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

Ontogenetic development of hearing sensitivity to airborne sound in the female red-eared slider, Trachemys scripta elegans

Ontogenetic development of hearing sensitivity has been verified in many groups of vertebrates, but not turtles. Turtles exhibit sexual dimorphism in hearing. To examine the development of hearing in female turtles, auditory brainstem responses (ABR) were compared by assessing the hearing-sensitivity bandwidth, ABR threshold, and latency of female Trachemys scripta elegans aged 1 week, 1 month, 1 yr, and 5 yr. The hearing-sensitivity bandwidths were 0.2-1.1, 0.2-1.1, 0.2-1.3, and 0.2-1.4 kHz in each age group, respectively. Below 0.6 kHz, the ABR threshold decreased from the 1-week to 1-yr age group, with a significant difference between age groups. No significant difference was detected between the 1- and 5-yr age groups (within a stimulus frequency of 0.2-0.6 kHz). Above 0.6 kHz, ABR thresholds decreased significantly from the 1-yr to 5-yr age group (within a stimulus frequency of 0.7-1.0 kHz). There was no significant difference between the 1-month and 1-yr age groups (within a stimulus frequency of 0.7-1.0 kHz), or between the 1-week and 1-month age groups (within a stimulus frequency of 0.7-1.0 kHz, except 0.9 kHz). Thus, female turtle hearing shows frequency-segmented development.

Development of hearing in the big brown bat

We studied the development of hearing in newborn pups of the big brown bat, Eptesicus fuscus. In the majority of pups, the opening of both outer auditory canals occurred on or before postnatal day (PND) 7, but in some, it extended to PND 11. Using repeated auditory brainstem response (ABR) recordings, we tracked the progressive development and maturation of auditory sensitivity in 22 E. fuscus pups every 3 days, from PND 10 to PND 31, with additional recordings in a subset of bats at 2 months, 3 months and 1 year of life. There was a profound increase in auditory sensitivity across development for frequencies between 4 and 100 kHz, with the largest threshold shifts occurring early in development between PND 10 and 19. Prior to PND 13-16 and when pups were still non-volant, most bats were unable to hear frequencies above 48 kHz; however, sensitivity to these higher ultrasonic frequencies increased with age. Notably, this change occurred near the age when young bats started learning how to fly and echolocate.

Post-natal development of the envelope following response to amplitude modulated sounds in the bat Phyllostomus discolor

Bats use a large repertoire of calls for social communication, which are often characterized by temporal amplitude and frequency modulations. As bats are considered to be among the few mammalian species capable of vocal learning, the perception of temporal sound modulations should be crucial for juvenile bats to develop social communication abilities. However, the post-natal development of auditory processing of temporal modulations has not been investigated in bats, so far. Here we use the minimally invasive technique of recording auditory brainstem responses to measure the envelope following response (EFR) to sinusoidally amplitude modulated noise (range of modulation frequencies: 11-130 Hz) in three juveniles (p8-p72) of the bat, Phyllostomus discolor. In two out of three animals, we show that although amplitude modulation processing is basically developed at p8, EFRs maturated further over a period of about two weeks until p33. Maturation of the EFR generally took longer for higher modulation frequencies (87-130 Hz) than for lower modulation frequencies (11-58 Hz).

Ontogeny of auditory brainstem responses in the bat, Phyllostomus discolor

Hearing is the primary sensory modality in bats, but its development is poorly studied. For newborns, hearing appears essential in maintaining contact with their mothers and to develop echolocation abilities. Here we measured auditory brainstem responses (ABRs) to clicks and narrowband tone pips covering a large frequency range (5-90 kHz) in juveniles (p7 to p200) and adults of the bat, Phyllostomus discolor. Tone-pip audiograms show that juveniles at p7 are already quite responsive, not only below 20 kHz but up to 90 kHz. Hearing sensitivity increases further until about p14 and is then refined, possibly correlated with growth and differentiation of the animals' outer ears. ABR amplitudes decrease within the first 3-4 months, inversely correlated with the bat weight and forearm length. ABR Wave I latency decreases with increasing stimulation level. ABR duration (measured between Waves I and V) is longer in juveniles and shortens with age which may reflect temporal refinement of auditory brainstem neurons to accommodate the exceptional temporal precision required for effective echolocation. Overall our data show that P. discolor bats have good hearing very early in life. The current method represents a fast and minimally invasive way of characterizing basic hearing in bats.

Conserved mechanisms of vocalization coding in mammalian and songbird auditory midbrain

The ubiquity of social vocalizations among animals provides the opportunity to identify conserved mechanisms of auditory processing that subserve communication. Identifying auditory coding properties that are shared across vocal communicators will provide insight into how human auditory processing leads to speech perception. Here, we compare auditory response properties and neural coding of social vocalizations in auditory midbrain neurons of mammalian and avian vocal communicators. The auditory midbrain is a nexus of auditory processing because it receives and integrates information from multiple parallel pathways and provides the ascending auditory input to the thalamus. The auditory midbrain is also the first region in the ascending auditory system where neurons show complex tuning properties that are correlated with the acoustics of social vocalizations. Single unit studies in mice, bats and zebra finches reveal shared principles of auditory coding including tonotopy, excitatory and inhibitory interactions that shape responses to vocal signals, nonlinear response properties that are important for auditory coding of social vocalizations and modulation tuning. Additionally, single neuron responses in the mouse and songbird midbrain are reliable, selective for specific syllables, and rely on spike timing for neural discrimination of distinct vocalizations. We propose that future research on auditory coding of vocalizations in mouse and songbird midbrain neurons adopt similar experimental and analytical approaches so that conserved principles of vocalization coding may be distinguished from those that are specialized for each species. This article is part of a Special Issue entitled "Communication Sounds and the Brain: New Directions and Perspectives".

High duty cycle echolocation and prey detection by bats

There are two very different approaches to laryngeal echolocation in bats. Although most bats separate pulse and echo in time by signalling at low duty cycles (LDCs), almost 20% of species produce calls at high duty cycles (HDCs) and separate pulse and echo in frequency. HDC echolocators are sensitive to Doppler shifts. HDC echolocation is well suited to detecting fluttering targets such as flying insects against a cluttered background. We used two complementary experiments to evaluate the relative effectiveness of LDC and HDC echolocation for detecting fluttering prey. We measured echoes from fluttering targets by broadcasting artificial bat calls, and found that echo amplitude was greatest for sounds similar to those used in HDC echolocation. We also collected field recordings of syntopic LDC and HDC bats approaching an insect-like fluttering target and found that HDC bats approached the target more often (18.6% of passes) than LDC bats (1.2% of passes). Our results suggest that some echolocation call characteristics, particularly duty cycle and pulse duration, translate into improved ability to detect fluttering targets in clutter, and that HDC echolocation confers a superior ability to detect fluttering prey in the forest understory compared with LDC echolocation. The prevalence of moths in the diets of HDC bats, which is often used as support for the allotonic frequency hypothesis, can therefore be partly explained by the better flutter detection ability of HDC bats.

Postnatal Maturation of Primary Auditory Cortex in the Mustached Bat, Pteronotus parnellii

The primary auditory cortex (AI) of adult Pteronotus parnellii features a foveal representation of the second harmonic constant frequency (CF2) echolocation call component. In the corresponding Doppler-shifted constant frequency (DSCF) area, the 61 kHz range is over-represented for extraction of frequency-shift information in CF2 echoes. To assess to which degree AI postnatal maturation depends on active echolocation or/and reflects ongoing cochlear maturation, cortical neurons were recorded in juveniles up to postnatal day P29, before the bats are capable of active foraging. At P1-2, neurons in posterior AI are tuned sensitively to low frequencies (22–45 dB SPL, 28–35 kHz). Within the prospective DSCF area, neurons had insensitive responses (>60 dB SPL) to frequencies <40 kHz and lacked sensitive tuning curve tips. Up to P10, when bats do not yet actively echolocate, tonotopy is further developed and DSCF neurons respond to frequencies of 51–57 kHz with maximum tuning sharpness ( Q10dB) of 57. Between P11 and 20, the frequency representation in AI includes higher frequencies anterior and dorsal to the DSCF area. More multipeaked neurons (33%) are found than at older age. In the oldest group, DSCF neurons are tuned to frequencies close to 61 kHz with Q10dB values ≤212, and threshold sensitivity, tuning sharpness and cortical latencies are adult-like. The data show that basic aspects of cortical tonotopy are established before the bats actively echolocate. Maturation of tonotopy, increase of tuning sharpness, and upward shift in the characteristic frequency of DSCF neurons appear to strongly reflect cochlear maturation.

Correlated evolution between hearing sensitivity and social calls in bats

Echolocating bats are auditory specialists, with exquisite hearing that spans several octaves. In the ultrasonic range, bat audiograms typically show highest sensitivity in the spectral region of their species-specific echolocation calls. Well-developed hearing in the audible range has been commonly attributed to a need to detect sounds produced by prey. However, bat pups often emit isolation calls with low-frequency components that facilitate mother-young reunions. In this study, we examine whether low-frequency hearing in bats exhibits correlated evolution with (i) body size; (ii) high-frequency hearing sensitivity or (iii) pup isolation call frequency. Using published audiograms, we found that low-frequency hearing sensitivity is not dependent on body size but is related to high-frequency hearing. After controlling for high-frequency hearing, we found that low-frequency hearing exhibits correlated evolution with isolation call frequency. We infer that detection and discrimination of isolation calls have favoured enhanced low-frequency hearing because accurate parental investment is critical: bats have low reproductive rates, non-volant altricial young and must often identify their pups within large crèches.

Sound azimuth selectivity of inferior collicular neurons in juvenile bats, Myotis chinensis

The directional selectivity of auditory neurons is one of the essential response properties that underlie sound localization, an important task performed by the mammalian auditory system. Here we evaluated the sound azimuth selectivity of inferior collicular neurons in juvenile bats, Myotis chinensis, at the age of 15 days under free-field stimulation conditions. Compared with those in adult bats, neurons in juvenile bats were broadly tuned to sound azimuth angles as indicated by both the type and width of the azimuth selectivity curves. Their best azimuth was distributed over a wide range and, moreover, the adult-like relationship between the best azimuth and the best frequency of these neurons was not developed. These data indicate that the directional selectivity of inferior collicular neurons, like other response properties examined previously, undergoes considerable change during postnatal maturation.

Development of echolocation calls in the mustached bat, Pteronotus parnellii

Adult mustached bats employ Doppler-sensitive sonar to hunt fluttering prey insects in acoustically cluttered habitats. The echolocation call consists of 4-5 harmonics, each composed of a long constant frequency (CF) component flanked by brief frequency modulations (FM). The 2nd harmonic CF component (CF2) at 61 kHz is the most intense, and analyzed by an exceptionally sharply tuned auditory system. The maturation of echolocation calls and the development of Doppler-shift compensation was studied in Cuba where large maternity colonies are found in hot caves. In the 1st postnatal week, infant bats did not echolocate spontaneously but could be induced to vocalize CF-FM signals by passive body motion. The CF2 frequency emitted by the smallest specimens was at 48 kHz (i.e., 0.4 octaves lower than the adult signal). CF-FM signals were spontaneously produced in the 2nd postnatal week at a CF2 frequency of 52 kHz. The CF2 frequencies of induced and spontaneous calls shifted upward to reach a value of 60.5 kHz in the 5th postnatal week. Standard deviations of CF2 frequency were large (up to +/-1.5 kHz) in the youngest bats and dropped to values of +/-250 Hz at the end of the 3rd postnatal week. Some individuals in the 4th and 5th postnatal weeks emitted with adultlike frequency precision of about +/-100 Hz. In the youngest bats, the 1st harmonic CF component (CF1) was up to 22 dB stronger than CF2. Adultlike relative levels of CF1 (-28 dB relative to CF2) were reached in the 5th postnatal week. In spontaneously emitted CF-FM calls, the duration of the CF2 component gradually increased with age from 5 ms to maximum values of 18 ms. Durations of the CF2 component in induced calls averaged 7 +/- 2.6 ms in the 1st postnatal week and 8.2 +/- 1.5 ms in the 5th postnatal week. There were no age-related changes in duration of the terminal FM sweep (3 +/- 0.4 ms) in both induced and spontaneous calls. The magnitude of the terminal FM sweep in spontaneous calls was not correlated with age (mean 13.5 +/- 2 kHz). Values for induced calls slightly increased with age from 11 +/- 2 to 13 +/- 2 kHz. The emission rate of induced CF-FM signals increased with age from values of 2.5 +/- 2 to 17 +/- 5 pulses/s. Values for spontaneously emitted calls were 4.4 +/- 3 and 9 +/- 4.5 pulses/s, respectively. Doppler-shift compensation, as tested in the pendulum task, emerged during the 4th postnatal week in young bats that were capable of very brief active flights, but before the time of active foraging outside the cave.

Influence of auditory deprivation upon the tonopic organization in the inferior colliculus: a Fos immunocytochemical study in the rat

The frequency organization in the inferior colliculus of neonatally-deafened rats was investigated using electrical stimulation of the cochlea and immunoreactivity for Fos as a marker of neuronal activity. An electrode implanted either at the base or at the apex of the right cochlea delivered a unique 45-min stimulation at two different level intensities and at two time points, i.e. either at 4 weeks or at 4 months. In 4-week-old rats stimulated at 5x threshold, a site-for-site organization was observed since basal or apical stimulation induced a strong labelling in the ventro-medial or in the dorsolateral part of the left inferior colliculus, respectively. In 4-month-old rats, stimulation of the base induced an extremely weak Fos labelling without any specific location in the left inferior colliculus while stimulation of the apex induced a diffuse labelling with two discrete bands being distinguishable in the left inferior colliculus. In 4-week-old rats stimulated at 15x threshold, basal stimulation elicited a diffuse Fos-like immunoreactivity in the left inferior colliculus while apical stimulation yielded a response restricted to the dorsal part of the left inferior colliculus. In 4-month-old rats, no response was detected in the left inferior colliculus after stimulation of the basal part of the cochlea. Stimulation of the apex could still induce a labelling in the dorsolateral left inferior colliculus. Thus, the inferior colliculus exhibits an adult-like tonotopic organization early on independently of any acoustic stimulation. Prolonged absence of auditory input dramatically alters this organization in the inferior colliculus, especially for high frequencies. From a clinical standpoint, these results could argue for early implantation in deaf children.

Tonotopic Order in the Adult and Developing Auditory System of the Rat as Shown by c-fos Immunocytochemistry

Immediate early genes such as the proto-oncogene c-fos can be expressed in neurons following synaptic excitation by sensory stimulation. C-fos immunocytochemistry has subsequently been shown to be a very sensitive marking technique for neuronal activity. Here, antibodies against the c-fos protein product Fos were used to map the tonotopic organization in the auditory system of adult and developing rats. After stimulating adult rats with pure-tone pulses, bands of Fos-immunoreactive neurons revealed the frequency representation in seven brainstem nuclei: all three subdivisions of the cochlear nucleus, the lateral superior olive, the medial nucleus of the trapezoid body, the ventral nucleus of the trapezoid body, the rostral periolivary nucleus, the dorsal nucleus of the lateral lemniscus and the inferior colliculus. With the exception of the dorsal cochlear nucleus and the inferior colliculus, tonotopicity has not been previously demonstrated in the brainstem nuclei of the rat. During development two striking results were obtained. First, beginning at postnatal day 14 (i.e. approximately 2 days after physiological hearing begins in rats), not only low but also high frequencies were able to induce strong Fos immunoreactivity, indicating that gradual recruitment of formerly unresponsive high-frequency sites does not occur in the rat. Second, a gradual age-related shift of the position of isofrequency bands was not seen in any of the nuclei, suggesting that changes in frequency - place code do not occur after 2 weeks postnatally. These results indicate that the rat's auditory brainstem nuclei achieve their adult-like tonotopic organization early on, implying a somewhat different developmental time course than is found in other mammalian species.

Development of single- and two-tone responses of anteroventral cochlear nucleus neurons in gerbil

Responses of anteroventral cochlear nucleus (AVCN) neurons in developing gerbils were obtained to single-tone stimuli, and two-tone stimuli elicited by best frequency probes presented over a range of intensities. Neurons displayed Type I, Type I/III, and Type III receptive field patterns. Best frequencies ranged from 1.5 to 10.0 kHz. Two-tone suppression (2TS) was first observed in 5 of 16 neurons examined at 14 dab. and in all neurons examined in gerbils aged 15 to 60 dab. Suppression areas grew larger, and discharge rate reductions became greater with age. Features of the two-tone responses that were highly correlated with single-tone responses across age groups include maximum rate reductions and suppression area thresholds. The intensity level of the CF probe-tone also influenced these features of 2TS. Maximum rate reductions to below spontaneous rate levels of activity were common across age groups. Results suggest that the cochlear amplifier is present and fundamentally adult-like by 15 dab for the regions of the cochlea coding the mid frequencies in gerbil. Over the subsequent week, contributions to the developing two-tone responses by the cochlear amplifier increase slightly. Two-tone responses are influenced by central inhibitory mechanisms as early as 14 dab.

Paradoxical relationship between frequency selectivity and threshold sensitivity during auditory-nerve fiber development

The acquisition of adult-like frequency selectivity is generally assumed to be the tightly coupled to improvements in threshold sensitivity during cochlear development. In this study, frequency versus threshold (tuning) curves obtained from 1108 auditory-nerve fibers were used to investigate the relationship between tuning and threshold at characteristic frequency (CF) during postnatal development in kittens. At the earliest ages included in this study, sharpness was within the adult range, but thresholds were significantly higher than adult values. Tuning and thresholds improved along different exponential time courses that varied with CF. For units with CFs below 1 kHz, tuning curve slopes below CF matured earliest, followed by CF threshold, and then by slopes above CF. In contrast, for CFs above 1 kHz, the high-frequency slopes matured first, followed by threshold and then by slope below CF. One interpretation of these results is that tuning and thresholds are not tightly coupled in immature animals. Paradoxically, however, high-frequency slopes were correlated with threshold for individual units at all ages, suggesting that the relationship between tuning and threshold is maintained during development. This contradiction can be resolved by a developmental model that features a functional separation between cochlear nonlinearities and mechanical/electrical conversion.

Ontogeny of vocal signals in the little brown bat, Myotis lucifugus

This study reports developmental changes in the vocal signals produced by wild-caught pre-volant and volant Myotis lucifugusAudio recordings were made from young animals (1-33 days old) and adults (over 1 year of age). The animals were removed from an attic maternity roost and studied individually in a room below. To stimulate flight-related behaviours, animals were released from a launching platform via a hinged floor, and their vocalizations were recorded as they approached a soft foam pad below. When the hinged floor opened, the youngest animals (1-4 days old) typically dropped onto the pad, but older animals either flapped their wings to achieve some horizontal displacement (>4 days) or sustained horizontal flight (>17 days). Vocalizations recorded under these conditions showed frequency modulation characteristic of adult echolocation sounds, even in animals as young as 4 days. Ontogenetic trends showed an increase in sound frequency, an increase in sound repetition rate and a decrease in sound duration as the animals matured. These data are discussed in the context of the development of echolocation behaviour in bats.

Modification of tonotopic representation in the auditory system during development

During the early development of the bird and the mammalian peripheral auditory system, a restricted range of low--mid frequencies is recorded in immature animals. These early recordings are correlated to the base or mid-basal region of the cochlea which codes high frequencies in the adult. In order to reconcile the functional observations with anatomical ones, two main hypotheses have been put forward: one called the development of the place principle derived from observations of acoustic trauma in chick cochlea and a second derived from auditory nerve fiber recordings in kittens. Whatever the theories, the tonotopic shift during development is a well-established phenomenon in both birds and mammals that could be explained by a synthetic theory including active and passive cochlear processes. The tonotopic shift observed in the central auditory system mimics quite closely the frequency representation of the peripheral auditory system. The same trend is observed in all auditory nuclei including the cortex, except that the frequency representation is more complex because it shows tonotopic maps that can be twisted in three dimensions. From current observations, there is a simultaneous onset of tonotopic maps across auditory nuclei up to the cortex. A hypothesis is presented related to the frequency changes observed in the cochlea that affect the central auditory pathway, along with possible consequences on auditory behavior.

Postmetamorphic changes in auditory sensitivity of the bullfrog midbrain

During metamorphosis, the lateral line system of ranid frogs (Rana catesbeiana) degenerates and an auditory system sensitive to airborne sounds develops. We examined the onset of function and developmental changes in the central auditory system by recording multi-unit activity from the principal nucleus of the torus semicircularis (TSp) of bullfrogs at different postmetamorphic stages in response to tympanically-presented auditory stimuli. No responses were recorded to stimuli of up to 95 dB SPL from late-metamorphic tadpoles, but auditory responses were recorded within 24 hours of completion of metamorphosis. Audiograms from froglets (SVL < 5.5 cm) were relatively flat in shape with high thresholds, and showed a decrease in most sensitive frequency (MSF) from about 2500 Hz to about 1500 Hz throughout the first 7-10 days after completion of metamorphosis. Audiograms from frogs larger than 5.5 cm showed continuous downward shifts in MSF and thresholds, and increases in sharpness around MSF until reaching adult-like values. Spontaneous activity in the TSp increased throughout postmetamorphic development. The torus increased in volume by approximately 50% throughout development and displayed changes in cell density and nuclear organization. These observations suggest that the onset of sensitivity to tympanically presented airborne sounds is limited by peripheral, rather than central, auditory maturation.

GABAergic terminals in nucleus magnocellularis and laminaris originate from the superior olivary nucleus

The auditory brainstem nuclei, angularis (NA), magnocellularis (NM), and laminaris (NL) of the chicken, Gallus, contain terminals that stain for antibodies against the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA). Some of these terminals originate from cells surrounding nucleus magnocellularis. Results from this study indicate that the majority of the GABAergic terminals found in NA, NM and NL originate from the superior olivary nucleus (SON). Injections of cholera toxin and horseradish peroxidase show that superior olivary nucleus (SON) neurons, which respond to pure tones, project bilaterally to NA, NM, and NL. NA and NL are reciprocally connected with the SON. More NA cells project to the SON than NL cells. While SON neurons project to NM, NM neurons do not project axons back to the SON. The configuration of SON terminals in NA, NM and NL matches the pattern of GABA-immunoreactive puncta seen in these three nuclei: they surround individual NM cells, congregate in the dendritic neuropil of NL, and blanket the NA. The data indicate that NA, NM and NL may be affected by two different inhibitory cell types: local interneurons and SON neurons. Patterns of connectivity described in this report suggest that the activity of NA cells could influence NM and NL cell physiology. Specifically, increases in NA cell activity could augment the effects of GABAergic SON neurons on NM and NL. Hence, binaural perception in the chicken may be more dependent upon changes in intensity cues than previously believed.

Development of tonotopy in the inferior colliculus II: 2-DG measurements in the kitten

The development of size and tonotopy in the inferior colliculus of the kitten was studied using the [14C]2-deoxyglucose technique and tone stimulation with 2 and 15 kHz at a maximum 110 dB sound pressure level. At 2 days of age, frequency-specific labelling cannot be detected. Two kilohertz labelling is distinctly visible in the rostral and central inferior colliculus at day 6; 15 kHz labelling occurs first at day 11. In the rostral and central inferior colliculus, 2 kHz labelling starts at a ventral and central position and shifts dorsalwards and to a more lateral location between postnatal days 6 and 21. Such a shift is not seen in the caudal inferior colliculus. There, the focus of 2 kHz labelling remains rather constant; only the extension of the labelling increases in the older animals. In all parts of the inferior colliculus, 15 kHz labelling starts at a ventromedial position and shifts to a more lateral location while extending also more dorsalwards as the age increases. These changes in 15 kHz labelling continue up to 3 months. In addition to the ventromedial-to-dorsolateral shift and expansion of labelling, there is also a rostral-to-caudal gradient of maturation, in that in older animals frequency-specific labelling reaches farther caudalwards. The reported changes in frequency representation in the inferior colliculus can be explained on the basis of a shift in frequency input and input sensitivity to the laminae of the inferior colliculus, mainly due to maturational changes within the cochlea and/or as a consequence of the increasing size of the inferior colliculus.

The postnatal development of frequency-place code and tuning characteristics in the auditory midbrain of the phyllostomid bat, Carollia perspicillata

This report describes the postnatal development of hearing range, auditory sensitivity and tonotopy within the inferior colliculus (IC) of a mammal specialized for ultrasonic hearing. The experimental animal, Carollia perspicillata, has an adult hearing range of 7-110 kHz (characteristic frequencies) but lack any significant overrepresentation of a limited frequency band as known for rhinolophoid bats and Pteronotus. The audiogram of the newborn Carollia includes characteristic frequencies from 8 to 76 kHz, which is about 65% of the adult hearing range. As in adults, low frequencies are represented in the dorsolateral portion of the IC. However, at birth the ventromedial IC is non-responsive to acoustic stimulation up to intensities of 90 dB SPL. During development there is a progressive conversion of non-responsive IC areas into acoustically responsive slabs with characteristic frequencies above 76 kHz along the dorsolateral to ventromedial (low-to-high frequency) IC axis. This development is superimposed by a non-uniform shift of characteristic frequency: a decrease of CFs in dorsolateral regions, and an increase of CFs in ventromedial areas. The results suggest a bidirectional shift of frequency representation along the cochlear tonotopic axis.

Postnatal development of central auditory frequency maps

In the early postnatal period of many mammals and in the perihatching period of chicks the auditory ranges are restricted to the species-specific low- and mid-frequency ranges. During subsequent development, the high frequency hearing expands (depending on the species) by 1-4 octaves. Adult-like audition is established between the 4th and the 7th week. It is still discussed controversially, how the extension of the auditory ranges relates to the maturation of orderly frequency representation in the cochleae of the respective species. The present review summarizes investigations of the development of tonotopy in nuclei of the central auditory system, and discusses how the centrally acquired data might contribute to the understanding of the maturation of cochlear stimulus transduction and to the development of frequency maps.

Developmental changes of frequency representation in the rat cochlea

The place-frequency map of the developing rat cochlea was measured by iontophoretic HRP-injections into the ventral cochlear nucleus at electrophysiologically characterized positions. Distribution of retrograde HRP transport in cochlear spiral ganglion cells was analysed by means of a three dimensional reconstruction of the cochlea. Cochlear place-frequency maps were derived in rats of two ages groups: 13 to 22 days, and 36/37 day old animals. These maps were compared with the place-frequency map of adult rats (Müller, 1991). No systematic difference in the place-frequency map between 36/37 day old and adult rats was observed. In animals of the younger age group the place-frequency map (for frequencies above 4 kHz) was shifted towards lower frequencies for a given place along the basilar membrane. The morphological and physiological basis for this frequency shift during development is discussed.

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

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-medio-caudal 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.

Audiovocal interactions during development? Vocalisation in deafened young horseshoe bats vs. audition in vocalisation-impaired bats

Horseshoe bats (Rhinolophus rouxi) were deafened in their 3rd-5th postnatal week. Subsequently their vocalisations were monitored to evaluate the impact of audition on the development of echolocation pulses. Hearing impairment affected the echolocation pulses as follows: the frequency of the constant frequency (CF) component was altered by between +4 kHz and -14 kHz, and the dominance of the second harmonic of the pulses was neutralised by a relative increase in intensity of the first and third harmonics. A second experiment focused on possible influences of acoustical self-stimulation with echolocation pulses on the establishment of auditory fovea representation in the inferior colliculus (IC). Frequency control of echolocation pulses was disrupted by larynx denervation. Thereafter, the bats produced multiharmonic echolocation signals (4-11 harmonics) varying in frequency. IC tonotopy, however, as monitored by stereotaxic electrophysiology, showed the same developmental dynamics as seen in control specimens (Fig. 10). Both experiments indicate that throughout postnatal development echolocation pulses are under auditory feedback control, whereas maturation of the auditory fovea and shifts in its frequency tuning represent an innate process. The significance of this postnatal development might be the adjustment of the vocal motor system of each bat to the frequency of its 'personal' auditory fovea.