A New Way to Measure Bat Hearing

Studying bats' hearing can be tricky due to their small size, making certain styles of measurement used for larger echolocating mammals unavailable to researchers. In this episode, we talk to Victoria Fouhy and Michael Smotherman (Texas A&M University) about their work to develop a noninvasive method to study cortical auditory evoked potentials in bats, thereby allowing scientists to better understand how these animals process echolocation information.

Associated paper:
- Victoria Fouhy, Sam Ellis, and Michael Smotherman. "Subcutaneous cortical auditory evoked potentials in echolocating bats." J. Acoust. Soc. Am. 158, 3390-3399 (2025). https://doi.org/10.1121/10.0039659.

Read more from The Journal of the Acoustical Society of America (JASA).
Learn more about Acoustical Society of America Publications.

Music Credit: Min 2019 by minwbu from Pixabay. 

Transcript

ASA Publications (00:26)

Even though both large and small animals echolocate, how we study individual species' hearing systems can differ based on the animal's size. Today we're going to be talking to Victoria Fouhy and Michael Smotherman, who developed a new technique for studying bat hearing systems similar to one used in larger mammals. They shared their research in their recently published article, “Subcutaneous Cortical Auditory Evoked Potentials in Echolocating Bats” in JASA. Thanks for taking the time to speak with me today. How are you?

Michael Smotherman (00:27)

Great, thank you.

Victoria Fouhy (00:53)

Good, thank you.

ASA Publications (00:54)

So first, tell us a bit about your research backgrounds.

Michael Smotherman (00:58)

Okay, well I'll go first. So I’m a sensory physiologist. I got my PhD at UCLA under Dr. Peter Narins, where I began by studying hearing in frogs, which was kind of a fun thing to do, but, you know, the frogs can be useful just for looking at the cell biology and the evolution of hearing.

You know, my whole life I've been really fascinated by just sensory ecology. Especially as a kid, I was fascinated by the idea that there was animals out there that could hear and see things that we couldn't hear and see. So that's kind of driven my curiosity in this field. But as a postdoc, I switched over to mammals, you know, mainly for funding reasons in order to get money for research, and that's how I ended up working on bats. Bats are just really great models for the mammalian hearing system.

ASA Publications (01:51)

Very cool.

Victoria Fouhy (01:52)

Well, I got my Bachelor's of Science from Texas A&M in biomedical science, and from there I went on and did microbiology research for the government, actually, for a few years. And then I knew I wanted to get my PhD, but after doing industry work in microbiology I knew I didn't want to do that. So when I got into my PhD program, I was very open to a bunch of different varieties of research, and I fell into Mike's lab, and I ended up loving it and so here I am.

ASA Publications (02:25)

Studying bats and their hearing. 

Victoria Fouhy (2:26)

Studying bats.

ASA Publications (02:27)

Awesome.

Okay. So to start, why are we interested in studying bat auditory systems?

Michael Smotherman (02:33)

Well that's a great question. Bats have been fascinating study animals for hearing for a long time, since the 1950s. Back when it really wasn't clear how the ear worked or how the auditory system worked, we used a thing called the comparative method.

And you had examples like bats where bats could hear very high frequencies that other animals couldn't hear. And then you could just look at the anatomy and see that the cochlea was bigger, that the parts of the brain that were processing sound were bigger in a bat compared to another animal. And so right from the start, scientists were attracted to bats as a way to simplify the questions and explore how the brain processes sounds.

So now, here I am looking at 75 years of really cool research to build on. And almost everything that we know about the mammalian auditory system was first discovered in a bat.

ASA Publications (03:30)

Very cool, interesting. So how has the bat auditory system typically been studied?

Michael Smotherman (03:37)

Because it's a small animal, it lends itself well to lab experiments. So, it’s the nervous system, and the currency of the nervous system is electrical activity and action potentials.  So historically we've used bats and electrophysiology to figure out what parts of the brain are active during listening and how the neural circuits of the brain are constructed to encode the sounds that the bats hear.

ASA Publications (04:05)

Okay, okay. So in your article, you mentioned that there is some differences in how researchers study the auditory system of bats versus those of larger echolocating animals, like cetaceans. What's different and why? How do these differences impact the type of research done on bats?

Michael Smotherman (04:22)

So the technology that we use in bats is identical to the technology that you might use in a lab mouse or a rat. And that's a benefit in that regard, because you can do a lot of things in those animals that you can't do in big animals. But it also has some limitations to it, right? If we want to study complex things, like how speech and language work in humans, for example, or if you want to just find ways to measure hearing processes in a human, you can't do that invasive research. You need to find non-invasive ways to do it. And they've come up with a lot of really cool ways to do that. The most common one is just what we call an electroencephalogram, or EEG, as most people have heard of it. But we do a lot of these non-invasive things routinely in humans. A thing called auditory brainstem responses, for example. Almost every baby that's born gets tested this way to test their hearing function as soon as they're born. But we use these non-invasive methods to test for hearing deficits, age-related hearing disorders, and those kind of things. And so consequently, the technology for doing these noninvasive measures in humans is pretty advanced. 

Another advantage is humans have big heads. And so if the goal is to figure out where in the brain an electrical signal is coming from, it helps a lot to have a big head. That's why with a human we can just cover it with a whole net of electrodes, dozens and dozens of electrodes, and then you can use some clever math to try to guess where the signal comes from. So because that technology is pretty advanced for humans, our colleagues that work with dolphins have been able to apply it to dolphins as well as a non-invasive tool. That's about the best technique they have available to study how the dolphin brain processes sounds for echolocation.

ASA Publications (06:20)

Okay, okay. But we don't have the teeny, teeny, tiny little electrodes for the bats.

Michael Smotherman (06:26)

Exactly. And even if you did, you still have the problem of such a small brain. It becomes very, very difficult to know where that signal came from.

ASA Publications (06:34)

Right, right, okay. So what are auditory brainstem responses and what are the benefits and limitations of using them to study an animal's auditory system?

Victoria Fouhy (06:44)

So auditory brainstem responses, or ABRs, are basically a way to look at how the early parts of an animal's auditory system are working. You play a sound and then measure the brain's electrical activity in response to that auditory sound. You usually see about five fast waves within the first 10 milliseconds after stimulus onset. Those represent activity from different stages of the brainstem and midbrain.

What's nice is that it's a pretty non-invasive technique, so you can get a snapshot of auditory processing without doing anything too invasive. The trade-off, though, is that ABRs only tell you about those early stages of processing. ABRs are widely used in both humans and animals to measure the sensitivity of your peripheral auditory systems, mainly the cochlea.

Clinically, they're used to diagnose hearing deficits, including universal screening of newborn infants, as Dr. Smotherman mentioned earlier. In animals, they are used to test hypotheses about how the cochlea works or to study things like age-related hearing disorders. You don't get too much insight into what's happening later in the auditory pathway, though, like in the cortex, where more complex sound processing is happening.

ASA Publications (08:00)

Okay, so it's like it helps with kind of figuring out if it's being heard, but not necessarily what the brain is doing with what it's hearing. Is that what I'm getting? Yeah, okay, got it. 

Victoria Fouhy (08:07)

Sure, yeah.

Michael Smotherman (08:09)

Yeah, that's a great way to put it.

ASA Publications (08:11)

So then what are cortical auditory evoked potentials and how do they compare to auditory brainstem responses?

Victoria Fouhy (08:20)

So cortical auditory evoke potentials, or CAEPs, are essentially an adaptation of the ABR technique that allows us to capture the more slowly evolving, longer-latency cortical responses to sounds. In addition to the activity in the brainstem or midbrain, these later signals represent electrical activity originating from the cortex. Methodologically, CAEPs and ABRs are quite similar. Both measure the brain's electrical responses to sound, but they differ in their timing and neuroanatomical sources. CAEPs are thought to reflect activation of higher order processing centers, such as the thalamus, primary auditory cortex, and other brain areas serving higher cognitive functions, like perception and decision making.

 In our study, we observed two late peaks in the bat's auditory evoked potential waveform that appear cortical in origin. The first, P1, peaked at around 23 milliseconds and had the same onset and duration as ensemble neuronal activity previously recorded in the auditory cortex. The second smaller wave component, P2, peaked around 37 milliseconds and corresponded to evoked neural activity in another brain area, called frontal auditory field. So in general, the added benefit of being able to record CAEPs is that it lets us study how sounds are being analyzed by the brain in higher cognitive centers, which isn't captured by the ABRs.

ASA Publications (09:48)

So what was the goal for this study?

Victoria Fouhy (09:51)

Our main goal for the study was to successfully classify and characterize cortical auditory evoke potentials in echolocating bats. Essentially, we wanted to understand what these cortical responses look like in species that rely so heavily on sound for navigation and perception of their world. Bats use continuous echo sequences to build what we call an auditory scene of their environment, and this only happens in those higher brain areas. 

Beyond that, a big part of our motivation was methodological. We hope to develop an approach that could act as a bridge between more invasive electrophysiological techniques that have traditionally been used in bats, rodents, and other small animals, and the non-invasive methods that are necessary for studying hearing in animals like humans and dolphins. The comparisons between bats and dolphins are important because they both use echolocation and they share similar specializations of their brain and auditory system. Comparisons between bats and humans can also be useful because both use long sequences of vocalizations, like speech in humans, to communicate. So there might be similarities in how brains are adapted to integrate and analyze long sequences of sounds over an extended time.

ASA Publications (11:09)

Okay, interesting, interesting. So you actually ended up doing five different experiments for this study. Can  you describe what you did, what you were hoping to learn, and what you found with each one?

Victoria Fouhy (11:20)

Yeah, so the first three were really about validation, making sure that the responses we were recording were coming from brain regions we intended and that our experimental setup was capable of capturing the cortical neural activity we were looking for. Most of our paper focuses on the first peak component, called P1, which is believed to originate from the auditory cortex. So in our first experiment, we wanted to confirm that our methods were reliable, that they produced reproducible responses consistent with CAEP signals previously reported in other species. Then in experiments two and three, we took that a step further to verify that what we were seeing was truly cortical in origin. The auditory cortex of our species has already been extensively studied using invasive methods, and there are some physiological features that distinguish cortical responses from other parts of the auditory system, which allowed us to make some predictions about how changing the acoustic stimulus parameters should change the results if they were coming from the cortex. The CAEPs we recorded in response to tone PIPs showed strong frequency dependence. Amplitude dropped sharply as the tone frequency increased from 20 to 50 kilohertz, which was consistent with the overall frequency sensitivity of the cortex. We also found that we increased the stimulus presentation rate, or the repetition rate, the response amplitude declined, which matched what's known about cortical processing limits. Because the cortex is thought to sum information from many sounds over time, it isn't able to respond to each individual sound when they start coming really fast. And that is what we saw in our measurements. The amplitude of the responses got smaller as the repetition rate got faster, just like what we saw with the more invasive intracranial methods.

So both of these patterns gave us confidence that we were indeed measuring cortical responses. So those are the first three. The last two experiments were where things got really interesting. Here we tested different types of behaviorally relevant auditory stimuli. Because bats naturally use downward frequency modulated sweeps, or downward FM sweeps, when they echolocate, we expected those to produce the strongest responses, and they did. Conversely, upward sweeps, which bats don't typically use for echolocation, although they do sometimes use them for communication, produced the weakest responses. So for the P1 component, both amplitude and latency, or timing, were significantly influenced by the stimulus type.

For P2, stimulus types strongly affected amplitude, but not latency. So that stability and latency across stimulus types at P2 suggests that there's some kind of active filtering mechanism happening. Basically, the brain is tuning out irrelevant noise and focusing in on behaviorally meaningful cues.

ASA Publications (14:26)

Okay, okay, that makes sense. Okay. So, what is the significance of the second peak in the cortical auditory evoked potential?

Victoria Fouhy (14:34)

Yeah, so the second peak, called P2, it's really exciting because it's thought to reflect activity coming from the frontal auditory field, which is a region that's believed to play a key role in sensory motor integration. Unlike the primary auditory cortex, which tends to respond to individual sounds or syllables, the frontal auditory field seems to care more about sequences of sounds, patterns over time, rather than isolated events.

So in our study, we did see evidence of that. When we presented five pulse sequences, which was intended to mimic what bats do when they rapidly approach a target, we observed a clear, though not simple, summation of information across those pulses. The individual waveforms started to blur together after the second pulse, producing a more complex CAEP waveform, that ultimately formed a large peak occurring around the same time as P2's response to a single pulse would. Now from this method alone, it's hard to pinpoint exactly what that means functionally, but the fact that P2 behaves this way hints that the frontal auditory field may be integrating temporal information, combining multiple sounds into a unified percept rather than responding to one of them alone.

ASA Publications (15:54)

Oh, that's interesting. Very cool. So how well does this method work for studying the bat auditory system?

Michael Smotherman (16:00)

Well, so far it's working great.

ASA Publications (16:02)

Awesome. We like to hear that.

Michael Smotherman (16:04)

I was really excited to see the way these experiments played out. Sometimes you start things and you hope that it works, and on paper everything should work, but you never know until you do it. But this gave us some really interesting results, and I'm excited about the future. So I think it blends really well with things we're already doing, but more importantly, it's really opening up a lot of new avenues for what we can do in the future.

ASA Publications (16:31)

Right. So part of these results were really unique to bats because they echolocate. What did you learn?

Michael Smotherman (16:37)

We were able to take advantage of what we already knew about specializations. And as Victoria mentioned, the bat auditory system is really, really adapted to being very sensitive to their own echolocation pulses, which follow a very specific acoustic pattern, these downward FM sweeps. And at the same time, we knew that the brain was built to filter out upward FM sweeps. And so this was a simple test of the hypothesis, but we saw that. That even at this superficial level, basically without going into the brain at all, you could still see that the bat's brain was very strongly responding to things they care about and filtering out or ignoring things that they don't normally care about.

ASA Publications (17:26)

Very cool, yeah. That's very interesting. So how can having a non-invasive method for studying cortical function in bats help with future research?

Michael Smotherman (17:34)

Well, we have a lot of plans for this. But the most important thing is when you're doing the invasive research where you're drilling a hole in the skull and you're recording from tingled neurons, you're not really able to study the natural behavior. There's just no way around that. So what you need is some kind of non-invasive method where you can pick up the activity you care about without really interfering with the animal's natural behavior. And so that's really our holy grail. Our long-term goal is to be able to study how the auditory system behaves while an animal is doing something really complicated and natural. And I think right now this is the best approach that we have.

ASA Publications (18:17)

That's amazing. That's so exciting.

Michael Smotherman (18:19)

I mean, I would love to be able to do this while a bat's flying around catching a bug, dodging trees. I think that's going to be possible in the near future.

ASA Publications (18:29)

That's so cool, yeah. Well, thank you again for sharing this exciting development, for helping us better understand how bats hear and process sound. I wish you the best of luck in your future research and have a great day!

Victoria Fouhy (18:42)

Thank you so much.

Michael Smotherman (18:43)

Thank you very much.