ABR frequency tuning curves in dolphins

Tone-tone masking was used to determine auditory brain-stem response tuning curves in dolphins (Tursiops truncatus) in a simultaneous-masking paradigm. The Q ₁₀ of the curves was as large as 16–19 in the frequency range 64–128 kHz. In the range 45–16 kHz, Q ₁₀ decreased proportionally to the frequency with the bandwidth of the curves being constant, about 3.5–4 kHz at the 10-dB level. Tuning curves below 45 kHz are supposed to reflect broad spectral bandwidth of the probe's effective part which is no longer than 0.5 ms, irrespective of actual probe duration. Tuning curves above 64 kHz are supposed to reflect the real frequency tuning of the dolphin's auditory system.Abbreviations ABR auditory brain stem response - AP action potential     

Multiple specializations in the peripheral auditory system of the CF-FM bat,Pteronotus parnellii

Summary Cochlear microphonic (CM) and evoked neural potentials (N₁) were recorded from the cochlear aqueduct of awakePteronotus parnellii. The CM audiograms obtained with continuous sounds had more or less uniform thresholds except for a sharp threshold notch at about 60 kHz (Fig. 1). When brief tone bursts were presented, the envelopes of the CM responses were always similar to the envelopes of the applied signals except when tone bursts having frequencies at or close to the frequency of the tuned sensitivity notch were presented (i.e., 59–63 kHz). The CM rise-decay times for frequencies around 60kHz were much longer than those of the presented signals (Fig. 2). The prolonged decay times are thought to be due to the ringing of the basilar membrane resulting from a mechanical resonance in the cochlea.The evoked neural potential audiograms (N₁-on and N₁-off responses) differed considerably from the CM audiogram. Of particular importance is the N₁-off audiogram which exhibited very sharp tuning in four frequency regions: 31–33 kHz, 60–63 kHz, 71–73 kHz, and 91–92 kHz (Fig. 5). The frequencies evoking the lowest thresholds of the CM and N₁-off (in the 60 kHz region) were either identical or differed by only 100–400 Hz.The sharp tuning in the 60 kHz region of both the CM and N₁ audiograms could be eliminated by presenting 90–100 dB continuous sounds for one min but only if the signal frequency was equal to the tuned frequency of the CM audiogram (Figs. 8–13). Presenting intense sounds having frequencies above or below the tuned 60kHz region had no effect on the audiogram. The overstimulation procedure had remarkably specific effects on the CM and N₁-off audiograms causing the greatest threshold increases at the 60 kHz tuned frequency and progressively smaller threshold changes on the slopes of the tuned notch.Assuming that the sharp changes of the N₁-off thresholds reflect some important underlying mechanism, the N₁-off audiograms demonstrate multiple specializations in the peripheral auditory system ofPteronotus with the bat possessing at least three and possibly four sharply tuned regions. With regard to mechanism, the tuned notch in the CM audiogram, the curious CM rise-decay times evoked by tone bursts, and the ease with which the 60 kHz sensitivity notch can be eliminated all argue strongly in favor of a mechanical resonance in the cochlea which is responsible for the sharp tuning around 60 kHz. On the other hand, the absence of tuned notches in the 30 kHz and 90 kHz regions of the CM audiogram together with the absence of any discernable ringing of the CM potentials evoked by 30 kHz and 90 kHz tone bursts both argue against a resonance mechanism for the tuning at these harmonically related frequency regions. Finally, the fact that overstimulating the 60 kHz region had no discernable effect on the N₁-off tuning at 30 kHz and 90 kHz demonstrates that the mechanism responsible for the tuned regions at 30 kHz and 90 kHz are independent of the resonance feature of the cochlea at 60 kHz.Abbreviations BF best frequency - CF constant frequency - CM cochlear microphonics - CM-aft after-response of the CM - FM frequency modulated - N ₁ evoked neural potentials We thank Professor Alvin Novick for the generous support provided during the conduct of these experiments. We also thank Professor Gerhard Neuweiler and Dr. Gerd Schuller for their helpful comments and suggestions. Supported by PHS Grant NB7616 11.     

Specializations for aerial hawking in the echolocation system of<Emphasis Type="Italic"> Molossus molossus</Emphasis> (Molossidae,Chiroptera)

While searching for prey, Molossus molossus broadcasts narrow-band calls of 11.42 ms organized in pairs of pulses that alternate in frequency. The first signal of the pair is at 34.5 kHz, the second at 39.6 kHz. Pairs of calls with changing frequencies were only emitted when the interpulse intervals were below 200 ms. Maximum duty cycles during search phase are close to 20%. Frequency alternation of search calls is interpreted as a mechanism for increasing duty cycle and thus the temporal continuity of scanning, as well as increasing the detection range. A neurophysiological correlate for the processing of search calls was found in the inferior colliculus. 64% of neurons respond to frequencies in the 30- to 40-kHz range and only in this frequency range were closed tuning curves found for levels below 40 dB SPL. In addition, 15% of the neurons have double-tuned frequency-threshold curves with best thresholds at 34 and 39 kHz. Differing from observations in other bats, approach calls of M. molossus are longer and of higher frequencies than search calls. Close to the roost, the call frequency is increased to 45.0–49.8 kHz and, in addition, extremely broadband signals are emitted. This demonstrates high plasticity of call design.Abbreviations BF best frequency - CF constant frequency - IC inferior colliculus - F_max maximal frequency - F_min minimal frequency - PF peak frequency - PSTH post-stimulus time histogram - QCF quasi-constant frequency - SPL sound pressure level     

Acoustic distortion products from the cochlea of the blind African mole rat, Cryptomys spec.

The measurement of distortion-product otoacoustic emissions is a noninvasive method that can be used for assessing the sensitivity and the frequency tuning of nonlinear cochlear mechanics. During stimulation with two pure tones f1 and f2, the acoustic 2f1-f2 distortion was recorded in the ear canal of Cryptomys spec. to study specializations in cochlear mechanics that could be associated with the presence of a frequency expanded cochlear region between 0.8–1 kHz. In addition, a distortion threshold curve was obtained which describes relative threshold of nonlinear cochlear mechanics. Sensitive distortion thresholds could be measured for stimulus frequencies between 0.4 to 18 kHz with a broad minimum between 0.75 to 2.5 kHz. The distortion threshold curve extends to higher frequencies than previous neuronal data indicated.As a measure of mechanical tuning sharpness in the cochlea, suppression tuning curves of 2f1-f2 were recorded. The tuning curves reflected the typical mammalian pattern with shallow low frequency and steep high frequency slopes. Their tuning sharpness was poor with Q10dB values between 0.3 and 1.88. In the range of the frequency expanded region, the Q10dB values were below 0.5. This finding emphasizes that the presence of frequency expansion does not necessarily lead to enhanced mechanical tuning in the cochlea and one has to consider if in certain bat species with cochlear frequency expansion and particularly sharp cochlear tuning, the two phenomena may not be interlinked.Abbreviations CF constant frequency component of echolocation call - STC suppression tuning curve     

Ontogenesis of auditory fovea representation in the inferior colliculus of the Sri Lankan rufous horseshoe bat,Rhinolophus rouxi

Summary This report describes the ontogenesis of tonotopy in the inferior colliculus (IC) of the rufous horseshoe bat (Rhinolophus rouxi). Horseshoe bats are deaf at birth, but consistent tonotopy with a low-to-high frequency gradient from dorsolateral to ventromedial develops from the 2nd up to the 5th week. The representation of the auditory fovea is established in ventro-mediocaudal parts of the IC during the 3rd postnatal week (Fig. 3). Then, a narrow frequency band 5 kHz in width, comprising 16% of the bat's auditory range, captures 50–60 vol% of the IC (Fig. 3c). However, foveal tuning is 10–12 kHz (1/3 octave) lower than in adults; foveal tuning in females (65–68 kHz) is 2–3 kHz higher than in males (62–65 kHz). Thereafter, foveal tuning increases by 1–1.5 kHz per day up to the 5th postnatal week, when the adult hearing range is established (Figs. 4, 5). The increase of sensitivity and of tuning sharpness of single units also follows a low-to-high frequency gradient (Fig. 6).Throughout this development the foveal tuning matches the second harmonic of the echolocation pulses vocalised by these young bats. The results confirm the hypothesis of developmental shifts in the frequency-place code for the foveal high frequency representation in the IC.Abbreviations BF best frequency - CF constant frequency - FM frequency modulation - IC inferior colliculus - IHC inner hair cell; - OHC outer hair cell - RR Rhinolophus rouxi     

Physiology and tonotopic organization of auditory receptors in the cricketGryllus bimaculatus DeGeer

Summary Physiological recordings were obtained from identified receptors in the tympanal organ ofGryllus bimaculatus. By immersing the prothoracic leg in Ringer solution and removing the anterior tympanic membrane the auditory receptors were exposed without significantly altering the frequency response of the auditory organ (Fig. 1). Each receptor was tuned to a specific sound frequency. For sound frequencies below this characteristic frequency the roll-off in sensitivity decreased from 20–30 dB/octave to 10–15 dB/octave as the characteristic frequency of receptors increased from 3–11 kHz (Fig. 4A). For each individual receptor the slope, dynamic range and maximum spike response were similar for different sound frequencies (Fig. 9A). The receptors were tonotopically organized with the characteristic frequency of the receptors increasing from the proximal to the distal end of the array (Figs. 5, 6). Several receptors had characteristic frequencies of 5 kHz. These receptors were divided into two groups on the basis of their maximum spike response produced in response to pure tones of increasing intensity (Fig. 7). Independent of the tuning of the receptor no two-tone inhibition was observed in the periphery, thus confirming that such interactions are a property of central integration.     

Discharge patterns of cochlear ganglion neurons in the chicken

Summary Physiological recordings were made of the compound action potential from the round window and single neurons in the cochlear ganglion of normal adult chickens (Gallus domesticus). The compound action potential threshold to tone bursts decreased from approximately 42 dB at 0.25 kHz to 30 dB between 1 and 2 kHz and then increased to 51 dB at 4 kHz. Most of the cochlear ganglion cells had characteristic frequencies below 2 kHz and the thresholds of most neurons were roughly 30–35 dB lower than the compound action potential thresholds. At any given characteristic frequency, thresholds varied by as much as 60 dB and units with the highest thresholds tended to have the lowest spontaneous rates. Spontaneous discharge rates ranged from 0 to 200 spikes/s with a mean rate of 86 spikes/s. Interspike interval histograms of spontaneous activity often contained regular peaks with the time interval between peaks approximately equal to 1/(characteristic frequency). Tuning curves were sharply tuned and V-shaped with approximately equal slopes to the curves above and below characteristic frequency. Q_10dB and Q_30dB values for the tuning curves increased with characteristic frequency. Post stimulus time histograms showed sustained firing during the stimulus and were characterized by a slight-to-moderate peak at stimulus onset. Most units showed vigorous phase-locking to tones at characteristic frequency although the degree of phase-locking declined sharply with increasing characteristic frequency. Discharge rate-level functions at characteristic frequency had a mean dynamic range of 42 dB and a mean saturation firing rate of 327 spikes/s. In general, the firing patterns of cochlear ganglion neurons are similar in most respects to those reported in other avians, but differ in several important respects from those seen in mammals.Abbreviations CF characteristic frequency - CAP compound action potential     

Ontogenesis of tonotopy in inferior colliculus of a hipposiderid bat reveals postnatal shift in frequency-place code

Summary The postnatal development of midbrain tonotopy was investigated in the inferior colliculus (IC) of the south Indian CF-FM batHipposideros speoris. The developmental progress of the three-dimensional frequency representation was determined by systematic stereotaxic recordings of multiunit clusters from the 1st up to the 7th postnatal week. Additional developmental measures included the tuning characteristics of single units (Figs. 3f; 4f; 5f), the analysis of the vocalised pulse repertoire (Figs. 3e, 4e, 5e), and morphometric reconstructions of the brains of all experimental animals (Fig. 1).The maturation of auditory processing could be divided into two distinct, possibly overlapping developmental periods: First, up to the 5th week, the orderly tonotopy in the IC developed, beginning with the low frequency representation and progressively adding the high frequency representation. With regard to the topology of isofrequency sheets within the IC, maturation progresses from dorsolateral to ventromedial (Figs. 3c, 4c). At the end of this phase the entire IC becomes specialised for narrowly tuned and sensitive frequency processing. This includes the establishment of the auditory fovea, i.e. the extensive spatial representation of a narrow band of behaviorally relevant frequencies in the ventromedial part of the IC. In the 5th postnatal week the auditory fovea is concerned with frequencies from 100–118 kHz (Fig. 4c, d). During subsequent development, the frequency tuning of the auditory fovea increases by 20–25 kHz and finally attains the adult range of ca. 125–140 kHz. During this process, neither the bandwidth of the auditory fovea (15–20 kHz) nor the absolute sensitivity of its units (ca. 50 dB SPL) were changed. Further maturation occurred at the single unit level : the sharpness of frequency tuning increased from the 5th to the 7th postnatal weeks (Q-₁₀-dB-values up to 30–60), and upper thresholds emerged (Figs. 4f, 5f).Although in the adult the frequency of the auditory fovea matches that of the vocalised pulses, none of the juvenile bats tested from the 5th to the 7th weeks showed such a frequency match between vocalisation and audition (Figs. 4e, 5e).The results show that postnatal maturation of audition in hipposiderid bats cannot be described by a model based on a single developmental parameter.Abbreviations BF best frequency - CF constant frequency - Cer cerebellum - CN cochlear nucleus - CO auditory cortex - CUF cuneiform nucleus - DAB days after birth - FAL forearm length - FM frequency modulation - IC inferior colliculus - NLL nucleus of the lateral lemniscus - PAG periaqueductal gray - SC superior colliculus     

Functional correlates of characteristic frequency in single cochlear nerve fibers of the Mongolian gerbil

Summary Single-unit recordings obtained from the auditory nerve of the Mongolian gerbil, Meriones unguiculatus, revealed functional differences in the response properties of neurons tuned to low and high frequencies. The distribution of neural thresholds displayed a distinct rise for auditory nerve fibers with characteristic frequencies] (CFs) between 3–5 kHz. This frequency band also marked abrupt changes in both the distribution of spontaneous discharge rates and the shape of the neural tuning curve. For neurons of all CFs, spontaneous firing rates were inversely related to neural threshold but unrelated to sharpness of neural tuning. The range of CF thresholds encountered, even when data from many animals were combined, rarely exceeded 20 dB, suggesting that cochlear nerve responses obtained from this species display little inter-animal variability. These results are compared with similar data from other species and discussed in terms of recent studies on sound communication and cochlear anatomy in gerbils.Abbreviations CF characteristic frequency - SR spontaneous discharge rate     

Effects of temperature on the properties of primary auditory fibres of the spectacled caiman,Caiman crocodilus (L.)

Summary The effect of temperature on the response properties of primary auditory fibres in caiman was studied. The head temperature was varied over the range of 10–35 ° C while the body was kept at a standard temperature of 27 °C (T_s). The temperature effects observed on auditory afferents were fully reversible. Below 11 °C the neural firing ceased.The mean spontaneous firing rate increased nearly linearly with temperature. The slopes in different fibres ranged from 0.2–3.5 imp s^–1 °C^–1. A bimodal distribution of mean spontaneous firing rate was found (<20 imp s^–1 and >20 imp s^–1 at T_s) at all temperatures.The frequency-intensity response area of the primary fibres shifted uniformly with temperature. The characteristic frequency (CF) increased nearly linearly with temperature. The slopes in different fibres ranged from 3–90 Hz °C^–1. Expressed in octaves the CF-change varied in each fibre from about O.14oct °C^–1 at 15 °C to about 0.06 oct °C^–1 at 30 °C, irrespective of the fibre's CF at T_s. Thresholds were lowest near T_s. Below T_s the thresholds decreased on average by 2dB°C^–1, above T_s the thresholds rose rapidly with temperature. The sharpness of tuning (Q_10db) showed no major change in the temperature range tested.Comparison of these findings with those from other lower vertebrates and from mammals shows that only mammalian auditory afferents do not shift their CF with temperature, suggesting that a fundamental difference in mammalian and submammalian tuning mechanisms exists. This does not necessarily imply that there is a single unifying tuning mechanism for all mammals and another one for non-mammals.Abbreviations BF best frequency: frequency of maximal response at an intensity 10 dB above the CF-threshold - CF characteristic frequency - FTC frequency threshold curve, tuning curve - T _s standard temperature of 27 °C     

Resonance phenomena in the cochlea of the mustache bat and their contribution to neuronal response characteristics in the cochlear nucleus

Summary The cochlea of the mustache bat, Pteronotus parnellii, is very sensitive and sharply tuned to the frequency range of the dominant second harmonic of the echolocation call around 61 kHz. About 900 Hz above this frequency the cochlear microphonic potential (CM) reaches its maximum amplitude and lowest threshold. At exactly the same frequency, pronounced evoked otoacoustic emissions (OAE) can be measured in the outer ear canal, indicating mechanical resonance. The CM amplitude maximum and the OAE are most severely masked by simultaneous exposure to tones within the range from about 61–62 kHz up to about 70 kHz. The data suggest that the mechanism of mechanical resonance involves cochlear loci basal to the 61 kHz position.The resonance contributes to auditory sensitivity and sharp tuning: At the frequency of the OAE, single unit responses in the cochlear nucleus have the lowest thresholds. Maximum tuning sharpness occurs at frequencies about 300 Hz below the OAE-frequency, where the threshold is about 10 dB less sensitive than at the OAE-frequency. In addition, in the frequency range around the OAE-frequency several specialized neuronal response features can be related to mechanical resonance: Long lasting excitation after the end of the stimulus, asymmetrical tuning curves with a shallow high frequency slope and phasic on-off neuronal response patterns. In particular the latter phenomenon indicates the occurrence of local mechanical cancellations in the cochlea.Abbreviations CF constant frequency component of echolocation calls - CM cochlear microphonic potential - FM frequency modulated component of echolocation calls - N1 compound action potential of the auditory nerve - OAE octoacoustic emission - SEOAE synchronous evoked OAE     

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     

Communication in the weakly electric fish Sternopygus macrurus

Summary A classical conditioning paradigm was used to test the ability of Sternopygus macrurus to detect EOD-like stimuli (sine waves) of different frequencies. The behavioral tuning curves were quite close in shape to tuning curves based on single-unit recordings of T units, although the sensitivity at all frequencies was much greater. The behavioral curves showed notches of greatly reduced sensitivity when the test frequency was equal to, or twice the EOD frequency. The EOD of each of the fish was eliminated by lesioning the medullary pacemaker nucleus, and the fish were retested. The resulting tuning curves were nearly the same in shape as those of the EOD-intact individuals, but the PMN-lesioned fish showed an overall reduction of sensitivity of 30 dB. The EOD appears to enhance sensitivity by placing the summed stimulus (test stimulus + fish's EOD) at an amplitude where T units are maximally sensitive to small temporal modulations in the fish's own EOD. Peripheral tuning appears to limit the ability of males to detect the EOD of females, since these are, on average, an octave higher in frequency than the male EOD, while the peak sensitivity of the male occurs 5–10 Hz above its own EOD frequency.Abbreviations EOD electric organ discharge - PMN pacemaker nucleus - BF best frequency - DF difference frequency     

Auditory sensitivity and psychophysical tuning curves in the elephant nose fish,Gnathonemus petersii

Summary Hearing sensitivity and psychophysical tuning curves were determined for the mormyridGnathonemus petersii. Pure tone hearing thresholds were determined from 100 Hz to 2,500 Hz, with best sensitivity being about –31 dB (re: 1 dyne/ cm²) from 300 Hz to 1,000 Hz. In order to determine frequency tuning of the auditory system, psychophysical tuning curves (PTC's) were measured with the masker presented simultaneously with, or just ahead of, the 500 Hz test signal. The sound level for different frequencies needed to just mask the test tone were determined from 100 to 800 Hz. Maximum masking occurred in both forward and simultaneous conditions when the masker and the test tone were at the same frequency. As the masker was moved in frequency from 500 Hz, higher sound levels of maskers were needed to afford masking of the test tone. The data were similar in simultaneous and forward masking, with theQ _{10 dB}, a measure of sharpness of tuning, being about 5 in both cases. Data were compared for other species for which behavioral thresholds and PTC's are available.Gnathonemus hears about as wide a range of frequencies as the goldfish,Carassius auratus, although the PTC's for the two species are strikingly different. The PTC's forGnathonemus resemble those determined in a forward-masking paradigm for the clown knife fish,Notopterus chitala, even thoughGnathonemus has a wider hearing bandwidth.Abbreviations AM amplitude modulated - EOD electric organ discharge - PTC psychophysical tuning curve     

Directional sensitivity of auditory neurons in the superior colliculus of the bat,Eptesicus fuscus,using free field sound stimulation

Summary Electrophysiological recordings were made from 130 single neurons of the superior colliculus (SC) of big brown bats,Eptesicus fuscus, in order to test their general as well as directional auditory response properties. Bursts of constant frequency and frequency modulated signals were broadcasted through a condenser loudspeaker, which could be placed at any azimuth and elevation on a hemisphere in front of the bat's head. The SC units responded equally well to both types of signals. The best frequencies of SC neurons ranged from 25 up to 82 kHz, and their tuning curves appeared to be broad (compared to those of the main auditory nuclei of the same and other bats) with Q_10dB values of 1.8–17.5. All units encountered revealed directional sensitivity and were classified in two groups: the majority of them had a constant best angle of response (BA) whatever the sound intensity was (unidirectional type); the others showed a shift in their BA towards the center of the stimulating hemisphere as the intensity decreased (pluridirectional type). In both response types, the BA results are corroborated by the properties of the receptive fields. The presence of an auditory space map in the horizontal plane is not obvious for the superior colliculus of this bat.Abbreviations BA best angle - BF best frequency - CF constant frequency - FM frequency modulated - FTC frequency threshold curve - IC inferior colliculus - MLD nucleus mesencephalicus dorsalis - MT minimum threshold - RF receptive field - RP reference point - SC superior colliculus - SPL sound pressure level re. 20 Pa·cm^–2     

‘Peripheral inhibition’ of vibration-sensitive units in the leg of the fiddler crabUca pugilator

Summary The activity of single vibration-sensitive neurons in the leg nerve of the fiddler crabUca pugilator was recorded extracellularly. All units recorded from fall into two groups according to basic differences in their spectral threshold curves. The first type of neuron can be excited over a broad frequency range (ca. 2–2,000 Hz) with minimal threshold at 15–30 Hz with 0.5–1.0 cm/s² (peak). The second type of neuron, in contrast to the first one spontaneously active, is excited only in the frequency range 2–100 Hz and shows a decrease in the nerve impulse rate at vibration frequencies up to 2 kHz. The intensity necessary for complete suppression of the firing activity is 80 cm/s² at 800 Hz, the range of frequency most sensitive for inhibition.     

Tonotopical organization and pure tone response characteristics of single units in the auditory cortex of the Greater Horseshoe Bat

Summary Tonotopical organization and frequency representation in the auditory cortex of Greater Horseshoe Bats was studied using multi-unit recordings.The auditory responsive cortical area can be divided into a primary and a secondary region on the basis of response characteristics forming a core/belt structure.In the primary area units with best frequencies in the range of echolocation signals are strongly overrepresented (Figs. 6–8). There are two separate large areas concerned with the processing of the two components of the echolocation signals. In one area frequencies between the individual resting frequency and about 2 kHz above are represented, which normally occur in the constant frequency (CF) part of the echoes (CF-area), in a second one best frequencies between resting frequency and about 8 kHz below are found (FM-area).In the CF-area tonotopical organization differs from the usual mammalian scheme of dorso-ventral isofrequency slabs. Here isofrequency contours are arranged in a semicircular pattern.The representation of the cochlear partition (cochleotopic organization) was calculated. In the inferior colliculus and auditory cortex there is a disproportionate representation of the basilar membrane. This finding is in contradiction to the current opinion that frequency representation in the auditory system of Horseshoe Bats is only determined by the mechanical tuning properties of the basilar membrane.Response characteristics for single units were studied using pure tone stimuli. Most units showed transient responses. In 25% of units response characteristics depended on the combination of frequency and sound pressure level used.Frequency selectivity of units with best frequencies in the range of echolocation sounds is very high. Q-10dB values of up to 400 were found in a small frequency band just above resting frequency.Abbreviations BF best frequency - CF constant frequency - FM frequency modulated - MT minimal threshold     

Echolocation and obstacle avoidance in the hipposiderid batAsellia tridens

Summary The echolocation sounds of the hipposiderid batAsellia tridens consist of a constant frequency (cf) component followed by a frequency modulated (fm) terminal downward sweep of 19–21 kHz. The cf-part constitutes about 7/10 of the entire signal. In individual roosting animals the frequencies of the cf-part of consecutive sounds (resting frequency) is kept very constant but varies from bat to bat. In 18Asellia tridens resting frequencies between 111–124 kHz have been measured.The sound duration in roosting and free flying bats is between 7–10 ms. In the approach and terminal phase of bats landing on a perch or flying through obstacles, the sound duration is reduced and the repetition rate increased the nearer the bat approaches the target. At the end of the terminal phase sound durations of a minimum of 3 ms have been measured. Flying bats lower their emission frequency in order to compensate for Doppler shifts caused by the flight movement. The echofrequency is therefore kept constant about 150–200 Hz above the resting frequency.In flights through obstacles consisting of vertically stretched wires with different diameters, the bats were able to avoid wires down to a diameter of 0.065 mm whereas at 0.05 mm the percentage of flights without collisions is far below the chance level. The results demonstrate that the echolocation behavior of the hipposiderid batAsellia tridens does not differ fundamentally from that of rhinolophid bats. As a result, a new suggestion for categorization of bats producing cf-fm orientation sounds is put forward.Abbreviations cf constant frequency component - fm frequency modulated component - P probability of collision-free flights through an obstacle of ertically tretched wires - I interval between wires - D minimal diameter of a bat with folded wings; , angle at which a bat approaches an obstacle - f _A frequency of the cf-component of the emitted sound - f _E frequency of the cf-component of the echo - f _M frequency of the cf-component of the sounds recorded with the microphone - c speed of sound Supported by the Deutsche Forschungsgemeinschaft grant no. Schn 138/6-9We thank W. Hollerbach for technical assistance.     

The directionality of the ear of the pigeon (Columba livia)

Summary The directionality of cochlear microphonic potentials in the azimuthal plane was investigated in the pigeon (Columba livia), using acoustic free-field stimulation (pure tones of 0.25–6 kHz).At high frequencies in the pigeon's hearing range (4–6 kHz), changing azimuth resulted in a maximum change of the cochlear microphonic amplitude by about 20 dB (SPL). The directionality decreased clearly with decreasing frequency.Acoustic blocking of the contralateral ear canal could reduce the directional sensitivity of the ipsilateral ear by maximally 8 dB. This indicates a significant sound transmission through the bird's interaural pathways. However, the magnitude of these effects compared to those obtained by sound diffraction (maximum > 15 dB) suggests that pressure gradients at the tympanic membrane are only of subordinate importance for the generation of directional cues.The comparison of interaural intensity differences with previous behavioral results confirms the hypothesis that interaural intensity difference is the primary directional cue of azimuthal sound localization in the high-frequency range (2–6 kHz).Abbreviations CM cochlear microphonic potential - IID interaural intensity difference - IID-MRA minimum resolvable angle calculated from interaural intensity difference - MRA minimum resolvable angle - OTD interaural ongoing time difference - RMS root mean square - SPL sound pressure level