Frequency selectivity of hearing in the green treefrog, Hyla cinerea

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.

Lung mediated auditory contrast enhancement improves the Signal-to-noise ratio for communication in frogs

Environmental noise is a major source of selection on animal sensory and communication systems. The acoustic signals of other animals represent particularly potent sources of noise for chorusing insects, frogs, and birds, which contend with a multi-species analog of the human "cocktail party problem" (i.e., our difficulty following speech in crowds). However, current knowledge of the diverse adaptations that function to solve noise problems in nonhuman animals remains limited. Here, we show that a lung-to-ear sound transmission pathway in frogs serves a heretofore unknown noise-control function in vertebrate hearing and sound communication. Inflated lungs improve the signal-to-noise ratio for communication by enhancing the spectral contrast in received vocalizations in ways analogous to signal processing algorithms used in hearing aids and cochlear implants. Laser vibrometry revealed that the resonance of inflated lungs selectively reduces the tympanum's sensitivity to frequencies between the two spectral peaks present in conspecific mating calls. Social network analysis of continent-scale citizen science data on frog calling behavior revealed that the calls of other frog species in multi-species choruses can be a prominent source of environmental noise attenuated by the lungs. Physiological modeling of peripheral frequency tuning indicated that inflated lungs could reduce both auditory masking and suppression of neural responses to mating calls by environmental noise. Together, these data suggest an ancient adaptation for detecting sound via the lungs has been evolutionarily co-opted to create auditory contrast enhancement that contributes to solving a multi-species cocktail party problem.

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.

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.

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.

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.

"To ear is human, to frogive is divine": Bob Capranica's legacy to auditory neuroethology

Bob Capranica was a towering figure in the field of auditory neuroethology. Among his many contributions are the exploitation of the anuran auditory system as a general vertebrate model for studying communication, the introduction of a signal processing approach for quantifying sender-receiver dynamics, and the concept of the matched filter for efficient neural processing of complex vocal signals. In this paper, meant to honor Bob on his election to Fellow of the International Society for Neuroethology, I provide a description and analysis of some of his most important research, and I highlight how the concepts and data he contributed still inspire neuroethology today.

Effects of noise bandwidth and amplitude modulation on masking in frog auditory midbrain neurons

Natural auditory scenes such as frog choruses consist of multiple sound sources (i.e., individual vocalizing males) producing sounds that overlap extensively in time and spectrum, often in the presence of other biotic and abiotic background noise. Detection of a signal in such environments is challenging, but it is facilitated when the noise shares common amplitude modulations across a wide frequency range, due to a phenomenon called comodulation masking release (CMR). Here, we examined how properties of the background noise, such as its bandwidth and amplitude modulation, influence the detection threshold of a target sound (pulsed amplitude modulated tones) by single neurons in the frog auditory midbrain. We found that for both modulated and unmodulated masking noise, masking was generally stronger with increasing bandwidth, but it was weakened for the widest bandwidths. Masking was less for modulated noise than for unmodulated noise for all bandwidths. However, responses were heterogeneous, and only for a subpopulation of neurons the detection of the probe was facilitated when the bandwidth of the modulated masker was increased beyond a certain bandwidth – such neurons might contribute to CMR. We observed evidence that suggests that the dips in the noise amplitude are exploited by TS neurons, and observed strong responses to target signals occurring during such dips. However, the interactions between the probe and masker responses were nonlinear, and other mechanisms, e.g., selective suppression of the response to the noise, may also be involved in the masking release.

A re-evaluation of auditory filter shape in delphinid odontocetes: evidence of constant-bandwidth filters

The auditory filter shape of delphinid odontocetes was previously considered to be typically mammalian constant-quality in which filter bandwidths increase proportionally with frequency. Recent studies with porpoises demonstrate constant-bandwidth portions of the auditory filter. The critical ratios for a bottlenose dolphin were measured between 40 and 120  kHz by behaviorally determining the subject's ability to detect pure tones in the presence of white noise. Critical ratios as a function of frequency were constant, indicating the auditory filter acts as a constant-bandwidth system in this frequency range. Re-analysis of past studies supports these findings, and suggests the delphinid auditory system is best characterized as a constant-Q system below 40  kHz and a constant-bandwidth-like system between 40  kHz and 120  kHz before returning to a constant Q pattern at the highest frequencies.

Environmental influences in the evolution of tetrapod hearing sensitivity and middle ear tuning

Vertebrates inhabit and communicate acoustically in most natural environments. We review the influence of environmental factors on the hearing sensitivity of terrestrial vertebrates, and on the anatomy and mechanics of the middle ears. Evidence suggests that both biotic and abiotic environmental factors affect the evolution of bandwidth and frequency of peak sensitivity of the hearing spectrum. Relevant abiotic factors include medium type, temperature, and noise produced by nonliving sources. Biotic factors include heterospecific, conspecific, or self-produced sounds that animals are selected to recognize, and acoustic interference by sounds that other animals generate. Within each class of tetrapods, the size of the middle ear structures correlates directly to body size and inversely to frequency of peak sensitivity. Adaptation to the underwater medium in cetaceans involved reorganization of the middle ear for novel acoustic pathways, whereas adaptation to subterranean life in several mammals resulted in hypertrophy of the middle ear ossicles to enhance their inertial mass for detection of seismic vibrations. The comparative approach has revealed a number of generalities about the effect of environmental factors on hearing performance and middle ear structure across species. The current taxonomic sampling of the major tetrapod groups is still highly unbalanced and incomplete. Future expansion of the comparative evidence should continue to reveal general patterns and novel mechanisms.

Hearing threshold and frequency discrimination in the purely aquatic frog Xenopus laevis (Pipidae): measurement by means of conditioning

Hearing threshold and frequency discrimination for underwater sound were measured in the clawed frog Xenopus laevis by means of conditioning. A go/no go discrimination procedure was used in which the test tone was presented concurrently with a wave on the surface of the water. The tone signalled whether or not the frog should respond to the wave. The hearing range of X. laevis was 200–4000 Hz. Similar thresholds of 92–96 dB re 1 microPa were found at 600 Hz, 1400–1800 Hz and 3200–3600 Hz. A high threshold at 1000–1300 Hz suggested that this was the frequency range between the sensitivities of the amphibian and basilar papillae. Relative frequency discrimination was approximately 5 % at 400–800 Hz, 45 % at 1000 Hz and 2.4-6 % at 1600–2500 Hz. This last range encompasses the dominant frequencies of the advertisement call of this species. High discrimination acuity at these frequencies may be used in distinguishing between calling males. The threshold for a one-third-octave bandpass noise centred at 600 Hz was 27.6 dB lower than that for a pure tone of 600 Hz, suggesting that sound intensity was integrated within this bandwidth, possibly by a critical-band mechanism.

Social signals influence hormones independently of calling behavior in the treefrog (Hyla cinerea)

Social signals play an important role in regulating hormone-behavior relationships. In anurans (frogs and toads), acoustic signals are an essential aspect of reproductive behavior; however, the physiological consequences of receiving social signals has remained largely undescribed. Each night for 5, 10, or 20 days, we presented acoustically isolated male treefrogs with a conspecific mating chorus, an array of tones, or no sound. We recorded calling rate of individuals throughout the experiment and collected blood before and after treatment. Days of stimulus exposure had no effect on any dependent measure. Acoustic treatment influenced steroid levels; testosterone, dihydrotestosterone, and corticosterone increased only in the group exposed to the chorus. Chorus-exposed males also showed an increase in stimulus-evoked calling. We found no correlation between androgens and calling within each treatment group. In addition, noncallers in the chorus group had higher levels of androgens than males in the tone or no sound groups. Further, chorus-exposed males with zero, low, or high rate of calling had similar levels of androgens. These data indicate that social signals increase circulating androgens independently of calling behavior. Elevated corticosterone associated with chorus reception did not inhibit calling behavior, and corticosterone showed no correlation with androgen levels.

Detection of auditory signals by frog inferior collicular neurons in the presence of spatially separated noise

Detection of auditory signals by frog inferior collicular neurons in the presence of spatially separated noise. J. Neurophysiol. 80: 2848-2859, 1998. Psychophysical studies have shown that the ability to detect auditory signals embedded in noise improves when signal and noise sources are widely separated in space; this allows humans to analyze complex auditory scenes, as in the cocktail-part effect. Although these studies established that improvements in detection threshold (DT) are due to binaural hearing, few physiological studies were undertaken, and very little is known about the response of single neurons to spatially separated signal and noise sources. To address this issue we examined the responses of neurons in the frog inferior colliculus (IC) to a probe stimulus embedded in a spatially separated masker. Frogs perform auditory scene analysis because females select mates in dense choruses by means of auditory cues. Results of the extracellular single-unit recordings demonstrate that 22% of neurons (A-type) exhibited improvements in signal DTs when probe and masker sources were progressively separated in azimuth. In contrast, 24% of neurons (V-type) showed the opposite pattern, namely, signal DTs were lowest when probe and masker were colocalized (in many instances lower than the DT to probe alone) and increased when the two sound sources were separated. The remaining neurons demonstrated a mix of these two types of patterns. An intriguing finding was the strong correlation between A-type masking release patterns and phasic neurons and a weaker correlation between V-type patterns and tonic neurons. Although not decisive, these results suggest that phasic units may play a role in release from masking observed psychophysically. Analysis of the data also revealed a strong and nonlinear interaction among probe, masker, and masker azimuth and that signal DTs were influenced by two factors: 1) the unit's sensitivity to probe in the presence of masker and 2) the criterion level for estimating DT. For some units, it was possible to examine the interaction between these two factors and gain insights into the variation of DTs with masker azimuth. The implications of these findings are discussed in relation to signal detection in the auditory system.

The neuroethology of frequency preferences in the spring peeper

We studied the relationship between auditory activity in the midbrain and selective phonotaxis in females of the treefrog, Pseudacris crucifer. Gravid females were tested in two-stimulus playback tests using synthetic advertisement calls of different frequencies (2600 versus 2875 Hz; 2800 versus 3500 Hz; 2600 versus 3500 Hz). Tests were conducted with and without a background of synthesized noise, which was filtered to resemble the spectrum of a chorus of spring peepers. There were no significant preferences for calls of any frequency in the absence of background noise. With background noise, females preferred calls of 3500 Hz to those of 2600 Hz. Multi-unit recordings of neural responses to synthetic sounds were made from the torus semicircularis of the same females following the tests of phonotaxis. We measured auditory threshold at 25 frequencies (1800-4200 Hz) as well as the magnitude of the neural response when stimulus amplitude was held constant and frequency was varied. This procedure yielded isointensity response contours, which we obtained at six amplitudes in the absence of noise and at the stimulus amplitude used during the phonotaxis tests with background noise. Individual differences in audiograms and isointensity responses were poorly correlated with behavioural data except for the test of 2600 Hz versus 3500 Hz calls in noise. The shape of the neural response contours changed with stimulus amplitude and in the presence of the simulated frog chorus. At 85 dB sound pressure level (SPL), the level at which females were tested, the contours of females were quite flat. The contours were more peaked at lower SPLs as well as during the broadcast of chorus noise and white noise at an equivalent spectrum level (45-46 dB/Hz). Peaks in the isointensity response plots of most females occurred at stimulus frequencies ranging from 3200 to 3400 Hz, frequencies close to the median best excitatory frequency (BEF) of 3357 Hz but higher than the mean of the mid-frequency of the male advertisement call (3011 Hz). Addition of background noise may cause a shift in the neural response-intensity level functions. Our results highlight the well-known nonlinearity of the auditory system and the danger inherent in focusing solely on threshold measures of auditory sensitivity when studying the proximate basis of female choice. The results also show an unexpected effect of the natural and noisy acoustic environment on behaviour and responses of the auditory system. Copyright 1998 The Association for the Study of Animal Behaviour.

Perception of complex sounds by the green treefrog, Hyla cinerea: envelope and fine-structure cues

1. The envelope periodicity of communication signals is an important feature distinguishing advertisement and aggressive calls for the green treefrog (Hyla cinerea). Envelope periodicity, a cue for periodicity pitch perception in humans, is affected by the fine-structure of the signal, a cue for timbre perception in humans. The present study examined perception of two acoustic features affecting waveform fine-structure--harmonic structure and phase structure--in male green treefrogs. 2. We analyzed evoked vocal responses of male green treefrogs living in laboratory arenas to playbacks of digitally-generated signals resembling either conspecific advertisement or aggressive calls in their first harmonic periodicity. Systematic changes in the harmonic structure of these signals were achieved by varying the harmonic relations between frequency components in the signals, and changes in phase structure were achieved by varying the starting phases of harmonically-related components. 3. Calling was significantly influenced by the first harmonic periodicity of the signals. Males vocalized more to signals with the periodicity of the advertisement than the aggressive call. There were no differences in response to harmonic and inharmonic signals with similar spectral content. Phase structure did not significantly influence vocal responses. 4. These results suggest that the fine-structure ("timbre") of complex acoustic signals is not a significant feature guiding behavior tested using a communication response in this species.

Periodicity extraction in the anuran auditory nerve. I. "Pitch-shift" effects

1. Activity of individual eight nerve fibers in the bullfrog, Rana catesbeiana, was measured in response to complex, multiple-frequency stimuli differing in both frequency composition and harmonic structure. Stimuli were chosen to parallel types of stimuli producing "pitch-shift" effects in humans. 2. The fundamental frequency of harmonic stimuli can be extracted from the autocorrelation of fiber firing, whether the fundamental is physically present in the stimulus or is a "missing" fundamental. The spectral fine-structure of harmonic stimuli is not robustly represented in fiber temporal response. These effects are seen in both AP and BP fibers. 3. The pseudoperiod of inharmonic stimuli is represented by synchronization to successive high-amplitude peaks in the stimulus envelope. Temporal responses to stimuli with high center frequencies are similar regardless of whether their frequency components are harmonically or inharmonically related. Responses remain dominated by the envelope periodicity, and no "pitch-shift" is signaled. In response to stimuli with low center frequencies, temporal responses signal a "pitch-shift" between harmonic and inharmonic complexes. Both AP and BP fibers show these effects. 4. These data suggest that bullfrog peripheral fibers extract the periodicity of complex stimuli by time-domain rather than frequency-domain coding.

Diversity of form in the amphibian papilla of Puerto Rican frogs

In modern frogs, the amphibian papilla exhibits a caudal extension whose shape, relative length, and proportion of hair cells vary markedly from species to species. Tuning in the caudal extension is organized tonotopically and evidently involves the tectorium. In terms of the proportion of amphibian-papillar hair cells in the caudal extension, we report more diversity among 8 species of a single genus (Eleutherodactylus) on a single island (Puerto Rico) than has been found so far among all of the (more than 50) other modern anurans examined for this feature from around the world. These 8 Puerto Rican species have overlapping habitat and conspicuous diversity in the male advertisement call. For 7 of the 8 species, we report that the call has transient spectral components in the frequency range of the amphibian papilla, and that the proportion of caudal extension hair cells and the frequency distribution of those components are correlated. Thus one might conclude that the selective pressures that led to diversity of calls among the 8 species also led to diversity in form of the amphibian papilla.

Female green treefrogs (Hyla cinerea) do not selectively respond to signals with a harmonic structure in noise

1. Females of the green treefrog, Hyla cinerea, communicate in noisy environments, with spectrally complicated signals. A previous study (Megela Simmons 1988), using the reflex modification technique, found that the masked threshold of green treefrogs to two-tone signals differed by about 10 dB depending on whether or not the two components were harmonically-related. The present study used the same two-component stimuli to test the prediction that gravid females would better detect harmonic sounds in noise than inharmonic ones. 2. We offered gravid treefrogs simultaneous choices between alternative two-component synthetic sounds: (1) an inharmonic sound of 831 + 3100 Hz, and a harmonic sound of 828 + 2760 Hz. We varied the sound pressure level (SPL in decibels [dB]) to which we equalized these alternatives at the female's release point (75 and 80 dB SPL), and we tested females in quiet conditions and in the presence of broadband background noise (52 dB/Hz at the female's release point). 3. At a signal playback level of 75 dB SPL, one-third of the females responded in the presence of background noise. Subtracting the spectrum level yields a critical ratio estimate of 23 dB, a value that is very similar to estimates for single pure tones in noise reported in other studies of this species (Ehret and Gerhardt 1980; Moss and Megela Simmons 1986). Females did not, however, choose the harmonic sound over the inharmonic sound in this condition, at the higher signal-to-noise ratio, or in either of the unmasked situations.(ABSTRACT TRUNCATED AT 250 WORDS)

Selectivity for harmonic structure in complex sounds by the green treefrog (Hyla cinerea)

1. A psychophysical technique based on reflex modification was used to study the detection of two-tone complexes in background noise by the green treefrog (Hyla cinerea). Three different two-tone complexes were synthesized and presented to measure detection thresholds--a harmonic complex of 900 + 3000 Hz (periodicity of 300 Hz, mimicking the structure of the natural advertisement call); an inharmonic complex of 830 + 3100 Hz; and a second harmonic complex of 828 + 2760 Hz (periodicity of 276 Hz). 2. Masked thresholds and 'critical ratios' (signal-to-noise ratios at threshold) were lowest for the two harmonic complexes (900 + 3000 Hz, mean 'critical ratio' of 16 dB; 828 + 2760 Hz, mean 'critical ratio' of 14 dB). For the inharmonic complex, for which there is no stable first-harmonic periodicity, the mean 'critical ratio' was 24 dB. These data suggest that the green treefrog is sensitive to the harmonic structure of complex sounds as a specific acoustic feature. 3. Because of the unique structure of the treefrog's inner ear, the heightened behavioral sensitivity to harmonic complexes must be due to processing in the central, rather than peripheral, auditory system.