Echo SPL, training experience, and experimental procedure influence the ranging performance in the big brown bat, Eptesicus fuscus

Four Eptesicus fuscus were trained in a range discrimination experiment to choose the closer of two phantom targets. Echo attenuation was roving between trials returning echoes ranging from −10 dB to −50 dB SPL (sound pressure level) relative to emission SPL. Discrimination thresholds were determined. After sufficient training, ranging performance was stable and about the same in the range between −20 dB and −50 dB with range difference thresholds around 300 μs. At −10 dB, performance was poor even after long training. After additional training at a constant relative echo SPL of −30 dB and a delay difference of 300 μs the performance measured with roving echo SPL improved at all relative echo SPL between −20 dB and −50 dB but not at −10 dB. The new experimental procedure improved the performance by additional learning, and the bats generalized over a wide range of relative echo SPL. Threshold improved to 100 μs when measured at a constant relative echo SPL of −30 dB, again indicating the influence of the experimental procedure. In correspondence to neurophysiological data the ranging performance deteriorates if the echo SPL is close to the emission SPL. Signal duration and emission SPL were variable during range discrimination. Accepted: 7 March 1998     

Arctiid moth clicks can degrade the accuracy of range difference discrimination in echolocating big brown bats,Eptesicus fuscus

Summary Four big brown bats (Eptesicus fuscus) born and raised in captivity were trained using the Yes/No psychophysical method to report whether a virtual sonar target was at a standard distance or not. At threshold bats were able to detect a minimum range difference of 6 mm (a t of 36 s).Following threshold determinations, a click burst 1.8 ms long containing 5 pulses from the ruby tiger moth, Phragmatobia fuliginosa (Arctiidae), was presented randomly after each phantom echo. The sound energy of the click burst was -4 dB relative to that of the phantom echo. Clicks presented for the very first time could startle naive bats to different degrees depending on the individual.The bats' performance deteriorated by as much as 4000% when the click burst started within a window of about 1.5 ms before the phantom echo (Fig. 4). Even when one of ten phantom echoes was preceded by a click burst, the range difference discrimination worsened by 200% (Fig. 9). Hence, clicks falling within the 1.5 ms time window seem to interfere with the bat's neural timing mechanism.The clicks of arctiid moths appear to serve 3 functions: they can startle naive bats, interfere with range difference determinations, or they can signal the moth's distastefulness, as shown in earlier studies.Abbreviations peSPL peak equivalent sound pressure level - sd standard deviation - FM frequency modulation - CF constant frequency - EPROM erasable programmable read only memory     

Calibrated earphones for the echolocating bat,Rhinolophus ferrumequinum

Summary A method to construct and calibrate earphones (physiologically) suitable for small bats and probably other small mammals is described. Particular emphasis was placed on getting a flat frequency response curve between 75kHz and 110kHz, the most important range forRhinolophus ferrumequinum. Below 60kHz the slope declined by about 30dB down to the audible frequency range. The maximal output without harmonic distortion (30dB down) was 80–90dB SPL, but up to 115dB SPL could be attained when accepting harmonics.Supported by the grants: Stiftung Volkswagenwerk, 111858; Deutsche Forschungsgemeinschaft, Schl 117/4; Schn 138/6     

Target-detection by the echolocating bat,Eptesicus fuscus

Summary The long-range echo-detection capabilities of echolocating bats (Eptesicus fuscus) were studied in a two-choice psychophysical procedure.E. fuscus can detect 4.8 mm diameter spheres at a distance of 2.9 m, and 19.1 mm diameter spheres at a distance of 5.1 m. The threshold of echo-detection corresponds to the distance at which a target returns an echo amplitude in the region of 0 dB SPL. The results demonstrate that the maximum effective range of bat sonar is greater than previously indicated by obstacleavoidance and target-interception tasks.     

Comparative collicular tonotopy in two bat species adapted to movement detection,Hipposideros speoris andMegaderma lyra

Summary The tonotopic organization of the inferior colliculus (IC) in two echolocating bats,Hipposideros speoris andMegaderma lyra, was studied by multiunit recordings.InHipposideros speoris frequencies below the range of the echolocation signals (i.e. below 120 kHz) are compressed into a dorsolateral cap about 400–600 m thick. Within this region, neuronal sheets of about 4–5 m thickness represent a 1 kHz-band.In contrast, the frequencies of the echolocation signals (120–140 kHz) are overrepresented and occupy the central and ventral parts of the IC (Fig. 3). In this region, neuronal sheets of about 80 m thickness represent a 1 kHz-band. The largest 1 kHz-slabs (400–600 m) represent frequencies of the pure tone components of the echolocation signals (130–140 kHz).The frequency of the pure tone echolocation component is specific for any given individual and always part of the overrepresented frequency range but did not necessarily coincide with the BF of the thickest isofrequency slab. Thus hipposiderid bats have an auditory fovea (Fig. 10).In the IC ofMegaderma lyra the complete range of audible frequencies, from a few kHz to 110 kHz, is represented in fairly equal proportions (Fig. 7). On the average, a neuronal sheet of 30 m thickness is dedicated to a 1 kHz-band, however, frequencies below 20 kHz, i.e. below the range of the echolocation signals, are overrepresented.Audiograms based on thresholds determined from multiunit recordings demonstrate the specific sensitivities of the two bat species. InHipposideros speoris the audiogram shows two sensitivity peaks, one in the nonecholocating frequency range (10–60 kHz) and one within the auditory fovea for echolocation (130–140 kHz).Megaderma lyra has extreme sensitivity between 15–20 kHz, with thresholds as low as –24 dB SPL, and a second sensitivity peak at 50 kHz (Fig. 8).InMegaderma lyra, as in common laboratory mammals, Q_10dB-values of single units do not exceed 30, whereas inHipposideros speoris units with BFs within the auditory fovea reach Q_10dB-values of up to 130.InMegaderma lyra, many single units and multiunit clusters with BFs below 30 kHz show upper thresholds of 40–50 dB SPL and respond most vigorously to sound intensities below 30 dB SPL (Fig. 9). Many of these units respond preferentially or exclusively to noise. These features are interpreted as adaptations to detection of prey-generated noises.The two different tonotopic arrangements (compare Figs. 3 and 7) in the ICs of the two species are correlated with their different foraging behaviours. It is suggested that pure tone echolocation and auditory foveae are primarily adaptations to echo clutter rejection for species foraging on the wing close to vegetation.Abbreviations BF Best frequency - CF constant frequency - FM frequency modulated - IC inferior colliculus - HS Hipposideros speoris     

Integration time for short broad band clicks in echolocating FM-bats (Eptesicus fuscus)

Vespertilionid FM-bats (four Eptesicus fuscus and one Vespertilio murinus) were trained in an electronic phantom target simulator to detect synthetic echoes consisting of either one or two clicks. The threshold sound pressure for single clicks was around 47 dB peSPL for all five bats corresponding to a threshold energy of -95 dB re 1 Pa² * s. By varying the interclick interval, T, for double clicks it was shown that the threshold intensity was around — 3 dB relative to the threshold for single clicks at T up to 2.4 ms, indicating perfect power summation of both clicks. A threshold shift of -13.5 dB for a 1 ms train of 20 clicks (0.05 ms interclick interval) confirmed that the bats integrated the power of the stimuli. At T longer than around 2.5 ms the threshold for double clicks was the same as for single clicks. Thus, the bats performed like perfect energy detectors with an integration time of approximately 2.4 ms. This integration time is an order of magnitude shorter than that reported for bats listening passively for pure tones. In our setup the bats emitted sonar signals with durations of 2–3 ms. Hence, the results may indicate that while echolocating the bats integration time is adapted to the duration of the sonar emissions.Abbreviations AGC automatic gain control - FM frequency modulated - peSPL peak equivalent sound pressure level - rms root mean square - SD standard deviation - SE standard error of mean - T interclick interval     

Stridulation and hearing in the noctuid mothThecophora fovea (Tr.)

Summary MaleThecophora fovea (Tr.) (Noctuidae) sing continuously for several minutes by rubbing the 1. tarsal segment of the metathoracic leg against a stridulatory swelling on the hindwing. In Northern Yugoslavia (Slovenia) the males emerge in late October and start stridulating about a week later when the females emerge.The sounds are pulse trains consisting of 10–12 ms long sound pulses with main energy around 32 kHz and a PRR of 20 pulses/s. The mechanics of the sound producing apparatus was studied by activating the stridulatory swelling with short sound impulses. The impulse response of the swelling was recorded by laser vibrometry and amplitude spectra of the vibrations showed maximum velocities between 25 and 35 kHz. Hence, it seems likely that the stridulatory swelling is driven as a mechanical oscillator with a resonance frequency which determines the carrier frequency of the sounds.Audiograms of both males and females showed peak sensitivities at 25–30 kHz. The median threshold at the BF was 36 dB SPL. The peak intensity of the sound pulses was 83 dB SPL at 1 m, which should enable the moths to hear each other at distances of around 30 m. Therefore sound production inT. fovea might function in long distance calling. It is argued thatT. fovea can survive making such a noise in spite of being palatable to bats because it flies so late in the year that it is temporally isolated from bats.Abbreviations PRR pulse repetition rate - SPL sound pressure level - BF best frequency     

Sensory basis for sound intensity discrimination in the bushcricket Requena verticalis (Tettigoniidae, Orthoptera)

The ability of the female bushcricket, Requena verticalis, to discriminate between two conspecific sound signals that differed in sound pressure level (SPL) was tested in a two-choice paradigm. Significant discrimination was achieved with a 2-dB difference. The property of each pair of receptors to establish binaural discharge differences was investigated in electrophysiological experiments. The threshold to the conspecific signal varies for each fibre from about 40 to 90 dB SPL, allowing for a range fractionation of the hearing organ. Each pair of receptors establishes significant binaural discharge differences only within a restricted intensity range about 10 dB above threshold. Based on a model of the intensity response function of a receptor the total discharge of the 22 receptors in both ears was calculated with monaural and binaural stimulation. The profile of receptors exhibiting significant discharge differences changes with increasing SPL, from the most sensitive fibres with a characteristic frequency between 12 kHz and 35 kHz at low SPLs to the least sensitive fibres at very low and high characteristic frequencies at medium to high SPLs. The discharge difference with an intensity difference of 2 dB is rather small (4% of the total receptor activity) and limited only to a few pairs of receptors. Accepted: 8 November 1997     

Target ranging and the role of time-frequency structure of synthetic echoes in big brown bats,Eptesicus fuscus

Summary Echolocating bats judge the distance to a target on basis of the delay between the emitted cry and the returning echo. In a phantom echo set-up it was investigated how changes in the time-frequency structure of synthetic echoes affect ranging accuracy of big brown bats, Eptesicus fuscus.A one channel phantom target simulator and a Y/N paradigm was used. Five Eptesicus fuscus were trained to discriminate between phantom targets with different virtual distances (delays). The phantom echo was stored in a memory and broadcast from a loudspeaker after a certain delay following the bat's triggering of the system via a trigger microphone. The ranging accuracy was compared using 5 different signals with equal energy as phantom echoes: a standard cry (a natural bat cry), two kinds of noise signals, a high pass, and a low pass filtered version of the standard cry.The standard cry was recorded from one of the bats while judging the distance to a real target. The duration was 1.1 ms, the first harmonic swept down from 55 to 25 kHz and there was energy also in the second and third harmonic. Both noise signals had the same duration, power spectrum, and energy as the standard cry. One noise signal was stored in a memory and hence was exactly the same each time the bat triggered the system. The other variable noise signal was produced by storing the envelope of the standard cry and multiplying on-line with band pass filtered noise. The time-frequency structure (e.g. rise time) of this noise signal changed from triggering to triggering. The filtered signals were produced by either 40 kHz high pass or 40 kHz low pass filtering of the standard cry.The range difference thresholds for the 5 bats were around 1–2 cm (51–119 us) using the standard cry as echo. The range difference threshold with both noise signals was 7–8 cm (around 450 s delay difference). The 40 kHz high pass filtered cry increased the threshold to approximately twice the threshold with the standard cry. With the 40 kHz low pass filtered cry the threshold was increased 2.5–3 times relative to the threshold with the standard cry. A single bat was tested with a signal filtered with a 55 kHz low pass filter leaving the whole first harmonic. The threshold was the same as that with the standard signal.The reduced ranging accuracy with the filtered signals indicates that the full band width of the first harmonic is utilised for ranging by the bats. The substantial reduction in accuracy with the noise signals indicates that not only the full band width but also the orderly time-frequency structure (the FM sweep) of the cry is important for ranging in echolocating bats.Abbreviations FM frequency modulated - CF constant frequency - peSPL peak equivalent sound pressure level - SD standard deviation - SE standard error of mean - EPROM erasable programmable read only memory - FFT fast Fourier transform - S/N signal-to-noise ratio     

Adaptive behavior for texture discrimination by the free-flying big brown bat, <Emphasis Type="Italic">Eptesicus fuscus</Emphasis>

This study examined behavioral strategies for texture discrimination by echolocation in free-flying bats. Big brown bats, Eptesicus fuscus, were trained to discriminate a smooth 16 mm diameter object (S+) from a size-matched textured object (S−), both of which were tethered in random locations in a flight room. The bat’s three-dimensional flight path was reconstructed using stereo images from high-speed video recordings, and the bat’s sonar vocalizations were recorded for each trial and analyzed off-line. A microphone array permitted reconstruction of the sonar beam pattern, allowing us to study the bat’s directional gaze and inspection of the objects. Bats learned the discrimination, but performance varied with S−. In acoustic studies of the objects, the S+ and S− stimuli were ensonified with frequency-modulated sonar pulses. Mean intensity differences between S+ and S− were within 4 dB. Performance data, combined with analyses of echo recordings, suggest that the big brown bat listens to changes in sound spectra from echo to echo to discriminate between objects. Bats adapted their sonar calls as they inspected the stimuli, and their sonar behavior resembled that of animals foraging for insects. Analysis of sonar beam-directing behavior in certain trials clearly showed that the bat sequentially inspected S+ and S−.     

The detection of phantom targets in noise by serotine bats; negative evidence for the coherent receiver

Summary Using a target simulator three serotine bats,Eptesicus serotinus, were trained to judge whether a phantom target was present or absent. The echolocation sounds emitted by the bats during the detection were intercepted by a microphone, amplified and returned by a loudspeaker as an artificial echo, with a delay of 3.2 ms and a sound level determined by the overall gain and cry amplitude. The cry level of each pulse was measured and the echo level received by the bat was calculated. The target was presented in 50% of the trials and the gain adjusted using conventional up/down procedures. Under these conditions between 40 and 48 dB peSPL were required for 50% detection (Figs. 2, 3).In a subsequent experiment the phantom target was masked with white noise (N_o) with a spectrum level of –113 dB re. 1 Pa·Hz^–1/2. The thresholds were increased by 7–14 dB. Energy density (S) of a single pulse was measured and used to estimate S/N_o, which ranged from 36–49 dB at threshold. Theoretically the coherent receiver model predicts the ratio between hits and false alarms observed for the bats at a S/N_o of ca. 1–2 dB. Since the bats require 40–50 dB higher S/N_o (Fig. 3), this is taken as negative evidence for coherent reception (cross correlation).Furthermore, a strong sensitivity to clutter was found since there seemed to exist a fixed relationship between thresholds and clutter level.Abbreviations C clutter - Nbw noise in a specified bandwidth - No noise in i Hz bandwidth - peSPL peak equivalent sound pressure level - S signal energy - SD standard deviation - Y/N Yes/No psychometry - 2AFC two alternative forced choice psychometry     

Phonotaxis in <Emphasis Type="Italic">Hyla versicolor</Emphasis> (Anura,Hylidae): the effect of absolute call amplitude

The influence of call amplitude on phonotaxis in female Hyla versicolor was studied using a no-choice paradigm. One set of experiments estimated effects of stimulus amplitude on phonotaxis toward a synthetic model of a conspecific call. The response strength increased with amplitude from the behavioral threshold (37–43 dB SPL) up to 79 dB SPL and then decreased at higher amplitudes. Females approached the loudspeaker with short walking bouts (1 s duration) occurring immediately after call presentations. Increase in response strength was attributed to an increasing proportion of calls that elicited such walking bouts, whereas the decrease at high amplitudes resulted from decreasing distance covered per bout. The quality of orientation remained constant for all above-threshold amplitudes. A second set of experiments tested the selectivity for interval duration and pulse duration at amplitudes of 55, 70, and 85 dB SPL. Selectivity for both parameters was similar at 70 and 85 dB SPL, but tended to increase at 55 dB SPL. The results suggest that selective phonotaxis in H. versicolor is not adapted for long-distance communication. This finding differs from those of comparable studies of acoustic insects.     

Detection of sonar signals in the presence of pulses of masking noise by the echolocating bat,Eptesicus fuscus

Summary Two big brown bats (Eptesicus fuscus) were trained to report the presence or absence of a virtual sonar target. The bats' sensitivity to transient masking was investigated by adding 5 ms pulses of white noise delayed from 0 to 16 ms relative to the target echo. When signal and masker occurred simultaneously, the bats required a signal energy to noise spectrum level ratio of 35 dB for 50% probability of detection. When the masker was delayed by 2 ms or more there was no significant masking and echo energy could be reduced by 30 dB for the same probability of detection. The average duration of the most energetic sonar signal of each trial was measured to be 1.7 ms and 2.4 ms for the two bats, but a simple relation between detection performance and pulse duration was not found.In a different experiment the masking noise pulses coincided with the echo, and the duration of the masker was varied from 2 to 37.5 ms. The duration of the masker had little or no effect on the probability of detection.The findings are consistent with an aural integration time constant of about 2 ms, which is comparable to the duration of the cries. This is an order of magnitude less than found in backward masking experiments with humans and may be an adaptation to the special constraints of echolocation. The short time of sensitivity to masking may indicate that the broad band clicks of arctiid moths produced as a countermeasure to bat predation are unlikely to function by masking the echo of the moth.Abbreviations SPL sound pressure level - SD standard deviation - SE standard error - BW bandwidth     

The Codacs? Direct Acoustic Cochlear Implant Actuator: Exploring Alternative Stimulation Sites and Their Stimulation Efficiency

This work assesses the efficiency of the Codacs system actuator (Cochlear Ltd., Sydney Australia) in different inner ear stimulation modalities. Originally the actuator was intended for direct perilymph stimulation after stapedotomy using a piston prosthesis. A possible alternative application is the stimulation of middle ear structures or the round window (RW). Here the perilymph stimulation with a K-piston through a stapes footplate (SFP) fenestration (N = 10) as well as stimulation of the stapes head (SH) with a Bell prosthesis (N = 9), SFP stimulation with an Omega/Aerial prosthesis (N = 8) and reverse RW stimulation (N = 10) were performed in cadaveric human temporal bones (TBs). Codacs actuator output is expressed as equivalent sound pressure level (eq. SPL) using RW and SFP displacement responses, measured by Laser Doppler velocimetry as reference. The axial actuator coupling force in stimulation of stapes and RW was adjusted to ~ 5 mN. The Bell prosthesis and Omega/Aerial prosthesis stimulation generated similar mean eq. SPLs (Bell: 127.5–141.8 eq. dB SPL; Omega/Aerial: 123.6–143.9 eq. dB SPL), being significantly more efficient than K-piston perilymph stimulation (108.6–131.6 eq. dB SPL) and RW stimulation (108.3–128.2 eq. dB SPL). Our results demonstrate that SH, SFP and RW are adequate alternative stimulation sites for the Codacs actuator using coupling prostheses and an axial coupling force of ~ 5 mN. Based on the eq. SPLs, all investigated methods were adequate for in vivo hearing aid applications, provided that experimental conditions including constant coupling force will be implemented.     

Target detection and range resolution by the big brown bat (Eptesicus fuscus) using normal and time-reversed model echoes

Summary Big brown bats (Eptesicus fuscus) were tested for their ability to detect an electronically simulated target, and to discriminate differences in range to two simulated targets, when receiving either a model of their own sonar emissions or the model reversed in time as the echo. The theory of matched detection predicts a large decrease in performance if bats use matched filtering, unless they are somehow able to adjust their filter to match the novel, time-reversed signal. The detection thresholds we obtained were much lower than Møhl's (1986), but like him we found no difference in threshold for reversed models (Table 2). This suggests either that bats do not use matched filtering for target detection, or, possibly, that they are able to adapt their filter to a highly unnatural signal in some way as yet unknown.Unlike detection, range discrimination was much poorer with reversed echoes (Table 3). Threshold increased from about 1 cm range difference with normal model echoes to 18 cm or more with reversed model echoes. This suggests that range determination, which is based on measuring the time of arrival of echoes, does involve matched filtering. Whether such filtering is ideal (i.e., equivalent to cross-correlation detection) cannot be decided by our results, but there are some indications that the match between an echo and the presumed internal template (the match of matched filtering) must be fairly precise. Also, since performance with phantom targets generated using model echoes was as good as has been found with real targets, the internal template is probably fixed (or only slowly modifiable) rather than re-programmed with each sonar emission. Finally, because synchronization of emission and model echo was not perfect, the apparent distance to targets probably varied by 2 to 4 cm from emission to emission, although both targets would appear to move together thus keeping the range difference constant. This suggests that bats determined range to the targets simultaneously rather than sequentially, as is usually assumed.Abbreviations BP bat-produced echo - FM frequency modulated - M _d detection model echo - M _d reversed detection model echo - M _rd range discrimination model echo - M _rd reversed range discrimination model echo - rms root mean square - SCR signal-to-clutter ratio - SNR signal-to-noise ratio - SPL sound pressure level - XCF crosscorrelation function     

Discrimination performance and echolocation signal integration requirements for target detection and distance determination in the CF/FM bat,Noctilio albiventris

Summary Bats of the speciesNoctilio albiventris were trained to detect the presence of a target or to discriminate differences in target distance by means of echolocation. During the discrimination trials, the bats emitted pairs of pulses at a rate of 7–10/s. The first was an 8 ms constant frequency (CF) signal at about 75 kHz. This was followed after about 28 ms by a short-constant frequency/ frequency modulated (short-CF/FM) signal composed of a 6 ms CF component at about 75 kHz terminating in a 2 ms FM component sweeping downward to about 57 kHz. There was no apparent difference in the pulse structure or emission pattern used for any of the tasks. The orientation sounds of bats flying in the laboratory and hunting prey under natural conditions follow the same general pattern but differ in interesting ways.The bats were able to discriminate a difference in target distance of 13 mm between two simultaneously presented targets and of 30 mm between single sequentially presented targets around an absolute distance of 35 cm, using a criterion of 75% correct responses.The bats were unable to detect the presence of the target or to discriminate distance in the presence of continuous white noise of 54 dB or higher SPL. Under conditions of continuous white noise, the bats increased their pulse repetition rate and the relative proportion of CF/FM pulses.The bats required a minimum of 1–2 successive CF/FM pulse-echo pairs for target detection and 2–3 to discriminate a 5 cm difference in distance. When the distance discrimination tasks were made more difficult by reducing the difference in distance between the two targets the bats needed to integrate information from a greater number of successive CF/FM pulse-echo pairs to make the discrimination.Abbreviations CF constant frequency - FM frequency modulation