver am I going to use? Lastly, the fly executes an escape sequence: lift wings, shift weight, extend legs rapidly, beat wings, bend legs.
Dr. Gwyneth Card at the Janelia Research Campus is systematically tracing neurons in the fly brain to answer one of the most fundamental questions in neuroscience: how we make decisions. When faced with a rapidly looming stimulus, essentially a large circle getting closer and closer, flies will execute a rapid escape maneuver 2. Getting off the ground as fast as possible is their priority. A fly won’t bother lifting its wings or precisely positioning itself, all of which taking up precious time. Dr. Card has traced back the decision for this rapid escape to a single signal fired by a crucial neuron called the giant fiber neuron. A tiny electrical impulse, called an action potential, in this giant fiber neuron can override the fly’s slower, precise escape response and replace it with the quick, imprecise one 3. For the fly, the decision between the fast and slow response makes the difference between life and death.
The giant fiber neuron bridges the junction between the brain and the ventral nerve cord in flies, making it a perfect candidate to understand how the brain sends messages to the muscles through the spinal cord and elicits a behavior in humans. Neurons connecting the brain and spinal cord are called descending neurons and are the focus of Dr. Card’s research. Descending neurons are analogous to a bottleneck of critical information in decision making. Specialized neurons carefully analyze features in the environment—how fast an object is approaching, how big it is, and from what direction it’s coming—and send it to one of these descending neurons. The descending neuron encodes this information and carries a message in the form of an electrical impulse, called an action potential. This action potential then tells the fly’s muscles what to do. Card studies these ‘secret messages’ descending neurons send by measuring the electrical impulses different cells send and receive during a behavior.
To unravel the circuits of neurons that underlie decision making in a dynamic environment, Card has created a novel apparatus that captures high speed videos of flies as they’re exposed to looming circles that mimic an approaching predator. Card and her colleagues analyze different escape sequences, breaking them down into individual components that come together to produce a concerted, complex behavior. Then, she toggles individual neurons one at a time to map out the circuits responsible for each part of the behavior.4
Though Card has made incredible progress in understanding the neural basis of escape responses, she’s only just begun to scratch the surface. Her work stands testament to how complex and fascinating our brains are. Understanding how fruit flies analyze and respond to their surroundings is one small part of a massive, global project to understand how the brain functions. Key to our sense of existence, this complex organ lies at the core of our being. Everything we see, think, and feel is orchestrated by this maze of neuronal circuitry, and untangling its inner workings is key to understanding how it defines us.
References
1. Card, G. M. (2012). Escape behaviors in insects. Current Opinion in Neurobiology, 22(2), 180–186.
2. Card, G., & Dickinson, M. (2008). Performance trade-offs in the flight initiation of Drosophila. Journal of Experimental Biology, 211(3), 341–353.
3. Reyn, C. R., Breads, P., Peek, M. Y., Zheng, G. Z., Williamson, W. R., Yee, A. L., … Card, G. M. (2014). A spike-timing mechanism for action selection. Nature Neuroscience, 17(7), 962–970.
4. Reyn, C. R. von, Nern, A., Williamson, W. R., Breads, P., Wu, M., Namiki, S., & Card, G. M. (2017). Feature Integration Drives Probabilistic Behavior in the Drosophila Escape Response. Neuron, 94(6), 1190–1204.e6.
