Michaela Warnecke

Dynamic Echo Information Guides Flight in the Big Brown Bat.

Animals rely on sensory feedback from their environment to guide locomotion. For instance, visually guided animals use patterns of optic flow to control their velocity and to estimate their distance to objects (e.g., Srinivasan et al., 1991, 1996). In this study, we investigated how acoustic information guides locomotion of animals that use hearing as a primary sensory modality to orient and navigate in the dark, where visual information is unavailable. We studied flight and echolocation behaviors of big brown bats as they flew under infrared illumination through a corridor with walls constructed from a series of individual vertical wooden poles. The spacing between poles on opposite walls of the corridor was experimentally manipulated to create dense/sparse and balanced/imbalanced spatial structure. The bats’ flight trajectories and echolocation signals were recorded with high-speed infrared motion-capture cameras and ultrasound microphones, respectively. As bats flew through the corridor, successive biosonar emissions returned cascades of echoes from the walls of the corridor. The bats flew through the center of the corridor when the pole spacing on opposite walls was balanced and closer to the side with wider pole spacing when opposite walls had an imbalanced density. Moreover, bats produced shorter duration echolocation calls when they flew through corridors with smaller spacing between poles, suggesting that clutter density influences features of the bat’s sonar signals. Flight speed and echolocation call rate did not, however, vary with dense and sparse spacing between the poles forming the corridor walls. Overall, these data demonstrate that bats adapt their flight and echolocation behavior dynamically when flying through acoustically complex environments.

Spatial release from simultaneous echo masking in bat sonar

Big brown bats (Eptesicus fuscus) use biosonar to navigate and locate objects in their surroundings. During natural foraging, they often encounter echoes returned by a target of interest located to the front while other, often stronger, clutter echoes are returned from objects, such as vegetation, located to the sides or above. Nevertheless, bats behave as if they do not suffer interference from this clutter. Using a two-choice delay discrimination procedure, bats were tested for the masking effectiveness of clutter echoes on target echoes when the target echoes were delivered from the bats front while clutter echoes were delivered from 90° overhead, a direction of lowpass filtering by the external ears. When clutter echoes are presented from the front at the same delay as target echoes, detection performance declines and clutter masking occurs. When the clutter echoes are presented at the same delay but from overhead, discrimination performance is unaffected and no masking occurs. Thus there is masking release for simultaneous off-axis lowpass clutter compared to masking by simultaneous clutter from the front. The bats performance for simultaneous target and clutter echoes indicates a new role for the mechanism that separates overlapping echoes by decomposing the bats auditory time-frequency representation.

Broadband noise exposure does not affect hearing sensitivity in big brown bats (Eptesicus fuscus).

ABSTRACT In many vertebrates, exposure to intense sounds under certain stimulus conditions can induce temporary threshold shifts that reduce hearing sensitivity. Susceptibility to these hearing losses may reflect the relatively quiet environments in which most of these species have evolved. Echolocating big brown bats (Eptesicus fuscus) live in extremely intense acoustic environments in which they navigate and forage successfully, both alone and in company with other bats. We hypothesized that bats may have evolved a mechanism to minimize noise-induced hearing losses that otherwise could impair natural echolocation behaviors. The hearing sensitivity of seven big brown bats was measured in active echolocation and passive hearing tasks, before and after exposure to broadband noise spanning their audiometric range (10–100 kHz, 116 dB SPL re. 20 µPa rms, 1 h duration; sound exposure level 152 dB). Detection thresholds measured 20 min, 2 h or 24 h after exposure did not vary significantly from pre-exposure thresholds or from thresholds in control (sham exposure) conditions. These results suggest that big brown bats may be less susceptible to temporary threshold shifts than are other terrestrial mammals after exposure to similarly intense broadband sounds. These experiments provide fertile ground for future research on possible mechanisms employed by echolocating bats to minimize hearing losses while orienting effectively in noisy biological soundscapes. Highlighted Article: Big brown bats are less susceptible to hearing losses after exposure to intense broadband noise than are terrestrial mammals.

Flow sensing in developing Xenopus laevis is disrupted by visual cues and ototoxin exposure

We explored how lateral line cues interact with visual cues to mediate flow sensing behaviors in the nocturnal developing frog, Xenopus laevis, by exposing animals to current flows under different lighting conditions and after exposure to the ototoxin gentamicin. Under dark conditions, Xenopus tadpoles move downstream at the onset of current flow, then turn, and orient toward the direction of the flow with high accuracy. Postmetamorphic froglets also exhibit positive rheotaxis but with less accuracy and longer latency. The addition of discrete light cues to an otherwise dark environment disrupts rheotaxis and positioning. Orientation is less accurate, latency to orient is longer, and animals do not move as far downstream in the presence of light. Compared with untreated tadpoles tested in the dark, tadpoles exposed to gentamicin show less accurate rheotaxis with longer latency and do not move as far downstream in response to flow. These effects are compounded by the presence of light cues. The disruptive effects of light on flow sensing in Xenopus emphasize the disturbances to natural behaviors that may be produced by anthropogenic illumination in nocturnal habitats.

Noise induced threshold shifts after noise exposure: Are bats special?

We conducted psychophysical and neurophysiological experiments to test the hypothesis that big brown bats suffer less severe temporary threshold shifts after noise exposure than other small mammals that also hear high frequency sounds. Five big brown bats were trained in psychophysical detection experiments to obtain thresholds to FM sweeps spanning the frequency range of their echolocation calls. Bats were then exposed to 115 dB of broadband noise for one hour, and thresholds re-measured 20 min and 24 hour after exposure. For all bats, threshold elevations at 20 min post-exposure were 3 dB or less, while at 24 hour post-exposure, thresholds were similar to those obtained pre-exposure. Local field potentials were recorded in the cochlear nucleus of anesthetized big brown bats before and after noise exposure. Neural thresholds to FM sweeps and to single tone frequencies were unaffected by noise exposure. These data suggest that big brown bats are an excellent model system to study naturally occurring immun...

Navigating the world using echo flow patterns

As an animal moves through the environment, it experiences flow of sensory information from stationary objects. Animals that relylargely on visual information to guide their movement experience optic flow patterns as they navigate, which can be used to measure the relative distance of objects in the environment (Gibson, 1958, Besl, 1988). For example, honeybees use optic flow patterns to center themselves in a flight corridor, and experimental manipulations of the visual patterns on the walls directly influence the animal’s navigation path (Srinivasan et al., 1996). Other animals instead show wall-following behavior, choosing to navigate close to visual obstacles (Scholtyssek et al., 2014). Here, we report on the navigation paths of animals that rely on acoustic signals to guide their movement. Echolocating bats emit ultrasonic signals that reflect from objects in the path of their sound beam, and we are studying how these animals use echo flow to guide their flight path through a corridor. In this study,...

High resolution acoustic measurement system and beam pattern reconstruction method for bat echolocation emissions

Measurements of the transmit beam patterns emitted by echolocating bats have previously been limited to cross-sectional planes or averaged over multiple signals using sparse microphone arrays. To date, no high-resolution measurements of individual bat transmit beams have been reported in the literature. Recent studies indicate that bats may change the time-frequency structure of their calls depending on the task, and suggest that their beam patterns are more dynamic than previously thought. To investigate beam pattern dynamics in a variety of bat species, a high-density reconfigurable microphone array was designed and constructed using low-cost ultrasonic microphones and custom electronic circuitry. The planar array is 1.83 m wide by 1.42 m tall with microphones positioned on a 2.54 cm square grid. The system can capture up to 228 channels simultaneously at a 500 kHz sampling rate. Beam patterns are reconstructed in azimuth, elevation, and frequency for visualization and further analysis. Validation of the array measurement system and post-processing functions is shown by reconstructing the beam pattern of a transducer with a fixed circular aperture and comparing the result with a theoretical model. To demonstrate the system in use, transmit beam patterns of the big brown bat, Eptesicus fuscus, are shown.

Echo interval and not echo intensity drives bat flight behavior in structured corridors

ABSTRACT To navigate in the natural environment, animals must adapt their locomotion in response to environmental stimuli. The echolocating bat relies on auditory processing of echo returns to represent its surroundings. Recent studies have shown that echo flow patterns influence bat navigation, but the acoustic basis for flight path selection remains unknown. To investigate this problem, we released bats in a flight corridor with walls constructed of adjacent individual wooden poles, which returned cascades of echoes to the flying bat. We manipulated the spacing and echo strength of the poles comprising each corridor side, and predicted that bats would adapt their flight paths to deviate toward the corridor side returning weaker echo cascades. Our results show that the bats trajectory through the corridor was not affected by the intensity of echo cascades. Instead, bats deviated toward the corridor wall with more sparsely spaced, highly reflective poles, suggesting that pole spacing, rather than echo intensity, influenced bat flight path selection. This result motivated investigation of the neural processing of echo cascades. We measured local evoked auditory responses in the bat inferior colliculus to echo playback recordings from corridor walls constructed of sparsely and densely spaced poles. We predicted that evoked neural responses would be discretely modulated by temporally distinct echoes recorded from the sparsely spaced pole corridor wall, but not by echoes from the more densely spaced corridor wall. The data confirm this prediction and suggest that the bats temporal resolution of echo cascades may drive its flight behavior in the corridor. Summary: The echolocating bat encounters echo cascades from objects at different relative positions: data show that the bats flight guidance is driven by the timing between echoes within cascades.

Do echo flow patterns guide flight behavior of the big brown bat

When animals move through the environment, they receive dynamic sensory information from surrounding objects. Past research has demonstrated that visually guided animals rely on optic flow to estimate their relative velocity and distance to objects. More recently, the flight and echolocation behavior of big brown bats was studied in a corridor constructed of poles of variable spacing. When the pole spacing on opposite walls was symmetric, animals centered themselves in the corridor, and when pole spacing was asymmetric, bats steered toward the less echoic wall. This finding raised the question whether bats adjusted their flight paths based on the flow of echoes returning from the corridor walls, or whether the animals steered away from the wall reflecting more intense echoes. To address this question, bats flew through the original corridor with the additional experimental condition of felt coverage on one corridor wall, reducing the reflectivity of pole echoes on that side, while retaining the flow patte...

Neural representation of sonar emissions and echoes in the auditory midbrain of the echolocating bat, Eptesicus fuscus

Local field potentials and multi-unit responses were recorded from the auditory brainstem of the anesthetized big brown bat, Eptesicus fuscus, in response to FM sweeps modeled on those the bat would receive from its broadcasts and also their echoes. Tungsten microelectrodes were inserted at an angle into the inferior colliculus (IC), and responses were recorded at progressively deeper sites in the IC and into the area of the cochlear nucleus (CN). Waveforms of the averaged field potentials varied with depth and recording site, with amplitude and latency of the largest peak in each response consistent with transmission time from the cochlea to the CN and then to the IC. For these responses, values of amplitude-latency trading are consistent with those from previous work. Simulated broadcasts followed by sequences of echoes that mimic reflections from complex sonar scenes evoked sequences of distinct responses. Their latencies were affected by amplitude and also by intersound intervals, creating a dynamic p...