Bats' amazing plan B for when they can't hear

They adapt immediately and robustly, suggesting for the first time that bats' brains are hard-wired with an ability to launch a Plan B in times of diminished hearing.

The Johns Hopkins University work, newly published in Current Biology, raises questions about whether other animals and even humans might be capable of such deft accommodations.

"Bats have this amazing flexible adaptive behavior that they can employ anytime," said senior author Cynthia F. Moss, a Johns Hopkins neuroscientist who studies bats. "Other mammals and humans also have these adaptive circuits that they can use to help make decisions and navigate their environment but what's striking here is that it's very fast, almost automatic."

All animals adapt in various ways as a response to sensory deprivation. People at a loud bar, might lean in to better hear what someone is saying. A dog might tilt its head toward a muted sound.

Here researchers wondered how hearing-dependent echolocating bats might adapt when a key auditory region in the brain was turned off.

They trained bats to fly from a platform, down a corridor, and through a window to get a treat. Researchers then had the same bats repeat the task but with a critical auditory pathway in the midbrain temporarily blocked. Disabling this brain region isn't like plugging your ears; it's preventing most auditory signals from reaching the deep brain. The drug-induced technique is reversible and lasts about 90 minutes.

With their hearing blocked, bats were able to navigate the course surprisingly well, even on the first try. They weren't as agile and ran into things, but every tested bat compensated immediately and effectively.

"They struggled but managed," Moss said.

The bats changed their flight path and vocalizations. They flew lower, oriented themselves along walls and increased both the number and length of their calls, which boosted the power of echo signals they use for navigation.

"Echolocation acts like strobes, so they were basically taking more snapshots to help them get the missing information," said co-author Clarice A. Diebold, a former Johns Hopkins graduate student who is now a postdoctoral student at Washington University in St. Louis. "We also found that they broadened the bandwidth on these calls. These adaptations are very interesting because we'd usually see them when bats are compensating for external noise but this is an internal processing deficit."

Although the team repeated the experiments, the compensation skills of the bats didn't improve over time. This means the adaptation behaviors the bats employed weren't learned; they were innate, latent and hard-wired into the bats' brain circuitry.

"It highlights how robust the brain is to manipulation and external noise," said co-author Jennifer Lawlor, a postdoctoral fellow at Johns Hopkins.

The team was surprised that the bats could hear at all with this region of their brain disabled. They believe bats either relied on a previously unknown auditory pathway or that unaffected neurons might support hearing in previously unknown ways.

"You'd think an animal wouldn't be able to hear at all," Moss said. "But it suggests that there might be multiple pathways for sound to travel to the auditory cortex."

The team would next like to determine to what degree the findings apply to other animals and humans.

"Can this work tell us something about auditory processing and adaptive responses in humans? Moss said. "Since no one has done this, we don't know. The findings raise important questions that will be exciting to pursue in other research models."

Authors include Kathryne Allen, Grace Capshaw, Megan G. Humphrey, Diego Cintron-De Leon and Kishore V. Kuchibhotla, all of Johns Hopkins.