Examining how bats distinguish different sounds

The Ceiba short-tailed bat (Caerulea perspicillata) lives in the subtropical and tropical forests of Central and South America, where it feeds mostly on black pepper fruit. These animals spend their days in groups of 10 to 100 individuals in hollow trunks and rock caves, and at night they forage together. They communicate using sounds that create distinct ambient noise in the colony — such as the sound of voices at a lively party.

At the same time, bats use sound to navigate their surroundings, a phenomenon known as echolocation, in which they emit ultrasonic sounds that reflect off solid surfaces. Animals then assemble these echoes into a “picture” of their surroundings.

But how does the ciba’s short-tailed bat manage to filter out important sounds from the constant ambient noise? A common explanation is that the brain constantly predicts the next signal and reacts more strongly to an unexpected signal than to an expected one.

It’s called deviance detection, and the method was developed by neuroscientists led by Johannes Vitacum and Prof. Manfred Kossel from the Neurobiology and Biosensors Working Group at the Institute of Cell Biology and Neuroscience at Goethe University Frankfurt. Looking for

Together with colleagues, they were already able to show this in 2021. Signal processing It does not start in the higher brain regions but is already in the brain, which is responsible for controlling vital functions such as breathing and heart rate. However, these studies used only artificial stimuli that were not meaningful to the animals.

In a recently published study, the team led by Wetekam and Kössl replicated experiments with natural communication and echolocation calls. “With our study, we wanted to find out what happens in deviance detection when, instead of meaningless stimuli, Ciba short-tailed bats are presented with stimuli that are actually their auditory cues,” says Withicum. happens in the world,” says Withicum.

To do this, two electrodes, about the thickness of a human hair, were inserted under the bats’ skulls to record their brain waves. Although it was painless for the animals, the measurements were performed under general anesthesia, as any movement could distort the results.

A bat’s brain reacts to sounds even when the animal is unconscious and fast asleep. Either echolocation or communication calls were then played to the animals, each paired with another vocalization, with a 10% probability of its occurrence.

From then on it was possible to read. Brain waves It was measured that the brainstem processes echolocation and communication calls differently. While infrequent echolocation sounds produced a stronger signal than frequent sounds—that is, detected deviations—in the case of communication sounds, their probability of occurrence did not affect the strength of the response.

Manfred Kossel believes that “bats probably need to react more quickly during echolocation than when communicating with conspecifics”. “The brainstem is the first station in the brain to receive sound signals, which is why it has to calculate its potential echo There the first calls may be necessary, and especially their echoes, so that the animal can clear obstacles in good time.” The stronger response to less frequent calls is probably due to better neural coordination.

The study also showed that the brainstem can use other features of bat calls to detect deviations, such as rapid changes in frequency or volume, in addition to differences in pitch. “This is surprising, because the brainstem is an ancient part of the brain that scientists didn’t previously think was capable of substantial involvement in signal processing,” says Withicum. “They saw his role as more of receiving signals Auditory nerve and transferring them to higher-level areas of the brain.”

These findings may also be important in relation to clinical applications in humans. For example, when studying diseases such as ADHD or schizophrenia, lower-level brain regions associated with impaired processing of external stimuli should be included. The fact is that bat Brain stem Processing different complex sound signals differently can also help scientists understand how the brain understands and processes complex human speech.

Here are the results. published I Journal of Neuroscience.