The Science of Value-Based Decision Making: Insights from Fruit Flies
Introduction
Like many LP collectors, James Fitzgerald’s brother-in-law has a favorite store where he always finds the best vinyl for his collection. However, there are times when he spends hours in the store and comes up empty-handed. He also knows that every once in a while he should venture to the record store across town, where he sometimes gets a hard-to-find gem that was in storage since his last visit.
Fitzgerald’s brother-in-law is making a calculation: weighing likely outcomes to guide his behavior. His favorite record store rewards him most often, so it’s the one he visits the most. The second-tier store is less likely to reward him, so he visits that store only occasionally.
This “record-hunting” habit of weighing probabilities and making decisions based on expected rewards is not unique to humans. In fact, it is a behavior called pairing that is ubiquitous in the animal kingdom, as neuroscientist Glenn Turner and his team at HHMI’s Janelia Research Campus have discovered.
The Ubiquity of Pairing Behavior
Paring behavior, similar to Fitzgerald’s brother-in-law’s record-hunting, is observed in non-hipster animals like mice and flies when they forage for food. They use sensory cues such as smells to evaluate the quality of the food from a distance. Coincidence, or the act of weighing probabilities and making value-based decisions, has been observed in various species. However, the mechanisms behind this decision-making process have remained unclear.
Researchers, including Fitzgerald, Turner, and their team at Janelia Research Campus, sought to understand how the brain carries out value-based decision making. They proposed a theory but had not tested it in the real world. Through a series of experiments conducted by Janelia graduate scholar Adithya Rajagopalan, the team confirmed that the proposed theory works in fruit flies.
Discovering Value-Based Decision Making in Fruit Flies
Rajagopalan’s experiments involved observing the decisions made by fruit flies in a symmetrical Y-shaped arena. The flies were presented with different odors that had varying probabilities of reward. One smell would lead to a reward 80 percent of the time, while the other smell would lead to a reward only 20 percent of the time. The researchers found that the flies learned to expect rewards in proportion to how they were presented and made decisions based on those expectations. The flies predominantly chose the odor that provided a higher probability of rewards.
By tracing the flies’ behavior to specific synapses in the mushroom body, a region of the fly brain responsible for learning and memory, the team created a model of how the brain carries out value-based decision making. According to the matching theory, the values associated with different choices are learned through changes in synaptic strength. Synaptic connections strengthen or weaken based on the difference between the expected and received reward.
Implications for Understanding Decision Making in Larger Animals
The discovery of value-based decision making in fruit flies provides valuable insights into how similar processes might occur in the brains of larger animals, including humans. Decision making plays a crucial role in various aspects of life, and understanding its mechanisms can help shed light on diseases like addiction, where decision making goes awry.
Rajagopalan and the team believe that the ideas and theoretical framework developed through their work have the potential to evolve in larger organisms, allowing for even more complex behaviors. By studying the simpler brains of flies and mice, researchers can uncover fundamental principles that apply to decision making in all species.
The Potential of Mechanistic Cognitive Neuroscience
The study of value-based decision making in fruit flies illustrates the power of mechanistic cognitive neuroscience. By observing the changes in synaptic strength and understanding how synapses are changing, researchers gain insights into the underlying mechanisms of decision making. The simplicity of the fruit fly brain allows for a clearer understanding of these mechanisms, paving the way for future research on decision making in more complex organisms.
Incorporating Insights into Larger Organisms
Understanding how value-based decision making occurs at the neural circuit level in fruit flies offers exciting possibilities for understanding decision making in larger organisms. By building on the knowledge gained from studying simpler brains, scientists can develop a more comprehensive understanding of decision making processes in humans and other animals.
Conclusion
The research conducted by Fitzgerald, Turner, Rajagopalan, and their team at Janelia Research Campus provides valuable insights into the mechanisms behind value-based decision making. By studying fruit flies and observing their behavior in response to different probabilities of rewards, the team demonstrated how the brain assigns value to choices based on expectations. This research has broader implications for understanding decision making in humans and other animals, as well as potential applications in addressing decision-making disorders.
The study serves as a testament to the power of mechanistic cognitive neuroscience and highlights the importance of experiment and theory working hand in hand to unravel the mysteries of the brain. While fruit flies may seem insignificant, their behavioral patterns and the underlying neural mechanisms can provide valuable insights into more complex decision-making processes. As researchers continue to explore the brain’s intricate workings, the field of neuroscience will undoubtedly uncover even more remarkable discoveries.
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Like many LP collectors, James Fitzgerald’s brother-in-law has a favorite store where he always finds the best vinyl for his collection. But there are times when he spends hours in the store and comes up empty-handed. He also knows that every once in a while he should venture to the record store across town, where he sometimes gets a hard-to-find gem that was in storage since his last visit.
Fitzgerald’s brother-in-law is making a calculation: weighing likely outcomes to guide his behavior. His favorite record store rewards him most often, so it’s the one he visits the most. The second-tier store is less likely to reward him, so he visits that store only occasionally.
Glenn Turner, who like Fitzgerald is a neuroscientist and group leader at HHMI’s Janelia Research Campus, says this “record-hunting” habit is a perfect example of a type of behavior called pairing that is ubiquitous in the animal kingdom. . Instead of vinyl, non-hipster animals like mice and flies forage for food, using sensory cues like smells to evaluate food quality from a distance.
But while coincidence has been observed in everything from pigeons to mice to humans, it was unclear how the brain carried out this value-based decision making. Researchers had previously proposed a theory about how that could happen, but the idea had not been tested in the real world.
Now, a team of Janelia researchers including Fitzgerald, Turner, Janelia graduate scholar Adithya Rajagopalan, former Janelia fellow Ran Darshan, and research specialist Karen Hibbard have confirmed that the proposed theory works. Rajagopalan’s experiments showed that, like Fitzgerald’s brother-in-law, fruit flies can make decisions based on their expectations about the probability of a reward. The team also identified the site in the fly brain where these value adjustments are made, allowing them to directly test this theory at the level of neural circuits.
“We found that flies use expectations to assign value to their world,” Turner says. “It also connects very well with this theoretical work that was so elegant and explains this widespread phenomenon.”
Discovering how the fly brain carries out this ubiquitous behavior could help scientists better understand how similar decision-making occurs in the brains of larger animals, including humans. Decision-making goes awry in diseases like addiction, so understanding how this process works in simpler brains has great value, researchers say.
“The types of ideas and theoretical framework we have identified in this paper feel like a seed for evolution to develop in larger organisms, where more layers are added to allow for more complex behaviors,” says Rajagopalan, the first author of a New article describing the work.
Investigate matching behavior
Fruit flies, whose brains have been well studied and mapped, were an attractive option to examine pairing and its underlying mechanisms. But first, the team had to devise a way to observe fruit flies’ decisions.
Rajagopalan, who came to the Turner Lab through a joint graduate program with Johns Hopkins University, spearheaded the project. He designed an experiment in which a single fly enters one arm of a symmetrical Y-shaped arena. Odors are pumped to the other two arms of the Y. The fly chooses to follow one odor or the other and is rewarded (in this case activating their sugar-sensitive neurons), but with different probabilities: one smell could result in a reward 80 percent of the time, while the other smell could result in a reward 20 percent of the time.
The researchers found that the fly learned to expect rewards in the same proportions in which they were presented and then made its decision based on those expectations. These actions give the pairing behavior its name: 80 percent of the time, the fly chooses the odor that provides 80 percent of the rewards. And 20 percent of the time, he chose the smell that generates 20 percent of the rewards.
The team traced the behavior to specific synapses in the mushroom body, a region of the fly brain responsible for learning and memory. This allowed them to create a model of how the brain carries out this behavior, based on matching theory. In this theory, the values associated with different choices are learned through changes in synaptic strength: synaptic connections strengthen or weaken in proportion to the difference between the expected and received reward. The team’s model based on this theory and fly behavior allowed them to demonstrate how individual synapses are changing to enable value-based decision making.
The new work emphasizes the important interaction between experiment and theory, converging on a description of the rules that govern how an animal learns, a result that, according to the researchers, is satisfactory on both a conceptual and mechanistic level.
“Being able to see that you can make these sophisticated economic decisions through this simple mechanistic explanation of how synapses are changing is a great illustration of what mechanistic cognitive neuroscience can mean,” Fitzgerald says. “We’re taking this universal property and using the strengths of these small animals to achieve it mechanically.”
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