Summary: Researchers have identified the factors that drive the brain to choose specific defensive strategies under threat and have implicated a specific pair of neurons in that decision process.
Source: Champalimaud Center for the Unknown.
Although threats from large predators are no longer a routine part of modern human life, the neural circuits that supported survival in those environments remain active in many animals today. “Like other animals, humans and other species respond to danger with one of three basic strategies: escape, fight, or freeze,” says Marta Moita, who together with Maria Luisa Vasconcelos led this study at the Champalimaud Centre for the Unknown in Lisbon, Portugal.
“These behaviors are fundamental, yet the rules that determine which strategy is chosen in a given situation are still poorly understood: how does the brain select among escape, fight or freeze, and how are these decisions translated into coordinated actions?” explains Ricardo Zacarias, the study’s first author. The full study appears in Nature Communications (published September 12, 2018).
Unexpectedly, important answers came from the common fruit fly, Drosophila melanogaster. “When we began, many assumed flies only try to escape threats. We asked whether flies might also use other defensive strategies when escape is not an option,” Moita says. The research team constrained flies in a confined arena and exposed them to an expanding dark circle that mimics an approaching predator. Instead of only fleeing, many flies froze: they became perfectly still for extended periods, sometimes holding awkward postures such as a partial crouch or legs raised in midair—behaviors strikingly similar to freezing in mammals.
Not all flies froze. Some fled. “That diversity was important,” Vasconcelos notes. “It showed flies can select between alternative defensive strategies rather than responding uniformly.” To quantify behavior precisely, the team used machine vision to track each fly’s movements at high resolution. The analysis revealed a simple but powerful rule: the fly’s walking speed at the moment the threat appeared strongly predicted its response. Flies moving slowly tended to freeze, while flies already moving quickly tended to run away. This demonstrates that the animal’s behavioral state directly influences the choice of defensive strategy.
Having established a behavioral rule, the researchers searched for the neuronal substrate responsible for the choice. Using genetic tools available in Drosophila, they screened candidate neurons and identified a single pair of descending neurons—one on each side of the brain—that are critical for freezing. Silencing these neurons abolished freezing while leaving fleeing intact, indicating those neurons are necessary for freezing but not for escape.
Further experiments used optogenetic activation to probe sufficiency. When the researchers activated this pair of neurons in the absence of any external threat, flies froze—but only if they were moving slowly at the time of activation. If the same activation occurred while the fly was moving quickly, freezing did not occur. This state-dependent effect places these descending neurons at a pivotal point where behavioral state and neural command signals converge to determine defensive actions.
These neurons are anatomically suited for both selection and execution: they descend from the brain to motor centers analogous to a spinal cord in vertebrates, implying a role not only in choosing freezing but also in initiating the motor pattern that produces it. “Finding two neurons among the hundreds of thousands in the fly brain that control freezing was remarkable,” Moita says. “Their properties suggest they form a key node in the circuitry that balances alternative defensive responses.”
The study opens new experimental opportunities. By establishing walking speed as a key factor shaping defensive decisions and identifying a defined neural element that gates freezing, the work provides a tractable model to study how brains choose among competing survival strategies. Because defensive behaviors such as freezing, fleeing and fighting are widespread across animals, these findings in flies offer a meaningful starting point for understanding similar decision rules in other species.
Funding: The research was supported by the Champalimaud Foundation, the Visiting Scientist Program of Janelia Research Campus, the European Research Council, and Fundação para a Ciência e Tecnologia.
Source: Maria João Soares – Champalimaud Center for the Unknown
Publisher: Organized by NeuroscienceNews.com.
Image Source: Image credited to Gil Costa, Champalimaud Centre for the Unknown.
Original Research: Open access research: “Speed dependent descending control of freezing behavior in Drosophila melanogaster” by Ricardo Zacarias, Shigehiro Namiki, Gwyneth M. Card, Maria Luisa Vasconcelos & Marta A. Moita in Nature Communications. Published September 12, 2018.
DOI: 10.1038/s41467-018-05875-1
Champalimaud Center for the Unknown. “To Flee or Not to Flee: How the Brain Decides What to Do in the Face of Danger.” NeuroscienceNews, 12 September 2018.
Abstract
Speed dependent descending control of freezing behavior in Drosophila melanogaster
The most fundamental decision an animal makes when confronting a threat is whether to freeze, which can reduce detection, or to flee to safety. This study shows that Drosophila melanogaster exposed to looming stimuli in a confined arena either freeze or flee, and that the probability of freezing versus fleeing is modulated by the fly’s walking speed at the time of threat. These results demonstrate that the freeze/flee decision depends on behavioral state. The authors describe a pair of descending neurons crucially implicated in freezing: genetic silencing of the DNp09 descending neurons disrupts freezing without preventing fleeing, while optogenetic activation of both DNp09 neurons induces running and freezing in a state-dependent manner. Together, these findings establish walking speed as a key factor in defensive response selection and identify a pair of descending neurons as a critical element of the circuitry that mediates the choice and execution of freezing or fleeing behaviors.