In Yartsev’s lab they built an artificial cave: a living room-size flight room designed to measure the bats’ brain activity while tracking their behavior. Five to seven bats at a time were free to fly about the room, which was lined with sound-dampening foam and equipped with perches and fruit to eat. To follow the bats’ precise 3D movements, the researchers outfitted collars with accelerometers and mobile tags — modified from systems used to track packages within warehouses — which communicated with sensors installed in the room’s walls. The team also implanted tiny electrodes into the bats’ brains to wirelessly record hippocampal neurons firing as the animals flew around their enclosure and interacted with each other.
The researchers put all this elaborate effort into their experimental setup so that they could investigate the bats’ spontaneous social interactions, which would presumably be like those experienced in the wild. That meant largely leaving the bats to their own devices without human meddling.
“The idea was just to take humans out of the room,” Yartsev said, and let bats do what bats normally do.
As expected, the place cells for a given bat changed their activity based on the bat’s location in the cave. Certain place cells fired more frequently when the bat was in a particular spot, while others increased their firing when the bat was someplace else.
The presence or absence of other bats also influenced the neurons’ firing. As a bat went in for a landing, the place cells behaved differently depending on whether there was a roostmate at the landing site. What’s more, the neurons appeared to encode the identity of specific bats, distinguishing friends from acquaintances. If a bat landed next to a close social contact, the neurons behaved differently than if it landed near a bat it didn’t spend much time with.
In short, the bats’ navigation system appeared to do double duty as a social map. The mammals weren’t just moving around their home — they were also using the exact same brain cells to track who was on the premises.
“[The researchers] knock it out of the park with respect to studying the neuroscience of a natural behavior,” said the behavioral neuroscientist Andy Alexander of the University of California, Santa Barbara, who was not involved in the study.
The discovery immediately spurred questions about whether this reuse of hippocampal place cells applies beyond Egyptian fruit bats in the evolution of the social brain. The hippocampus is an ancient brain structure: It is highly conserved among mammals with diverse lifestyles and degrees of sociality, from largely solitary platypuses to highly communal people. It’s possible that the hippocampus’s navigation system logs the social environment similarly across species. However, it’s equally possible that the place circuitry evolved this dual purpose only in the Egyptian fruit bat. Only additional research can fill in the gaps.
The findings go beyond the social map. They also fit with the concept of mixed selectivity, Alexander said: the idea that it’s more computationally efficient for single neurons to encode multiple features of the environment.
In that sense, Forli said, the hippocampus may be like a powerful graphics card in a computer, which can have many uses, from rendering graphics for video games to performing machine learning computations. The hippocampus may be great at particular kinds of computations and may have the ability to be modified or programmed by evolution.
“We’ve classically thought of the hippocampus as having these [place] cells that code particular locations in space,” Alexander said. “But I think more and more we’re discovering that it’s actually highly adaptive and flexible, and that the hippocampus will code for all sorts of things depending on what you present to it.”
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