Otoacoustic emissions from the cochlea of the 'constant frequency' bats, Pteronotus parnellii and Rhinolophus rouxi

Transmitter and receiver of the low frequency horseshoe bat Rhinolophus paradoxolophus are functionally matched for fluttering target detection

Flutter-detecting foragers require specific adaptations of the transmitter and the receiver of their echolocation systems to detect and evaluate flutter information in the echoes of potential prey. These adaptations include Doppler shift compensation (DSC), which keeps the echo frequency from targets ahead constant at a reference frequency (f_ref), and an auditory fovea in the cochlea, which results in foveal areas in the hearing system with many sharply tuned neurons with best frequencies near f_ref. So far, this functional match has been verified only for a very few key species, but is postulated for all flutter-detecting foragers. In this study we determined both, the transmitter and receiver properties within individuals of the Bourret's horseshoe bat (Rhinolophus paradoxolophus), an allometric outlier in the rhinolophid family. Here we show that the transmitter and receiver are functionally matched in a similar way as postulated for all flutter-detecting foragers. The performance of DSC, measured as the ability to keep the echo frequency constant at f_ref, had a precision similar to that found in other flutter-detecting foragers, and the audiogram showed the characteristic course with a minimum at f_ref. Furthermore, we show for a rhinolophid bat a variation over time of the coupled resting frequency and f_ref. Finally, we discuss the tight match between transmitter and receiver properties, which is guaranteed by the link between the foveal areas of the receiver and the audio-vocal control system for DSC.

The resting frequency of echolocation signals changes with body temperature in the hipposiderid bat Hipposideros armiger

Doppler shift (DS) compensating bats adjust in flight the second harmonic of the constant-frequency component (CF2) of their echolocation signals so that the frequency of the Doppler-shifted echoes returning from ahead is kept constant with high precision (0.1-0.2%) at the so-called reference frequency (fref). This feedback adjustment is mediated by an audio-vocal control system that correlates with a maximal activation of the foveal resonance area in the cochlea. Stationary bats adjust the average CF2 with similar precision at the resting frequency (frest), which is slightly below the fref. Over a range of time periods (from minutes up to years), variations of the coupled fref and frest have been observed, and were attributed to age, social influences and behavioural situations in rhinolophids and hipposiderids, and to body temperature effects and flight activity in Pteronotus parnellii. We assume that, for all DS-compensating bats, a change in body temperature has a strong effect on the activation state of the foveal resonance area in the cochlea, which leads to a concomitant change in emission frequency. We tested our hypothesis in a hipposiderid bat, Hipposideros armiger, and measured how the circadian variation of body temperature at activation phases affected frest. With a miniature temperature logger, we recorded the skin temperature on the back of the bats simultaneously with echolocation signals produced. During warm-up from torpor, strong temperature increases were accompanied by an increase in frest, of up to 1.44 kHz. We discuss the implications of our results for the organization and function of the audio-vocal control systems of all DS-compensating bats.

Correlating Cochlear Morphometrics from Parnell's Mustached Bat (Pteronotus parnellii) with Hearing

Morphometric analysis of the inner ear of mammals can provide information for cochlear frequency mapping, a species-specific designation of locations in the cochlea at which different sound frequencies are encoded. Morphometric variation occurs in the hair cells of the organ of Corti along the cochlea, with the base encoding the highest frequency sounds and the apex encoding the lowest frequencies. Changes in cell shape and spacing can yield additional information about the biophysical basis of cochlear tuning mechanisms. Here, we investigate how morphometric analysis of hair cells in mammals can be used to predict the relationship between frequency and cochlear location. We used linear and geometric morphometrics to analyze scanning electron micrographs of the hair cells of the cochleae in Parnell's mustached bat (Pteronotus parnellii) and Wistar rat (Rattus norvegicus) and determined a relationship between cochlear morphometrics and their frequency map. Sixteen of twenty-two of the morphometric parameters analyzed showed a significant change along the cochlea, including the distance between the rows of hair cells, outer hair cell width, and gap width between hair cells. A multiple linear regression model revealed that nine of these parameters are responsible for 86.9 % of the variation in these morphometric data. Determining the most biologically relevant measurements related to frequency detection can give us a greater understanding of the essential biomechanical characteristics for frequency selectivity during sound transduction in a diversity of animals.

The rate of cochlear compression in a dolphin: a forward-masking evoked-potential study

The "active" cochlear mechanism of hearing manifests in the cochlear compression. Investigations of compression in odontocetes help to determine the frequency limit of the active mechanism. The compression may be evaluated by comparison of low- and on-frequency masking. In a bottlenose dolphin, forward masking of auditory evoked potentials to tonal pips was investigated. Measurements were performed for test frequencies of 45 and 90 kHz. The low-frequency maskers were - 0.25 to - 0.75 oct relative the test. Masking efficiency was varied by masker-to-test delay variation from 2 to 20 ms, and masker levels at threshold (MLTs) were evaluated at each of the delays. It was assumed that low-frequency maskers were not subjected or little subjected to compression whereas on-frequency maskers were subjected equally to the test. Therefore, the compression rate was assessed as the slope of low-frequency MLT dependence on on-frequency MLT. For the 90-kHz test, the slopes were 0.63 and 0.18 dB/dB for masker of - 0.25 and - 0.5 oct, respectively. For the 45 kHz test, the slopes were 0.69 and 0.39 dB/dB for maskers of - 0.25 and - 0.5 oct. So, compression did not decay at the upper boundary of the hearing frequency range in the dolphin.

Integrating the active process of hair cells with cochlear function

Uniquely among human senses, hearing is not simply a passive response to stimulation. Our auditory system is instead enhanced by an active process in cochlear hair cells that amplifies acoustic signals several hundred-fold, sharpens frequency selectivity and broadens the ear's dynamic range. Active motility of the mechanoreceptive hair bundles underlies the active process in amphibians and some reptiles; in mammals, this mechanism operates in conjunction with prestin-based somatic motility. Both individual hair bundles and the cochlea as a whole operate near a dynamical instability, the Hopf bifurcation, which accounts for the cardinal features of the active process.

Influence of ketamine-xylazine anaesthesia on cubic and quadratic high-frequency distortion-product otoacoustic emissions

Ketamine is a dissociative anaesthetic, analgesic drug as well as an N-methyl-D-aspartate receptor antagonist and has been reported to influence otoacoustic emission amplitudes. In the present study, we assess the effect of ketamine-xylazine on high-frequency distortion-product otoacoustic emissions (DPOAE) in the bat species Carollia perspicillata, which serves as model for sensitive high-frequency hearing. Cubic DPOAE provide information about the nonlinear gain of the cochlear amplifier, whereas quadratic DPOAE are used to assess the symmetry of cochlear amplification and potential efferent influence on the operating state of the cochlear amplifier. During anaesthesia, maximum cubic DPOAE levels can increase by up to 35 dB within a medium stimulus level range from 35 to 60 dB SPL. Close to the -10 dB SPL threshold, at stimulus levels below about 20-30 dB SPL, anaesthesia reduces cubic DPOAE amplitudes and raises cubic DPOAE thresholds. This makes DPOAE growth functions steeper. Additionally, ketamine increases the optimum stimulus frequency ratio which is indicative of a reduction of cochlear tuning sharpness. The effect of ketamine on cubic DPOAE thresholds becomes stronger at higher stimulus frequencies and is highly significant for f2 frequencies above 40 kHz. Quadratic DPOAE levels are increased by up to 25 dB by ketamine at medium stimulus levels. In contrast to cubic DPOAEs, quadratic DPOAE threshold changes are variable and there is no significant loss of sensitivity during anaesthesia. We discuss that ketamine effects could be caused by modulation of middle ear function or a release from ipsilateral efferent modulation that mainly affects the gain of cochlear amplification.

Information generated by the moving pinnae of Rhinolophus rouxi: tuning of the morphology at different harmonics

Bats typically emit multi harmonic calls. Their head morphology shapes the emission and hearing sound fields as a function of frequency. Therefore, the sound fields are markedly different for the various harmonics. As the sound field provides bats with all necessary cues to locate objects in space, different harmonics might provide them with variable amounts of information about the location of objects. Also, the ability to locate objects in different parts of the frontal hemisphere might vary across harmonics. This paper evaluates this hypothesis in R. rouxi, using an information theoretic framework. We estimate the reflector position information transfer in the echolocation system of R. rouxi as a function of frequency. This analysis shows that localization performance reaches a global minimum and a global maximum at the two most energetic frequency components of R. rouxi call indicating tuning of morphology and harmonic structure. Using the fundamental the bat is able to locate objects in a large portion of the frontal hemisphere. In contrast, using the 1 overtone, it can only locate objects, albeit with a slightly higher accuracy, in a small portion of the frontal hemisphere by reducing sensitivity to echoes from outside this region of interest. Hence, different harmonic components provide the bat either with a wide view or a focused view of its environment. We propose these findings can be interpreted in the context of the foraging behaviour of R. rouxi, i.e., hunting in cluttered environments. Indeed, the focused view provided by the 1 overtone suggests that at this frequency its morphology is tuned for clutter rejection and accurate localization in a small region of interest while the finding that overall localization performance is best at the fundamental indicates that the morphology is simultaneously tuned to optimize overall localization performance at this frequency.

A system for measuring distortion product otoacoustic emissions at ultra-sonic frequencies in rodents Un sistema para medida emisiones otoacústicas por productos de distorsión en roedores a frecuencias ultra-sónicas

The aim of this work has been to construct and evaluate a system for recording distortion product otoacoustic emissions in rats at ultrasonic frequencies up to at least 50 kHz. The paper primarily describes the design of the acoustic probe system, as this is the most critical part. An acoustic ear simulator was developed and used for the subsequent calibration of the stimulus signals. A detachable probe system was provided in order to allow for visual inspection of the probe fitting in the ear canal prior to the final placement of the acoustic probe. Test/retest performance was evaluated by comparing DP-grams and I/O curves in 12 anaesthetized Wistar rats in two sessions separated approximately by one week. The between subject variance of the 12 tested rats appeared to be very modest, thus making the setup suitable for testing, for instance, ototoxicity of drugs or detection of cochlear damage due to noise exposures in rodents.

Power efficiency of outer hair cell somatic electromotility

Cochlear outer hair cells (OHCs) are fast biological motors that serve to enhance the vibration of the organ of Corti and increase the sensitivity of the inner ear to sound. Exactly how OHCs produce useful mechanical power at auditory frequencies, given their intrinsic biophysical properties, has been a subject of considerable debate. To address this we formulated a mathematical model of the OHC based on first principles and analyzed the power conversion efficiency in the frequency domain. The model includes a mixture-composite constitutive model of the active lateral wall and spatially distributed electro-mechanical fields. The analysis predicts that: 1) the peak power efficiency is likely to be tuned to a specific frequency, dependent upon OHC length, and this tuning may contribute to the place principle and frequency selectivity in the cochlea; 2) the OHC power output can be detuned and attenuated by increasing the basal conductance of the cell, a parameter likely controlled by the brain via the efferent system; and 3) power output efficiency is limited by mechanical properties of the load, thus suggesting that impedance of the organ of Corti may be matched regionally to the OHC. The high power efficiency, tuning, and efferent control of outer hair cells are the direct result of biophysical properties of the cells, thus providing the physical basis for the remarkable sensitivity and selectivity of hearing.

The auditory cortex of the bat Molossus molossus: disproportionate search call frequency representation

The extent of the auditory cortex in the bat Molossus molossus was electrophysiologically investigated. Best frequencies and minimum thresholds of neural tuning curves were analyzed to define the topography of the auditory cortex. The auditory cortex encompasses an average cortical surface area of 5mm(2). Characteristic frequencies are tonotopically organized with low frequencies being represented caudally and high frequencies rostrally. However, a large interindividual variability in the tonotopic organization was found. In most animals, the caudal 50% was tonotopically organized. More anterior, a variable area was found. A distinct field with reversed topography was not consistently found. Within the demarcated auditory cortex, frequencies of 30-40 kHz, which correspond to the frequency range of search calls emitted during hunting, are overrepresented, occupying 49% of the auditory cortex surface. High minimum thresholds >50 dB SPL were found in a narrow dorsal narrow area. Neurons with multipeaked tuning curves (20%) preferentially were located in the dorsal part of the auditory cortex. In accordance with studies in other bat species, the auditory cortex of M. molossus is highly sensitive to the dominant frequencies of biosonar search calls.

Theoretical conditions for high-frequency hair bundle oscillations in auditory hair cells

Substantial evidence exists for spontaneous oscillations of hair cell stereociliary bundles in the lower vertebrate inner ear. Since the oscillations are larger than expected from Brownian motion, they must result from an active process in the stereociliary bundle suggested to underlie amplification of the sensory input as well as spontaneous otoacoustic emissions. However, their low frequency (<100 Hz) makes them unsuitable for amplification in birds and mammals that hear up to 5 kHz or higher. To examine the possibility of high-frequency oscillations, we used a finite-element model of the outer hair cell bundle incorporating previously measured mechanical parameters. Bundle motion was assumed to activate mechanotransducer channels according to the gating spring hypothesis, and the channels were regulated adaptively by Ca(2+) binding. The model generated oscillations of freestanding bundles at 4 kHz whose sharpness of tuning depended on the mechanotransducer channel number and location, and the Ca(2+) concentration. Entrainment of the oscillations by external stimuli was used to demonstrate nonlinear amplification. The oscillation frequency depended on channel parameters and was increased to 23 kHz principally by accelerating Ca(2+) binding kinetics. Spontaneous oscillations persisted, becoming very narrow-band, when the hair bundle was loaded with a tectorial membrane mass.

The auditory cortex of the bat Phyllostomus discolor: Localization and organization of basic response properties

BACKGROUND

The mammalian auditory cortex can be subdivided into various fields characterized by neurophysiological and neuroarchitectural properties and by connections with different nuclei of the thalamus. Besides the primary auditory cortex, echolocating bats have cortical fields for the processing of temporal and spectral features of the echolocation pulses. This paper reports on location, neuroarchitecture and basic functional organization of the auditory cortex of the microchiropteran bat Phyllostomus discolor (family: Phyllostomidae).

RESULTS

The auditory cortical area of P. discolor is located at parieto-temporal portions of the neocortex. It covers a rostro-caudal range of about 4800 mum and a medio-lateral distance of about 7000 mum on the flattened cortical surface. The auditory cortices of ten adult P. discolor were electrophysiologically mapped in detail. Responses of 849 units (single neurons and neuronal clusters up to three neurons) to pure tone stimulation were recorded extracellularly. Cortical units were characterized and classified depending on their response properties such as best frequency, auditory threshold, first spike latency, response duration, width and shape of the frequency response area and binaural interactions. Based on neurophysiological and neuroanatomical criteria, the auditory cortex of P. discolor could be subdivided into anterior and posterior ventral fields and anterior and posterior dorsal fields. The representation of response properties within the different auditory cortical fields was analyzed in detail. The two ventral fields were distinguished by their tonotopic organization with opposing frequency gradients. The dorsal cortical fields were not tonotopically organized but contained neurons that were responsive to high frequencies only.

CONCLUSION

The auditory cortex of P. discolor resembles the auditory cortex of other phyllostomid bats in size and basic functional organization. The tonotopically organized posterior ventral field might represent the primary auditory cortex and the tonotopically organized anterior ventral field seems to be similar to the anterior auditory field of other mammals. As most energy of the echolocation pulse of P. discolor is contained in the high-frequency range, the non-tonotopically organized high-frequency dorsal region seems to be particularly important for echolocation.

Otoacoustic emissions from insect ears: evidence of active hearing?

Sensitive hearing organs often employ nonlinear mechanical sound processing which generates distortion-product otoacoustic emissions (DPOAE). Such emissions are also recordable from tympanal organs of insects. In vertebrates (including humans), otoacoustic emissions are considered by-products of active sound amplification through specialized sensory receptor cells in the inner ear. Force generated by these cells primarily augments the displacement amplitude of the basilar membrane and thus increases auditory sensitivity. As in vertebrates, the emissions from insect ears are based on nonlinear mechanical properties of the sense organ. Apparently, to achieve maximum sensitivity, convergent evolutionary principles have been realized in the micromechanics of these hearing organs-although vertebrates and insects possess quite different types of receptor cells in their ears. Just as in vertebrates, otoacoustic emissions from insects ears are vulnerable and depend on an intact metabolism, but so far in tympanal organs, it is not clear if auditory nonlinearity is achieved by active motility of the sensory neurons or if passive cellular characteristics cause the nonlinear behavior. In the antennal ears of flies and mosquitoes, however, active vibrations of the flagellum have been demonstrated. Our review concentrates on experiments studying the tympanal organs of grasshoppers and moths; we show that their otoacoustic emissions are produced in a frequency-specific way and can be modified by electrical stimulation of the sensory cells. Even the simple ears of notodontid moths produce distinct emissions, although they have just one auditory neuron. At present it is still uncertain, both in vertebrates and in insects, if the nonlinear amplification so essential for sensitive sound processing is primarily due to motility of the somata of specialized sensory cells or to active movement of their (stereo-)cilia. We anticipate that further experiments with the relatively simple ears of insects will help answer these questions.

Distortion-product otoacoustic emissions in the common marmoset (Callithrix jacchus): parameter optimization

Distortion-product otoacoustic emissions (DPOAEs) were measured in a New World primate, the common marmoset (Callithrix jacchus). We determined the optimal primary-tone frequency ratio (f(2)/f(1)) to generate DPOAEs of maximal amplitude between 3 and 24 kHz. The optimal f(2)/f(1), determined by varying f(2)/f(1) from 1.02 to 1.40 using equilevel primary tones, decreased with increasing f(2) frequency between 3 and 17 kHz, and increased at 24 kHz. The optimal f(2)/f(1) ratio increased with increasing primary-tone levels from 50 to 74 dB SPL. When all stimulus parameters were considered, the mean optimal f(2)/f(1) was 1.224-1.226. Additionally, we determined the effect of reducing L(2) below L(1). Decreasing L(2) below L(1) by 0, 5, and 10 dB (f(2)/f(1)=1.21) minimally affected DPOAE strength. DPOAE levels were stronger in females than males and stronger in the right ear than the left, just as in humans. This study is the first to measure OAEs in the marmoset, and the results indicate that the effect of varying the frequency ratio and primary-tone level difference on marmoset DPOAEs is similar to the reported effects in humans and Old World primates.

Numerical study of the effect of the noseleaf on biosonar beamforming in a horseshoe bat

Around 300 bat species are known to emit their ultrasonic biosonar pulses through the nostrils. This nasal emission coincides with the presence of intricately shaped baffle structures surrounding the nostrils. Some prior experimental evidence indicates that these "noseleaves" have an effect on the shape of the animals' radiation patterns. Here, we present a numerical acoustical analysis of the noseleaf of a horseshoe bat species. We show that all three distinctive parts of its noseleaf ("lancet," "sella," "anterior leaf") have an effect on the acoustic near field as well as on the directivity pattern. Furthermore, we show that furrows in one of the parts (the lancet) also exert such an influence. The underlying physical mechanisms suggested by the properties of the estimated near field are cavity resonance, as well as reflection and shadowing of the sound waves emitted by the nostrils. In their effects on the near field, the noseleaf parts showed a tendency toward spatial partitioning with the effects due to each part dominating a certain region. However, interactions between the acoustic effects of the parts were also evident, most notably, a synergism between two frequency-dependent effects (cavity resonance and shadowing) to produce an even stronger frequency selectivity.

Noseleaf furrows in a horseshoe bat act as resonance cavities shaping the biosonar beam

Horseshoe bats emit their ultrasonic biosonar pulses through nostrils surrounded by intricately shaped protuberances (noseleaves). While these noseleaves have been hypothesized to affect the sonar beam, their physical function has never been analyzed. Using numerical methods, we show that conspicuous furrows in the noseleaf act as resonance cavities shaping the sonar beam. This demonstrates that (a) animals can use resonances in external, half-open cavities to direct sound emissions, (b) structural detail in the faces of bats can have acoustic effects even if it is not adjacent to the emission sites, and (c) specializations in the biosonar system of horseshoe bats allow for differential processing of subbands of the pulse in the acoustic domain.

Threshold minima and maxima in the behavioral audiograms of the bats Artibeus jamaicensis and Eptesicus fuscus are not produced by cochlear mechanics

Behavioral audiograms of Artibeus jamaicensis and Eptesicus fuscus are characterized by two threshold minima separated by a threshold maximum at 40 kHz, for A. jamaicensis, and 45 kHz, for E. fuscus [Koay, G., Heffner, H.E., Heffner R.S., 1997. Audiogram of the big brown bat (Eptesicus fuscus). Hear. Res. 105, 202-210; Heffner, R.S., Koay, G., Heffner H.E., 2003. Hearing in American leaf-nosed bats. III: Artibeus jamaicensis. Hear. Res. 184, 113-122.]. To investigate whether these characteristics are due to cochlear properties, we recorded distortion product otoacoustic emissions (DPOAEs) and calculated DPOAE threshold curves. We found that in both species cochlear sensitivity, assessed by DPOAE recordings, does not show local threshold maxima. The DPOAE threshold curve calculated for A. jamaicensis reveals a broadly tuned minimum for frequencies between 20 and 50 kHz and the threshold curve of E. fuscus shows a broad sensitive area for frequencies between 15 and 60 kHz. In none of the two species any pronounced threshold irregularities were found. The characteristic pattern of a threshold maximum followed by a minimum observed in behavioral studies seems to be shaped by transfer characteristics of the outer ear and/or neuronal processing in the ascending auditory pathway rather than by cochlear mechanics.

Auditory responses in the cochlear nucleus of awake mustached bats: precursors to spectral integration in the auditory midbrain

In the cochlear nucleus (CN) of awake mustached bats, single- and two-tone stimuli were used to examine how responses in major CN subdivisions contribute to spectrotemporal integrative features in the inferior colliculus (IC). Across CN subdivisions, the proportional representation of frequencies differed. A striking result was the substantial number of units tuned to frequencies <23 kHz. Across frequency bands, temporal response patterns, latency, and spontaneous discharge differed. For example, the 23- to 30-kHz representation, which comprises the fundamental of the sonar call, had an unusually high proportion of units with onset responses (39%) and low spontaneous rates (53%). Units tuned to 58-59 kHz, corresponding to the sharply tuned cochlear resonance, had slightly but significantly longer latencies than other bands. In units tuned to frequencies >30 kHz, 31% displayed a secondary excitatory peak, usually between 10 and 22 kHz. The secondary peak may originate in cochlear mechanisms for some units, but in others it may result from convergent input onto CN neurons. In 20% of units tested with two-tone stimuli, suppression of best frequency (BF) responses was tuned at least an octave below BF. These properties may underlie similar IC responses. However, other forms of spectral interaction present in IC were absent in CN: we found no facilitatory combination-sensitive interactions and very few combination-sensitive inhibitory interactions of the dominant IC type in which inhibition was tuned to 23-30 kHz. Such interactions arise above CN. Distinct forms of spectral integration thus originate at different levels of the ascending auditory pathway.

Isoflurane increases amplitude and incidence of evoked and spontaneous otoacoustic emissions

The volatile anesthetic isoflurane was tested for its effect on cochlear function by means of measuring distortion product otoacoustic emissions (DPOAE) and spontaneous otoacoustic emissions (SOAE) in the mustached bat (Pteronotus parnellii parnellii). Averaged growth functions of DPOAE and spontaneous otoacoustic emissions were assessed and compared between the control group (no isoflurane application) and the isoflurane group (application of isoflurane at vaporizer settings sof about 1.5-2%). Isoflurane significantly increases the DPOAE amplitude, e.g. at a primary tone level l2 of 40 dB SPL by 10.7 dB. Additionally, the incidence of SOAEs was highly increased during application of isoflurane. The sound-evoked efferent effect on the generation of otoacoustic emissions was significantly reduced in the isoflurane group. We suggest that isoflurane might affect the postsynaptic action of acetylcholine (ACh) released by the efferent terminals of outer hair cells (OHCs). This could lead to the observed decrease of efferent suppression and to a disinhibition of cochlear amplification.

Level dependence of distortion-product otoacoustic emissions measured at high frequencies in humans

Given that high-frequency hearing is most vulnerable to cochlear pathology, it is important to characterize distortion-product otoacoustic emissions (DPOAEs) measured with higher-frequency stimuli in order to utilize these measures in clinical applications. The purpose of this study was to explore the dependence of DPOAE amplitude on the levels of the evoking stimuli at frequencies greater than 8 kHz, and make comparisons with those data that have been extensively measured with lower-frequency stimuli. To accomplish this, DPOAE amplitudes were measured at six different f2 frequencies (2, 5, 10, 12, 14, and 16 kHz), with a frequency ratio (f2/f1) of 1.2, at five fixed levels (30 to 70 dB SPL) of one primary (either f1 or f2), while the other primary was varied in level (30 to 70 dB SPL). Generally, the level separation between the two primary tones (L1 > L2) generating the largest DPOAE amplitude (referred to as the "optimal level separation") decreased as the level of the fixed primary increased. Additionally, the optimal level separation was frequency dependent, especially at the lower fixed primary tone levels ( < or = 50 dB SPL). In agreement with previous studies, the DPOAE level exhibited greater dependence on L1 than on L2.

Directionality of hearing in two CF/FM bats, Pteronotus parnellii and Rhinolophus rouxi

The head-related transfer function (HRTF) has been measured in two CF/FM bats, Pteronotus parnellii and Rhinolophus rouxi from 575 positions in the frontal hemisphere. P. parnellii showed an increase of the elevation angle of the axis of highest pinna gain with increasing frequency followed by a specific decrease at 75 kHz. Such a drop of elevation angle of the acoustic axis was not seen in R. rouxi. The HRTF further showed a spectral notch dependent on elevation and frequency in P. parnellii, but not in R. rouxi. The functional implications of this difference between both bat species are discussed. Frequencies at maximum pinna gain values did not clearly match the frequencies of the harmonics of the echolocation calls whereas spatial resolution of interaural intensity differences was best in a frequency range that included the higher harmonics of the echolocation calls in both bat species. However, specializations of HRTF patterns matching the exact frequencies of the harmonics of the echolocation calls could not be observed in both bat species.

Cochlear sensitivity in the lesser spear-nosed bat, Phyllostomus discolor

Behavioral auditory thresholds of Phyllostomus discolor are characterized by two threshold minima separated by an insensitive region at about 55 kHz (Esser and Daucher 1996). To investigate whether these characteristics are due to cochlear properties, we recorded distortion product otoacoustic emissions (DPOAEs) and calculated relative DPOAE threshold curves, which proved to be a good measure of cochlear sensitivity. Our results indicate that in P. discolor, cochlear sensitivity, as assessed by DPOAE recordings, does not show a threshold maximum at 55 kHz. The DPOAE threshold curves display an absolute minimum at approximately 30 kHz, and from that frequency region, the threshold continuously increases without any pronounced irregularities. The frequency tuning properties of the cochlea, as assessed by DPOAE suppression tuning curves (STCs) reveal broad filter bandwidths with Q10dB values between 3.4 and 10.7. There are no frequency-specific specializations of cochlear tuning. The characteristic pattern of subsequent threshold maxima and minima at high frequencies observed in behavioral studies seems to be shaped by transfer characteristics of the outer ear and/or neuronal processing in the ascending auditory pathway rather than by cochlear mechanics.

Relationship of neural and otoacoustic emission thresholds during endocochlear potential development in the gerbil

Distortion product otoacoustic emissions and auditory brainstem responses (ABRs) were measured in neonatal gerbils at three ages: at 15-16 days after birth (dab), near the onset of hearing when the endocochlear potential (EP) is known to be still immature; at 22 dab, when the EP first reaches mature levels; and at 30 dab. Comparing individual 15-16 dab animals to the 22 dab group, ABR threshold changes were typically larger than those for cubic distortion tone (CDT, 2f1-f2) emission thresholds which were, in turn, larger than those for the simple difference tone (DT, f2-f1). In contrast, from 22 to 30 dab there were no important changes in CDT or DT emission thresholds. Observed threshold-change relationships were very similar to those found in differential diagnosis investigations, where the EP was experimentally decreased using a chronic furosemide application. Therefore, most of the change in cochlear function over the two week period studied could be attributed to the maturation of EP during the first week. Model calculations further show that relative changes in CDT and DT emission thresholds are compatible with a movement of the operating point of the cochlear amplifier toward its symmetrical "central" point as the EP reaches mature levels.

Sound-evoked efferent effects on cochlear mechanics of the mustached bat

The influence of the crossed medial efferent system on cochlear mechanics of the mustached bat was tested by measuring delayed evoked otoacoustic emissions (DEOAEs), cochlear microphonics, distortion product otoacoustic emissions (DPOAEs) and stimulus frequency otoacoustic emissions. Contralaterally delivered sinusoids, broadband noise and bat echolocation calls were used for acoustic stimulation of the efferent system. With all four measures we found a level-dependent suppression under stimulation with both broadband noise and echolocation calls. In addition, the sharply tuned cochlear resonance of the mustached bat which is involved in processing echolocation signals at 61 kHz shifted upward in frequency by several 100 Hz. Presentation of sinusoids did not have any significant effect. DEOAEs and DPOAEs were in some cases enhanced during contralateral presentation of the bat calls at moderate intensities. The most important function of the efferent system in the mustached bat might be the control of the extraordinarily fine-tuned resonator of this species, which is close to instability as evident from the very pronounced evoked otoacoustic emissions which sometimes convert into spontaneous otoacoustic emissions of high level.

Low-frequency distortion product otoacoustic emissions in two species of kangaroo rats: implications for auditory sensitivity

Low-frequency distortion-product otoacoustic emissions were measured in two species of kangaroo rats to test the prediction that a large footdrumming species (e.g., Dipodomys spectabilis) would have greater distortion-product otoacoustic emission amplitude than a small non-footdrumming species (e.g., Dipodomys merriami), indicating better hearing sensitivity at low frequencies. Equal-level (65 dB SPL) stimulus tones ( f(1), f(2)), presented over a ( f(1)) range of 200-1000 Hz, were used to evoke the 2 f(1)- f(2) distortion-product otoacoustic emission. Mean 2 f(1)- f(2) levels for D. merriami showed good correspondence to previously published audiograms for that species. Mean 2 f(1)- f(2) levels and 95% confidence intervals indicated species differences below 400 Hz, supporting the theory that low-frequency hearing sensitivity is better in large kangaroo rat species. These results suggest that the size-related divergence in footdrumming behavior may be related to differential auditory sensitivity.

Distortion product otoacoustic emissions and auditory evoked potentials in the hedgehog tenrec, Echinops telfairi

The hedgehog tenrec, Echinops telfairi, has certain basal mammalian features, like a cloaca and a sparsely differentiated brain with smooth cerebral hemispheres. The peripheral auditory capabilities of this species were investigated by means of distortion product otoacoustic emissions (DPOAE). For comparison, we determined auditory evoked potentials (AEP) in the inferior colliculus and the auditory cortex. Both methods show that the auditory range of E. telfairi extends well into ultrasonic frequencies, with a region of highest sensitivity at around 16 kHz. The total auditory range spans about 4 octaves at 40 dB SPL. The low-frequency limit of auditory processing is found at frequencies of about 2-3 kHz. The DPOAE and the AEP thresholds of E. telfairi do not run fully parallel in the high-frequency range. For a threshold value of 40 dB SPL, cochlear mechanical thresholds as measured with DPOAE extend up to 50 kHz, whereas neuronal thresholds reach the high-frequency limit at about 30 kHz. Frequency tuning, as assessed from DPOAE suppression tuning curves, was low to moderate with Q(10 dB) values ranging from 1.7 to 8. The lack of discontinuity in the group delay (derived from DPOAE measurements) reveals that cochlear frequency representation is tonotopic without any region of specialized mechanical tuning.

Postnatal Development of Cochlear Function in the Mustached Bat,Pteronotus parnellii

Postnatal development of the mustached bat's cochlea was studied by measuring cochlear microphonic and compound action potentials. In adults, a cochlear resonance is involved in enhanced tuning to the second harmonic constant frequency component (CF2) of their echolocation calls at ∼61 kHz This resonance is present immediately after birth in bats that do not yet echolocate. Its frequency is lower (46 kHz) and the corresponding threshold minimum of cochlear microphonic potentials is broader than in adults. Long-lasting ringing of the cochlear microphonic potential after tone stimulus offset that characterizes the adult auditory response close to CF2 is absent in newborns. In the course of the first 5 postnatal weeks, there is a concomitant upward shift of CF2 and the frequency of cochlear threshold minima. Up to the end of the third postnatal week, sensitivity of auditory threshold minima and the Q value of the cochlear resonance increase at a fast rate. Between 2 and 4 wk of age, two cochlear microphonic threshold minima are found consistently in the CF2 range that differ in their level-dependent dynamic growth behavior and are 1.5–5.7 kHz apart from each other. In older animals, there is a single minimum that approaches adult tuning in its sharpness. The data provide evidence to show that during maturation of the cochlea, the frequency and the sensitivity of the threshold minimum associated with CF2 increases and that these increases are associated with the fusion of two resonances that are partly dissociated in developing animals.

Sources and Mechanisms of DPOAE Generation: Implications for the Prediction of Auditory Sensitivity

Otoacoustic emissions (OAEs) have become a commonly used clinical tool for assessing cochlear health status, in particular, the integrity of the cochlear amplifier or motor component of cochlear function. Predicting hearing thresholds from OAEs, however, remains a research challenge. Models and experimental data suggest that there are two mechanisms involved in the generation of OAEs. For distortion product, transient, and high-level stimulus frequency emissions, the interaction of multiple sources of emissions in the cochlea leads to amplitude variation in the composite ear canal signal. Multiple sources of emissions complicate simple correlations between audiometric test frequencies and otoacoustic emission frequencies. Current research offers new methods for estimating the individual components of OAE generation. Input-output functions and DP-grams of the nonlinear component of the 2f2-f2 DPOAE may ultimately show better correlations with hearing thresholds. This paper reviews models of OAE generation and methods for estimating the contribution of source components to the composite emission that is recorded in the ear canal. The clinical implications of multiple source components are discussed.

Distortion-product otoacoustic emissions measured at high frequencies in humans

Distortion-product otoacoustic emissions (DPOAEs) elicited with stimulus frequencies less than or equal to 8 kHz have been used in hearing clinics to assess whether the middle ear and cochlea are normal, but high-frequency hearing (>4 kHz) is most vulnerable to cochlear pathology. It might prove useful to measure DPOAEs with even higher frequency stimuli (>8 kHz), but there have been few reports of such studies in humans. DPOAEs have been measured in other mammals to the upper range of hearing sensitivity. The purpose of this study was to compare some characteristics of DPOAEs in human subjects elicited with high-frequency stimuli with those that have been extensively measured with lower-frequency stimuli. The primary goal was to establish if the same phenomenon responsible for the behavior of low-frequency DPOAEs is responsible for the behavior of high-frequency DPOAEs. Specifically, the DPOAE level with stimuli varied from 2 to 20 kHz, growth functions of DPOAEs, effects of varying the primary frequency ratio (f2/f1) on the DPOAE level, and DPOAE group delay were determined. Because the behaviors appeared to vary smoothly with stimulus frequency, the study suggests that emissions measured from 2 to 20 kHz were the product of the same biological process.

Micromechanical responses to tones in the auditory fovea of the greater mustached bat's cochlea

An extended region of the greater mustached bat's cochlea, the sparsely innervated (SI) zone, is located just basally to the frequency place of the dominant 61-kHz component of the echolocation signal (CF2). Anatomic adaptations in the SI zone are thought to provide the basis for cochlear resonance to the CF2 echoes and for the extremely sharp tuning throughout the auditory system that allows these bats to detect Doppler shifts in the echoes caused by insect wing beat. We measured basilar membrane (BM) displacements in the SI zone with a laser interferometer and recorded acoustic distortion products at the ear drum at frequencies represented in the SI zone. The basilar membrane in the SI region was tuned both to its characteristic frequency (62-72 kHz) and to the resonance frequency (61-62 kHz). With increasing stimulus levels, the displacement growth functions are compressive curves with initial slopes close to unity, and their properties are consistent with the mammalian cochlear amplifier working at high sound frequencies. The sharp basilar membrane resonance is associated with a phase lag of 180 degrees and with a shift of the peak resonance to lower frequencies for high stimulus levels. Within the range of the resonance, the distortion product otoacoustic emissions, which have been attributed to the resonance of the tectorial membrane in the SI region, are associated with an abrupt phase change of 360 degrees. It is proposed that a standing wave resonance of the tectorial membrane drives the BM in the SI region and that the outer hair cells enhance, fine tune, and control the resonance. In the SI region, cochlear micromechanics appear to be able to work in two different modes: a conventional traveling wave leads to shear displacement between basilar and tectorial membrane and to neuronal excitation for 62-70 kHz. In addition, the SI region responds to 61-62 kHz with a resonance based on standing waves and thus preprocesses signals which are represented more apically in the CF2 region of the cochlea.

On inner ear function and the origin of oto-acoustic emissions

The extremely low hearing threshold of the mammalian ear suggests the presence of a special amplifying mechanism, because the stereocilia of the outer hair cells (OHCs) are not likely to be sensitive enough themselves, although their mechanical embedding may provide some amplification. In the past decades, biophysicists have increasingly turned to the chaos theory for explanation. a theory the implications of which are considerable. One of its major tenets, self-organization, is not easily understood at first glance, but is easily reproducible mathematically. With self-organization, the processes involving the OHCs can readily be simulated: Self-organization can help to explain why OHCs vibrate at amplitudes much higher than those of the exciting stimulus. To further our understanding of the process of hearing, vibratory processes, which presumably occur in normal and damaged OHC clusters, are described and compared with a mathematical analysis of data sets obtained from normal subjects using an extremely sensitive microphone.

Frequency tuning and spontaneous activity in the auditory nerve and cochlear nucleus magnocellularis of the barn owl Tyto alba

Single-unit recordings were obtained from the brain stem of the barn owl at the level of entrance of the auditory nerve. Auditory nerve and nucleus magnocellularis units were distinguished by physiological criteria, with the use of the response latency to clicks, the spontaneous discharge rate, and the pattern of characteristic frequencies encountered along an electrode track. The response latency to click stimulation decreased in a logarithmic fashion with increasing characteristic frequency for both auditory nerve and nucleus magnocellularis units. The average difference between these populations was 0.4-0.55 ms. The average most sensitive thresholds were approximately 0 dB SPL and varied little between 0.5 and 9 kHz. Frequency-threshold curves showed the simple V shape that is typical for birds, with no indication of a low-frequency tail. Frequency selectivity increased in a gradual, power-law fashion with increasing characteristic frequency. There was no reflection of the unusual and greatly expanded mapping of higher frequencies on the basilar papilla of the owl. This observation is contrary to the equal-distance hypothesis that relates frequency selectivity to the spatial representation in the cochlea. On the basis of spontaneous rates and/or sensitivity there was no evidence for distinct subpopulations of auditory nerve fibers, such as the well-known type I afferent response classes in mammals. On the whole, barn owl auditory nerve physiology conformed entirely to the typical patterns seen in other bird species. The only exception was a remarkably small spread of thresholds at any one frequency, this being only 10-15 dB in individual owls. Average spontaneous rate was 72.2 spikes/s in the auditory nerve and 219.4 spikes/s for nucleus magnocellularis. This large difference, together with the known properties of endbulb-of-Held synapses, suggests a convergence of approximately 2-4 auditory nerve fibers onto one nucleus magnocellularis neuron. Some auditory nerve fibers as well as nucleus magnocellularis units showed a quasiperiodic spontaneous discharge with preferred intervals in the time-interval histogram. This phenomenon was observed at frequencies as high as 4.7 kHz.

The acoustic two-tone distortions 2f1-f2 and f2-f1 and their possible relation to changes in the operating point of the cochlear amplifier

Acoustic two-tone distortions are generated during non-linear mechanical amplification in the cochlea. Generation of the cubic distortion 2f1-f2 depends on asymmetric components of a non-linear transfer function whereas the difference tone f2-f1 relies on symmetric components. Therefore, a change of the operating point and hence the symmetry of the cochlear amplifier could be strongly reflected in the level of the f2-f1 distortion. To test this hypothesis, low-frequency tones (5 Hz) were used to bias the position of the cochlear partition in the gerbil. Phase-correlated changes of f2-f1 occurred at bias tone levels where there were almost no effects on 2f1-f2. Higher levels of the bias tone induced pronounced changes of both distortions. These results are qualitatively in good agreement with the results of a simulation in which the operating point of a Boltzman function was shifted. This function is similar to those used to describe outer hair cell (OHC) transduction. To influence OHC motility, salicylate was injected. It caused a decrease of the 2f1-f2 level and an increase in the level of f2-f1. Such reciprocal changes of both distortions, again, can be interpreted in terms of a shift of the operating point of the cochlear amplifier along a non-linear transfer characteristic. To directly influence the cochlear amplifier, DC current was injected into the scala media. Large negative currents (> -2 microA) caused a pronounced decrease of 2f1-f2 (> 15 dB) and positive currents had more complex effects with increasing and/or decreasing 2f1-f2 distortion level. The effects were time and primary level dependent. Changes of f2-f1 for DC currents > magnitude of mu 2A were in most cases larger compared to 2f1-f2 and reversed for certain primary levels. The current effects probably result from a combination of changing the endocochlear potential and shifting the operating point along a non-linear transfer function.

Further studies on the mechanics of the cochlear partition in the mustached bat. II. A second cochlear frequency map derived from acoustic distortion products

It has been proposed that acoustic 2f1-f2 distortions reflect the frequency characteristics of a secondary cochlear filter mechanism (Brown et al., 1992; Allen and Fahey, 1993). This concept was used to construct a second cochlear frequency map that may represent aspects of tectorial membrane (TM) tuning. Within the frequency range of 15-105 kHz, for a given f2 frequency, f1 was varied and the frequency ratio f2/f1 determined that produced maximum levels of the 2f1-f2 distortion (best ratio). The second cochlear frequency map was derived by plotting the distortion frequency that corresponded to the best ratio f2/f1 against the cochlear place of f2 which was obtained from the HRP-frequency map of Pteronotus (Kössl and Vater, 1985b). Minimum best ratios of 1.0005 and hence practically identical characteristic frequencies of the putative tuning of basilar membrane (HRP) and TM (2f1-f2) were found at about 45% distance from the base, a point at which 62 kHz are represented on the BM. This frequency is associated with strong cochlear resonance and large evoked and spontaneous otoacoustic emissions. Between 45% and 20% distance from the base, the basilar membrane (BM) tuning progressively increases to about 70 kHz whereas the calculated TM tuning remains constant at a frequency close to 62 kHz. The range of constant TM tuning coincides with the sparsely innervated cochlear region of Pteronotus where BM thickness is maximal and TM mass and limbal attachment are reduced (Vater and Kössl, 1996). We suggest that here the TM oscillates strongly at 62 kHz and may carry most of the energy of cochlear resonance which is transferred into movement of the organ of Corti at and apical to the 45% location where the BM is tuned to 62 kHz.

Analysis of non-linear cochlear mechanics in the marsupial Monodelphis domestica: ancestral and modern mammalian features

The acoustic distortion product 2f1-f2 was measured in the ear canal of the gray short-tailed opossum. Monodelphis domestica, during stimulation with two pure tone stimuli of the frequencies f1 and f2 (f1 < f2). From 2f1-2f growth functions for different primary frequencies from 4 to 85 kHz, a distortion threshold curve was obtained describing the relative threshold of non-linear mechanical processes in the cochlea. Distortion products could be measured over the entire investigated frequency range and the distortion audiogram proved sensitive (thresholds < 20 dB SPL) over a wide range from 8 to 55 kHz with a broad minimum around 17.5 kHz. Thresholds steeply increased for frequencies below 8 kHz which can be regarded as an original mammalian feature (Heffner and Masterton, 1980). To assess the tuning characteristics of the cochlea, suppression tuning curves of 2f1-2f were measured. The shape and tuning sharpness of these curves resembled that of modern eutherian mammals with a shallower low-frequency and a steeper high-frequency flank and Q10dB values between 2.4 and 7.12. The optimum primary ratio f2/f1 (best-ratio) was on average 1.172. For f2 frequencies between 20 and 30 kHz the ration curves displayed multiple peaks in periodic intervals. The same peaks were also evident for higher-order distortions (3f1-2f2, 4f1-3f2). It is discussed to which extent this finding is consistent with the second filter theory of Brown et al. (1992) and Allen and Fahey (1993) and how in Monodelphis domestica this filter could work differently, possibly representing a less evolved stage, than in eutherian mammals.

The effect of contralateral stimulation on cochlear resonance and damping in the mustached bat: the role of the medial efferent system

In the unanesthetized mustached bat, stimulation of the ear with an acoustic transient produces damped oscillations which are evident in the cochlear microphonic potential. In this report we demonstrate how the decay time of these oscillations is affected by broadband noise presented to the contralateral ear (CLN). In the absence of CLN, the mean decay time was 1.94 +/- 0.23 ms, but during the presentation of CLN the decay time consistently decreased. The changes were finely graded, the higher the CLN, the greater the change. The effect could be maintained at a constant level for extended periods of time and this was evident when the CLN exceeded 40 dB SPL. The latency of the reflex for 64 dB noise was about 11 ms and near maximum changes occurred within 15 ms of CLN onset. Sectioning medial efferent nerve fibers in the floor of the fourth ventricle or the administration of a single dose of gentamicin eliminated changes produced by CLN. The prominence of CM responses to damped oscillations and the robust changes in response to CLN make the mustached bat an excellent model for studying the influence of the medial efferent system on cochlear mechanics.

Distortion product otoacoustic emissions in rhesus (Macaca mulatta) monkey ears: normative findings

Distortion product otoacoustic emissions (DPOAEs) in rhesus monkeys were characterized and the optimal parameters for their generation were determined. Robust DPOAEs were readily measurable from the ear canals of six rhesus monkeys (n = 12 ears). The nonmonotonic behavior of the f2/f1 ratio functions in rhesus monkeys was found to be similar to other animals and humans. The optimal mean f2/f1 ratio of 1.21 and the effect of the primary frequency and level on the optimal f2/f1 ratios were also similar to human measurements. The contour of the rhesus monkey DPOAE 'audiograms' and their behavior were also comparable to human measurements with slight differences in peak frequencies. The rhesus monkey DPOAE input/output (I/O) functions were generally monotonic with a slope approaching unity with increasing frequency. Therefore, our study shows that many basic DPOAE characteristics are remarkably similar in the two species and emphasizes the appropriateness of the rhesus monkey as a model for DPOAE research. Detailed studies of the behavior of DPOAEs can be carried out in a model that is phylogenetically close to human both in hearing and in the gross structure and histology of the inner ear.

The shape of 2f1-f2 suppression tuning curves reflects basilar membrane specializations in the mustached bat, Pteronotus parnellii

Iso-suppression tuning curves (STCs) of the 2f1-f2 distortion product (dp) were measured over a primary frequency range of 20 to 93 kHz in mustached bats, Pteronotus parnellii parnellii. Primary levels were chosen to produce dp levels between 0 and 7 dB SPL. At frequencies outside the ranges of 60-72 kHz and 90-93 kHz the shapes of the STCs were symmetrical or asymmetrical with a steep high frequency slope. In the vicinity of 61 kHz where a strong stimulus frequency otoacoustic emission (SFOAE) is present, the asymmetry of the STCs was inverted with a very steep low frequency slope (max. -89 dB/kHz) and a shallow high frequency slope. The inverted STCs resemble neuronal tuning curves of the same species with best frequencies at about 61 kHz. Close to 61 kHz the STCs were sharply tuned with Q10dB values up to 177. The STC-thresholds were about 20 dB above the neuronal thresholds. Thickenings of the basilar membrane located just basal to the cochlear place of the SFOAE frequency are probably involved in creating the asymmetric STCs. Cochlear resonance at the SFOAE frequency and an increased longitudinal coupling within the thickened basilar membrane region are thought to contribute to the specialized STC shape. In the range of 90-93 kHz, the STCs are also sharply tuned with inverted asymmetry which is probably not due to an harmonic effect of the specialized cochlear mechanics in the 60 kHz region but may be caused by an independent mechanism.

Evidence for a mechanical filter in the cochlea of the 'constant frequency' bats, Rhinolophus rouxi and Pteronotus parnellii

To investigate the function of basilar membrane (BM) thickenings in the cochlea of bats which use constant frequency (CF) echolocation calls, acoustic distortion products were measured while placing the primary stimuli f1 and f2 at frequencies which are represented in the thickened BM regions. In Rhinolophus, for primary stimuli between about 80-100 kHz, pronounced maxima of the level of distortion products (2f1-f2, 3f1-2f2, 4f1-3f2) can be measured if the frequency separation between the two primary tones is chosen so that the resulting distortion frequency matches the dominant CF frequency (resting frequency, RF). The distortion maxima extend from the individual RF down to frequencies which are 2-4 kHz lower. The data indicate that the thickened BM region in the basal halfturn of the cochlea strongly oscillates at the bats' RF and slightly below. The hearing threshold, however, is at a maximum at the RF (see Kössl 1993). Therefore, the RF oscillations are thought to be involved in absorbing the respective frequency such that the more apically located frequency place of the RF is insensitive and a mechanical notch filter is established. In Pteronotus, there are maxima of the level of 2f1-f2 at distortion frequencies around the RF of about 61.5 kHz up to the frequency of a strong stimulus-frequency otoacoustic emission (SFOAE) which is a few hundred Hz higher. Pronounced distortions in the RF range can only be elicited when the stimulus frequencies are between about 62 to 72 kHz. Similar to the situation in Rhinolophus, this frequency band is represented on a stretch of thickened BM.