Representation of auditory distance within a central neuropil of the bushcricketMygalopsis marki

Local prothoracic auditory neurons in Ensifera

A new method for individually staining insect neurons with metal ions was described in the late 60s, closely followed by the introduction of the first bright fluorescent dye, Lucifer Yellow, for the same purpose. These milestones enabled an unprecedented level of detail regarding the neuronal basis of sensory processes such as hearing. Due to their conspicuous auditory behavior, orthopterans rapidly established themselves as a popular model for studies on hearing (first identified auditory neuron: 1974; first local auditory interneuron: 1977). Although crickets (Ensifera, Gryllidae) surpassed grasshoppers (Caelifera) as the main model taxon, surprisingly few neuronal elements have been described in crickets. More auditory neurons are described for bush crickets (Ensifera, Tettigoniidae), but due to their great biodiversity, the described auditory neurons in bush crickets are scattered over distantly related groups, hence being confounded by potential differences in the neuronal pathways themselves. Our review will outline all local auditory elements described in ensiferans so far. We will focus on one bush cricket species, Ancistrura nigrovittata (Phaneropterinae), which has the so-far highest diversity of identified auditory interneurons within Ensifera. We will present one novel and three previously described local prothoracic auditory neuron classes, comparing their morphology and aspects of sensory processing. Finally, we will hypothesize about their functions and evolutionary connections between ensiferan insects.

Innate releasing mechanisms and fixed action patterns: basic ethological concepts as drivers for neuroethological studies on acoustic communication in Orthoptera

This review addresses the history of neuroethological studies on acoustic communication in insects. One objective is to reveal how basic ethological concepts developed in the 1930s, such as innate releasing mechanisms and fixed action patterns, have influenced the experimental and theoretical approaches to studying acoustic communication systems in Orthopteran insects. The idea of innateness of behaviors has directly fostered the search for central pattern generators that govern the stridulation patterns of crickets, katydids or grasshoppers. A central question pervading 50 years of research is how the essential match between signal features and receiver characteristics has evolved and is maintained during evolution. As in other disciplines, the tight interplay between technological developments and experimental and theoretical advances becomes evident throughout this review. While early neuroethological studies focused primarily on proximate questions such as the implementation of feature detectors or central pattern generators, later the interest shifted more towards ultimate questions. Orthoptera offer the advantage that both proximate and ultimate questions can be tackled in the same system. An important advance was the transition from laboratory studies under well-defined acoustic conditions to field studies that allowed to measure costs and benefits of acoustic signaling as well as constraints on song evolution.

Experimental and Theoretical Explorations of Traveling Waves and Tuning in the Bushcricket Ear

The ability to detect airborne sound is essential for many animals. Examples from the inner ear of mammals and bushcrickets demonstrate that similar detection strategies evolved in taxonomically distant species. Both mammalian and bushcricket ears possess a narrow strip of sensory tissue that exhibits an anatomical gradient and traveling wave motion responses used for frequency discrimination. We measured pressure and motion in the bushcricket ear to investigate physical properties, stiffness, and mass, which govern the mechanical responses to sound. As in the mammalian cochlea, sound-induced fluid pressure and motion responses were tonotopically organized along the longitudinal axis of the crista acustica, the bushcricket's hearing organ. The fluid pressure at the crista and crista motion were used to calculate the acoustic impedance of the organ-bounded fluid mass (Z_mass). We used a theoretical wave analysis of wavelength data from a previous study to predict the crista acustica stiffness. The wave analysis also predicts Z_mass, and that result agreed reasonably well with the directly measured Z_mass, lending support to the theoretical wave analysis. The magnitude of the crista stiffness was similar to basilar membrane stiffness in mammals, and as in mammals, the stiffness decreased from the high-frequency to the low-frequency region. At a given location, the stiffness increased with increasing frequency, corresponding to increasing curvature of the traveling wave (decreasing wavelength), indicating that longitudinal coupling plays a substantial role in determining crista stiffness. This is in contrast to the mammalian ear, in which stiffness is independent of frequency and longitudinal coupling is relatively small.

Auditory DUM neurons in a bush-cricket: A filter bank for carrier frequency

In bush-crickets the first stage of central auditory processing occurs in the prothoracic ganglion. About 15 to 50 different auditory dorsal unpaired median neurons (DUM neurons) exist but they have not been studied in any detail. These DUM neurons may be classified into seven different morphological types, although, there is only limited correlation between morphology and physiological responses. Ninety seven percent of the stained neurons were local, 3% were intersegmental. About 90% project nearly exclusively into the auditory neuropile, and 45% into restricted areas therein. Lateral extensions overlap with the axons of primary auditory sensory neurons close to their branching point. DUM neurons are typically tuned to frequencies covering the range between 2 and 50 kHz and thereby may establish a filter bank for carrier frequency. Less than 10% of DUM neurons have their branches in adjacent and more posterior regions of the auditory neuropile and are mostly tuned to low frequencies, less sensitive than the other types and respond to vibration. Thirty five percent of DUM show indications of inhibition, either through reduced responses at higher intensities, or by hyperpolarizing responses to sound. Most DUM neurons produce phasic spike responses preferably at higher intensities. Spikes may be elicited by intracellular current injection. Preliminary data suggest that auditory DUM neurons have GABA as transmitter and therefore may inhibit other auditory interneurons. From all known local auditory neurons, only DUM neurons have frequency specific responses which appear suited for local processing relevant for acoustic communication in bush crickets.

Listening in the bog: II. Neural correlates for acoustic interactions and spacing between Sphagniana sphagnorum males

Males of the katydid Sphagniana sphagnorum maintain inter-male distances from one another using agonistic song interactions with a frequency-modulated song that consists of alternating audio and ultrasonic parts. We studied the neuronal representation of this song in auditory receptors and interneurons of receivers, using playbacks of songs that mimicked the absolute and relative sound pressure levels of the two song modes varying with distance. The tuning and sensitivity of both receptors and interneurons strongly determine their responses to the two song modes at different distances. Low-frequency interneurons respond preferentially to the audio mode of the song at larger distances. High-frequency (HF) interneurons respond preferentially to the HF component of the song at close range. ‘Switch interneurons’ are sensitive to both spectral song components, but exhibit a typical activity switch towards the high-frequency mode at distances nearer than 3–6 m. The activity of the latter two groups of interneurons correlates with the distance in the field at which males begin to interact acoustically with their neighbours. Important information about the rate of changes in the song mode is represented by the afferent activity despite the influence of the masking song produced by a sympatric katydid species.

Morphological basis for a tonotopic design of an insect ear

The tonotopically organized hearing organs of bushcrickets provide the opportunity for a detailed correlation of morphological and structural properties within hearing organs that are needed to establish tonotopic gradients. In the present study of a tonotopic insect hearing organ, we combine mechanical measurements of sound-induced hearing organ motion and detailed anatomical investigations to explore the anatomical basis of tonotopy. We compare mechanical data of frequency responses along the auditory organ to several anatomical parameters. Low frequency responses are related to larger organ and cap cell size in the proximal part of the hearing organ while in the distal part of the organ, small organ and cap cell size is related to high-frequency representation. However, the correlation between organ and cap cell size with continuous frequency representation along the organ is not very tight. Instead, the height of the organ and the corresponding length of the sensory dendrites are best correlated to tonotopic frequency representation. The sensory dendrite contains a ciliary root with a pronounced cross-banding of electron-dense material that should be important for the stiffness of the dendrite. The geometry of surrounding structures like the hemolymph channel and the acoustic trachea as well as the extension of the tectorial membrane are not correlated to the tonotopy. We provide evidence that tonotopy in the bushcricket hearing organ may depend on the size of ciliary structures. In particular, the ciliary root of the sensory cells is a likely cellular basis of tonotopy.

Parasitoid flies exploiting acoustic communication of insects-comparative aspects of independent functional adaptations

Two taxa of parasitoid Diptera have independently evolved tympanal hearing organs to locate sound producing host insects. Here we review and compare functional adaptations in both groups of parasitoids, Ormiini and Emblemasomatini. Tympanal organs in both groups originate from a common precursor organ and are somewhat similar in morphology and physiology. In terms of functional adaptations, the hearing thresholds are largely adapted to the frequency spectra of the calling song of the hosts. The large host ranges of some parasitoids indicate that their neuronal filter for the temporal patterns of the calling songs are broader than those found in intraspecific communication. For host localization the night active Ormia ochracea and the day active E. auditrix are able to locate a sound source precisely in space. For phonotaxis flight and walking phases are used, whereby O. ochracea approaches hosts during flight while E. auditrix employs intermediate landings and re-orientation, apparently separating azimuthal and vertical angles. The consequences of the parasitoid pressure are discussed for signal evolution and intraspecific communication of the host species. This natural selection pressure might have led to different avoidance strategies in the hosts: silent males in crickets, shorter signals in tettigoniids and fluctuating population abundances in cicadas.

Computational themes of peripheral processing in the auditory pathway of insects

Hearing in insects serves to gain information in the context of mate finding, predator avoidance or host localization. For these goals, the auditory pathways of insects represent the computational substrate for object recognition and localization. Before these higher level computations can be executed in more central parts of the nervous system, the signals need to be preprocessed in the auditory periphery. Here, we review peripheral preprocessing along four computational themes rather than discussing specific physiological mechanisms: (1) control of sensitivity by adaptation, (2) recoding of amplitude modulations of an acoustic signal into a labeled-line code (3) frequency processing and (4) conditioning for binaural processing. Along these lines, we review evidence for canonical computations carried out in the peripheral auditory pathway and show that despite the vast diversity of insect hearing, signal processing is governed by common computational motifs and principles.

Lateralization of travelling wave response in the hearing organ of bushcrickets

Travelling waves are the physical basis of frequency discrimination in many vertebrate and invertebrate taxa, including mammals, birds, and some insects. In bushcrickets (Tettigoniidae), the crista acustica is the hearing organ that has been shown to use sound-induced travelling waves. Up to now, data on mechanical characteristics of sound-induced travelling waves were only available along the longitudinal (proximal-distal) direction. In this study, we use laser Doppler vibrometry to investigate in-vivo radial (anterior-posterior) features of travelling waves in the tropical bushcricket Mecopoda elongata. Our results demonstrate that the maximum of sound-induced travelling wave amplitude response is always shifted towards the anterior part of the crista acustica. This lateralization of the travelling wave response induces a tilt in the motion of the crista acustica, which presumably optimizes sensory transduction by exerting a shear motion on the sensory cilia in this hearing organ.

Intensity invariance properties of auditory neurons compared to the statistics of relevant natural signals in grasshoppers

The temporal pattern of amplitude modulations (AM) is often used to recognize acoustic objects. To identify objects reliably, intensity invariant representations have to be formed. We approached this problem within the auditory pathway of grasshoppers. We presented AM patterns modulated at different time scales and intensities. Metric space analysis of neuronal responses allowed us to determine how well, how invariantly, and at which time scales AM frequency is encoded. We find that in some neurons spike-count cues contribute substantially (20-60%) to the decoding of AM frequency at a single intensity. However, such cues are not robust when intensity varies. The general intensity invariance of the system is poor. However, there exists a range of AM frequencies around 83 Hz where intensity invariance of local interneurons is relatively high. In this range, natural communication signals exhibit much variation between species, suggesting an important behavioral role for this frequency band. We hypothesize, just as has been proposed for human speech, that the communication signals might have evolved to match the processing properties of the receivers. This contrasts with optimal coding theory, which postulates that neuronal systems are adapted to the statistics of the relevant signals.

Sensory-encoding differences contribute to species-specific call recognition mechanisms

Object recognition is a fundamental function of the auditory system, but the underlying mechanisms are not well understood. Acoustic communication in the Tettigoniid genus Neoconocephalus provides a useful system for studying these mechanisms. We examined the ascending interneuron pathway in three Neoconocephalus species with diverse calls and recognition mechanisms. This pathway processes spectral information and transmits call temporal patterns to the supraesophageal ganglion where the recognition circuits reside. For each species, we describe one local auditory interneuron (ON) and three with ascending projections (AN-1, AN-2, TN-1), which were physiologically and morphologically similar to those described in other Tettigoniids. TN-1 responded only to the beginning of call models. For AN-1, each call model pulse elicited a single action potential in N. robustus and N. bivocatus, whereas every other pulse elicited an action potential in N. triops. Individual pulses did not reliably evoke AN-2 responses in all three species. AN-1 responses were limited to frequencies<20 kHz. AN-1 tuning differed among the three species, reflecting their differences in the dominant frequency of the calls. AN-2 was broadly tuned, and responses increased with intensity in all three species. In behavioral experiments, N. robustus showed greater spectral selectivity than the other two species. Adding the second harmonic to the spectrum of call models suppressed phonotaxis in N. robustus, but not N. triops or N. bivocatus. Adding the second harmonic reduced AN-1 responses in N. robustus but not in the other two species. We discuss the potential function of the ascending neurons for call recognition.

Listening for males and bats: spectral processing in the hearing organ of Neoconocephalus bivocatus (Orthoptera: Tettigoniidae)

Tettigoniids use hearing for mate finding and the avoidance of predators (mainly bats). Using intracellular recordings, we studied the response properties of auditory receptor cells of Neoconocephalus bivocatus to different sound frequencies, with a special focus on the frequency ranges representative of male calls and bat cries. We found several response properties that may represent adaptations for hearing in both contexts. Receptor cells with characteristic frequencies close to the dominant frequency of the communication signal were more broadly tuned, thus extending their range of high sensitivity. This increases the number of cells responding to the dominant frequency of the male call at low signal amplitudes, which should improve long distance call localization. Many cells tuned to audio frequencies had intermediate thresholds for ultrasound. As a consequence, a large number of receptors should be recruited at intermediate amplitudes of bat cries. This collective response of many receptors may function to emphasize predator information in the sensory system, and correlates with the amplitude range at which ultrasound elicits evasive behavior in tettigoniids. We compare our results with spectral processing in crickets, and discuss that both groups evolved different adaptations for the perceptual tasks of mate and predator detection.

Tuning the drum: the mechanical basis for frequency discrimination in a Mediterranean cicada

Cicadas are known to use sound to find a mate. While the mechanism employed by male cicadas to generate loud calling songs has been described in detail,little information exists to explain how their ears work. Using microscanning laser Doppler vibrometry, the tympanal vibrations in the cicada Cicadatra atra are measured in response to acoustic playbacks. The topographically accurate optical measurements reveal the vibrational behaviour of the anatomically complex tympanal membrane. Notably, the tympanal ridge, a distinct structural element of the tympanum that is a link to the receptor cells, undergoes mechanical vibrations reminiscent of a travelling wave. In effect, the frequency for which the maximum deflection amplitude is observed regularly decreases from the apex to the base of the ridge. It is also shown that whilst female ears are mechanically tuned to the male's song, the male's tympanum is only partially tuned to its own song. This study establishes the presence of a peripheral auditory mechanism that can potentially process auditory frequency analysis. In view of the importance of acoustic signalling in cicadas, this unconventional tympanal mechanism may be employed in the context of species recognition and sexual selection.

Hearing and frequency dependence of auditory interneurons in the parasitoid fly Homotrixa alleni (Tachinidae: Ormiini)

The parasitoid tachinid fly Homotrixa alleni detects its hosts by their acoustic signals. The tympanal organ of the fly is located at the prothorax and contains scolopidial sensory units of different size and orientation. The tympanal membrane vibrates in the frequency range of approximately 4-35 kHz, which is also reflected in the hearing threshold measured at the neck connective. The auditory organ is not tuned to the peak frequency (5 kHz) of the main host, the bush cricket Sciarasaga quadrata. Auditory afferents project in the three thoracic neuromeres. Most of the ascending interneurons branch in all thoracic neuromeres and terminate in the deutocerebrum of the brain. The interneurons do not differ considerably in frequency tuning, but in their sensitivity with lowest thresholds around 30 dB SPL. Suprathreshold responses of most neurons depend on frequency and intensity, indicating inhibitory influence at higher intensities. Some neurons respond particularly well at low frequency sounds (around 5 kHz) and high intensities (80-90 dB SPL), and thus may be involved in detection of the primary host, S. quadrata. The auditory system of H. alleni contains auditory interneurons reacting in a wide range of temporal patterns from strictly phasic to tonic and with clear differences in frequency responses.

Diversity of intersegmental auditory neurons in a bush cricket

Various auditory interneurons of the duetting bush cricket Ancistrura nigrovittata with axons ascending to the brain are presented. In this species, more intersegmental sound-activated neurons have been identified than in any other bush cricket so far, among them a new type of ascending neuron with posterior soma in the prothoracic ganglion (AN4). These interneurons show not only morphological differences in the prothoracic ganglion and the brain, but also respond differently to carrier frequencies, intensity and direction. As a set of neurons, they show graded differences for all of these parameters. A response type not described among intersegmental neurons of crickets and other bush crickets so far is found in the AN3 neuron with a tonic response, broad frequency tuning and little directional dependence. All neurons, with the exception of AN3, respond in a relatively similar manner to the temporal patterns of the male song: phasically to high syllable repetitions and rhythmically to low syllable repetitions. The strongest coupling to the temporal pattern is found in TN1. In contrast to behavior the neuronal responses depend little on syllable duration. AN4, AN5 and TN1 respond well to the female song. AN4 (at higher intensities) and TN1 respond well to a complete duet.

The physiology of insect auditory afferents

This review presents an overview of the physiology of primary receptors serving tympanal hearing in insects. Auditory receptor responses vary with frequency, intensity, and temporal characteristics of sound stimuli. Various insect species exploit each of these parameters to differing degrees in the neural coding of auditory information, depending on the nature of the relevant stimuli. Frequency analysis depends on selective tuning in individual auditory receptors. In those insect groups that have individually tuned receptors, differences in physiology are correlated with structural differences among receptors and with the anatomical arrangement of receptors within the ear. Intensity coding is through the rate-level characteristics of tonically active auditory receptors and through variation in the absolute sensitivities of individual receptors (range fractionation). Temporal features of acoustic stimuli may be copied directly in the timing of afferent responses. Salient signal characteristics may also be represented by variation in the timing of afferent responses on a finer temporal scale, or by the synchrony of responses across a population of receptors.

Processing of auditory information in insects

Insects exhibit an astonishing diversity in the design of their ears and the subsequent processing of information within their auditory pathways. The aim of this review is to summarize and compare the present concepts of auditory processing by relating behavioral performance to known neuronal mechanisms. We focus on three general aspects, that is frequency, directional, and temporal processing. The first part compares the capacity (in some insects high) for frequency analysis in the ear with the rather low specificity of tuning in interneurons by looking at Q10dB values and frequency dependent inhibition of interneurons. Since sharpening of frequency does not seem to be the prime task of a set of differently tuned receptors, alternative hypotheses are discussed. Moreover, the physiological correspondence between tonotopic projections of receptors and dendritic organization of interneurons is not in all cases strong. The second part is concerned with directional hearing and thus with the ability for angular resolution of insects. The present concepts, as derived from behavioral performances, for angular resolution versus lateralization and serial versus parallel processing of directional and pattern information can be traced to the thoracic level of neuronal processing. Contralateral inhibition, a mechanism for enhancing directional tuning, appears to be most effective in parallel pathways, whereas in serial processing it may have detrimental effects on pattern processing. The third part, after some considerations of signal analysis in the temporal domain, demonstrates that closely related species often use different combinations of temporal parameters in their recognition systems. On the thoracic level, analysis of temporal modulation functions and effects of inhibition on spiking patterns reveals relatively simple processing, whereas brain neurons may exhibit more complex properties.

Morphology and physiology of auditory and vibratory ascending interneurones in bushcrickets

Auditory/vibratory interneurones of the bushcricket species Decticus albifrons and Decticus verrucivorus were studied with intracellular dye injection and electrophysiology. The morphologies of five physiologically characterised auditory/vibratory interneurones are shown in the brain, subesophageal and prothoracic ganglia. Based on their physiology, these five interneurones fall into three groups, the purely auditory or sound neurones: S-neurones, the purely vibratory V-neurones, and the bimodal vibrosensitive VS-neurones. The S1-neurones respond phasically to airborne sound whereas the S4-neurones exhibit a tonic spike pattern. Their somata are located in the prothoracic ganglion and they show an ascending axon with dendrites located in the prothoracic, subesophageal ganglia, and the brain. The VS3-neurone, responding to both auditory and vibratory stimuli in a tonic manner, has its axon traversing the brain, the suboesophageal ganglion and the prothoracic ganglion although with dendrites only in the brain. The V1- and V2-neurones respond to vibratory stimulation of the fore- and midlegs with a tonic discharge pattern, and our data show that they receive inhibitory input suppressing their spontaneous activity. Their axon transverses the prothoracic ganglion, subesophageal ganglion and terminate in the brain with dendritic branching. Thus the auditory S-neurones have dendritic arborizations in all three ganglia (prothoracic, subesophageal, and brain) compared to the vibratory (V) and vibrosensitive (VS) neurones, which have dendrites almost only in the brain. The dendrites of the S-neurones are also more extensive than those of the V-, VS-neurones. V- and VS-neurones terminate more laterally in the brain. Due to an interspecific comparison of the identified auditory interneurones the S1-neurone is found to be homologous to the TN1 of crickets and other bushcrickets, and the S4-neurone also can be called AN2. J. Exp. Zool. 286:219-230, 2000.

Modular construction of nervous systems: a basic principle of design for invertebrates and vertebrates

The modular construction of brain tissue is not solely a feature of vertebrate nervous tissue, but is characteristic of many invertebrate nervous systems as well. Modern vertebrate and invertebrate modules vary over several orders of magnitude in volume but vary less in diameter. Although the physiological and anatomical differences between the modules discussed herein are overpowering, their importance to nervous system functions are similar. Modules are the serial and parallel processing units that have allowed large-brained animals to evolve. Many invertebrate modules are discrete, hemispherical lobes, visible on the surface of the brain or nerve cord, whereas most mammalian modules are columnar or ellipsoidal tissue compartments that can only be visualized with specific anatomical methods. Lobes from the largest invertebrates can be more voluminous than any neocortical compartments, but these large lobes are usually not single modules. Large invertebrate lobes contain internal compartments that are single modules and of similar size to their vertebrate analogs. However, vertebrate cortical modules or columns, are far more numerous than the compartments in invertebrate brains and in several cases are known to be adjoined laterally into slabs of tissue that extend for several millimeters. Physiological data support the idea that neural modules are not just anatomical entities, but are active local circuits. The specific activities within each type of module will depend upon its neuronal components, both intrinsic and extrinsic, its functional roles and phylogenetic history. Many cellular and intercellular phenomena common to vertebrates and invertebrates underlie the development of modules. Neuronal and glial interactions and their interplay with the extracellular environment depend upon families of molecules with broad phyletic occurrences. The commonalities of growth mechanisms may to a large degree account for the widespread incidence of neuronal processing units. The strategy of enlarging a nervous system through the replication of the basic units is thought to be advantageous for several reasons. This plan allows nervous systems to economize on the branch sizes and lengths needed for interconnections, to ensure that appropriate targets are reached during development and to modulate specific circuits within a larger network.