How fruit flies use internal representations of head direction to support goal-directed navigation

Animal behavior is known to rely on the transformation of sensory information into motor commands, often influenced by the animal’s internal needs. While in mammals and other large animals this process is supported by complex brain processes, simpler versions of this process may also guide the behavior of smaller living organisms, including insects.

To plan their actions when sensory signals are unavailable, some animals have evolved to rely on internal representations of their relationship to their environment. These representations may include information related to the direction of their head or collected by place cells, neurons in the hippocampus that form internal “maps” of environments.

Researchers at the Howard Hughes Medical Institute recently studied how fruit flies simultaneously map new environments and use these internal representations to determine which goals to pursue. Their paper, published in Neuronoffers new insight into how internal representations can guide goal-directed behavior in animals.

“Anchoring goals to spatial representations enables flexible navigation, but is challenging in novel environments where both representations must be acquired simultaneously,” Chuntao Dan, Brad K. Hulse, and colleagues wrote in their paper. “We propose a framework for how Drosophila uses internal representations of head direction (HD) to construct goal representations upon selective thermal reinforcement.”

The researchers conducted experiments on common fruit flies (Drosophila melanogaster). The flies’ behavior was conditioned by heat, which was associated with different instances of repetitive visual patterns.

These visual patterns altered the flies’ internal representations of HD, allowing the researchers to observe how evolving HD representations interacted with the flies’ goals to ultimately shape their behavior. Using machine learning algorithms and data collected from previous studies of fruit flies, the researchers set out to determine how these processes might be carried out in a region of the insect brain known as the central complex (CX).

“We show that flies use stochastically generated fixations and directed saccades to express heading preferences in a visual operant learning paradigm and that HD neurons are required to modify these preferences as a function of reinforcement,” Dan, Hulse, and colleagues wrote. “We used a symmetric visual framework to expose how flies’ HD and goal representations co-evolve and how the reliability of these interacting representations impacts behavior.”

The results of this recent study offer new insight into how flies simultaneously map their environment and link internal goals to the maps created while initially learning about a new environment. By analyzing their experimental data and previously collected results using computational models, the researchers then created a framework for how the fruit fly brain supports the insects’ goal-directed behavior in new environments.

“We describe how rapid learning of new goals can rely on a behavioral policy whose parameters are flexible but whose form is genetically encoded in the circuit architecture,” Dan, Hulse, and colleagues wrote. “Such evolutionarily structured architectures, which enable rapidly adaptive behavior guided by internal representations, may be relevant across species.”

The researchers conducted their experiments in fruit flies, but they found that circuit architectures and processes similar to those they discovered could also exist in other species. Future studies using genetic techniques could shed additional light on the cells involved in the goal-directed learning process the researchers identified or could help identify analogous processes in other animals.

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