AM representation in green treefrog auditory nerve fibers: neuroethological implications for pattern recognition and sound localization

Lung-to-ear sound transmission does not improve directional hearing in green treefrogs (Hyla cinerea)

Amphibians are unique among extant vertebrates in having middle ear cavities that are internally coupled to each other and to the lungs. In frogs, the lung-to-ear sound transmission pathway can influence the tympanum's inherent directionality, but what role such effects might play in directional hearing remains unclear. In this study of the American green treefrog (Hyla cinerea), we tested the hypothesis that the lung-to-ear sound transmission pathway functions to improve directional hearing, particularly in the context of intraspecific sexual communication. Using laser vibrometry, we measured the tympanum's vibration amplitude in females in response to a frequency modulated sweep presented from 12 sound incidence angles in azimuth. Tympanum directionality was determined across three states of lung inflation (inflated, deflated, reinflated) both for a single tympanum in the form of the vibration amplitude difference (VAD) and for binaural comparisons in the form of the interaural vibration amplitude difference (IVAD). The state of lung inflation had negligible effects (typically less than 0.5 dB) on both VADs and IVADs at frequencies emphasized in the advertisement calls produced by conspecific males (834 and 2730 Hz). Directionality at the peak resonance frequency of the lungs (1558 Hz) was improved by ∼3 dB for a single tympanum when the lungs were inflated versus deflated, but IVADs were not impacted by the state of lung inflation. Based on these results, we reject the hypothesis that the lung-to-ear sound transmission pathway functions to improve directional hearing in frogs.

Crocodiles use both interaural level differences and interaural time differences to locate a sound source

To explore how crocodilians locate a sound source, two Nile crocodiles (Crocodylus niloticus) were trained to swim towards an acoustic target. Using filtered versions of synthesized stimuli, the respective roles of interaural level differences (ILDs) and interaural time differences (ITDs), which are the two main cues providing information on sound source position, were tested. This study shows that crocodiles rely on both ILDs and ITDs to locate the spatial direction of a sound source and that their performance is lower when one of the cues is lacking.

Sound source localization and segregation with internally coupled ears: the treefrog model

Acoustic signaling plays key roles in mediating many of the reproductive and social behaviors of anurans (frogs and toads). Moreover, acoustic signaling often occurs at night, in structurally complex habitats, such as densely vegetated ponds, and in dense breeding choruses characterized by high levels of background noise and acoustic clutter. Fundamental to anuran behavior is the ability of the auditory system to determine accurately the location from where sounds originate in space (sound source localization) and to assign specific sounds in the complex acoustic milieu of a chorus to their correct sources (sound source segregation). Here, we review anatomical, biophysical, neurophysiological, and behavioral studies aimed at identifying how the internally coupled ears of frogs contribute to sound source localization and segregation. Our review focuses on treefrogs in the genus Hyla, as they are the most thoroughly studied frogs in terms of sound source localization and segregation. They also represent promising model systems for future work aimed at understanding better how internally coupled ears contribute to sound source localization and segregation. We conclude our review by enumerating directions for future research on these animals that will require the collaborative efforts of biologists, physicists, and roboticists.

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.

Sex differences and endocrine regulation of auditory-evoked, neural responses in African clawed frogs (Xenopus)

Mating depends on the accurate detection of signals that convey species identity and reproductive state. In African clawed frogs, Xenopus, this information is conveyed by vocal signals that differ in temporal patterns and spectral features between sexes and across species. We characterized spectral sensitivity using auditory-evoked potentials (AEPs), commonly known as the auditory brainstem response, in males and females of four Xenopus species. In female X. amieti, X. petersii, and X. laevis, peripheral auditory sensitivity to their species own dyad-two, species-specific dominant frequencies in the male advertisement call-is enhanced relative to males. Males were most sensitive to lower frequencies including those in the male-directed release calls. Frequency sensitivity was influenced by endocrine state; ovariectomized females had male-like auditory tuning while dihydrotestosterone-treated, ovariectomized females maintained female-like tuning. Thus, adult, female Xenopus demonstrate an endocrine-dependent sensitivity to the spectral features of conspecific male advertisement calls that could facilitate mating. Xenopus AEPs resemble those of other species in stimulus and level dependence, and in sensitivity to anesthetic (MS222). AEPs were correlated with body size and sex within some species. A frequency following response, probably encoded by the amphibian papilla, might facilitate dyad source localization via interaural time differences.

Evolutionary adaptations for the temporal processing of natural sounds by the anuran peripheral auditory system

Sensory systems function most efficiently when processing natural stimuli, such as vocalizations, and it is thought that this reflects evolutionary adaptation. Among the best-described examples of evolutionary adaptation in the auditory system are the frequent matches between spectral tuning in both the peripheral and central auditory systems of anurans (frogs and toads) and the frequency spectra of conspecific calls. Tuning to the temporal properties of conspecific calls is less well established, and in anurans has so far been documented only in the central auditory system. Using auditory-evoked potentials, we asked whether there are species-specific or sex-specific adaptations of the auditory systems of gray treefrogs (Hyla chrysoscelis) and green treefrogs (H. cinerea) to the temporal modulations present in conspecific calls. Modulation rate transfer functions (MRTFs) constructed from auditory steady-state responses revealed that each species was more sensitive than the other to the modulation rates typical of conspecific advertisement calls. In addition, auditory brainstem responses (ABRs) to paired clicks indicated relatively better temporal resolution in green treefrogs, which could represent an adaptation to the faster modulation rates present in the calls of this species. MRTFs and recovery of ABRs to paired clicks were generally similar between the sexes, and we found no evidence that males were more sensitive than females to the temporal modulation patterns characteristic of the aggressive calls used in male-male competition. Together, our results suggest that efficient processing of the temporal properties of behaviorally relevant sounds begins at potentially very early stages of the anuran auditory system that include the periphery.

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.

Spatial hearing in Cope's gray treefrog: II. Frequency-dependent directionality in the amplitude and phase of tympanum vibrations

Anuran ears function as pressure difference receivers, and the amplitude and phase of tympanum vibrations are inherently directional, varying with sound incident angle. We quantified the nature of this directionality for Cope's gray treefrog, Hyla chrysoscelis. We presented subjects with pure tones, advertisement calls, and frequency-modulated sweeps to examine the influence of frequency, signal level, lung inflation, and sex on ear directionality. Interaural differences in the amplitude of tympanum vibrations were 1-4 dB greater than sound pressure differences adjacent to the two tympana, while interaural differences in the phase of tympanum vibration were similar to or smaller than those in sound phase. Directionality in the amplitude and phase of tympanum vibration were highly dependent on sound frequency, and directionality in amplitude varied slightly with signal level. Directionality in the amplitude and phase of tone- and call-evoked responses did not differ between sexes. Lung inflation strongly affected tympanum directionality over a narrow frequency range that, in females, included call frequencies. This study provides a foundation for further work on the biomechanics and neural mechanisms of spatial hearing in H. chrysoscelis, and lends valuable perspective to behavioral studies on the use of spatial information by this species and other frogs.

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.

Dip listening or modulation masking? Call recognition by green treefrogs (Hyla cinerea) in temporally fluctuating noise

Despite the importance of perceptually separating signals from background noise, we still know little about how nonhuman animals solve this problem. Dip listening, an ability to catch meaningful 'acoustic glimpses' of a target signal when fluctuating background noise levels momentarily drop, constitutes one possible solution. Amplitude-modulated noises, however, can sometimes impair signal recognition through a process known as modulation masking. We asked whether fluctuating noise simulating a breeding chorus affects the ability of female green treefrogs (Hyla cinerea) to recognize male advertisement calls. Our analysis of recordings of the sounds of green treefrog choruses reveal that their levels fluctuate primarily at rates below 10 Hz. In laboratory phonotaxis tests, we found no evidence for dip listening or modulation masking. Mean signal recognition thresholds in the presence of fluctuating chorus-like noises were never statistically different from those in the presence of a non-fluctuating control. An analysis of statistical effects sizes indicates that masker fluctuation rates, and the presence versus absence of fluctuations, had negligible effects on subject behavior. Together, our results suggest that females listening in natural settings should receive no benefits, nor experience any additional constraints, as a result of level fluctuations in the soundscape of green treefrog choruses.

Coding of amplitude modulation in primary auditory cortex

Conflicting results have led to different views about how temporal modulation is encoded in primary auditory cortex (A1). Some studies find a substantial population of neurons that change firing rate without synchronizing to temporal modulation, whereas other studies fail to see these nonsynchronized neurons. As a result, the role and scope of synchronized temporal and nonsynchronized rate codes in AM processing in A1 remains unresolved. We recorded A1 neurons' responses in awake macaques to sinusoidal AM noise. We find most (37-78%) neurons synchronize to at least one modulation frequency (MF) without exhibiting nonsynchronized responses. However, we find both exclusively nonsynchronized neurons (7-29%) and "mixed-mode" neurons (13-40%) that synchronize to at least one MF and fire nonsynchronously to at least one other. We introduce new measures for modulation encoding and temporal synchrony that can improve the analysis of how neurons encode temporal modulation. These include comparing AM responses to the responses to unmodulated sounds, and a vector strength measure that is suitable for single-trial analysis. Our data support a transformation from a temporally based population code of AM to a rate-based code as information ascends the auditory pathway. The number of mixed-mode neurons found in A1 indicates this transformation is not yet complete, and A1 neurons may carry multiplexed temporal and rate codes.

Tone and call responses of units in the auditory nerve and dorsal medullary nucleus of Xenopus laevis

The clawed frog Xenopus laevis produces vocalizations consisting of distinct patterns of clicks. This study provides the first description of spontaneous, pure-tone and communication-signal evoked discharge properties of auditory nerve (n.VIII) fibers and dorsal medullary nucleus (DMN) cells in an obligatorily aquatic anuran. Responses of 297 n.VIII and 253 DMN units are analyzed for spontaneous rates (SR), frequency tuning, rate-intensity functions, and firing rate adaptation, with a view to how these basic characteristics shape responses to recorded call stimuli. Response properties generally resemble those in partially terrestrial anurans. Broad tuning exists across characteristic frequencies (CFs). Threshold minima are -101 dB re 1 mm/s at 675 Hz; -87 dB at 1,600 Hz; and -61 dB at 3,000 Hz (-90, -77, and -44 dB re 1 Pa, respectively), paralleling the peak frequency of vocalizations at 1.2-1.6 kHz with approximately 500 Hz in 3 dB bandwidth. SRs range from 0 to 80 (n.VIII) and 0 to 73 spikes/s (DMN). Nerve and DMN units of all CFs follow click rates in natural calls, < or =67 clicks/s and faster. Units encode clicks with a single spike, double spikes, or bursts. Spike times correlate closely with click envelopes. No temporal filtering for communicative click rates occurs in either n.VIII or the DMN.

Mid-frequency suppression in the green treefrog (Hyla cinerea): mechanisms and implications for the evolution of acoustic communication

Advertisement calls of green treefrogs (Hyla cinerea) have two spectral peaks centered at about 1 kHz and 3 kHz. Addition of a component of intermediate frequency (1.8 kHz) to a synthetic call reduced its attractiveness to females relative to an alternative lacking this component. This mid-frequency suppression occurred over a 20-dB range of playback levels. Addition of other intermediate frequencies had weak effects on preferences at some playback levels, in some localities, and at lower-than-normal temperatures. These effects correlate well with the response properties of a population of low-frequency-tuned auditory neurons innervating the amphibian papilla. Males of a closely related species (H. gratiosa) produce calls with emphasized frequencies within the range of suppression in H. cinerea; however, suppression also occurred in localities well outside the area of geographical overlap with this species. Thus, previous speculation that mid-frequency suppression evolved to enhance species discrimination is probably incorrect. This phenomenon is more likely to reflect a general sensory bias in anurans and other vertebrates, tone-on-tone inhibition. Such negative biases, and other inhibitory mechanisms, almost certainly play an important role in the evolution of communication systems but have received far less attention than positive biases that enhance signal attractiveness.