Female reproductive state influences the auditory midbrain response

Auditory sensitivity and tympanic middle ear in a vocal and a non-vocal frog

The tympanic middle ear is important for anuran hearing on land. However, many species have partly or entirely lost their tympanic apparatus. Previous studies have compared hearing sensitivities in species that possess and lack tympanic membranes capable of sound production and acoustic communication. However, little is known about how these hearing abilities are comparable to those of mutant species. Here, we compared the eardrum and middle ear anatomies of two sympatric sibling species from a noisy stream habitat, namely the "non-vocal" Hainan torrent frog (Amolops hainanensis) and the "vocal" little torrent frog (Amolops torrentis), the latter of which is capable of acoustic communication. Our results showed that the relative (to head size) eardrum diameter of A. hainanensis was smaller than that of A. torrentis, although the absolute size was not smaller. Unlike A. torrentis, the tympanic membrane area of A. hainanensis was not clearly differentiated from the surrounding skin. The middle ear, however, was well-developed in both species. We measured the auditory brainstem responses (ABRs) of A. hainanensis and compared the ABR thresholds and latencies to those previously obtained for A. torrentis. Our results suggested that these two species exhibited significant differences in hearing sensitivity. A. hainanensis (smaller relative eardrum, nonvocal) had higher ABR thresholds and longer initial response times than A. torrentis (larger relative eardrum, vocal) at lower frequencies. Neurophysiological responses from the brain were obtained for tone pips between 800 Hz and 7,000 Hz, with peak sensitivities found at 3,000 Hz (73 dB SPL) for A. hainanensis, and at 1,800 Hz (61 dB SPL) for A. torrentis. Our results suggest that the non-vocal A. hainanensis has lower hearing sensitivity than its vocal sister species (i.e., A. torrentis), which may be related to differences in tympanic or inner ear structure and morphology.

Behind the mask(ing): how frogs cope with noise

Albert Feng was a pioneer in the field of auditory neuroethology who used frogs to investigate the neural basis of spectral and temporal processing and directional hearing. Among his many contributions was connecting neural mechanisms for sound pattern recognition and localization to the problems of auditory masking that frogs encounter when communicating in noisy, real-world environments. Feng's neurophysiological studies of auditory processing foreshadowed and inspired subsequent behavioral investigations of auditory masking in frogs. For frogs, vocal communication frequently occurs in breeding choruses, where males form dense aggregations and produce loud species-specific advertisement calls to attract potential mates and repel competitive rivals. In this review, we aim to highlight how Feng's research advanced our understanding of how frogs cope with noise. We structure our narrative around three themes woven throughout Feng's research-spectral, temporal, and directional processing-to illustrate how frogs can mitigate problems of auditory masking by exploiting frequency separation between signals and noise, temporal fluctuations in noise amplitude, and spatial separation between signals and noise. We conclude by proposing future research that would build on Feng's considerable legacy to advance our understanding of hearing and sound communication in frogs and other vertebrates.

Reproductive state modulates utricular auditory sensitivity in a vocal fish

The plainfin midshipman, Porichthys notatus, is a seasonally breeding vocal fish that relies on acoustic communication to mediate nocturnal reproductive behaviors. Reproductive females use their auditory senses to detect and localize "singing" males that produce multiharmonic advertisement (mate) calls during the breeding season. Previous work showed that the midshipman saccule, which is considered the primary end organ used for hearing in midshipman and most other fishes, exhibits reproductive state and hormone-dependent changes that enhance saccular auditory sensitivity. In contrast, the utricle was previously posited to serve primarily a vestibular function, but recent evidence in midshipman and related toadfish suggests that it may also serve an auditory function and aid in the detection of behaviorally relevant acoustic stimuli. Here, we characterized the auditory-evoked potentials recorded from utricular hair cells in reproductive and nonreproductive female midshipman in response to underwater sound to test the hypothesis that variation in reproductive state affects utricular auditory sensitivity. We show that utricular hair cells in reproductive females exhibit up to a sixfold increase in the utricular potential magnitude and have thresholds based on measures of particle acceleration (re: 1 ms-2) that are 7-10 dB lower than nonreproductive females across a broad range of frequencies, which include the dominant harmonics of male advertisement calls. This enhanced auditory sensitivity of the utricle likely plays an essential role in facilitating midshipman social and reproductive acoustic communication.NEW & NOTEWORTHY In many animals, vocal-acoustic communication is fundamental for facilitating social behaviors. For the vocal plainfin midshipman fish, the detection and localization of social acoustic signals are critical to the species' reproductive success. Here, we show that the utricle, an inner ear end organ often thought to primarily serve a vestibular function, serves an auditory function that is seasonally plastic and modulated by the animal's reproductive state effectively enhancing auditory sensitivity to courting male advertisement calls.

Tuned in to communication sounds: Neuronal sensitivity in the túngara frog midbrain to frequency modulated signals

For complex communication signals, it is often difficult to identify the information-bearing elements and their parameters necessary to elicit functional behavior. Consequently, it may be difficult to design stimuli that test how neurons contribute to communicative processing. For túngara frogs (Physalaemus pustulosus), however, previous behavioral testing with numerous stimuli showed that a particular frequency modulated (FM) transition in the male call is required to elicit phonotaxis and vocal responses. Modeled on such behavioral experiments, we used awake in vivo recordings of single units in the midbrain to determine if their excitation was biased to behaviorally important FM parameters. Comparisons of stimulus driven action potentials revealed greatest excitation to the behaviorally important FM transition: a downward FM sweep or step that crosses ~600 Hz. Previous studies using long-duration acoustic exposure found immediate early gene expression in many midbrain neurons to be most sensitive to similar FM. However, those data could not determine if FM coding was accomplished by the population and/or individual neurons. Our data suggest both coding schemes could operate, as 1) individual neurons are more sensitive to the behaviorally significant FM transition and 2) when single unit recordings are analytically combined across cells, the combined code can produce high stimulus discrimination (FM vs. noise driven excitation), approaching that found in behavioral discrimination of call vs. noise.

Evolutionary and Allometric Insights into Anuran Auditory Sensitivity and Morphology

As species change through evolutionary time, the neurological and morphological structures that underlie behavioral systems typically remain coordinated. This is especially important for communication systems, in which these structures must remain coordinated both within and between senders and receivers for successful information transfer. The acoustic communication of anurans ("frogs") offers an excellent system to ask when and how such coordination is maintained, and to allow researchers to dissociate allometric effects from independent correlated evolution. Anurans constitute one of the most speciose groups of vocalizing vertebrates, and females typically rely on vocalizations to localize males for reproduction. Here, we compile and compare data on various aspects of auditory morphology, hearing sensitivity, and call-dominant frequency across 81 species of anurans. We find robust, phylogenetically independent scaling effects of body size for all features measured. Furthermore, after accounting for body size, we find preliminary evidence that morphological evolution beyond allometry can correlate with hearing sensitivity and dominant frequency. These data provide foundational results regarding constraints imposed by body size on communication systems and motivate further data collection and analysis using comparative approaches across the numerous anuran species.

Social Communication across Reproductive Boundaries: Hormones and the Auditory Periphery of Songbirds and Frogs

Most animals experience reproductive transitions in their lives; for example, reaching reproductive maturity or cycling in and out of breeding condition. Some reproductive transitions are abrupt, while others are more gradual. In most cases, changes in communication between the sexes follow the time course of these reproductive transitions and are typically thought to be coordinated by steroid hormones. We know a great deal about hormonal control of communication behaviors in birds and frogs, as well as the central neural control of these behaviors. There has also been significant interest in the effects of steroid hormones on central nervous system structures that control both the production and reception of communication signals associated with reproductive behaviors. However, peripheral sensory structures have typically received less attention, although there has been growing interest in recent years. It is becoming clear that peripheral sensory systems play an important role in reproductive communication, are plastic across reproductive conditions, and, in some cases, this plasticity may be mediated by steroid hormones. In this article, we discuss recent evidence for the role of peripheral auditory structures in reproductive communication in birds and frogs, the plasticity of the peripheral auditory system, and the role of steroid hormones in mediating the effects of the peripheral auditory system on reproductive communication. We focus on both seasonal and acute reproductive transitions, introduce new data on the role of hormones in modulating seasonal patterns, and make recommendations for future work.

Endocrine Modulation of Sending and Receiving Signals in Context-Dependent Social Communication

Animal communication requires senders to transmit signals through the environment to conspecific receivers, which then leads to context-dependent behavioral decisions. Sending and receiving sensory information in social contexts, however, can be dramatically influenced by an individual’s internal state, particularly in species that cycle in and out of breeding or other physiological condition like nutritional state or social status. Modulatory substances like steroids, peptides, and biogenic amines can influence both the substrates used for sending social signals (e.g., motivation centers, sensorimotor pathways, and muscles) as well as the peripheral sensory organs and central neural circuitry involved in the reception of this information and subsequent execution of behavioral responses. This issue highlights research from neuroethologists on the topic of modulation of sending and receiving social signals and demonstrates that it can occur in both males and females, in different senses at both peripheral sensory organs and the brain, at different levels of biological organization, on different temporal scales, in various social contexts, and across many diverse vertebrate taxa. Modifying a signal produced by a sender or how that signal is perceived in a receiver provides flexibility in communication and has broad implications for influencing social decisions like mate choice, which ultimately affects reproductive fitness and species persistence. This phenomenon of modulators and internal physiological state impacting communication abilities is likely more widespread than currently realized and we hope this issue inspires others working on diverse systems to examine this topic from different perspectives. An integrative and comparative approach will advance discovery in this field and is needed to better understand how endocrine modulation contributes to sexual selection and the evolution of animal communication in general.

Reproductive- and Social-State Plasticity of Multiple Sensory Systems in a Cichlid Fish

Intra- and inter-sexual communications are vital to the survival and reproductive success of animals. In species that cycle in and out of breeding or other physiological condition, sensory function can be modulated to optimize communication at crucial times. Little is known, however, about how widespread this sensory plasticity is across taxa, whether it occurs in multiple senses or both sexes within a species, and what potential modulatory substances and substrates are involved. Thus, studying modulation of sensory communication in a single species can provide valuable insights for understanding how sensory abilities can be altered to optimize detection of salient signals in different sensory channels and social contexts. The African cichlid fish Astatotilapia burtoni uses multimodal communication in social contexts such as courtship, territoriality, and parental care and shows plasticity in sensory abilities. In this review, we synthesize what is known about how visual, acoustic, and chemosensory communication is used in A. burtoni in inter- and intra-specific social contexts, how sensory funtion is modulated by an individual's reproductive, metabolic, and social state, and discuss evidence for plasticity in potential modulators that may contribute to changes in sensory abilities and behaviors. Sensory plasticity in females is primarily associated with the natural reproductive cycle and functions to improve detection of courtship signals (visual, auditory, chemosensory, and likely mechanosensory) from high-quality males for reproduction. Plasticity in male sensory abilities seems to function in altering their ability to detect the status of other males in the service of territory ownership and future reproductive opportunities. Changes in different classes of potential modulators or their receptors (steroids, neuropeptides, and biogenic amines) occur at both peripheral sensory organs (eye, inner ear, and olfactory epithelium) and central visual, olfactory, and auditory processing regions, suggesting complex mechanisms contributing to plasticity of sensory function. This type of sensory plasticity revealed in males and females of A. burtoni is likely more widespread among diverse animals than currently realized, and future studies should take an integrative and comparative approach to better understand the proximate and ultimate mechanisms modulating communication abilities across taxa.

Reproductive state-dependent plasticity in the visual system of an African cichlid fish

Visual communication is used widely across the animal kingdom to convey crucial information about an animals' identity, reproductive status, and sex. Although it is well-demonstrated that auditory and olfactory sensitivity can change with reproductive state, fewer studies have tested for plasticity in the visual system, a surprising detail since courtship and mate choice behaviors in many species are largely dependent on visual signals. Here, we tested for reproductive state-dependent plasticity in the eye of the cichlid fish Astatotilapia burtoni using behavioral, gene expression, neural activation, and electrophysiology techniques. Males court ovulated females more intensely than gravid females, and ovulated females were more responsive to male courtship behaviors than gravid females. Using electroretinography to measure visual sensitivity in dark-adapted fish, we revealed that gravid, reproductively-ready females have increased visual sensitivity at wavelengths associated with male courtship coloration compared to non-gravid females. After ovulation was hormonally induced, female's spectral sensitivity further increased compared to pre-injection measurements. This increased sensitivity after hormone injection was absent in non-gravid females and in males, suggesting an ovulation-triggered increase in visual sensitivity. Ovulated females had higher mRNA expression levels of reproductive neuromodulatory receptors (sex-steroids; gonadotropins) in the eye than nonovulated females, whereas males had similar expression levels independent of reproductive/social state. In addition, female mate choice-like behaviors positively correlated with expression of gonadotropin system receptors in the eye. Collectively, these data provide crucial evidence linking endocrine modulation of visual plasticity to mate choice behaviors in females.

The difference a day makes: Breeding remodels hearing, hormones and behavior in female Cope's gray treefrogs (Hyla chrysoscelis)

In seasonal breeders, there are behavioral, endocrine, and neural adaptations that promote the sexual receptivity of females and tune their sensory systems to detect and discriminate among advertising males and to successfully copulate. What happens immediately after this key life history event is unclear, but this transitional moment offers a window into the mechanisms that remodel sexual phenotypes. In this study of wild female Cope's gray treefrogs (Hyla chrysoscelis), we tested the hypothesis that oviposition results in a suite of coordinated changes in the sexual phenotype. Specifically, we predicted that sexual receptivity and discrimination behaviors would decline along with circulating concentrations of steroid hormones (corticosterone, estradiol, testosterone) and auditory sensitivity to the acoustic frequencies emphasized in male advertisement calls. We conducted these trait measurements before and after oviposition (ca. 24-h period). There was a 100% decrease in behavioral responsiveness after oviposition, and the concentrations of all three steroids plummeted during this brief window of time, especially testosterone. Moreover, higher concentrations of corticosterone-an important component of the endocrine stress response-were associated with longer response latencies, suggesting that adrenal hormones should be considered in future studies on the hormonal basis of mate choice. Counter to our prediction, auditory sensitivity increased following oviposition, and the amplitude of the auditory brainstem response was influenced by concentrations of estradiol. In pre-oviposition females auditory sensitivity diminished with increasing estradiol concentrations, while sensitivity increased with increasing estradiol concentrations in post-oviposition females, suggesting non-linear estrogenic modulation of peripheral auditory neural recruitment. Overall, our results indicate that there is considerable remodeling of behavioral output following oviposition that co-occurs with changes in both endocrine and sensory physiology.

Auditory perception exhibits sexual dimorphism and left telencephalic dominance in Xenopus laevis

Sex differences in both vocalization and auditory processing have been commonly found in vocal animals, although the underlying neural mechanisms associated with sexual dimorphism of auditory processing are not well understood. In this study we investigated whether auditory perception exhibits sexual dimorphism in Xenopus laevis. To do this we measured event-related potentials (ERPs) evoked by white noise (WN) and conspecific calls in the telencephalon, diencephalon and mesencephalon respectively. Results showed that (1) the N1 amplitudes evoked in the right telencephalon and right diencephalon of males by WN are significantly different from those evoked in females; (2) in males the N1 amplitudes evoked by conspecific calls are significantly different from those evoked by WN; (3) in females the N1 amplitude for the left mesencephalon was significantly lower than for other brain areas, while the P2 and P3 amplitudes for the right mesencephalon were the smallest; in contrast these amplitudes for the left mesencephalon were the smallest in males. These results suggest auditory perception is sexually dimorphic. Moreover, the amplitude of each ERP component (N1, P2 and P3) for the left telencephalon was the largest in females and/or males, suggesting that left telencephalic dominance exists for auditory perception in Xenopus.

Summary: Investigation of auditory neural mechanisms in the South African clawed frog (Xenopus laevis) indicates that auditory perception exhibits sexual dimorphism and left telencephalic advantage.

Serotonin, estrus, and social context influence c-Fos immunoreactivity in the inferior colliculus

A fundamental task of sensory systems is to extract relevant social information from a range of environmental stimuli in the face of changing behavioral contexts and reproductive states. Neuromodulatory pathways that interact with such contextual variables are 1 mechanism for achieving this. In the mouse inferior colliculus (IC), a midbrain auditory region, the neuromodulator serotonin increases in females interacting with courting males, but events downstream of serotonin release have not been investigated. Here, we manipulated serotonin levels in female mice with the serotonin releaser fenfluramine or the serotonin depleter para-chlorophenylalaninemethyl ester (pCPA). Females were then exposed to an empty cage, a male partner, or a playback of courtship vocalizations, and the numbers of neurons in the IC with positive immunoreactivity for the immediate early gene product c-Fos were measured. The effects of drug treatments depended on social context and estrous state. Fenfluramine had greater effects in the nonsocial than in the partner social treatments. Females in proestrus or estrus and given fenfluramine had higher densities of c-Fos immunoreactive neurons, while females in diestrus had fewer immunoreactive neurons. The drug pCPA had the expected opposite effect of fenfluramine, causing a decreased response in pro/estrus females and an increased response in diestrus females. These findings show that the effects of serotonin on c-Fos activity in the IC of females is dependent on both external context and reproductive state, and suggest that these effects occur downstream of serotonin release. (PsycINFO Database Record

Do Green Treefrogs Use Social Information to Orient Outside the Breeding Season?

Gerlinde Höbel and Ashley Christie (2016) To decide efficiently where to forage, rest or breed, animals need information about their environment, which they may gather by monitoring the behavior of others. For example, attending to the signals of conspecifics or heterospecifics with similar habitat requirements may facilitate habitat choice. Such social information use seems taxonomically widespread, yet there is currently a dearth of information for amphibians. Anuran amphibians, with their highly developed auditory system and robust phonotaxis towards advertisement calls when searching for mates seem predisposed to use this hearing capability in other behavioral contexts. We conducted playback experiments to test whether anurans exploit acoustic signals in a non-reproductive context. In our experiments female Green Treefrogs did not show phonotaxis to signals associated with the presence of other frogs, and the orientation and speed of their movement was not different from animals randomly moving inside a silent arena. Previous studies documenting social information use in anurans have tested reproductively active frogs during the breeding season. By contrast, our study examined non-reproductive animals, and these did not approach social signals. We propose two non-exclusive hypotheses for this observed difference in phonotaxis behavior: (1) attending to social signals is restricted to ecologically most relevant time periods in a frogs life (i.e., finding breeding sites during the mating season), or (2) the ability of acoustic signals to stimulate the auditory system may be influenced by hormone levels regulating the reproductive state.

Hearing conspecific vocal signals alters peripheral auditory sensitivity

We investigated whether hearing advertisement calls over several nights, as happens in natural frog choruses, modified the responses of the peripheral auditory system in the green treefrog, Hyla cinerea. Using auditory evoked potentials (AEP), we found that exposure to 10 nights of a simulated male chorus lowered auditory thresholds in males and females, while exposure to random tones had no effect in males, but did result in lower thresholds in females. The threshold change was larger at the lower frequencies stimulating the amphibian papilla than at higher frequencies stimulating the basilar papilla. Suprathreshold responses to tonal stimuli were assessed for two peaks in the AEP recordings. For the peak P1 (assessed for 0.8-1.25 kHz), peak amplitude increased following chorus exposure. For peak P2 (assessed for 2-4 kHz), peak amplitude decreased at frequencies between 2.5 and 4.0 kHz, but remained unaltered at 2.0 kHz. Our results show for the first time, to our knowledge, that hearing dynamic social stimuli, like frog choruses, can alter the responses of the auditory periphery in a way that could enhance the detection of and response to conspecific acoustic communication signals.

Socially induced serotonergic fluctuations in the male auditory midbrain correlate with female behavior during courtship

Cues from social partners trigger the activation of socially responsive neuromodulatory systems, priming brain regions including sensory systems to process these cues appropriately. The fidelity with which neuromodulators reflect the qualities of ongoing social interactions in sensory regions is unclear. We addressed this issue by using voltammetry to monitor serotonergic fluctuations in an auditory midbrain nucleus, the inferior colliculus (IC), of male mice (Mus musculus) paired with females, and by concurrently measuring behaviors of both social partners. Serotonergic activity strongly increased in male mice as they courted females, relative to serotonergic activity in the same males during trials with no social partners. Across individual males, average changes in serotonergic activity were negatively correlated with behaviors exhibited by female partners, including broadband squeaks, which relate to rejection of males. In contrast, serotonergic activity did not correlate with male behaviors, including ultrasonic vocalizations. These findings suggest that during courtship, the level of serotonergic activity in the IC of males reflects the valence of the social interaction from the perspective of the male (i.e., whether the female rejects the male or not). As a result, our findings are consistent with the hypothesis that neuromodulatory effects on neural responses in the IC may reflect the reception, rather than the production, of vocal signals.

Progesterone and prostaglandin F2α induce species-typical female preferences for male sexual displays in Cope's gray treefrog (Hyla chrysoscelis)

Endocrine systems play critical roles in facilitating sexual behavior in seasonally breeding vertebrates. Much of the research exploring this topic has focused on the endocrine correlates of signaling behavior in males and sexual proceptivity in females. What is less understood is how hormones promote the expression of the often complex and highly selective set of stimulus-response behaviors that are observed in naturally breeding animals. In female frogs, phonotaxis is a robust and sensitive bioassay of mate choice and is exhibited by gravid females during the breeding season. In stark contrast, females exhibit low phonotactic responsiveness outside the breeding season, but the administration of hormones can induce sexual proceptivity. Here we test the hypothesis that manipulation of a minimal set of reproductive hormones-progesterone and prostaglandin F2α-are capable of evoking not only proceptive behavior in non-breeding females, but also the patterns of intraspecific selectivity for male sexual displays observed in gravid females tested during the breeding season. Specifically, we investigated whether preferences for faster call rates, longer call durations, and higher call efforts were similar between breeding and hormone-treated females of Cope's gray treefrog (Hyla chrysoscelis). Hormone injections induced patterns of selective phonotaxis in non-breeding females that were remarkably similar to those observed in breeding females. These results suggest that there may be an important contribution of hormonal pleiotropy in regulating this complex, acoustically-guided sexual behavior. Our findings also support the idea that hormonal induction could be used to evaluate hypotheses about selective mate choice, and its underlying mechanisms, using non-breeding females.

Effects of estradiol on neural responses to social signals in female túngara frogs

Estradiol plays an important role in mediating changes in female sexual behavior across reproductive cycles. In the túngara frog [Physalaemus (=Engystomops) pustulosus], the relationship between gonadal activity and female sexual behavior, as expressed by phonotaxis, is mediated primarily by estradiol. Estradiol receptors are expressed in auditory and motivational brain areas and the hormone could serve as an important modulator of neural responses to conspecific calls. To better understand how estradiol modifies neural responses to conspecific social signals, we manipulated estradiol levels and measured expression of the immediate early gene egr-1 in the auditory midbrain, thalamus and limbic forebrain in response to conspecific or heterospecific calls. We found that estradiol and conspecific calls increased egr-1 expression in the auditory midbrain and limbic forebrain, but in the thalamus, only conspecific calls were effective. In the preoptic area, estradiol enhanced the effect of the conspecific call on egr-1 expression, suggesting that the preoptic area could act as a hormonal gatekeeper to phonotaxis. Overall, the results suggest that estradiol has broad influences on the neural circuit involved in female reproduction, particularly those implicated in phonotaxis.

Assessing stimulus and subject influences on auditory evoked potentials and their relation to peripheral physiology in green treefrogs (Hyla cinerea)

Anurans (frogs and toads) are important models for comparative studies of communication, auditory physiology, and neuroethology, but to date, most of our knowledge comes from in-depth studies of a relatively small number of model species. Using the well-studied green treefrog (Hyla cinerea), this study sought to develop and evaluate the use of auditory evoked potentials (AEPs) as a minimally invasive tool for investigating auditory sensitivity in a larger diversity of anuran species. The goals of the study were to assess the effects of frequency, signal level, sex, and body size on auditory brainstem response (ABR) amplitudes and latencies, characterize gross ABR morphology, and generate an audiogram that could be compared to several previously published audiograms for green treefrogs. Increasing signal level resulted in larger ABR amplitudes and shorter latencies, and these effects were frequency dependent. There was little evidence for an effect of sex or size on ABRs. Analyses consistently distinguished between responses to stimuli in the frequency ranges of the three previously-described populations of afferents that innervate the two auditory end organs in anurans. The overall shape of the audiogram shared prominent features with previously published audiograms. This study highlights the utility of AEPs as a valuable tool for the study of anuran auditory sensitivity.

Sound-by-sound thalamic stimulation modulates midbrain auditory excitability and relative binaural sensitivity in frogs

Descending circuitry can modulate auditory processing, biasing sensitivity to particular stimulus parameters and locations. Using awake in vivo single unit recordings, this study tested whether electrical stimulation of the thalamus modulates auditory excitability and relative binaural sensitivity in neurons of the amphibian midbrain. In addition, by using electrical stimuli that were either longer than the acoustic stimuli (i.e., seconds) or presented on a sound-by-sound basis (ms), experiments addressed whether the form of modulation depended on the temporal structure of the electrical stimulus. Following long duration electrical stimulation (3-10 s of 20 Hz square pulses), excitability (spikes/acoustic stimulus) to free-field noise stimuli decreased by 32%, but returned over 600 s. In contrast, sound-by-sound electrical stimulation using a single 2 ms duration electrical pulse 25 ms before each noise stimulus caused faster and varied forms of modulation: modulation lasted <2 s and, in different cells, excitability either decreased, increased or shifted in latency. Within cells, the modulatory effect of sound-by-sound electrical stimulation varied between different acoustic stimuli, including for different male calls, suggesting modulation is specific to certain stimulus attributes. For binaural units, modulation depended on the ear of input, as sound-by-sound electrical stimulation preceding dichotic acoustic stimulation caused asymmetric modulatory effects: sensitivity shifted for sounds at only one ear, or by different relative amounts for both ears. This caused a change in the relative difference in binaural sensitivity. Thus, sound-by-sound electrical stimulation revealed fast and ear-specific (i.e., lateralized) auditory modulation that is potentially suited to shifts in auditory attention during sound segregation in the auditory scene.

Prior experience with conspecific signals enhances auditory midbrain responsiveness to conspecific vocalizations

There is a long history in neuroethology of investigating how communication signals influence the brain and behavior. It has become increasingly clear that brain areas associated with sensory processing are plastic in adults and that this plasticity is related to reproductive condition. However, the role of communication signal reception in adult auditory plasticity has received relatively little attention. Here, we investigated whether the reception of communication signals (a frog chorus) could enhance the responsiveness of the auditory system to future reception of communication signals (a single male call). We found that animals that had been exposed to 10 days of a male chorus had stronger auditory midbrain immediate early gene expression than animals that had been exposed to 10 days of random tones when tested with 30 min of male calls or 30 min of tones. Our results suggest that exposure to dynamic social stimuli, like frog choruses, may play an important role in shaping the neural and behavioral responses to communication signals.

Auditory brainstem responses in Cope's gray treefrog (Hyla chrysoscelis): effects of frequency, level, sex and size

Our knowledge of the hearing abilities of frogs and toads is largely defined by work with a few well-studied species. One way to further advance comparative work on anuran hearing would be greater use of minimally invasive electrophysiological measures, such as the auditory brainstem response (ABR). This study used the ABR evoked by tones and clicks to investigate hearing in Cope's gray treefrog (Hyla chrysoscelis). The objectives were to characterize the effects of sound frequency, sound pressure level, and subject sex and body size on ABRs. The ABR in gray treefrogs bore striking resemblance to ABRs measured in other animals. As stimulus level increased, ABR amplitude increased and latency decreased, and for responses to tones, these effects depended on stimulus frequency. Frequency-dependent differences in ABRs were correlated with expected differences in the tuning of two sensory end organs in the anuran inner ear (the amphibian and basilar papillae). The ABR audiogram indicated two frequency regions of increased sensitivity corresponding to the expected tuning of the two papillae. Overall, there was no effect of subject size and only small effects related to subject sex. Together, these results indicate the ABR is an effective method to study audition in anurans.

Treefrogs as animal models for research on auditory scene analysis and the cocktail party problem

The perceptual analysis of acoustic scenes involves binding together sounds from the same source and separating them from other sounds in the environment. In large social groups, listeners experience increased difficulty performing these tasks due to high noise levels and interference from the concurrent signals of multiple individuals. While a substantial body of literature on these issues pertains to human hearing and speech communication, few studies have investigated how nonhuman animals may be evolutionarily adapted to solve biologically analogous communication problems. Here, I review recent and ongoing work aimed at testing hypotheses about perceptual mechanisms that enable treefrogs in the genus Hyla to communicate vocally in noisy, multi-source social environments. After briefly introducing the genus and the methods used to study hearing in frogs, I outline several functional constraints on communication posed by the acoustic environment of breeding "choruses". Then, I review studies of sound source perception aimed at uncovering how treefrog listeners may be adapted to cope with these constraints. Specifically, this review covers research on the acoustic cues used in sequential and simultaneous auditory grouping, spatial release from masking, and dip listening. Throughout the paper, I attempt to illustrate how broad-scale, comparative studies of carefully considered animal models may ultimately reveal an evolutionary diversity of underlying mechanisms for solving cocktail-party-like problems in communication.

Electroencephalographic signals synchronize with behaviors and are sexually dimorphic during the light-dark cycle in reproductive frogs

Male frogs behave differently from females during the breeding season, particularly with respect to courtship displays and in response to mating signals. In search of physiological correlates of these differences, the present study measured changes in baseline electroencephalogram (EEG) power output within four frequency bands in the telencephalon and mesencephalon, together with changes in locomotor activity as a function of the light-dark cycle in male and female Emei music frogs (Babina daunchina) at the reproductive stage. Previous studies have shown that male vocal activity varies both seasonally and daily in this species and that females use male advertisement calls to locate and select mates. The present results show that both EEG and locomotor activity exhibit highly correlated circadian patterns with peaks around light onset and offset. Importantly, during the reproductive stage, statistically significant sex differences in EEG output across brain regions during the light and dark phases were found indicating that sexual dimorphism exists for EEG activity which may underlie sexually specific information processing and behavioral activities.

Context-dependent fluctuation of serotonin in the auditory midbrain: the influence of sex, reproductive state and experience

In the face of changing behavioral situations, plasticity of sensory systems can be a valuable mechanism to facilitate appropriate behavioral responses. In the auditory system, the neurotransmitter serotonin is an important messenger for context-dependent regulation because it is sensitive to both external events and internal state, and it modulates neural activity. In male mice, serotonin increases in the auditory midbrain region, the inferior colliculus (IC), in response to changes in behavioral context such as restriction stress and social contact. Female mice have not been measured in similar contexts, although the serotonergic system is sexually dimorphic in many ways. In the present study, we investigated the effects of sex, experience and estrous state on the fluctuation of serotonin in the IC across contexts, as well as potential relationships between behavior and serotonin. Contrary to our expectation, there were no sex differences in increases of serotonin in response to a restriction stimulus. Both sexes had larger increases in second exposures, suggesting experience plays a role in serotonergic release in the IC. In females, serotonin increased during both restriction and interactions with males; however, the increase was more rapid during restriction. There was no effect of female estrous phase on the serotonergic change for either context, but serotonin was related to behavioral activity in females interacting with males. These results show that changes in behavioral context induce increases in serotonin in the IC by a mechanism that appears to be uninfluenced by sex or estrous state, but may depend on experience and behavioral activity.

Estrogenic modulation of auditory processing: a vertebrate comparison

Sex-steroid hormones are well-known regulators of vocal motor behavior in several organisms. A large body of evidence now indicates that these same hormones modulate processing at multiple levels of the ascending auditory pathway. The goal of this review is to provide a comparative analysis of the role of estrogens in vertebrate auditory function. Four major conclusions can be drawn from the literature: First, estrogens may influence the development of the mammalian auditory system. Second, estrogenic signaling protects the mammalian auditory system from noise- and age-related damage. Third, estrogens optimize auditory processing during periods of reproductive readiness in multiple vertebrate lineages. Finally, brain-derived estrogens can act locally to enhance auditory response properties in at least one avian species. This comparative examination may lead to a better appreciation of the role of estrogens in the processing of natural vocalizations and mayprovide useful insights toward alleviating auditory dysfunctions emanating from hormonal imbalances.

Songbird frequency selectivity and temporal resolution vary with sex and season

Many species of songbirds exhibit dramatic seasonal variation in song output. Recent evidence suggests that seasonal changes in auditory processing are coincident with seasonal variation in vocal output. Here, we show, for the first time, that frequency selectivity and temporal resolution of the songbird auditory periphery change seasonally and in a sex-specific manner. Male and female house sparrows (Passer domesticus) did not differ in their frequency sensitivity during the non-breeding season, nor did they differ in their temporal resolution. By contrast, female house sparrows showed enhanced frequency selectivity during the breeding season, which was matched by a concomitant reduction of temporal resolution. However, males failed to show seasonal plasticity in either of these auditory properties. We discuss potential mechanisms generating these seasonal patterns and the implications of sex-specific seasonal changes in auditory processing for vocal communication.

Plasticity in ion channel expression underlies variation in hearing during reproductive cycles

Sensory plasticity related to reproductive state, hormonal profiles, and experience is widespread among vertebrates, including humans. Improvements in audio-vocal coupling that heighten the detection of conspecifics are part of the reproductive strategy of many nonmammalian vertebrates. Although seasonal changes in hearing are known, molecular mechanisms determining this form of adult sensory plasticity remain elusive. Among both nonmammals and mammals, large-conductance, calcium-activated potassium (BK) channels underlie a primary outward current having a predominant influence on frequency tuning in auditory hair cells. We now report an example from fish showing that increased BK channel abundance can improve an individual's ability to hear vocalizations during the breeding season. Pharmacological manipulations targeting BK channels, together with measures of BK transcript abundance, can explain the seasonal enhancement of auditory hair cell sensitivity to the frequency content of calls. Plasticity in ion channel expression is a simple, evolutionarily labile solution for sculpting sensory bandwidth to maximize the detection of conspecific signals during reproductive cycles.

From behavioral context to receptors: serotonergic modulatory pathways in the IC

In addition to ascending, descending, and lateral auditory projections, inputs extrinsic to the auditory system also influence neural processing in the inferior colliculus (IC). These types of inputs often have an important role in signaling salient factors such as behavioral context or internal state. One route for such extrinsic information is through centralized neuromodulatory networks like the serotonergic system. Serotonergic inputs to the IC originate from centralized raphe nuclei, release serotonin in the IC, and activate serotonin receptors expressed by auditory neurons. Different types of serotonin receptors act as parallel pathways regulating specific features of circuitry within the IC. This results from variation in subcellular localizations and effector pathways of different receptors, which consequently influence auditory responses in distinct ways. Serotonin receptors may regulate GABAergic inhibition, influence response gain, alter spike timing, or have effects that are dependent on the level of activity. Serotonin receptor types additionally interact in nonadditive ways to produce distinct combinatorial effects. This array of effects of serotonin is likely to depend on behavioral context, since the levels of serotonin in the IC transiently increase during behavioral events including stressful situations and social interaction. These studies support a broad model of serotonin receptors as a link between behavioral context and reconfiguration of circuitry in the IC, and the resulting possibility that plasticity at the level of specific receptor types could alter the relationship between context and circuit function.

Organization of Estrogen-Associated Circuits in the Mouse Primary Auditory Cortex

Sex steroid hormones influence the perceptual processing of sensory signals in vertebrates. In particular, decades of research have shown that circulating levels of estrogen correlate with hearing function. The mechanisms and sites of action supporting this sensory-neuroendocrine modulation, however, remain unknown. Here we combined a molecular cloning strategy, fluorescence in-situ hybridization and unbiased quantification methods to show that estrogen-producing and -sensitive neurons heavily populate the adult mouse primary auditory cortex (AI). We also show that auditory experience in freely-behaving animals engages estrogen-producing and -sensitive neurons in AI. These estrogen-associated networks are greatly stable, and do not quantitatively change as a result of acute episodes of sensory experience. We further demonstrate the neurochemical identity of estrogen-producing and estrogen-sensitive neurons in AI and show that these cell populations are phenotypically distinct. Our findings provide the first direct demonstration that estrogen-associated circuits are highly prevalent and engaged by sensory experience in the mouse auditory cortex, and suggest that previous correlations between estrogen levels and hearing function may be related to brain-generated hormone production. Finally, our findings suggest that estrogenic modulation may be a central component of the operational framework of central auditory networks.

Contextual modulation of behavioral choice

We review the influence of context on behavioral choice. Context can refer to external (environmental) factors such as the season or presence of predators and it can also refer to the internal or behavioral state of an animal. Usually, animals make decisions in the midst of other ongoing behaviors. We discuss recent findings on the impact of both types of contexts, focusing on how context gets encoded at the intersection between the sensory and motor systems, emphasizing the role of neuromodulators. We also review recent technological advances that have made feasible the exploration of neural correlates of decision making in freely moving, behaving animals.

Female sexual arousal in amphibians

Rather than being a static, species specific trait, reproductive behavior in female amphibians is variable within an individual during the breeding season when females are capable of reproductive activity. Changes in receptivity coincide with changes in circulating estrogen. Estrogen is highest at the point when females are ready to choose a male and lay eggs. At this time female receptivity (her probability of responding to a male vocal signal) is highest and her selectivity among conspecific calls (measured by her probability of responding to a degraded or otherwise usually unattractive male signal) is lowest. These changes occur even though females retain the ability to discriminate different acoustic characteristics of various conspecific calls. After releasing her eggs, female amphibians quickly become less receptive and more choosy in terms of their responses to male sexual advertisement signals. Male vocal signals stimulate both behavior and estrogen changes in amphibian females making mating more probable. The changes in female reproductive behavior are the same as those generally accepted as indicative of a change in female sexual arousal leading to copulation. They are situationally triggered, gated by interactions with males, and decline with the consummation of sexual reproduction with a chosen male. The changes can be triggered by either internal physiological state or by the presence of stimuli presented by males, and the same stimuli change both behavior and physiological (endocrine) state in such a way as to make acceptance of a male more likely. Thus amphibian females demonstrate many of the same general characteristics of changing female sexual state that in mammals indicate sexual arousal.

The behavioral neuroscience of anuran social signal processing

Acoustic communication is the major component of social behavior in anuran amphibians (frogs and toads) and has served as a neuroethological model for the nervous system's processing of social signals related to mate choice decisions. The male's advertisement or mating call is its most conspicuous social signal, and the nervous system's analysis of the call is a progressive process. As processing proceeds through neural systems, response properties become more specific to the signal and, in addition, neural activity gradually shifts from representing sensory (auditory periphery and brainstem) to sensorimotor (diencephalon) to motor (forebrain) components of a behavioral response. A comparative analysis of many anuran species shows that the first stage in biasing responses toward conspecific signals over heterospecific signals, and toward particular features of conspecific signals, lies in the tuning of the peripheral auditory system. Biases in processing signals are apparent through the brainstem auditory system, where additional feature detection neurons are added by the time processing reaches the level of the midbrain. Recent work using immediate early gene expression as a marker of neural activity suggests that by the level of the midbrain and forebrain, the differential neural representation of conspecific and heterospecific signals involves both changes in mean activity levels across multiple subnuclei, and in the functional correlations among acoustically active areas. Our data show that in frogs the auditory midbrain appears to play an important role in controlling behavioral responses to acoustic social signals by acting as a regulatory gateway between the stimulus analysis of the brainstem and the behavioral and physiological control centers of the forebrain. We predict that this will hold true for other vertebrate groups such as birds and fish that produce acoustic social signals, and perhaps also in fish where electroreception or vibratory sensing through the lateral line systems plays a role in social signaling, as in all these cases ascending sensory information converges onto midbrain nuclei which relay information to higher brain centers.

Peripheral auditory processing changes seasonally in Gambel's white-crowned sparrow

Song in oscine birds is a learned behavior that plays important roles in breeding. Pronounced seasonal differences in song behavior and in the morphology and physiology of the neural circuit underlying song production are well documented in many songbird species. Androgenic and estrogenic hormones largely mediate these seasonal changes. Although much work has focused on the hormonal mechanisms underlying seasonal plasticity in songbird vocal production, relatively less work has investigated seasonal and hormonal effects on songbird auditory processing, particularly at a peripheral level. We addressed this issue in Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii), a highly seasonal breeder. Photoperiod and hormone levels were manipulated in the laboratory to simulate natural breeding and non-breeding conditions. Peripheral auditory function was assessed by measuring the auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOAEs) of males and females in both conditions. Birds exposed to breeding-like conditions demonstrated elevated thresholds and prolonged peak latencies when compared with birds housed under non-breeding-like conditions. There were no changes in DPOAEs, however, which indicates that the seasonal differences in ABRs do not arise from changes in hair cell function. These results suggest that seasons and hormones impact auditory processing as well as vocal production in wild songbirds.

Dissecting natural sensory plasticity: hormones and experience in a maternal context

There is a growing consensus that the auditory system is dynamic in its representation of behaviorally relevant sounds. The auditory cortex in particular seems to be an important locus for plasticity that may reflect the memory of such sounds, or functionally improve their processing. The mechanisms that underlie these changes may be either intrinsic because they depend on the receiver's physiological state, or extrinsic because they arise from the context in which behavioral relevance is gained. Research in a mouse model of acoustic communication between offspring and adult females offers the opportunity to explore both of these contributions to auditory cortical plasticity in a natural context. Recent works have found that after the vocalizations of infant mice become behaviorally relevant to mothers, auditory cortical activity is significantly changed in a way that may improve their processing. Here we consider the hypothesis that maternal hormones (intrinsic factor) and sensory experience (extrinsic factor) contribute together to drive these changes, focusing specifically on the evidence that well-known experience-dependent mechanisms of cortical plasticity can be modulated by hormones.