Search

The plainfin midshipman fish, Porichthys notatus, is a nocturnal marine teleost that uses social acoustic signals for communication during the breeding season. Nesting type I males produce multiharmonic advertisement calls by contracting their swim bladder sonic muscles to attract females for courtship and spawning while subsequently attracting cuckholding type II males. Here, we report intra- and intersexual dimorphisms of the swim bladder in a vocal teleost fish and detail the swim bladder dimorphisms in the three sexual phenotypes (females, type I and II males) of plainfin midshipman fish. Micro-computerized tomography revealed that females and type II males have prominent, horn-like rostral swim bladder extensions that project toward the inner ear end organs (saccule, lagena, and utricle). The rostral swim bladder extensions were longer, and the distance between these swim bladder extensions and each inner-ear end organ type was significantly shorter in both females and type II males compared to that in type I males. Our results revealed that the normalized swim bladder length of females and type II males was longer than that in type I males while there was no difference in normalized swim bladder width among the three sexual phenotypes. We predict that these intrasexual and intersexual differences in swim bladder morphology among midshipman sexual phenotypes will afford greater sound pressure sensitivity and higher frequency detection in females and type II males and facilitate the detection and localization of conspecifics in shallow water environments, like those in which midshipman breed and nest.

Introduction to the Cocktail Party.- Auditory Object Formation and Selection.- Spatial Mechanisms for Scene Analysis.- Informational masking and masking release.- Models of Stream Segregation.- Spatial Stream Segregation in the Auditory Cortex.- Neurophysiology and Neuroimaging of Auditory Stream Segregation in Humans.- Development.- Aging.- Cochlear implants and hearing aids.

Previous work on auditory processing in Opsanus tau has focused on the descending octaval nucleus; however, the magnocellular octaval nucleus receives similar inputs from the otolithic endorgans. The purpose of this study was to assess whether cells in any of the three subdivisions of the magnocellular nucleus respond to auditory frequencies and encode sound source direction. Extracellular recording sites were chosen based on anatomical landmarks, and neurobiotin injections confirmed the location of auditory sites in subdivisions of the magnocellular nucleus. In general, the auditory cells in M2 and M3 responded best to frequencies at or below 100 Hz. Most auditory cells responded well to directional stimuli presented along axes in the horizontal plane. Cells in M3 (not M2) also responded to lateral line stimulation, consistent with otolithic endorgan and lateral line inputs to M3. The convergence of auditory and lateral line inputs in M3, the lack of Mauthner cells in this species, and previous evidence that the magnocellular nucleus does not contribute to ascending auditory pathways suggest to us that the large cells of M3 may play a role in rapid behavioral responses to particle motion stimuli in oyster toadfish.

Researchers often perform hearing studies on fish in small tanks. The acoustic field in such a tank is considerably different from the acoustic field that occurs in the animal’s natural environment. The significance of these differences is magnified by the nature of the fish’s auditory system where either acoustic pressure (a scalar), acoustic particle velocity (a vector), or both may serve as the stimulus. It is essential for the underwater acoustician to understand the acoustics of small tanks to be able to carry out valid auditory research in the laboratory and to properly compare and interpret the results of others.

Georg von Békésy was one of the first comparative auditory researchers. He not only studied basilar membrane (BM) movements in a range of mammals of widely different sizes, he also worked on the chicken basilar papilla and the frog middle ear. We show that, in mammals, at least, his data do not differ from those that could be collected using modern techniques but with the same, very loud sounds. There is in all cases a major difference to frequency maps collected using low-level sounds. In contrast, the same cannot be said of his chicken data, perhaps due to the different roles played by the BM in mammals and birds. In lizards, the BM is not tuned and it is perhaps good that Békésy did not begin with those species and get discouraged in his seminal comparative work.

The lake sturgeon, Acipenser fulvescens, belongs to one of the few extant nonteleost ray-finned fishes and diverged from the main vertebrate lineage about 250 million years ago. The aim of this study was to use this species to explore the peripheral neural coding strategies for sound direction and compare these results to modern bony fishes (teleosts). Extracellular recordings were made from afferent neurons innervating the saccule and lagena of the inner ear while the fish was stimulated using a shaker system. Afferents were highly directional and strongly phase locked to the stimulus. Directional response profiles resembled cosine functions, and directional preferences occurred at a wide range of stimulus intensities (spanning at least 60 dB re 1 nm displacement). Seventy-six percent of afferents were directionally selective for stimuli in the vertical plane near 90° (up down) and did not respond to horizontal stimulation. Sixty-two percent of afferents responsive to horizontal stimulation had their best axis in azimuths near 0° (front back). These findings suggest that in the lake sturgeon, in contrast to teleosts, the saccule and lagena may convey more limited information about the direction of a sound source, raising the possibility that this species uses a different mechanism for localizing sound. For azimuth, a mechanism could involve the utricle or perhaps the computation of arrival time differences. For elevation, behavioral strategies such as directing the head to maximize input to the area of best sensitivity may be used. Alternatively, the lake sturgeon may have a more limited ability for sound source localization compared with teleosts.

SUMMARY Acipenser fulvescens, the lake sturgeon, belongs to one of the few extant non-teleost ray-finned (bony) fishes. The sturgeons (family Acipenseridae) have a phylogenetic history that dates back about 250 million years. The study reported here is the first investigation of peripheral coding strategies for spectral analysis in the auditory system in a non-teleost bony fish. We used a shaker system to simulate the particle motion component of sound during electrophysiological recordings of isolated single units from the eighth nerve innervating the saccule and lagena. Background activity and response characteristics of saccular and lagenar afferents (such as thresholds, response–level functions and temporal firing) resembled the ones found in teleosts. The distribution of best frequencies also resembled data in teleosts (except for Carassius auratus, goldfish) tested with the same stimulation method. The saccule and lagena in A. fulvescens contain otoconia, in contrast to the solid otoliths found in teleosts, however, this difference in otolith structure did not appear to affect threshold, frequency tuning, intensity- or temporal responses of auditory afferents. In general, the physiological characteristics common to A. fulvescens, teleosts and land vertebrates reflect important functions of the auditory system that may have been conserved throughout the evolution of vertebrates.

Binaural computations involving the convergence of excitatory and inhibitory inputs have been proposed to explain directional sharpening and frequency tuning documented in the brainstem of a teleost fish, the oyster toadfish (Opsanus tau). To assess the presence of inhibitory neurons in the ascending auditory circuit, we used a monoclonal antibody to GABA to evaluate immunoreactivity at three levels of the circuit: the first order descending octaval nucleus (DON), the secondary octaval population (dorsal division), and the midbrain torus semicircularis. We observed a subset of immunoreactive (IR) cells and puncta distributed throughout the neuropil at all three locations. To assess whether contralateral inhibition is present, fluorescent dextran crystals were inserted into dorsal DON to fill contralateral, commissural inputs retrogradely prior to GABA immunohistochemistry. GABA-IR somata and puncta co-occurred with retrogradely filled, GABA-negative auditory projection cells. GABA-IR projection cells were more common in the dorsolateral DON than in the dorsomedial DON, but GABA-IR puncta were common in both dorsolateral and dorsomedial divisions. Our findings demonstrate that GABA is present in the ascending auditory circuit in the brainstem of the toadfish, indicating that GABA-mediated inhibition participates in shaping auditory response characteristics in a teleost fish as in other vertebrates.

SUMMARYComparisons of left and right auditory input are required for sound source localization in most terrestrial vertebrates. Previous physiological and neuroanatomical studies have indicated that binaural convergence is present in the ascending auditory system of the toadfish. In this study, we introduce a new technique, otolith tipping, to reversibly alter directional auditory input to the central nervous system of a fish. The normal directional response pattern (DRP) was recorded extracellularly for auditory cells in the first-order descending octaval nucleus (DON) or the midbrain torus semicircularis (TS) using particle motion stimuli in the horizontal and mid-sagittal planes. The same stimuli were used during tipping of the saccular otolith to evaluate changes in the DRPs. Post-tipping DRPs were generated and compared with the pre-tipping DRPs to ensure that the data had been collected consistently from the same unit. In the DON, ipsilateral or contralateral tipping most often eliminated spike activity, but changes in spike rate(±) and DRP shape were also documented. In the TS, tipping most often caused a change in spike rate (±) and altered the shape or best axis of the DRP. The data indicate that there are complex interactions of excitatory and inhibitory inputs in the DON and TS resulting from the convergence of binaural inputs. As in anurans, but unlike other terrestrial vertebrates,binaural processing associated with encoding the direction of a sound source begins in the first-order auditory nucleus of this teleost.

SUMMARY Zebrafish (Danio rerio) were placed in small wells that could be driven vertically with a series of calibrated sinusoids. Video images of the fish were obtained and analyzed to determine the levels and frequencies at which the fish responded to the stimulus tones. It was found that fish 4 days post fertilization (dpf) did not respond to the stimulus tones, whereas fish 5 dpf to adult did respond. It was further found that the stimulus thresholds and frequency bandwidth to which the fish responded did not change from 5 dpf to adult; indicating that the otolithic organ adaptations for high-frequency hearing are already present in larval fish. Deflating the swimbladders in adult fish eliminated their response, which is consistent with sensing sound pressure. Deflating the swimbladder in larval fish did not affect their thresholds, which is consistent with sensing the particle motion of the fluid directly. Because adult fish with Weberian ossicles have a greater input to the inner ear for a given sound pressure level (SPL), the finding that the adult and larval fish respond at the same SPL with intact swimbladders suggests that the acoustic startle response threshold is adjusted as the fish develop in order to maintain appropriate reactions to relevant stimuli.

Bimodal cells in the torus semicircularis of the toadfish respond to both directional acoustic stimuli and hydrodynamic stimuli. Our previous physiological work indicated that bimodal cells may be distributed throughout the torus semicircularis. In this study, neurobiotin was used to compare the distribution of auditory-only and bimodal sites and to assess the inputs to those sites. A brief neurobiotin injection with short survival time was used to document the recording location. In other fish, a longer injection and survival time was used at an auditory-only or a bimodal site to fill the axons of the medullary inputs. Auditory-only sites were located in the most dorsal and medial sites in nucleus centralis. Bimodal sites were identified within both nucleus centralis and nucleus ventrolateralis. The greatest number of retrogradely filled cell bodies was found in the descending octaval nucleus following injection at auditory-only recording sites in nucleus centralis. In contrast, retrogradely filled cell bodies were found in both the descending octaval nucleus and the lateral line nucleus medialis following injection at bimodal sites in nucleus centralis or nucleus ventrolateralis. Retrogradely filled cell bodies were located in the dorsal and ventral divisions of the secondary octaval population from injections at either bimodal or auditory-only sites. The secondary octaval population has been implicated in auditory processing based on previous studies of both auditory specialist and auditory generalist fishes; however, this study is the first to reveal the potential role of the secondary octaval population in directional hearing in a fish. Relatively large numbers of retrogradely filled cells around the lateral lemniscus at consistent locations in the medulla indicate that a perilemniscal cell group also might be a component of the directional hearing circuit.

Single-unit recordings were made from areas in the midbrain (torus semicircularis) of the oyster toadfish. We evaluated frequency tuning and directional responses using whole-body oscillation to simulate auditory stimulation by particle motion along axes in the horizontal and mid-sagittal planes. We also tested for bimodality in responses to auditory and hydrodynamic stimuli. One recording location in each animal was marked by a neurobiotin injection to confirm the recording site. Recordings were made in nucleus centralis, nucleus ventrolateralis, and the deep cell layer. Most units were frequency-selective with best frequencies between 50 and 141 Hz. Suppression of activity was apparent in 10% of the cells. Bimodality was common, including inhibition and suppression of background activity by auditory or hydrodynamic stimulation. The majority of the cells were directionally selective with directional response patterns that were sharpened compared with those of primary saccular afferents. The best directional axes were arrayed widely in spherical space, covering most azimuths and elevations. This representation is adequate for the computation of the motional axis of an auditory stimulus for sound source localization.

While particle motion is thought to directly stimulate the inner ear of most fish species, it is difficult to measure and might not be predictable from pressure measurements in a small tank. It is therefore important to replicate experiments conducted relative to pressure measurements using stimuli of known particle motion, to ensure that unmeasured components of the stimulus field do not produce misleading frequency response profiles. The frequency sensitivity of the inner ear of the plainfin midshipman fish, Porichthys notatus, in response to isopressure stimuli has been described. This study now examines the frequency and directional response properties of midshipman saccular afferents in response to whole-body displacements simulating acoustic particle motion. Best frequencies were distributed bimodally, with peaks at 50 Hz and 100 Hz. Most units had cosinusoidally shaped directional response profiles in the horizontal and vertical planes, though some units showed slight deviations from this pattern. A few units (probably saccular efferents) had omnidirectional directional response profiles and did not phase lock to the stimulus waveform. These results are consistent with responses of the midshipman saccular nerve to isopressure stimuli, and strengthen the hypothesis that the frequency sensitivity of the midshipman ear matches the frequency content of behaviorally relevant vocalizations.

The neurocranium of the toadfish (Opsanus tau) exhibits a distinct translucent region in the otic capsule (OC) that may have functional significance for the auditory pathway. This study used ultrahigh resolution computerized tomography (100 µm voxels) to compare the relative density of three sites along the OC (dorsolateral, midlateral, and ventromedial) and two reference sites (dorsal: supraoccipital crest; ventral: parasphenoid bone) in the neurocranium. Higher attenuation occurs where structural density is greater; thus, we compared the X-ray attenuations measured, which provided a measure of relative density. The maximum attenuation value was recorded for each of the five sites (x and y) on consecutive sections throughout the OC and for each of the three calcareous otoliths associated with the sensory maculae (lagena, saccule, and utricle) in the OC. All three otoliths had higher attenuations than any sites in the neurocranium. Both dorsal and ventral reference sites (supraoccipital crest and parasphenoid bone, respectively) had attenuation levels consistent with calcified bone and had relatively small, irregular variations along the length of the OC in all individuals. The lowest relative attenuations (lowest densities) occurred consistently at the three sites along the OC. In addition, the lowest attenuations measured along the OC occurred at the ventromedial site around the saccular otolith for all seven fish. The decrease in bone density along the OC is consistent with the hypothesis that there is a low-density channel in the skull to facilitate transmission of acoustic stimuli to the auditory endorgans of the ear.

Chapters cannot be read stand-alone. Please see complete SpringerBrief at: http://link.springer.com/book/10.1007/978-3-319-06659-2.

A Brief History of SHAR.- Structures, Mechanisms and Energetics in Temporal Processing.- The Human Auditory Cortex: In Search of the Flying Dutchman.- From Cajal to the Connectome: Building a Neuroanatomical Framework for Understanding the Auditory System.- Recording from Hair Cells.- Three Decades of Tinnitus Related Research.- The Sense of Hearing in Fishes.- A Quarter-Century's Perspective on a Psychoacoustical Approach to Loudness.- Nonsyndromic Deafness: It Ain't Necessarily So.- Evolving Mechanosensory Hair Cells to Hearing Organs by Altering Genes and Their Expression.- The Implications of Discharge Regularity-My Forty-Year Peek into the Vestibular System.- Aging, Hearing Loss and Speech Recognition: Stop Shouting, I Can't Understand You.- Cochlear Mechanics, Otoacoustic Emissions and Medial Olivocochlear Efferents: 20 Years of Advances and Controversies Along with Areas Ripe for New Work.- Examining Fish in the Sea: A European Perspective on Fish Hearing Experiments.- The Behavioral Study of Mammalian Hearing.- Hearing in Insects: The Why, When and How.- The Cognitive Auditory System.- Fundamentals of Hearing in Amniote Vertebrate.- Directional Hearing in Insects and Other Small Animals: The Physics of Pressure-Difference Receiving Ears.- Cortical Representation of Sound Locations: Distributed Representation by Magnitude and Timing of Neural Spike Patterns.- Mechanisms Underlying the Pitch of Pure and Complex Tones.- Unavoidably Delayed: A Personal Perspective of Twenty Years of Research on a Sound Localization Cue.- Size Matters in Hearing: How the Auditory System Normalizes the Sounds of Speech and Music for Source Size.- A Changing View of the Auditory System Obtained from the Ears of Bats.- From Cave Fish to Pile Driving: A Tail of Fish Bioacoustics.- Current Topics in the Study of Sound Conduction to the Inner Ear.- From Degenerative Debris to Neuronal Tracing: An Anterograde View of Auditory Circuits.- Adventures in Bionic Hearing.- My Dull Deaf Ears: Four Millennia of Acquired Hearing Loss.- What's the Use of Genetics?.- Advances in the Understanding of Binaural Information Processing: Consideration of the Stimulus as Processed.- Temporal Processing: Observations on the Psychophysics and Modeling of Temporal Integration and Temporal Resolution.- Psychoacoustics and Auditory Perception.- APPENDIX: Table of Contents from SHAR volumes 1-49.

This paper considers aspects of the evolution of the vertebrate auditory system from an 'ichthyocentric' perspective. It is argued that all vertebrate auditory systems are required to do certain basic tasks including acoustic feature discrimination, sound source localization, frequency analysis, and auditory scene analysis, among others. These sorts of capabilities arose very early in the evolution of the vertebrates and have been modified by selection in different species. In some cases the same structures have been involved in detection and analysis throughout the vertebrates, while in other cases the mechanism by which the same type of analysis takes place may have changed.

This paper reviews and discusses several investigations of the peripheral neural code for the directional axis of acoustical particle motion in the saccule of two fishes: goldfish ( Carassius auratus ) and toadfish ( Opsanus tau ). Most saccular afferents are directional in the manner of hair cells, having a cosine–shaped directional response pattern. The saccular sensory epithelia are orientated almost vertically in a parasagittal plane. In the horizontal plane, these epithelia are orientated obliquely with respect to the midline. Hair–cell stereocilia project perpendicularly. Thus, directional response patterns of saccular afferents tend to be orientated in azimuth parallel to the orientation of the epithelia in the head. The oblique angle of the toadfish saccule is greater than that of the goldfish, and the range of best directions in the horizontal plane for each species reflects those differing orientations. The azimuth of acoustical particle motion could be computed by comparing the relative activation of the two saccules, as is the case for the ears of most terrestrial vertebrates. The spatial patterns of saccular hair–cell orientation of most fishes thus appear to have little function in azimuthal source location, but for toadfish are probably most important for determining the elevation of monopole sources.

Neurobiotin was injected iontophoretically into saccular afferents of toadfish (Opsanus tau) after intracellular recording to examine dendritic arbors and central projections with respect to the physiological and directional response properties of the cells. Dendritic arbors of 36 afferents were examined in detail. Maximum diameter of the arbor and the number of terminal points were positively correlated with each other, but neither was predictive of spontaneous activity or sensitivity. Best azimuths were centered around 30 degrees -40 degrees, which corresponds to the angle of the saccule with respect to the fish's midline. In general, best elevations for afferents corresponded to hair cell orientations in the region innervated; unexpectedly low elevations obtained from afferents innervating the middle saccule may reflect curvature of the sensory epithelium against the otolith. Three efferent cells were filled partially. The location and large size of the efferent projections indicate that activity along the saccule could be modulated by a single efferent. All afferents projected to the dorsal zone of the descending octaval nucleus (dDON); many afferents bifurcated to terminate in the anterior octaval nucleus, and a few of those also had terminal fields in the medial zone of DON. All afferent projections into the dDON consisted of multiple axon collaterals projecting to numerous sites along the rostral-caudal extent of the nucleus. Variation in terminal field sites also was noted in the medial to lateral axis of the dDON; however, there were no consistent correlations between terminal field locations, physiology, and best directions of the saccular afferents.

Turtles, like other amphibious animals, face a trade-off between terrestrial and aquatic hearing. We used laser vibrometry and auditory brainstem responses to measure their sensitivity to vibration stimuli and to airborne versus underwater sound. Turtles are most sensitive to sound underwater, and their sensitivity depends on the large middle ear, which has a compliant tympanic disc attached to the columella. Behind the disc, the middle ear is a large air-filled cavity with a volume of approximately 0.5 ml and a resonance frequency of approximately 500 Hz underwater. Laser vibrometry measurements underwater showed peak vibrations at 500–600 Hz with a maximum of 300 µm s−1Pa−1, approximately 100 times more than the surrounding water. In air, the auditory brainstem response audiogram showed a best sensitivity to sound of 300–500 Hz. Audiograms before and after removing the skin covering reveal that the cartilaginous tympanic disc shows unchanged sensitivity, indicating that the tympanic disc, and not the overlying skin, is the key sound receiver. If air and water thresholds are compared in terms of sound intensity, thresholds in water are approximately 20–30 dB lower than in air. Therefore, this tympanic ear is specialized for underwater hearing, most probably because sound-induced pulsations of the air in the middle ear cavity drive the tympanic disc.

Introduction and overview.- Neuronal response properties and voltage-gated ion channels.- The hair cell synapse.- The endbulbs of Held.- The calyces of Held.- Synaptic coincidence detection.- Synaptic inhibition in the auditory system.- Modulatory mechanisms for controlling auditory processing.- Long-term plasticity in the auditory system.

Overview.- Olivocochlear Efferents: Structure of the medial and lateral olivocochlear systems.- Olivocochlear Efferents: Physiology of the medial and lateral olivocochlear systems.- Olivocochlear Efferents: Chemistry of the medial and lateral olivocochlear systems.- Cholinergic inhibition of hair cells in the inner ear.- Vestibular Efferents.- Development of efferent systems.- Exuberant efferent innervation of the auditory receptors.- Comparative studies of the efferent system.- Descending projections.- Efferent modulation of brainstem activity.- Efferent effects on the central auditory system.- Index.

SUMMARYIn goldfish and other otophysans, the Weberian ossicles mechanically link the saccule of the inner ear to the anterior swimbladder chamber (ASB). These structures are correlated with enhanced sound-pressure sensitivity and greater sensitivity at high frequencies (600–2000 Hz). However, surprisingly little is known about the potential impact of the ASB on other otolithic organs and about how auditory responses are modulated by discrete sources that change their location or orientation with respect to the ASB. In this study, saccular and lagenar nerve fiber responses and conditioned behaviors of goldfish were measured to a small, low-frequency (50 Hz) vibrating sphere (dipole) source as a function of its location along the body and its orientation with respect to the ASB. Conditioned behaviors and saccular nerve fiber activity exhibited response characteristics nearly identical to those measured from a hydrophone in the same relative position as the ASB. By contrast, response patterns from lagena fibers could not be predicted by pressure inputs to the ASB. Deflation of the ASB abolished the characteristic spatial response pattern of saccular but not lagena fibers. These results show that: (1) the lagena is not driven by ASB-mediated pressure inputs to the ear; (2) the ASB–saccule pathway dominates behavioral responsiveness, operating effectively at frequencies as low as 50 Hz; and (3) behavioral and neural (saccular) responses are strongly modulated by the position and orientation of the dipole with respect to the ASB.

In this paper we reconsider the designation of fishes as being either "hearing specialists" or "hearing generalists," and recommend dropping the terms. We argue that this classification is only vaguely and variously defined in the literature, and that these terms often have unclear and different meaning to different investigators. Furthermore, we make the argument that the ancestral, and most common, mode of hearing in fishes involves sensitivity to acoustic particle motion via direct inertial stimulation of the otolith organ(s). Moreover, any possible pressure sensitivity is the result of the presence of an air bubble (e.g., the swim bladder), and that hearing sensitivity may be enhanced by the fish having a specific connection between the inner ear to a bubble of air. There are data showing that some fish species have a sensitivity to both pressure and motion that is frequency dependent. Thus such species could not possibly be termed as either hearing "generalists" or specialists," and many more species probably could be classified in this way as well. Furthermore, we propose that the term "specialization" be reserved for cases in which a species has some kind of morphological connection or close continuity between the inner ear and an air bubble that affects behavioral sensitivity to sound pressure (i.e., an otophysic connection).

This study is a continuation of a long-term investigation of the auditory circuit in the oyster toadfish, Opsanus tau. Input from the auditory periphery projects to the ipsilateral descending octaval nucleus (DON). Ipsilateral and contralateral DONs project to the auditory midbrain, where a previous study indicated that both frequency tuning and directional sharpening are present. To better understand the transformation of auditory information along the auditory pathway, we have examined over 400 units in the DON to characterize frequency and directional information encoded in the dorsolateral division of the nucleus. Background activity was primarily low (

This chapter, written from the perspective of four authors who have been studying fish bioacoustics for over 120 years (cumulative!), examines the major issues of the field. Each topic is put in some historical perspective, but the chapter emphasizes current thinking about acoustic communication, hearing (including bandwidth, sensitivity, detection of signals in noise, discrimination, and sound source localization), the functions of the ear (both auditory and vestibular, and including the role(s) of the otoliths and sensory hair cells) and their relationships to peripheral structures such as the swim bladder, and the interactions between the ear and the lateral line. Hearing in fishes is not only for acoustic communication and detection of sound-emitting predators and prey but can also play a major role in telling fishes about the acoustic scene at distances well beyond the range of vision. The chapter concludes with the personal views of the authors as to the major challenges and questions for future study. There are still many gaps in our knowledge of fish bioacoustics, including questions on ear function and the significance of interspecific differences in otolith size and shape and hair cell orientation, the role of the lateral line vis-a-vis the ear, the mechanisms of central processing of acoustic (and lateral line) signals, the mechanisms of sound source localization and whether fishes can determine source distance as well as direction, the evolution and functional significance of hearing specializations in taxonomically diverse fish species, and the origins of fish (and vertebrate) hearing and hearing organs.

Otoacoustic Emissions: Concepts and Origins.- Traveling Waves, Second Filters, and Physiological Vulnerability: A Short History of the Discovery of Active Processes in Hearing.- Critical Oscillators as Active Elements in Hearing.- Active Hair-Bundle Motility of the Hair Cells of Vestibular and Auditory Organs.- The Morphological Specializations and Electromotility of the Mammalian Outer Hair Cell.- Active Processes in Insect Hearing.- Otoacoustic Emissions in Amphibians, Lepidosaurs, and Archosaurs.- Otoacoustic Emissions: Basic Studies in Mammalian Models.- Mechanisms of Mammalian Otoacoustic Emission.- Cellular and Molecular Mechanisms in the Efferent Control of Cochlear Nonlinearities.- Cochlear Models Incorporating Active Processes.- Relationships Between Otoacoustic and Psychophysical Measures of Cochlear Function.- Otoacoustic Emissions as a Diagnostic Tool in a Clinical Context.- Future Directions in the Study of Active Processes and Otoacoustic Emissions.

Overview: Regeneration and Repair.- Morphological Correlates of Regeneration and Repair in the Inner Ear.- Recovery of Function in the Avian Auditory System After Ototrauma.- Functional Recovery After Hair Cell Regeneration in Birds.- Hair Cell Regeneration: Mechanisms Guiding Cellular Proliferation and Differentiation.- Protection and Repair of Inner Ear Sensory Cells.- Gene Arrays, Cell Lines, Stem Cells, and Sensory Regeneration in Mammalian Ears.

The suppression of background spike activity in the absence of deliberate acoustic stimulation occurs in fibers of the goldfish saccular nerve tuned in the region of 250 Hz. Suppression is most robust in the frequency range between 450 and 1050 Hz, the range of CF for the mid- and high-frequency saccular fibers. Suppression of background activity tends to occur following the suppressor tone offset ('off-suppression'), even though the spike response during the suppressor is below the background rate. This suggests that the suppressor tone is excitatory at the level of the hair cells and their synapses onto saccular afferents. Tones at the low- frequency edge of the suppression region may show net excitation at low intensity levels, and net suppression at higher levels. This suggests that the spike response observed is the result of the relative strengths of excitatory and suppressive effects which operate simultaneously. The magnitude and frequency of best suppression tends to increase with stimulus intensity. A suppressing tone produces transient excitation at onset. In fibers with high levels of spontaneous activity, a spike response 'rebound' often occurs 20 to 50 ms following the suppressing tone offset. These 'on' and 'off' effects are not due to energy 'splatter' in the stimulus domain. Suppression by tones can also be observed in non-spontaneous fibers when the background spike activity is evoked by noise. In these cases, however, off-suppression following a suppressed response and the 'rebound' seldom occurs. Possible sites of suppression are the hair cells and their synapses, the spike-initiation zones of the saccular afferents, and efferent inhibition. The most likely site seems to be the spike-initiation zones of saccular afferents. An important consequence of suppression for hearing is the sharpening of frequency response areas for low frequency fibers, and the partial preservation of frequency analysis in saccular fibers stimulated well above threshold.

Vertebrate Hair Cells: Modern and Historic Perspectives.- The Development of Hair Cells in the Inner Ear.- The Structure and Composition of the Stereociliary Bundle of Vertebrate Hair Cells.- Mechanoelectrical Transduction in Auditory Hair Cells.- Contribution of Ionic Currents to Tuning in Auditory Hair Cells.- The Synaptic Physiology of Hair Cells.- The Piezoelectric Outer Hair Cell.- Mammalian Vestibular Hair Cells.

Our previous studies have shown that the peripheral auditory system of the toadfish encodes the direction of a sound source. Here, we compare directional responses of peripheral saccular afferents, cells in the descending octaval nucleus (DON) of the medulla, and the torus semicircularis (TS) of the midbrain. Recording locations in the brain were labeled with neurobiotin to confirm the site. To compare directional responses among cells, we calculated an index [sharpening ratio (SR)] that weights the relative strength of responses to the best direction for that cell and to the adjacent stimulus angles tested. Unsharpened saccular afferents tend to have a cosinusoidal directional response pattern (DRP) with an expected SR of 0.87. In DON, more than 60% of the cells exhibited directional sharpening (defined as SR

The octavolateralis system consists of several submodalities, including the inertial-sensitive inner ear, the pressure-sensitive ear/air cavity complex (when present), and acceleration- and velocity-sensitive components of the lateral line system (canal and superficial neuromasts, respectively). All four of these channels are responsive to many of the same stimulus sources, particularly moving or vibrating objects within a short distance from the receiver. We therefore argue that the octavolateralis system is an excellent model for the study of multisensory interactions. We focus on the possible ways in which these channels may contribute to source localization mechanisms and to the multisensory guidance of behaviors with strong directional components (e.g., predator avoidance, prey capture and mate attraction). Finally, we define four ways in which information from multiple senses might interact. These include fractionation, synergy, accessory stimulation, and complementation. Although evidence for all types of octavolateralis interactions can be found, the primary modes of interaction appear to be complementation and fractionation. For example, the inertial and pressure-sensitive submodalities of the auditory system provide complementary pieces of information about the direction (e.g., left/right) and polarity (advancing or receding) of a moving source. In contrast, the lateral line canal system subserves short-range localization tasks, whereas the auditory system may subserve longer-range detection and localization tasks.

Experiments examined differential coding of acoustic particle motion axis in the auditory midbrain of goldfish. Animals were exposed to vibratory stimuli varying in axis orientation as action potentials were recorded from single units in the central neuropil of nucleus centralis in the torus semicircularis. Response magnitudes as a function of stimulation axis were visualized in three dimensional plots called directional response profiles. These are generally comparable to directional responses observed among primary saccular afferents in having substantially vertical orientations. Distortions in shape from the peripheral patterns indicate neural information processing. A three-dimensional model was used to evaluate the hypothesis that responses in the auditory midbrain reflect the convergence of excitatory and inhibitory primary afferent-like responses. Model afferent inputs were generated and combined arithmetically. This analysis gives insight into the mechanisms of information processing that appear to occur in brainstem nuclei. The lack of diversity in best axis directions suggests that this mechanism alone cannot account for directional hearing abilities in this species. The roles that this directional representation and processing may play in directional hearing and sound source localization are not yet clear. Implications of these data on current models of fish directional hearing are discussed.

Introduction by Donata Oertel * From the cochlea to the cortex and back by Philip H. Smith and George Spirou * Cellular mechanisms for information coding in auditory brain stem nuclei by Laurence Trussell * Detection of interaural time and intensity differences for localization in the binaural pathways through the brainstem by Tom Yin * Circuitry and function of the dorsal cochlear nucleus by Eric D. Young and Kevin A. Davis * Ascending pathways through the ventral nuclei of the lateral lemniscus nuclei and their possible role in pattern recognition of natural sounds by Donata Oertel and Robert E. Wickesberg * The inferior colliculus: a hub for the central auditory system by John H. Casseday, Thane Fremouw, and Ellen Covey * Localizing sound by cortical neurons by John C. Middlebrooks, Li Xu, Shigeto Furukawa, and Brian J. Mickey * Feature detection by the auditory cortex by Israel Nelken

1 Introduction.- 2 The Middle Ear of Reptiles and Birds.- 3 The Hearing Organ of Birds and Crocodilia.- 4 The Hearing Organs of Lizards.- 5 The Central Auditory System of Reptiles and Birds.- 6 Sound Localization in Birds.- 7 Hearing in Birds and Reptiles.

As noted in the Preface to this volume, a major goal of hearing research is to explain how the human auditory system normally functions and to help identify the causes of, and treatments for, hearing impairment. Animal models are used extensively in this research, and valid generalizations from these models are required for progress to be made in understanding the human auditory system. In general, comparative hearing research establishes the biological and evolutionary context within which animal models can be developed, evaluated, validated, and successfully applied, and is therefore of fundamental importance to hearing research. The confidence that allows us to generalize some observations from one species to another arises from comparative research that investigates the structures, physiological functions, and hearing capabilities of various species in order to determine the fundamental principles by which structures determine functions in all vertebrate auditory systems.

Fossil evidence shows that an inner ear is found in the most primitive of jawless vertebrates (Stensio 1927; Jarvick 1980; Long 1995). It may never be known whether these vertebrates actually were able to “hear” or whether the earliest ear may have been only a vestibular organ for the detection of angular and linear accelerations of the head. However, it is not hard to imagine that such a system could have ultimately evolved into a system for detection of somewhat higher frequency sounds during early vertebrate evolution (Van Bergeijk 1967; Poper and Fay 1997). Although some might argue that sound detection would not have evolved until fish, or predators, started to make sounds, this may not be a valid argument. In fact, it is very likely that the earliest role, and still the most general role, for sound detection is to enable a fish to gain information about its environment from the environment’s acoustic signature (Popper and Fay 1993). Such a signature results from the ways a sound field produced by sources such as surface waves, wind, rain, and moving animals is scattered by things like the water surface, bottom, and other objects.

1 Acute as a Bug's Ear: An Informal Discussion of Hearing in Insects.- 2 Biophysics of Sound Localization in Insects.- 3 The Sensory Ecology of Acoustic Communication in Insects.- 4 Development of the Insect Auditory System.- 5 Neural Processing of Acoustic Signals.- 6 The Evolutionary Innovation of Tympanal Hearing in Diptera.- 7 The Vibrational Sense of Spiders.- 8 The Sensory Coevolution of Moths and Bats.

1 Overview: Personal Views on the Study of Auditory System Development.- 2 Behavioral Studies of Hearing Development.- 3 Early Embryology of the Vertebrate Ear.- 4 Development of Sensory and Neural Structures in the Mammalian Cochlea.- 5 The Development of Cochlear Function.- 6 The Development of Central Auditory Processing.- 7 Structural Development of the Mammalian Central Auditory Pathways.

The frequency response of primary saccular afferents of toadfish (Opsanus tau) was studied in the time and frequency domains using the reverse correlation (revcor) method. Stimuli were noise bands with flat acceleration spectra delivered as whole-body motion. The recorded acceleration waveform was averaged over epochs preceding and following each spike. This average, termed the revcor, is an estimate of the response of an equivalent linear filter intervening between body motion and spike initiation. The spectrum of the revcor estimates the shape of the equivalent linear filter. Revcor responses were brief, damped oscillations indicative of relatively broadly tuned filters. Filter shapes were generally band-pass and differed in bandwidth, band edge slope, and characteristic frequency (74 Hz to 140 Hz). Filter shapes tend to be independent of stimulus level. Afferents can be placed into two groups with respect to characteristic frequency (74-88 Hz and 140 Hz). Some high-frequency afferents share a secondary peak at the characteristic frequency of low-frequency afferents, suggesting that an afferent may receive differently tuned peripheral inputs. For some afferents having similar filter shapes, revcor responses often differ only in polarity, probably reflecting inputs from hair cells oriented in opposite directions. The origin of frequency selectivity and its diversity among saccular afferents may arise from a combination of hair cell resonance and micromechanical processes. The resulting frequency analysis is the simplest yet observed among vertebrate animals. During courtship, male toadfish produce the 'boatwhistle' call, a periodic vocalization having several harmonics of a 130 Hz fundamental frequency. The saccule encodes the waveform of acoustic particle acceleration between < 50 and about 250 Hz. Thus, the fundamental frequency component of the boatwhistle is well encoded, but the successive higher harmonics are filtered out. The boatwhistle is thus encoded as a time-domain representation of its fundamental frequency or pulse repetition rate.

The ear appears to have arisen early in the evolution of the vertebrates. While there are significant interspecific differences in ear structure, it appears that receptor cell structure and the basic function of the ear and auditory system are similar among all vertebrate groups. In this paper we present the evolution of the sensory hair cells of the ear, the origins of the ear itself, and selected functions of the sense of hearing. We argue that there have been strong selective pressures in most vertebrate groups for the sorts of sound encoding and processing abilities that result in the efficient detection, localization, and identification of sound sources in noisy environments. Many of the encoding and processing strategies underlying these functions are shared as well.

Overview Development of the vestibular system The molecular biological approach to studies of the vestibular system Biomechanics of the semicircular canals and otolith organs Cellular properties of the vestibular sensory periphery Morphophysiology of the vestibular sensory periphery The vestibular nuclei The angular and linear vestibulo-ocular reflexes, the perception of linear motion, and velocity storage The vestibulo-colic and vestibulo-spinal reflexes The vestibular cerebellum Motor learning in the vestibular system The vestibular control of the autonomic system Clinical applications of vestibular basic research.

Acoustic responses were recorded extracellularly from single neurons in the thalamic central posterior nucleus (CP). Spontaneous activity, best sensitivity, and sharpness of tuning (Q10dB) of CP neurons ranged from 0 to 36 spikes/s, -40 to 5 dB re: 1 dyne/cm2, and 0.18 to 1.80, respectively. The distribution of characteristic frequency (CF) was nonuniform with a mode at 195 Hz. Temporal response patterns of CP neurons (N = 60) were categorized into three groups: phasic (25%), tonic chopper-like (22%), and tonic nonchopper-like (53%) on the basis of peri-stimulus time and inter-spike interval histograms. Most CP neurons (90%) did not phase-lock to tones, and none phase-locked strongly. The properties of CP neurons are similar to those of the midbrain torus semicircularis neurons in spontaneous rates, best sensitivities, nonuniform CF distributions, and in exhibiting level-independent best frequencies. Both CP and toral neurons show a diversity of response patterns resembling those found in the mammalian central auditory system. However, CP neurons have broader tuning and less phase-locking than toral neurons, suggesting different roles in auditory processing. While peripheral frequency analysis is enhanced at the midbrain level, the integration of frequency-selective channels in the thalamus may function in the processing of wideband spectra characteristic of natural sound sources.

1 Hearing in Bats: An Overview.- 2 Natural History and Biosonar Signals.- 3 Behavioral Studies of Auditory Informatio Processing.- 4 Auditory Dimensions of Acoustic Images in Echolocation.- 5 Cochlear Structure and Function in Bats.- 6 The Lower Brainstem Auditory Pathways.- 7 The Inferior Colliculus.- 8 The Auditory Thalamus in Bats.- 9 The Bat Auditory Cortex.- 10 Perspectives on the Functional Organization of the Mammalian Auditory System: Why Bats Are Good Models.

Two features distinguish mammalian hearing from auditory reception in fishes, amphibians, reptiles, and birds. First, the audible frequency range is significantly broader in mammals than in other vertebrate taxa due to the responsiveness of the mammalian ear to higher frequency sounds. Second, when compared with other vertebrates, mammals show considerably more species diversity in particular attributes of hearing such as the lower frequency limit of hearing, the upper frequency range of hearing, and the frequency of maximum sensitivity (Fay 1988). Some mammals, which might be termed hearing generalists, perceive a broad range of sound frequencies and show little variation in threshold sensitivity throughout a large portion of their hearing range. In contrast, other species, which could be called hearing specialists, respond to sounds within a more restricted bandwidth and generally display a greater sensitivity for particular frequencies, often those found within behaviorally relevant acoustic signals.

Although a number of previous behavioral studies have demonstrated that elasmobranch fishes can detect and are attracted to low frequency sounds, few physiological studies have characterized the auditory response properties of the elasmobranch inner ear to such low frequency sounds. In this study, we examined the directional and frequency responses of the inner ear saccule in the little skate, Raja erinacea to low frequency stimuli. Evoked microphonic potentials were recorded from the middle region of the saccule while sound was generated using a shaker table designed to mimic the particle motion vector component of sound. 8 test frequencies (50, 64, 84, 100, 140, 185, 243, and 303 Hz) and 11 directions were used to characterize the displacement sensitivity, frequency response and directional response properties of the skate saccule. Saccular potentials were evoked and measured at twice the stimulus frequency in vivo using a wave analyzer while stimuli were generated via the shaker table system. The right...