In the tropical and subtropical forests of Central and South America, there is a short-tailed bat named Seba (Carollia perspicillata), which mainly feeds on pepper fruits. These animals often gather in groups in tree holes or rock caves during the day and go out together to forage at night. They communicate using sounds, creating a unique ambient noise in the colony—like the din of a lively party.
At the same time, these bats also use vocalizations to navigate, a phenomenon known as echolocation. They emit ultrasonic waves that reflect off solid surfaces. The animals then combine these echoes into “images” of their surroundings.
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But how does the Seba short-tailed bat filter out important sounds from constant ambient noise? One common explanation is that the brain is constantly anticipating the next signal and reacting more strongly to unexpected signals. This is called “bias detection” and a team of neurobiologists led by Professors Johannes Wetkam and Manfred Kosl are exploring its mechanism.
In collaboration with colleagues, they have shown in 2021 that signal processing does not start in higher regions of the brain, but already in the brainstem, which is responsible for controlling vital functions such as breathing and heart rate. However, these studies only used artificial stimuli that were not meaningful to the animals.
In the latest study, Wetkam and Kosl’s research team conducted a series of experiments using the humming and echolocation sounds that Seba short-tailed bats would hear in their natural environment. “We wanted to figure out how the brains of these bats responded when they heard the sounds they normally hear, rather than some meaningless sounds,” Wetkam explained.
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To do this, they inserted two electrodes about the thickness of a human hair under the bats’ scalp to record their brain waves. Although this is painless for the animal, the measurements are performed under general anesthesia as any movement could distort the results.
Even when anesthetized and asleep, the bat’s brain responds to sound. They then played the animals either echolocation calls or communication calls, each sound mixed with the other, with a 10 percent chance of occurring.
The brainstem can distinguish the difference between “echo” and “hum”
The Seba short-tailed bat (Carollia perspicillata) can filter important signals from environmental sounds and distinguish between echolocation and communication calls. (Image credit: Julio Hechavarría)
It can be read from measured brain waves that the brainstem processes echolocation and communication calls differently. While infrequent echolocation sounds did elicit relatively frequent strong signals – that is, showing “biased detection” – in the case of communication sounds, their probability of occurrence did not affect the strength of the response.
“Bats may need to react faster when echolocating than when communicating,” Cosl speculates. “The brainstem is the first station where the brain receives acoustic signals, which is why it may be necessary to first calculate the probability of echolocation calls there, especially their echoes, so that the animal can avoid obstacles in time.” Right. The stronger response to common calls may be due to better neural synchronization.
The study also shows that the brainstem can exploit other features of bat calls in addition to pitch differences for deviation detection, such as rapid changes in frequency or volume. “This is surprising because the brainstem is a relatively primitive part of the brain, and scientists had not previously thought it had any significant involvement in signal processing,” Wetkam said. “They think its role is more to receive signals from the auditory nerve and pass them on to higher areas of the brain.”
The findings may also be important for medical applications in humans. For example, when studying disorders associated with impaired processing of external stimuli, such as ADHD or schizophrenia, lower regions of the brain should be included. The fact that the bat brainstem processes different complex acoustic signals differently could also help scientists understand how the brain deciphers and processes complex human language.
The findings were published in Neuroscience.
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First image source: Julio Hechavarría cc By4.0
Image source: Julio Hechavarría cc By4.0
Reference papers:
1.Deviance detection to natural stimuli in population responses of the brainstem of batsThe Journal of Neuroscience
Further reading:
1.Professional night hunter?How did this bat become a master of “sneaking”?
2024-02-25 06:49:43
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