Neurophysiology goes wild: from exploring sensory coding in sound proof rooms to natural environments.

To perform adaptive behaviours, animals have to establish a representation of the physical "outside" world. How these representations are created by sensory systems is a central issue in sensory physiology. This review addresses the history of experimental approaches toward ideas about sensory coding, using the relatively simple auditory system of acoustic insects. I will discuss the empirical evidence in support of Barlow's "efficient coding hypothesis", which argues that the coding properties of neurons undergo specific adaptations that allow insects to detect biologically important acoustic stimuli. This hypothesis opposes the view that the sensory systems of receivers are biased as a result of their phylogeny, which finally determine whether a sound stimulus elicits a behavioural response. Acoustic signals are often transmitted over considerable distances in complex physical environments with high noise levels, resulting in degradation of the temporal pattern of stimuli, unpredictable attenuation, reduced signal-to-noise levels, and degradation of cues used for sound localisation. Thus, a more naturalistic view of sensory coding must be taken, since the signals as broadcast by signallers are rarely equivalent to the effective stimuli encoded by the sensory system of receivers. The consequences of the environmental conditions for sensory coding are discussed.

Directional hearing in insects: biophysical, physiological and ecological challenges.

Sound localisation is a fundamental attribute of the way that animals perceive their external world. It enables them to locate mates or prey, determine the direction from which a predator is approaching and initiate adaptive behaviours. Evidence from different biological disciplines that has accumulated over the last two decades indicates how small insects with body sizes much smaller than the wavelength of the sound of interest achieve a localisation performance that is similar to that of mammals. This Review starts by describing the distinction between tympanal ears (as in grasshoppers, crickets, cicadas, moths or mantids) and flagellar ears (specifically antennae in mosquitoes and fruit flies). The challenges faced by insects when receiving directional cues differ depending on whether they have tympanal or flagellar years, because the latter respond to the particle velocity component (a vector quantity) of the sound field, whereas the former respond to the pressure component (a scalar quantity). Insects have evolved sophisticated biophysical solutions to meet these challenges, which provide binaural cues for directional hearing. The physiological challenge is to reliably encode these cues in the neuronal activity of the afferent auditory system, a non-trivial problem in particular for those insect systems composed of only few nerve cells which exhibit a considerable amount of intrinsic and extrinsic response variability. To provide an integrative view of directional hearing, I complement the description of these biophysical and physiological solutions by presenting findings on localisation in real-world situations, including evidence for localisation in the vertical plane.

Decision making in the face of a deadly predator: high-amplitude behavioural thresholds can be adaptive for rainforest crickets under high background noise levels.

Many insect families have evolved ears that are adapted to detect ultrasonic calls of bats. The acoustic sensory cues indicating the presence of a bat are then used to initiate bat avoidance behaviours. Background noise, in particular at ultrasonic frequencies, complicates these decisions, since a response to the background may result in costly false alarms. Here, we quantify bat avoidance responses of small rainforest crickets (Gryllidae, Trigoniinae), which live under conditions of high levels of ultrasonic background noise. Their bat avoidance behaviour exhibits markedly higher thresholds than most other studied eared insects. Their responses do not qualitatively differ at suprathreshold amplitudes up to sound pressure levels of 105 dB. Moreover, they also exhibit evasive responses to single, high-frequency events and do not require the repetitive sequence of ultrasonic calls typical for the search phase of bat echolocation calls. Analysis of bat and katydid sound amplitudes and peak frequencies in the crickets' rainforest habitat revealed that the cricket's behavioural threshold would successfully reject the katydid background noise. Using measurements of the crickets' echo target strength for bat predators, we calculated the detection distances for both predators and prey. Despite their high behavioural threshold, the cricket prey still has a significant detection advantage at frequencies between 20 and 40 kHz. The low-amplitude bat calls they ignore are no predation threat because even much louder calls would be detected before the bat would hear the cricket echo. This leaves ample time for evasive actions. Thus, a simple decision criterion based on a high-amplitude behavioural threshold can be adaptive under the high background noise levels in nocturnal rainforests, in avoiding false alarms and only missing detection for bat calls too far away to pose a risk. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Evolutionarily conserved coding properties favour the neuronal representation of heterospecific signals of a sympatric katydid species.

To function as a mechanism in premating isolation, the divergent and species-specific calling songs of acoustic insects must be reliably processed by the afferent auditory pathway of receivers. Here, we analysed the responses of interneurons in a katydid species that uses long-lasting acoustic trills and compared these with previously reported data for homologous interneurons of a sympatric species that uses short chirps as acoustic signals. Some interneurons of the trilling species respond exclusively to the heterospecific chirp due to selective, low-frequency tuning and "novelty detection". These properties have been considered as evolutionary adaptations in the sensory system of the chirper, which allow it to detect signals effectively during the simultaneous calling of the sympatric sibling species. We propose that these two mechanisms, shared by the interneurons of both species, did not evolve in the chirper to guarantee its ability to detect the chirp under masking conditions. Instead we suggest that chirpers evolved an additional, 2-kHz component in their song and exploited pre-existing neuronal properties for detecting their song under masking noise. The failure of some interneurons to respond to the conspecific song in trillers does not prevent intraspecific communication, as other interneurons respond to the trill.

Listening in the bog: I. Acoustic interactions and spacing between males of Sphagniana sphagnorum.

Males of the katydid Sphagniana sphagnorum form calling aggregations in boreal sphagnum bogs to attract mates. They broadcast frequency-modulated (FM) songs in steady series, each song comprised of two wing-stroking modes that alternate audio and ultrasonic spectra. NN analysis of three populations found mean distances between 5.1 and 8.4 m, but failed to find spacing regularity: in one males spaced randomly, in another they were clumped, but within the clumps spaced at random. We tested a mechanism for maintaining inter-male distances by playback of conspecific song to resident males and analysing song interactions between neighbouring males in the field. The results indicate that the song rate is an important cue for males. Information coded in song rates is confounded by variation in bog temperatures and by the linear correlation of song rates with temperature. The ultrasonic and audio spectral modes suffer different excess attenuation: the ultrasonic mode is favoured at shorter distances (< 6 m), the audio mode at longer distances (> 6 m), supporting a hypothesized function in distance estimation. Another katydid, Conocephalus fasciatus, shares habitat with S. sphagnorum and could mask its ultrasonic mode; however, mapping of both species indicate the spacing of S. sphagnorum is unaffected by the sympatric species.

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.

Functional relevance of acoustic tracheal design in directional hearing in crickets.

Internally coupled ears (ICEs) allow small animals to reliably determine the direction of a sound source. ICEs are found in a variety of taxa, but crickets have evolved the most complex arrangement of coupled ears: an acoustic tracheal system composed of a large cross-body trachea that connects two entry points for sound in the thorax with the leg trachea of both ears. The key structure that allows for the tuned directionality of the ear is a tracheal inflation (acoustic vesicle) in the midline of the cross-body trachea holding a thin membrane (septum). Crickets are known to display a wide variety of acoustic tracheal morphologies, most importantly with respect to the presence of a single or double acoustic vesicle. However, the functional relevance of this variation is still not known. In this study, we investigated the peripheral directionality of three co-occurring, closely related cricket species of the subfamily Gryllinae. No support could be found for the hypothesis that a double vesicle should be regarded as an evolutionary innovation to (1) increase interaural directional cues, (2) increase the selectivity of the directional filter or (3) provide a better match between directional and sensitivity tuning. Nonetheless, by manipulating the double acoustic vesicle in the rainforest cricket Paroecanthus podagrosus, selectively eliminating the sound-transmitting pathways, we revealed that these pathways contribute almost equally to the total amount of interaural intensity differences, emphasizing their functional relevance in the system.

Rhythm Generation and Rhythm Perception in Insects: The Evolution of Synchronous Choruses.

Insect sounds dominate the acoustic environment in many natural habitats such as rainforests or meadows on a warm summer day. Among acoustic insects, usually males are the calling sex; they generate signals that transmit information about the species-identity, sex, location, or even sender quality to conspecific receivers. Males of some insect species generate signals at distinct time intervals, and other males adjust their own rhythm relative to that of their conspecific neighbors, which leads to fascinating acoustic group displays. Although signal timing in a chorus can have important consequences for the calling energetics, reproductive success and predation risk of individuals, still little is known about the selective forces that favor the evolution of insect choruses. Here, we review recent advances in our understanding of the neuronal network responsible for acoustic pattern generation of a signaler, and pattern recognition in receivers. We also describe different proximate mechanisms that facilitate the synchronous generation of signals in a chorus and provide examples of suggested hypotheses to explain the evolution of chorus synchrony in insects. Some hypotheses are related to sexual selection and inter-male cooperation or competition, whereas others refer to the selection pressure exerted by natural predators. In this article, we summarize the results of studies that address chorus synchrony in the tropical katydid Mecopoda elongata, where some males persistently signal as followers although this reduces their mating success.

Directional hearing in insects with internally coupled ears.

Compared to all other hearing animals, insects are the smallest ones, both in absolute terms and in relation to the wavelength of most biologically relevant sounds. The ears of insects can be located at almost any possible body part, such as wings, legs, mouthparts, thorax or abdomen. The interaural distances are generally so small that cues for directional hearing such as interaural time and intensity differences (IITs and IIDs) are also incredibly small, so that the small body size should be a strong constraint for directional hearing. Yet, when tested in behavioral essays for the precision of sound source localization, some species demonstrate hyperacuity in directional hearing and can track a sound source deviating from the midline by only [Formula: see text]-[Formula: see text]. They can do so by using internally coupled ears, where sound pressure can act on both sides of a tympanic membrane. Here we describe their varying anatomy and mode of operation for some insect groups, with a special focus on crickets, exhibiting probably one of the most sophisticated of all internally coupled ears in the animal kingdom.

Neural Mechanisms for Acoustic Signal Detection under Strong Masking in an Insect.

Communication is fundamental for our understanding of behavior. In the acoustic modality, natural scenes for communication in humans and animals are often very noisy, decreasing the chances for signal detection and discrimination. We investigated the mechanisms enabling selective hearing under natural noisy conditions for auditory receptors and interneurons of an insect. In the studied katydid Mecopoda elongata species-specific calling songs (chirps) are strongly masked by signals of another species, both communicating in sympatry. The spectral properties of the two signals are similar and differ only in a small frequency band at 2 kHz present in the chirping species. Receptors sharply tuned to 2 kHz are completely unaffected by the masking signal of the other species, whereas receptors tuned to higher audio and ultrasonic frequencies show complete masking. Intracellular recordings of identified interneurons revealed two mechanisms providing response selectivity to the chirp. (1) Response selectivity is when several identified interneurons exhibit remarkably selective responses to the chirps, even at signal-to-noise ratios of -21 dB, since they are sharply tuned to 2 kHz. Their dendritic arborizations indicate selective connectivity with low-frequency receptors tuned to 2 kHz. (2) Novelty detection is when a second group of interneurons is broadly tuned but, because of strong stimulus-specific adaptation to the masker spectrum and "novelty detection" to the 2 kHz band present only in the conspecific signal, these interneurons start to respond selectively to the chirp shortly after the onset of the continuous masker. Both mechanisms provide the sensory basis for hearing at unfavorable signal-to-noise ratios. Significance statement: Animal and human acoustic communication may suffer from the same "cocktail party problem," when communication happens in noisy social groups. We address solutions for this problem in a model system of two katydids, where one species produces an extremely noisy sound, yet the second species still detects its own song. Using intracellular recording techniques we identified two neural mechanisms underlying the surprising behavioral signal detection at the level of single identified interneurons. These neural mechanisms for signal detection are likely to be important for other sensory modalities as well, where noise in the communication channel creates similar problems. Also, they may be used for the development of algorithms for the filtering of specific signals in technical microphones or hearing aids.

Directional hearing: from biophysical binaural cues to directional hearing outdoors.

When insects communicate by sound, or use acoustic cues to escape predators or detect prey or hosts they have to localize the sound in most cases, to perform adaptive behavioral responses. In the case of particle velocity receivers such as the antennae of mosquitoes, directionality is no problem because such receivers are inherently directional. Insects equipped with bilateral pairs of tympanate ears could principally make use of binaural cues for sound localization, like all other animals with two ears. However, their small size is a major problem to create sufficiently large binaural cues, with respect to both interaural time differences (ITDs, because interaural distances are so small), but also with respect to interaural intensity differences (IIDs), since the ratio of body size to the wavelength of sound is rather unfavorable for diffractive effects. In my review, I will only shortly cover these biophysical aspects of directional hearing. Instead, I will focus on aspects of directional hearing which received relatively little attention previously, the evolution of a pressure difference receiver, 3D-hearing, directional hearing outdoors, and directional hearing for auditory scene analysis.

From microseconds to seconds and minutes-time computation in insect hearing.

The computation of time in the auditory system of insects is of relevance at rather different time scales, covering a large range from microseconds to several minutes. At the one end of this range, only a few microseconds of interaural time differences are available for directional hearing, due to the small distance between the ears, usually considered too small to be processed reliably by simple nervous systems. Synapses of interneurons in the afferent auditory pathway are, however, very sensitive to a time difference of only 1-2 ms provided by the latency shift of afferent activity with changing sound direction. At a much larger time scale of several tens of milliseconds to seconds, time processing is important in the context species recognition, but also for those insects where males produce acoustic signals within choruses, and the temporal relationship between song elements strongly deviates from a random distribution. In these situations, some species exhibit a more or less strict phase relationship of song elements, based on phase response properties of their song oscillator. Here we review evidence on how this may influence mate choice decisions. In the same dimension of some tens of milliseconds we find species of katydids with a duetting communication scheme, where one sex only performs phonotaxis to the other sex if the acoustic response falls within a very short time window after its own call. Such time windows show some features unique to insects, and although its neuronal implementation is unknown so far, the similarity with time processing for target range detection in bat echolocation will be discussed. Finally, the time scale being processed must be extended into the range of many minutes, since some acoustic insects produce singing bouts lasting quite long, and female preferences may be based on total signaling time.

Selective phonotaxis of female crickets under natural outdoor conditions.

Acoustic mate choice in insects has been extensively studied under laboratory conditions, using different behavioural paradigms. Ideally, however, mate choice designs should reflect natural conditions, including the physical properties of the transmission channel for the signal. Since little is known about the discrimination ability of females between male song variants under natural conditions, we performed phonotaxis experiments with female field crickets (Gryllus bimaculatus) outdoors, using two-choice decisions based on differences in carrier frequency, sound pressure level, and chirp rate. For all three song parameters, minimum differences necessary for a significant preference between two song models were considerably larger outdoors compared to laboratory conditions. A minimum amplitude difference of 5 dB was required for a significant choice in the field, compared to only 1-2 dB reported for lab-based experiments. Due to the tuned receiver system, differences in carrier frequency equal differences in perceived loudness, and the results on choice for differences in carrier frequency corroborate those in amplitude. Similarly, chirp rate differences of 50 chirps/min were required outdoors compared to only 20 chirps/min in the lab. For predictions about patterns of sexual selection, future studies need to consider the different outcomes of mate choice decisions in lab and field trials.

Out of phase: relevance of the medial septum for directional hearing and phonotaxis in the natural habitat of field crickets.

A modified tracheal system is the anatomical basis for a pressure difference receiver in field crickets, where sound has access to the inner and outer side of the tympanum of the ear in the forelegs. A thin septum in the midline of a connecting trachea coupling both ears is regarded to be important in producing frequency-dependent interaural intensity differences (IIDs) for sound localization. However, the fundamental role of the septum in directional hearing has recently been challenged by the finding that the localization ability is ensured even with a perforated septum, at least under controlled laboratory conditions. Here, we investigated the influence of the medial septum on phonotaxis of female Gryllus bimaculatus under natural conditions. Surprisingly, even with a perforated septum, females reliably tracked a male calling song in the field. Although reduced by 5.2 dB, IIDs still averaged at 7.9 dB and provided a reliable proximate basis for the observed behavioural performance of operated females in the field. In contrast, in the closely related species Gryllus campestris the same septum perforation caused a dramatic decline in IIDs over all frequencies tested. We discuss this discrepancy with respect to a difference in the phenotype of their tracheal systems.

Diversity of acoustic tracheal system and its role for directional hearing in crickets.

BACKGROUND: Sound localization in small insects can be a challenging task due to physical constraints in deriving sufficiently large interaural intensity differences (IIDs) between both ears. In crickets, sound source localization is achieved by a complex type of pressure difference receiver consisting of four potential sound inputs. Sound acts on the external side of two tympana but additionally reaches the internal tympanal surface via two external sound entrances. Conduction of internal sound is realized by the anatomical arrangement of connecting trachea. A key structure is a trachea coupling both ears which is characterized by an enlarged part in its midline (i.e., the acoustic vesicle) accompanied with a thin membrane (septum). This facilitates directional sensitivity despite an unfavorable relationship between wavelength of sound and body size. Here we studied the morphological differences of the acoustic tracheal system in 40 cricket species (Gryllidae, Mogoplistidae) and species of outgroup taxa (Gryllotalpidae, Rhaphidophoridae, Gryllacrididae) of the suborder Ensifera comprising hearing and non hearing species. RESULTS: We found a surprisingly high variation of acoustic tracheal systems and almost all investigated species using intraspecific acoustic communication were characterized by an acoustic vesicle associated with a medial septum. The relative size of the acoustic vesicle - a structure most crucial for deriving high IIDs - implies an important role for sound localization. Most remarkable in this respect was the size difference of the acoustic vesicle between species; those with a more unfavorable ratio of body size to sound wavelength tend to exhibit a larger acoustic vesicle. On the other hand, secondary loss of acoustic signaling was nearly exclusively associated with the absence of both acoustic vesicle and septum. CONCLUSION: The high diversity of acoustic tracheal morphology observed between species might reflect different steps in the evolution of the pressure difference receiver; with a precursor structure already present in ancestral non-hearing species. In addition, morphological transitions of the acoustic vesicle suggest a possible adaptive role for the generation of binaural directional cues.

Asymmetry in cricket song: female preference and proximate mechanism of discrimination.

Subtle random deviations from perfect symmetry in bilateral traits are suggested to signal reduced phenotypic and genetic quality of a sender, but little is known about the related receiver mechanisms for discriminating symmetrical from asymmetrical traits. Here, we investigated these mechanisms in behavioural and neurophysiological experiments in the Mediterranean field cricket, Gryllus bimaculatus. A downward frequency modulation at the end of each syllable in the calling song has been suggested to indicate morphological asymmetry in sound radiating structures between left and right forewings. Even under ideal laboratory conditions on a trackball system, female crickets only discriminated between songs of symmetrical and asymmetrical males in two-choice experiments at carrier frequencies of 4.4 kHz and a large modulation depth of 600 and 800 Hz. Under these conditions they preferred the pure-tone calling songs over the modulated (asymmetrical) alternative, whereas no preference was observed at carrier frequencies of 4.9 and 5.2 kHz. These preferences correlate well with the responses of a pair of identified auditory interneurons (AN1), known for their importance in female phonotaxis. The AN1 interneuron is tuned to an average frequency of 4.9 kHz, and the roll-off towards lower and higher frequencies determines the magnitude of responses to pure-tone and frequency-modulated calling songs. The difference in response magnitude between the two neurons appears to drive the decision of females towards the song alternatives. We discuss the relevance of song differences based on asymmetry in the morphology of song-producing structures under natural conditions.

Solutions to the cocktail party problem in insects: selective filters, spatial release from masking and gain control in tropical crickets.

BACKGROUND: Insects often communicate by sound in mixed species choruses; like humans and many vertebrates in crowded social environments they thus have to solve cocktail-party-like problems in order to ensure successful communication with conspecifics. This is even more a problem in species-rich environments like tropical rainforests, where background noise levels of up to 60 dB SPL have been measured. PRINCIPAL FINDINGS: Using neurophysiological methods we investigated the effect of natural background noise (masker) on signal detection thresholds in two tropical cricket species Paroecanthus podagrosus and Diatrypa sp., both in the laboratory and outdoors. We identified three 'bottom-up' mechanisms which contribute to an excellent neuronal representation of conspecific signals despite the masking background. First, the sharply tuned frequency selectivity of the receiver reduces the amount of masking energy around the species-specific calling song frequency. Laboratory experiments yielded an average signal-to-noise ratio (SNR) of -8 dB, when masker and signal were broadcast from the same side. Secondly, displacing the masker by 180° from the signal improved SNRs by further 6 to 9 dB, a phenomenon known as spatial release from masking. Surprisingly, experiments carried out directly in the nocturnal rainforest yielded SNRs of about -23 dB compared with those in the laboratory with the same masker, where SNRs reached only -14.5 and -16 dB in both species. Finally, a neuronal gain control mechanism enhances the contrast between the responses to signals and the masker, by inhibition of neuronal activity in interstimulus intervals. CONCLUSIONS: Thus, conventional speaker playbacks in the lab apparently do not properly reconstruct the masking noise situation in a spatially realistic manner, since under real world conditions multiple sound sources are spatially distributed in space. Our results also indicate that without knowledge of the receiver properties and the spatial release mechanisms the detrimental effect of noise may be strongly overestimated.

A cost-benefit analysis of public and private communication.

Understanding the diversity of animal signals requires knowledge of factors which may influence the different stages of communication, from the production of a signal by the sender up to the detection, identification and final decision-making in the receiver. We studied a Neotropical katydid (Docidocercus gigliotosi) which uses airborne sound for long distance communication, but also an alternative form of private signalling through substrate vibration. Males spend more time with private signalling under full moon conditions, when the nocturnal rainforest favours predation by visually hunting predators. For either type of signal we measured the energetic costs of producing it, its active space, and the background noise levels in both transmission channels. Signal perception was studied using neurophysiological methods under outdoor conditions, which is more reliable for the private mode of communication. Our results demonstrate the complex effects of ecological conditions, such as predation, nocturnal ambient light levels and masking noise on the performance of receivers in detecting mating signals, affecting the net advantage or disadvantage of either mode of communication.

High background noise shapes selective auditory filters in a tropical cricket.

Because of call frequency overlap and masking interference, the airborne sound channel represents a limited resource for communication in a species-rich cricket community like the tropical rainforest. Here we studied the frequency tuning of an auditory neuron mediating phonotaxis in the rainforest cricket Paroecanthus podagrosus, suffering from strong competition, in comparison with the same homologous neuron in two species of European field crickets, where such competition does not exist. As predicted, the rainforest species exhibited a more selective tuning compared with the European counterparts. The filter reduced background nocturnal noise levels by 26 dB, compared with only 16 and 10 dB in the two European species. We also quantified the performance of the sensory filter under the different filter regimes by examining the representation of the species-specific amplitude modulation of the male calling song, when embedded in background noise. Again, the filter of the rainforest cricket performed significantly better in terms of representing this important signal parameter. The neuronal representation of the calling song pattern within receivers was maintained for a wide range of signal-to-noise ratios because of the more sharply tuned sensory system and selective attention mechanisms. Finally, the rainforest cricket also showed an almost perfect match between the filter for sensitivity and the peripheral filter for directional hearing, in contrast to its European counterparts. We discuss the consequences of these adaptations for intraspecific acoustic communication and reproductive isolation between species.