Forward masking enhances the auditory brainstem response in the free-tailed bat, Tadarida brasiliensis, during a critical time window for sonar reception

Effect of Level and Frequency of Forward Masker on Auditory Brainstem Response

PURPOSE

Forward masking (FM) is characterized by the perception of a signal being reduced or wholly masked due to a preceding sound (masker) of the same or different frequencies that offers a challenge for the auditory system to resolve. Considering that the off-frequency masker is expected to undergo linear processing compared to the on-frequency masker at the signal place, it reflects the peripheral auditory systems' compressive response. Thus, the present study focused on employing FM electrophysiological analogous such as auditory brainstem responses (ABR) to the behavioral masking experiments to objectively measure the frequency and level of processing in the auditory system, from the periphery to the brainstem level.

METHOD

The study was an observational research on 21 female volunteers. ABR was obtained using a tone-on-tone FM paradigm for 1000- and 4000-Hz probe stimuli. An experiment used two forward maskers, on-frequency and off-frequency, with varying levels from 50 to 70 dB SPL.

RESULTS

A progressive shift for Vth peak latency and reduction in response amplitude was observed in proportion to the increase of masker level for both the probe stimuli and the masking experiments. However, ABR responses in neither masking condition were observed to differ between 60 and 70 dB SPL.

CONCLUSION

FM ABR experiments are an assessment tool for estimating frequency and level processing in the auditory system, providing good efficiency, reliability, and less subject bias compared to behavioral measures.

Echolocating bats show species-specific variation in susceptibility to acoustic forward masking

Echolocating bats rely on precise auditory temporal processing to detect echoes generated by calls that may be emitted at rates reaching 150-200 Hz. High call rates can introduce forward masking perceptual effects that interfere with echo detection; however, bats may have evolved specializations to prevent repetition suppression of auditory responses and facilitate detection of sounds separated by brief intervals. Recovery of the auditory brainstem response (ABR) was assessed in two species that differ in the temporal characteristics of their echolocation behaviors: Eptesicus fuscus, which uses high call rates to capture prey, and Carollia perspicillata, which uses lower call rates to avoid obstacles and forage for fruit. We observed significant species differences in the effects of forward masking on ABR wave 1, in which E. fuscus maintained comparable ABR wave 1 amplitudes when stimulated at intervals of <3 ms, whereas post-stimulus recovery in C. perspicillata required 12 ms. When the intensity of the second stimulus was reduced by 20-30 dB relative to the first, however, C. perspicillata showed greater recovery of wave 1 amplitudes. The results demonstrate that species differences in temporal resolution are established at early levels of the auditory pathway and that these differences reflect auditory processing requirements of species-specific echolocation behaviors.

Representation of frequency-modulated sweeps in the cochlear nucleus of the big brown bat

The cochlear nucleus (CN) receives ipsilateral input from the auditory nerve and projects to other auditory brainstem nuclei. Little is known about CN processing of signals used for echolocation. This study recorded multiple unit activity in the CN of anesthetized big brown bats (Eptesicus fuscus) to ultrasonic frequency-modulated (FM) sweeps differing in sweep direction. FM up-sweeps evoke larger peak amplitudes at shorter onset latencies and with smaller amplitude-latency trading ratios than FM down-sweeps. Variability of onset latencies is in the tens of microsecond ranges, indicating sharp temporal precision in the CN for coding of FM signals.

Bats experience age-related hearing loss (presbycusis)

Hearing loss is a hallmark of aging, typically initially affecting the higher frequencies. In echolocating bats, the ability to discern high frequencies is essential. However, nothing is known about age-related hearing loss in bats, and they are often assumed to be immune to it. We tested the hearing of 47 wild Egyptian fruit bats by recording their auditory brainstem response and cochlear microphonics, and we also assessed the cochlear histology in four of these bats. We used the bats' DNA methylation profile to evaluate their age and found that bats exhibit age-related hearing loss, with more prominent deterioration at the higher frequencies. The rate of the deterioration was ∼1 dB per year, comparable to the hearing loss observed in humans. Assessing the noise in the fruit bat roost revealed that these bats are exposed to continuous immense noise-mostly of social vocalizations-supporting the assumption that bats might be partially resistant to loud noise. Thus, in contrast to previous assumptions, our results suggest that bats constitute a model animal for the study of age-related hearing loss.

Echo feedback mediates noise-induced vocal modifications in flying bats

Diverse animal taxa are capable of rapidly modifying vocalizations to mitigate interference from environmental noise. Echolocating bats, for example, must frequently perform sonar tasks in the presence of interfering sounds. Numerous studies have documented sound production flexibility in echolocating bats; however, it remains unknown whether noise-induced vocal modifications (NIVMs) mitigate interference effects on echoes or calls. In this study, we leverage echo level compensation behavior of echolocating bats to answer this question. Using a microphone array, we recorded echolocation calls of Hipposideros pratti trained to approach and land on a perch in the laboratory under quiet and noise conditions. We found that H. pratti exhibited echo level compensation behavior during approaching flights, which depended critically on distance to the landing perch. Broadcast noise delayed and affected the rate of echo level compensation in H. pratti. Moreover, H. pratti increased vocalization amplitude, i.e., exhibited the Lombard effect, while also adjusting call duration and bandwidth with increasing noise levels. Quantitative analyses of the data show that H. pratti relies on echo feedback, not vocal feedback, to adjust signals in the presence of noise. These findings provide compelling evidence that NIVMs in echolocating animals and non-echolocating animals operate through different mechanisms.

Perceptual hearing sensitivity during vocal production

Vocalization, such as speaking, inevitably generates sensory feedback that can cause self-generated masking. However, perceptual hearing sensitivity during vocal production is poorly understood. Using an adaptive psychophysical method, we measured the perceptual hearing sensitivity of an echolocating bat, Hipposideros pratti, in a passive listening (PL) task to detect pure tones, an active listening (AL) task to detect pure tones triggered by its vocalization, and a phantom echo task. We found that hanging H. pratti had the best hearing sensitivity of approximately 0 dB sound pressure level (SPL) in the PL task but much lower hearing sensitivity (nearly 40 dB worse) in the echo task. In the AL task, all bats gradually increased call frequency by 0.8–1.1 kHz, which improved their hearing sensitivity by 25–29 dB. This study underscores the need for studying the sensory capability of subjects engaged in active behaviors.

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Vocal production strongly affects the perceptual hearing sensitivity of bats

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Forward masking explains the reduced hearing sensitivity during vocalization

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Long-term vocal plasticity enables bats to overcome self-generated auditory masking

Hearing sensitivity: An underlying mechanism for niche differentiation in gleaning bats

Tropical ecosystems are known for high species diversity. Adaptations permitting niche differentiation enable species to coexist. Historically, research focused primarily on morphological and behavioral adaptations for foraging, roosting, and other basic ecological factors. Another important factor, however, is differences in sensory capabilities. So far, studies mainly have focused on the output of behavioral strategies of predators and their prey preference. Understanding the coexistence of different foraging strategies, however, requires understanding underlying cognitive and neural mechanisms. In this study, we investigate hearing in bats and how it shapes bat species coexistence. We present the hearing thresholds and echolocation calls of 12 different gleaning bats from the ecologically diverse Phyllostomid family. We measured their auditory brainstem responses to assess their hearing sensitivity. The audiograms of these species had similar overall shapes but differed substantially for frequencies below 9 kHz and in the frequency range of their echolocation calls. Our results suggest that differences among bats in hearing abilities contribute to the diversity in foraging strategies of gleaning bats. We argue that differences in auditory sensitivity could be important mechanisms shaping diversity in sensory niches and coexistence of species.

Hearing sensitivity and amplitude coding in bats are differentially shaped by echolocation calls and social calls

Differences in auditory perception between species are influenced by phylogenetic origin and the perceptual challenges imposed by the natural environment, such as detecting prey- or predator-generated sounds and communication signals. Bats are well suited for comparative studies on auditory perception since they predominantly rely on echolocation to perceive the world, while their social calls and most environmental sounds have low frequencies. We tested if hearing sensitivity and stimulus level coding in bats differ between high and low-frequency ranges by measuring auditory brainstem responses (ABRs) of 86 bats belonging to 11 species. In most species, auditory sensitivity was equally good at both high- and low-frequency ranges, while amplitude was more finely coded for higher frequency ranges. Additionally, we conducted a phylogenetic comparative analysis by combining our ABR data with published data on 27 species. Species-specific peaks in hearing sensitivity correlated with peak frequencies of echolocation calls and pup isolation calls, suggesting that changes in hearing sensitivity evolved in response to frequency changes of echolocation and social calls. Overall, our study provides the most comprehensive comparative assessment of bat hearing capacities to date and highlights the evolutionary pressures acting on their sensory perception.

Development of hearing in the big brown bat

We studied the development of hearing in newborn pups of the big brown bat, Eptesicus fuscus. In the majority of pups, the opening of both outer auditory canals occurred on or before postnatal day (PND) 7, but in some, it extended to PND 11. Using repeated auditory brainstem response (ABR) recordings, we tracked the progressive development and maturation of auditory sensitivity in 22 E. fuscus pups every 3 days, from PND 10 to PND 31, with additional recordings in a subset of bats at 2 months, 3 months and 1 year of life. There was a profound increase in auditory sensitivity across development for frequencies between 4 and 100 kHz, with the largest threshold shifts occurring early in development between PND 10 and 19. Prior to PND 13-16 and when pups were still non-volant, most bats were unable to hear frequencies above 48 kHz; however, sensitivity to these higher ultrasonic frequencies increased with age. Notably, this change occurred near the age when young bats started learning how to fly and echolocate.

Functional organization of the primary auditory cortex of the free-tailed bat Tadarida brasiliensis

The Mexican free-tailed bat, Tadarida brasiliensis, is a fast-flying bat that hunts by biosonar at high altitudes in open space. The auditory periphery and ascending auditory pathways have been described in great detail for this species, but nothing is yet known about its auditory cortex. Here we describe the topographical organization of response properties in the primary auditory cortex (AC) of the Mexican free-tailed bat with emphasis on the sensitivity for FM sweeps and echo-delay tuning. Responses of 716 units to pure tones and of 373 units to FM sweeps and FM-FM pairs were recorded extracellularly using multielectrode arrays in anesthetized bats. A general tonotopy was confirmed with low frequencies represented caudally and high frequencies represented rostrally. Characteristic frequencies (CF) ranged from 15 to 70 kHz, and fifty percent of CFs fell between 20 and 30 kHz, reflecting a hyper-representation of a bandwidth corresponding to search-phase echolocation pulses. Most units showed a stronger response to downward rather than upward FM sweeps and forty percent of the neurons interspersed throughout AC (150/371) showed echo-delay sensitivity to FM-FM pairs. Overall, the results illustrate that the free-tailed bat auditory cortex is organized similarly to that of other FM-type insectivorous bats.

Frequency-modulated up-chirps produce larger evoked responses than down-chirps in the big brown bat auditory brainstem

In many mammals, upward-sweeping frequency-modulated (FM) sounds (up-chirps) evoke larger auditory brainstem responses than downward-sweeping sounds (down-chirps). To determine if similar effects occur in FM echolocating bats, auditory evoked responses (AERs) in big brown bats in response to up-chirps and down-chirps at different chirp durations and levels were recorded. Even though down-chirps are the biologically relevant stimulus for big brown bats, up-chirps typically evoked larger peaks in the AER, but with some exceptions at the shortest chirp durations. The up-chirp duration that produced the largest AERs and the greatest differences between up-chirps and down-chirps varied between individual bats and stimulus levels. Cross-covariance analyses using the entire AER waveform confirmed that amplitudes were typically larger to up-chirps than down-chirps at supra-threshold levels, with optimal durations around 0.5-1 ms. Changes in response latencies with stimulus levels were consistent with previous estimates of amplitude-latency trading. Latencies tended to decrease with increasing up-chirp duration and increase with increasing down-chirp duration. The effects of chirp direction on AER waveforms are generally consistent with those seen in other mammals but with small differences in response patterns that may reflect specializations for FM echolocation.