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     

Coding of sinusoidally amplitude modulated acoustic stimuli in the inferior colliculus of the rufous horseshoe bat,Rhinolophus rouxi

Summary Single neuron responses to sinusoidally amplitude modulated (SAM) signals were studied in the inferior colliculus of the horseshoe bat,Rhinolophus rouxi.57% of the neurons responded to SAM stimuli with periodical discharges synchronized to the modulation cycle. The proportion of cells driven by amplitude modulated signals was independent of the best frequency of the neurons. Best modulation frequencies were at or below 100 Hz in about 70% of the neurons. Synchronized activity could be elicited by modulation frequencies up to 400 Hz.Best SAM responses were observed at stimulus intensities 10 dB above threshold. Generally the BMF of a neuron did not change with intensity. The BMF decreased with decreasing modulation depth of the amplitude modulation.A trend for a topographical organization of neurons according to best modulation frequencies was detected. The results did not reveal any significant specialization of the bat's auditory system for coding of amplitude modulations as compared to other mammals.Abbreviations BF best frequency - BMF best modulation frequency - CF constant frequency - FM frequency modulation - IC inferior colliculus - SAM sinusoidal amplitude modulation - SFM sinusoidal frequency modulation     

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.     

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     

Frequency tuning and response latencies at three levels in the brainstem of the echolocating bat,Eptesicus fuscus

To determine the level at which certain response characteristics originate, we compared monaural auditory responses of neurons in ventral cochlear nucleus, nuclei of lateral lemniscus and inferior colliculus. Characteristics examined were sharpness of frequency tuning, latency variability for individual neurons and range of latencies across neurons.Exceptionally broad tuning curves were found in the nuclei of the lateral lemniscus, while exceptionally narrow tuning curves were found in the inferior colliculus. Neither specialized tuning characteristic was found in the ventral cochlear nuclei.All neurons in the columnar division of the ventral nucleus of the lateral lemniscus maintained low variability of latency over a broad range of stimulus conditions. Some neurons in the cochlear nucleus (12%) and some in the inferior colliculus (15%) had low variability in latency but only at best frequency.Range of latencies across neurons was small in the ventral cochlear nucleus (1.3–5.7 ms), intermediate in the nuclei of the lateral lemniscus (1.7–19.8 ms) and greatest in the inferior colliculus (2.9–42.0 ms).We conclude that, in the nuclei of the lateral lemniscus and in the inferior colliculus, unique tuning and timing properties are built up from ascending inputs.Abbreviations AVCN anteroventral cochlear nucleus - BF best frequency - CV coefficient of variation - DCN dorsal cochlear nucleus - FM frequency modulation - IC inferior colliculus - NLL nuclei of lateral lemniscus - PSTH post stimulus time histogram - PVCN posteroventral cochlear nucleus - SD standard deviation - SPL sound pressure level - VCN ventral cochlear nuclei - VNLLc ventral nucleus of the lateral lemniscus, columnar division     

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     

Electrical responses of the superior olivary complex in Vespertilionidae and Rhinolophidae to ultrasonic stimuli with different fill frequency

Global and single unit responses of the superior olivary complex were investigated during ultrasonic stimulation at different frequencies in two species of bats from the Vespertilionidae, which emit frequency-modulated signals and the Rhinolophidae, which utilize almost monochromatic (80 ± 1 kHz) echolocation cries. Maximal sensitivity to ultrasound in the Vespertilionidae was found at frequencies of 10–40 kHz, and in the Rhinolophidae also within the range 10–40 kHz but with a second increase in sensitivity in the region 82–86 kHz. Sharply tuned neurons were more numerous in the Rhinolophidae than in the Vespertilionidae. Neurons whose response in the echolocation frequency band changed in character depending on the fill frequency of the stimulus were found in Rhinolophidae: a phasic discharge occurs over a wide range of frequencies and a tonic discharge at the characteristic frequency; the latter was also observed over a limited range of intensities.A. A. Zhdanov Leningrad State University. Translated from Neirofiziologiya, Vol. 5, No. 1, pp. 33–39, January–February, 1973.     

Ontogenesis of the echolocation system in the rufous horseshoe bat,Rhinolophus rouxi (Audition and vocalization in early postnatal development)

1. The development of vocalization and hearing was studied in Sri Lankan horseshoe bats (Rhinolophus rouxi) during the first postnatal month. The young bats were caught in a nursing colony of rhinolophids in which birth took place within a two week period. 2. The new-born bats emitted isolation calls through the mouth. At the beginning these calls consisted of pure tones with frequencies below 10 kHz (Fig. 1). During the first postnatal week the call frequency increased to about 15 kHz, and the fundamental was augmented by two to four harmonics. No evoked potentials to pure tone stimuli could be elicited in the inferior colliculus of this age group, i.e., auditory processing at the midbrain level was not demonstrable. 3. Evoked potentials were first recorded in the second week, broadly tuned to 15-45 kHz, with a maximum sensitivity between 15-25 kHz. In the course of the second week, however, higher frequencies up to 60 kHz became progressively incorporated into the audiogram (Fig. 3). The fundamental frequency of the multiharmonic isolation calls, emitted strictly through the mouth, increased to about 20 kHz. 4. In the bats' third postnatal week an increased hearing sensitivity (auditory filter) emerged, sharply tuned at frequencies between 57 and 60 kHz (Fig. 4e). The same individuals were also the first to emit long constant frequency echolocation calls through the nostrils (Fig. 4c). The energy of the calls was arranged in harmonic frequency bands with the second harmonic exactly tuned to the auditory filter. These young bats continued to emit isolation calls through the mouth, which were, however, not harmonically related to the echolocation calls (Fig. 4b, d). 5. During the fourth week, both the auditory filter and the matched echolocation pulses (the second harmonic) shifted towards higher frequencies (Fig. 5). During the fifth week the fundamental frequency of the calls was progressively attenuated, and both the second harmonic of the pulses and the auditory filter reached the frequency range typical for adult bats of 73-78 kHz (Fig. 6). 6. The development of audition and vocalization is discussed with regard to possible interactions of both subsystems, and their incorporation into the active orientation system of echolocation.     

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     

Electric reactions of the auditory cortex areas in bats to ultrasonic stimuli with various fill frequencies

The overall electric reactions and action potentials of single neurons in the auditory cortex were investigated for Vespertilionidae (Myotis oxygnathus) and Rhinolophidae (Rhinolophus ferrum equinum) narcotized with Hexenal. In the Vespertilionidae the greatest sensitivity to ultrasound is manifest at frequencies from 10 to 50 kHz, and in the Rhinolophidae for the ranges from 10 to 40 and from 82 to 84 kHz. The shapes of the response areas of single neurons in both types of bats are similar except for neurons discovered in Rhinolophidae that have three response areas with characteristic frequencies in the ranges 27–28, 40–42, and 80–84kHz. Narrow response areas with characteristic frequencies in the range from 70 to 90kHz appear on a considerable proportion of the neurons in the Rhinolophidae, but not the Vespertilionidae. Low thresholds are recorded to the stimulus cutoff in the range from 76 to 86 kHz.A. A. Zhdanov Leningrad State University. Translated from Neirofiziologiya, Vol. 3, No. 5, pp. 526–532, September–October, 1971.     

Spectral and temporal gating mechanisms enhance the clutter rejection in the echolocating bat,Rhinolophus rouxi

Summary Doppler shift compensation behaviour in horseshoe bats, Rhinolophus rouxi, was used to test the interference of pure tones and narrow band noise with compensation performance. The distortions in Doppler shift compensation to sinusoidally frequency shifted echoes (modulation frequency: 0.1 Hz, maximum frequency shift: 3 kHz) consisted of a reduced compensation amplitude and/or a shift of the emitted frequency to lower frequencies (Fig. 1).Pure tones at frequencies between 200 and 900 Hz above the bat's resting frequency (RF) disturbed the Doppler shift compensation, with a maximum of intererence between 400 and 550 Hz (Fig. 2). Minimum duration of pure tones for interference was 20 ms and durations above 40 ms were most effective (Fig. 3). Interfering pure tones arriving later than about 10 ms after the onset of the echolocation call showed markedly reduced interference (Fig. 4). Doppler shift compensation was affected by pure tones at the optimum interfering frequency with sound pressure levels down to –48 dB rel the intensity level of the emitted call (Figs. 5, 6).Narrow bandwidth noise (bandwidth from ± 100 Hz to ± 800 Hz) disturbed Doppler shift compensation at carrier frequencies between –250 Hz below and 800 Hz above RF with a maximum of interference between 250 and 500 Hz above resting frequency (Fig. 7). The duration and delay of the noise had similar influences on interference with Doppler shift compensation as did pure tones (Figs. 8, 9). Intensity dependence for noise interference was more variable than for pure tones (-32 dB to -45 dB rel emitted sound pressure level, Fig. 10).The temporal and spectral gating in Doppler shift compensation behaviour is discussed as an effective mechanism for clutter rejection by improving the processing of frequency and amplitude transients in the echoes of horseshoe bats.Abbreviations CF constant frequency - FM frequency modulation - RF resting frequency - SPL sound pressure level     

Echolocation sound features processed to provide distance information in the CF/FM bat,Noctilio albiventris: evidence for a gated time window utilizing both CF and FM components

Summary Bats of the speciesNoctilio albiventris, trained to discriminate differences in target distance, emitted pairs of pulses at a rate of 7–10/s, the first a constant frequency (CF) pulse of about 8 ms duration and 75 kHz frequency, followed after about 28 ms by a CF/FM pulse having a 6 ms, 75 kHz CF component that terminates in a 2 ms FM sweep to about 57 kHz.Loud free-running artificial pulses, simulating the bat's natural CF/FM echolocation sound, interfered with distance discrimination at repetition rates exceeding 5/s. Systematic modifications in the temporal and frequency structure of the artificial pulses resulted in orderly changes in the degree of interference. Artificial pulses simulating the natural CF or FM components alone had no effect, nor did 10/s white noise pulses, although constant white noise of the same intensity masked the behavior.Interference occurred when the CF of the artificial pulses was between 52 and 77 kHz, ending with a downward FM sweep of 25 kHz from the CF. For interference to occur there was a much more critical requirement that the FM sweep begin at approximately the frequency of the CF component. The FM sweep needed to be 11 kHz or greater bandwidth. Interference occurred when the duration of the CF component of the CF/FM artificial pulse was between 2 and 30 ms, with maximal effect between 10 and 20 ms. However, a brief (2.0 ms) CF signal 2–27 ms before an isolated FM signal was as effective as a continuous CF component of the same duration.When coupled with the bat's own emissions, artificial CF/FM pulses interfered if they occurred after the bat's CF/FM pulse and before the next natural emission. A 2 ms FM sweep alone was effective in interfering with distance discrimination when it came 8–27 ms after the onset of the bat's own CF/FM pulse. Neither CF/FM nor FM artificial pulses interfered when they began during the bat's own emission. A 10 ms CF pulse alone had no effect at any time.These findings indicate thatN. albiventris uses both the CF and FM components of its short-CF/FM echolocation sound for distance discrimination. The CF onset activates a gating mechanism that, during a narrowly defined subsequent time window, enables the nervous system to process FM pulse-echo pairs for distance information, within a fairly broad frequency range, as long as the frequencies of the CF and the beginning of the FM sweep are nearly identical.Abbreviations CF constant frequency - FM frequency modulation     

Reflection of the release from masking effect in unit activity of the cat inferior colliculus

The effect of release from masking in unit activity of the inferior colliculus was studied in anesthetized cats. When series of cophasic and antiphasic clicks with varied interaural delay were used as stimuli, sensitivity to changes in the interaural phase shift within a narrow band of the spectrum of the signal close to the characteristic frequency was discovered in neurons with low characteristic frequencies (up to 2.6 kHz). The masking effect with cophasic and antiphasic noise was found to depend on the phase sensitivity of the neurons. The neurophysiological mechanisms of the release from masking effect are discussed.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 17, No. 4, pp. 463–468, July–August, 1985.     

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     

Peripheral auditory tuning for fine frequency analysis by the CF-FM bat,Rhinolophus ferrumequinum

Summary For echolocation,Rhinolophus ferrumequinum emits orientation sounds, each of which consists of a long constant-frequency (CF) component and short frequency-modulated (FM) components. The CF component is about 83 kHz and is used for Doppler-shift compensation. In this bat, single auditory nerve fibers and cochlear nuclear neurons tuned at about 83 kHz show low threshold and very sharp filter characteristics. The slopes of their tuning curves ranged between 1,000 and 3,500 dB/octave and their Q-10 dB values were between 20 and 400, 140 on the average (Figs. 3–5). The peripheral auditory system is apparently specialized for the reception and fine frequency analysis of the CF component in orientation sounds and Doppler-shift compensated echoes. This specialization is not due to suppression or inhibition comparable to lateral inhibition, but due to the mechanical specialization of the cochlea. Peripheral auditory neurons with the best frequency between 77 and 87 kHz showed not only on-responses, but also off-responses to tonal stimuli (Figs. 1, 2, and 6). The off-responses with a latency comparable to that of N₁-off were not due to a rebound from either suppression or inhibition, but probably due to a mechanical transient occurring in the cochlea at the cessation of a tone burst.We thank Alexander von Humboldt Stiftung, Deutsche Forschungsgemeinschaft (Grant No. Ne146/6-8), Stiftung Volkswagenwerk (Grant No. 111858), and American National Science Foundation (Grant No. 40018 and BMS 75-17077) for their support for our cooperative work.     

Unit responses of the superior olives in bats to single and paired ultrasonic stimuli

Single unit responses in the superior olive of the greater horseshoe bat to ultrasonic stimuli with a filling frequency within the echolocation range were investigated. Some neurons were found to have three completely unconnected response regions with characteristic frequencies of 1/2 and 1/3 of the basic frequency, which was within the 80–86 kHz band. An increase in strength of the stimulus with filling frequency equal to the characteristic frequency of the neuron changed the tonic regime of activity into phasic. Presentation of two stimuli, overlapping in time, replaced the phasic regime by tonic. The frequency of the tonic response corresponded exactly to the beating frequency up to 1200 Hz (synchronization of unit discharges with each beating cycle). The synchronized tonic regime was preserved to definite strengths and filling frequencies of the two stimuli.A. A. Zhdanov State University, Leningrad. Translated from Neirofiziologiya, Vol. 8, No. 1, pp. 30–38, January–February, 1976.     

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.     

Characteristics of auditory brainstem responses in ground squirrels

Summary Auditory brainstem responses (ABRs) were characterized at 37 °C in ground squirrels (Citellus lateralis) which were implanted with recording screws to record ABRs, and a thermistor to record brain temperature. After two weeks ground squirrels were reanesthetized and tone pips and clicks were delivered through a TDH-49 headphone.Recorded ABRs were found to vary in a predictable manner as a function of stimulus frequency and intensity. At intensities above 50 dB SPL, ABRs could be recorded over the range tested (2–32 kHz). An 8 kHz tone pip was the best frequency for recording ABRs at the lowest stimulus intensities. Latencies decreased as stimulus frequencies increased from 4 kHz to 32 kHz.