How bats are helping us rethink how the brain hears

June 3rd, 2025

Written by: Abby Lieberman

Every day, without even thinking about it, you can instantly tell the difference between a question and a statement or between laughter and anger in someone’s voice. This skill is called auditory categorization¹, and it’s a key part of how we make sense of the world. Our brains are constantly taking in a flood of sounds and quickly deciding what they mean–who’s talking, how they feel, and whether we need to react. But how exactly does the brain do this? And could it be happening in a different part of the brain than scientists previously thought?

Where sounds get their meaning

When a sound enters your ear, it sets off on a journey through many parts of the brain before arriving in the auditory cortex. Scientists have long believed that neurons in the auditory cortex give sounds their meaning and that auditory categorization happens in the cortex as well. This means that your brain waits until sound reaches the cortex to decide whether someone is laughing or angry. But, an exciting new study in big brown bats (yes, that’s really the name of the species!) is challenging this idea by showing that the brain starts sorting sounds into categories well before they reach the cortex. The results from the bat study are important because they suggest that auditory categorization may be more automatic than we thought. If categorization happens before sound reaches the cortex, it gives our brains a kind of shortcut to quickly understand what is happening around us, which is important for survival and communication. 

What can we learn from Bat (Brain) Signals? 

Why study auditory categorization in big brown bats? Bats are ideal for this research question because they rely heavily on sound for both social communication (like mating or competing with other bats) and navigation (using echolocation to avoid obstacles or track prey). As a consequence, bats have to consistently tell the difference between different types of calls (and do it quickly!). Also, though we seem quite different now, humans and bats actually share a common ancestor and many brain structures, including those important for auditory processing and categorization. This means that studying how bats process sound can give us clues about how our brains do too.  

In the new big brown bat study, scientists visualized neural activity in a part of the brain called the dorsal cortex of the inferior colliculus (DCIC) while bats listened to different sounds. The researchers chose to monitor neurons in this part of the brain because of their location in the auditory pathway (Figure 1). After sound waves enter your ear, they reach the cochlea (Fig 1, Box 1), which converts sound vibrations into electrical signals. These signals travel along the cochlear nerve (Fig 1, Box 2) to the brainstem (Fig 1, Box 3), where early sound processing begins. Next, the signals move to the inferior colliculus (which contains the DCIC; Fig 1, Box 4). From there, sound information is directed through the thalamus (Fig 1, Box 5) to the auditory cortex (Fig 1, Box 6). Critically, sound information passes through the DCIC on the way to the auditory cortex.

During the experiment, bats listened to pure tones, white noise, mouse vocalizations, as well as social and navigation bat vocalizations. Most of the DCIC neurons responded more to bat vocalizations than to the other sounds. This means that these neurons help the bats process sound information that is important for communication and navigation as opposed to other common noises, including the vocalizations of other small mammals. Excitingly, some of the individual DCIC neurons visualized during the experiment were more active during social vocalizations while others were more active during navigation vocalizations. This result suggests that the DCIC is categorizing social and navigation vocalizations into two separate streams of information that can then flow to the thalamus and cortex.

After identifying neurons that prefer social or navigation vocalizations, the researchers examined where these neurons are located within the DCIC of each bat. Remarkably, they found that the neurons weren’t randomly scattered. Instead, the two kinds of neurons were grouped into their own “neighborhoods” in the DCIC. Neurons that responded to social vocalizations tended to be in the same neighborhood, while neurons that responded to navigation vocalizations were in a separate neighborhood. This pattern suggests that the bat brain may sort different types of communication signals well before the information reaches the auditory cortex.

Why do the results of this study matter? 

The results from this study shift our understanding of how the brain processes sound. These discoveries suggest that the work of categorizing vocalizations, often thought to happen only once sound information reaches the cortex, starts much earlier in the brain. If the brain starts categorizing sounds in the DCIC, this could make sound processing faster and more efficient in situations where reacting quickly matters (like avoiding danger)! 

While this study focused on bats, the importance of the findings extend beyond the cave. Any species that relies on complex vocal communication, including humans, could benefit from having early, efficient ways of sorting sounds. The results from this study suggest that our brains might be getting a head start on what sounds mean, long before we’re even aware of what we are hearing. So, the next time you instantly sense someone’s mood just by hearing their voice, it may be due to an efficient system deep in your brain that’s doing the hard work of categorizing sounds before you’re even aware of it. And thanks to some flying fuzzy mammals we are starting to uncover how that system really works.

References

1. Tsunada, J., & Cohen, Y. E. (2014). Neural mechanisms of auditory categorization: from across brain areas to within local microcircuits. Frontiers in neuroscience, 8, 161. https://doi.org/10.3389/fnins.2014.00161
2. Pickles J. O. (2015). Auditory pathways: anatomy and physiology. Handbook of clinical neurology, 129, 3–25. https://doi.org/10.1016/B978-0-444-62630-1.00001-9
3. Lawlor, J., Wohlgemuth, M. J., Moss, C. F., & Kuchibhotla, K. V. (2025). Spatially clustered neurons in the bat midbrain encode vocalization categories. Nature neuroscience, 28(5), 1038–1047. https://doi.org/10.1038/s41593-025-01932-3
4. Salles, A., Loscalzo, E., Montoya, J., Mendoza, R., Boergens, K. M., & Moss, C. F. (2024). Auditory processing of communication calls in interacting bats. iScience, 27(6), 109872. https://doi.org/10.1016/j.isci.2024.109872
5. Warnecke, M., Simmons, J. A., & Simmons, A. M. (2021). Population registration of echo flow in the big brown bat’s auditory midbrain. Journal of neurophysiology, 126(4), 1314–1325. https://doi.org/10.1152/jn.00013.2021

Cover photo by Rhododendrites from Wikimedia Commons.

Figure 1 by Shaishyy on Wikimedia Commons, adapted by Abby Lieberman.