Directional hearing in the grassfrog (Rana temporaria L.). II. Acoustics and modelling of the auditory periphery

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.

ICE on the road to auditory sensitivity reduction and sound localization in the frog

Frogs and toads are capable of producing calls at potentially damaging levels that exceed 110 dB SPL at 50 cm. Most frog species have internally coupled ears (ICE) in which the tympanic membranes (TyMs) communicate directly via the large, permanently open Eustachian tubes, resulting in an inherently directional asymmetrical pressure-difference receiver. One active mechanism for auditory sensitivity reduction involves the pressure increase during vocalization that distends the TyM, reducing its low-frequency airborne sound sensitivity. Moreover, if sounds generated by the vocal folds arrive at both surfaces of the TyM with nearly equal amplitudes and phases, the net motion of the eardrum would be greatly attenuated. Both of these processes appear to reduce the motion of the frog's TyM during vocalizations. The implications of ICE in amphibians with respect to sound localizations are discussed, and the particularly interesting case of frogs that use ultrasound for communication yet exhibit exquisitely small localization jump errors is brought to light.

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.

Acoustic analysis of the frequency-dependent coupling between the frog's ears

The ears of anurans are coupled through the Eustachian tubes and mouth cavity. The degree of coupling varies with frequency showing a bandpass characteristic, but the characteristics differ between empirically measured data based on auditory nerve responses and tympanic membrane vibration. In the present study, the coupling was modeled acoustically as a tube connected with a side branch. This tube corresponds to the Eustachian tubes, whereas the side branch corresponds to the mouth cavity and nares. The analysis accounts for the frequency dependency shown by the empirical data and reconciles the differences observed between the coupling as measured by tympanic membrane vibration and auditory nerve responses.

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.

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.

How minute sooglossid frogs hear without a middle ear

Acoustic communication is widespread in animals. According to the sensory drive hypothesis [Endler JA (1993) Philos Trans R Soc Lond B Biol Sci 340(1292):215-225], communication signals and perceptual systems have coevolved. A clear illustration of this is the evolution of the tetrapod middle ear, adapted to life on land. Here we report the discovery of a bone conduction-mediated stimulation of the ear by wave propagation in Sechellophryne gardineri, one of the world's smallest terrestrial tetrapods, which lacks a middle ear yet produces acoustic signals. Based on X-ray synchrotron holotomography, we measured the biomechanical properties of the otic tissues and modeled the acoustic propagation. Our models show how bone conduction enhanced by the resonating role of the mouth allows these seemingly deaf frogs to communicate effectively without a middle ear.

Vertebrate pressure-gradient receivers

The eardrums of all terrestrial vertebrates (tetrapods) are connected through Eustachian tubes or interaural canals. In some of the animals, these connections create pressure-gradient directionality, an enhanced directionality by interaction of sound arriving at both sides of the eardrum and strongly dependent on interaural transmission attenuation. Even though the tympanic middle ear has originated independently in the major tetrapod groups, in each group the ancestral condition probably was that the two middle ears were exposed in the mouth cavity with relatively high interaural transmission. Recent vertebrates form a continuum from perfect interaural transmission (0 dB in a certain frequency band) and pronounced eardrum directionality (30-40 dB) in the lizards, over somewhat attenuated transmission and limited directionality in birds and frogs, to the strongly attenuated interaural transmission and functionally isolated pressure receiver ears in the mammals. Since some of the binaural interaction already takes place at the eardrum in animals with strongly coupled ears, producing enhanced interaural time and level differences, the subsequent neural processing may be simpler. In robotic simulations of lizards, simple binaural subtraction (EI cells, found in brainstem nuclei of both frogs and lizards) produces strongly lateralized responses that are sufficient for steering the animal robustly to sound sources.

Acoustical coupling of lizard eardrums

Lizard ears are clear examples of two-input pressure-difference receivers, with up to 40-dB differences in eardrum vibration amplitude in response to ipsi- and contralateral stimulus directions. The directionality is created by acoustical coupling of the eardrums and interaction of the direct and indirect sound components on the eardrum. The ensuing pressure-difference characteristics generate the highest directionality of any similar-sized terrestrial vertebrate ear. The aim of the present study was to measure the gain of the direct and indirect sound components in three lizard species: Anolis sagrei and Basiliscus vittatus (iguanids) and Hemidactylus frenatus (gekkonid) by laser vibrometry, using either free-field sound or a headphone and coupler for stimulation. The directivity of the ear of these lizards is pronounced in the frequency range from 2 to 5 kHz. The directivity is ovoidal, asymmetrical across the midline, but largely symmetrical across the interaural axis (i.e., front-back). Occlusion of the contralateral ear abolishes the directionality. We stimulated the two eardrums with a coupler close to the eardrum to measure the gain of the sound pathways. Within the frequency range of maximal directionality, the interaural transmission gain (compared to sound arriving directly) is close to or even exceeds unity, indicating a pronounced acoustical transparency of the lizard head and resonances in the interaural cavities. Our results show that the directionality of the lizard ear is caused by the acoustic interaction of the two eardrums. The results can be largely explained by a simple acoustical model based on an electrical analog circuit.

Directionality of the pressure-difference receiver ears in the northern leopard frog, Rana pipiens pipiens

We studied the directional response of the coupled-eardrum system in the northern leopard frog, Rana pipiens pipiens. Eardrum behavior closely approximates a linear time-invariant system, with a highly correlated input-output relationship between the eardrum pressure difference and the eardrum velocity. Variations in the eardrum transfer function at frequencies below 800 Hz indicate the existence of an extratympanic sound transmission pathway which can interfere with eardrum motions. The eardrum velocity was shown to shift in phase as a function of sound incident angle, which was a direct result of the phase-shift of the eardrum pressure difference. We used two laser-Doppler vibrometers to measure the interaural vibration time difference (IVTD) and the interaural vibration amplitude difference (IVAD) between the motions of the two eardrums. The coupled-eardrum system enhanced the IVTD and IVAD by a factor of 3 and 3 dB, respectively, when compared to an isolated-eardrum system of the same size. Our findings are consistent with the time-delay sensitivity of other coupled-eardrum systems such as those found in crickets and flies.

Distortion product otoacoustic emissions in frogs: correlation with middle and inner ear properties

Four frog species, Rana pipiens, Scaphiopus couchii, Xenopus laevis and Bombina orientalis, were examined for distortion product otoacoustic emissions (DPOAE). These species were chosen for their diverse otic morphologies. Rana has a well-developed caudal extension of the amphibian papilla within the inner ear; Scaphiopus, Xenopus and Bombina do not. Rana and Scaphiopus have a tympanic middle ear, Xenopus has a subcutaneous tympanic disk and Bombina has only an operculum. DPOAEs were present in Rana and Xenopus, with amplitudes up to 55 and 20 dB SPL, respectively. DPOAEs could be detected in neither Scaphiopus nor Bombina. These results show that (1) a well-developed caudal extension is not necessary for generation of DPOAEs, and (2) a tympanic middle ear is neither required nor sufficient to have DPOAEs.

Selectivity for temporal characteristics of sound and interaural time difference of auditory midbrain neurons in the grassfrog: a system theoretical approach

The selectivity for temporal characteristics of sound and interaural time difference (ITD) was investigated in the torus semicircularis (TS) of the grassfrog. Stimuli were delivered by means of a closed sound system and consisted of binaurally presented Poisson distributed condensation clicks, and pseudo-random (RAN) or equidistant (EQU) click trains of which ITD was varied. With RAN and EQU trains, 86% of the TS units demonstrated a clear selectivity for ITD. Most commonly, these units had monotonically increasing ITD-rate functions. In general, units responding to Poisson clicks, responded also to RAN and EQU trains. One category of units which showed strong time-locking had comparable selectivities for ITD with both stimulus ensembles. A second category of units showed a combined selectivity for temporal structure and ITD. These units responded exclusively to EQU trains in a nonsynchronized way. From the responses obtained with the Poisson click ensemble so-called Poisson system kernels were determined, in analogy to the Wiener-Volterra functional expansion for nonlinear systems. The kernel analysis was performed up to second order. Contralateral (CL) first order kernels usually had positive or combinations of positive and negative regions, indicating that the contralateral ear exerted an excitatory or combined excitatory-inhibitory influence upon the neural response. Ipsilateral (IL), units were characterized by first order kernels which were not significantly different from zero, or kernels in which a single negative region was present. A large variety of CL second order kernels has been observed whereas rarely IL second order kernels were encountered. About 35% of the units possessed nonzero second order cross kernels, which indicates that CL and IL neural processes are interacting in a nonlinear way. Units demonstrating a pronounced selectivity for ITD, were generally characterized by positive CL combined with negative IL first order kernels. Findings suggested that, in the grassfrog, neural selectivity for ITD mainly is established by linear interaction of excitatory and inhibitory processes originating from the CL and IL ear, respectively. Units exhibiting strong time-locking to Poisson clicks and RAN and EQU trains had significantly shorter response latencies than moderately time-locking units. In the first category of units, a substantial higher number of nonzero first and second order kernels was observed. It was concluded that nonlinear response properties, as observed in TS units, most likely have to be ascribed to nonlinear characteristics of neural components located in the auditory nervous system peripheral to the torus semicircularis.

Directionality of phase locking in auditory nerve fibers of the leopard frog Rana pipiens pipiens

A dorsal approach to the eighth nerve and free-field stimulation were used to investigate the effect of sound direction and intensity on phase locking in auditory nerve fibers of the leopard frog Rana pipiens pipiens. Tuning curves of 75 auditory neurons were analyzed (Fig. 2). Amphibian papillar neurons, but not basilar papillar neurons, exhibit significant phase locking to short tone bursts at the characteristic frequency (CF), the degree of phase locking (vector strength) decreasing with the neuron's CF (Figs. 3, 4 and 10E). Vector strength increases with sound pressure level to saturate about 20 dB above threshold, while the preferred firing phase is only slightly affected (Figs. 5 and 6). In contrast, sound direction hardly affects vector strength (Figs. 7, 8, 9A and 10A and C), but has a strong influence on the preferred firing phase (Figs. 7, 8, 9B and C, 10B and D): With respect to anterior tone presentation there are phase lags for ipsilateral and phase leads for posterior and contralateral presentation. Phase differences between both ears show a sinusoidal or cardioid/ovoidal directional characteristic; maximum differences are found with antero-lateral tone presentation (Fig. 11). The directionality of phase locking decreases with the neuron's CF (Fig. 10F) and only slightly changes with sound pressure level (Fig. 12). Thus, phase locking of amphibian papilla neurons can potentially provide intensity-independent information for sound localization.

Middle-ear transmission: acoustic versus ossicular coupling in cat and human

Otologic surgeons consider the action of sound pressure on the cochlear windows to be of major importance in certain cases of middle-ear pathology, yet previously published network models of mammalian middle ears do not include such a mechanism. A unified middle-ear model is developed in which it is assumed that the difference of acoustic pressures acting on the windows adds to the ossicular-chain pressure to produce cochlear input. From a network model of the cat middle-ear cavities we estimate the contributions of pressures on the cochlear windows for both normal and abnormal cat ears. For the human ear we use the model of Kringlebotn (1988) and measurements of Békésy (1947). We determine that the pressure difference across the cochlear windows is negligibly small in normal cat and human ears. Thus, it is a reasonable approximation to ignore this mechanism in normal ears. For ears with a drastically altered tympanic membrane and/or ossicular chain, acoustic coupling to the cochlear windows can--to a considerable extent--explain residual hearing in human. The model predicts hearing levels for type IV tympanoplastic reconstructions that agree with the best results obtained surgically.

Directional hearing in the gray tree frog Hyla versicolor: eardrum vibrations and phonotaxis

1. We used laser vibrometry to study the vibrational frequency response of the eardrum of female gray tree frogs for different positions of the sound source in three-dimensional space. Furthermore, we studied the accuracy of 3-D phonotaxis in the same species for sounds with different frequency contents. 2. The directionality of the eardrum was most pronounced in a narrow frequency range between 1.3 and 1.8 kHz. 3. The average 3-D, horizontal and vertical jump error angles for phonotactic approaches with a sound similar to the natural advertisement call (1.1 and 2.2 kHz frequency components) were 23 degrees, 19 degrees and 12 degrees, respectively. 4. 3-D jump error angle distributions for the 1.4 + 2.2 kHz, 1.0 kHz and 2.0 kHz sounds were not significantly different from that for the 1.1 + 2.2 kHz sound. 5. The average 3-D jump error angle for the 1.4 kHz sound was 36 degrees, and the distribution was significantly different from that for the 1.1 + 2.2 kHz sound. Hence, phonotactic accuracy was poorer in the frequency range of maximum eardrum directionality. 6. Head scanning was not observed and is apparently unnecessary for accurate sound localization in three-dimensional space. 7. Changes in overall sound pressure level experienced by the frog during phonotactic approaches are not an important cue for sound localization.

Measuring and modelling the response of auditory midbrain neurons in the grassfrog to temporally structured binaural stimuli

The combined selectivity for amplitude modulation frequency (AMF) and interaural time difference (ITD) was investigated for single units in the auditory midbrain of the grassfrog. Stimuli were presented by means of a closed sound system. A large number of units was found to be selective for AMF (95%) or ITD (85%) and mostly, these selectivities were intricately coupled. At zero ITD most units showed a band-pass (54%) or bimodal (24%) AMF-rate histogram. At an AMF of 36 Hz, which is equal to the pulse repetition rate of the mating call, 70% of the units possessed an asymmetrical ITD-rate histogram, whereas about 15% showed a symmetrically peaked histogram. With binaural stimulation more units appeared to be selective for AMF (95%) as was the case with monaural stimulation (85%). A large fraction of the units appeared to be most selective for ITD at AMFs of 36 and 72 Hz, whereas units seldomly exhibited ITD selectivity with unmodulated tones. Based upon previous papers (Melssen et al., 1990; Van Stokkum, 1990) a binaural model is proposed to explain these findings. An auditory midbrain neuron is modelled as a third order neuron which receives excitatory input from second order neurons. Furthermore the model neuron receives inputs from the other ear, which may be either excitatory or inhibitory. Spatiotemporal integration of inputs from both ears, followed by action potential generation, produces a combined selectivity for AMF and ITD. In particular the responses of an experimentally observed EI neuron to a set of stimuli are reproduced well by the model.

Sensitivity for interaural time and intensity difference of auditory midbrain neurons in the grassfrog

The sensitivity for interaural time (ITD) and intensity (IID) difference was investigated for single units in the auditory midbrain of the grassfrog. A temporally structured stimulus was used which was presented by means of a closed sound system. At best frequency (BF) the majority of units was selective for ITD as indicated by an asymmetrically (73%) or symmetrically (7%) shaped ITD-rate histogram. About 20% appeared to be nonselective. Units with a symmetrical rate histogram had BFs well above 0.9 kHz, whereas for the other categories no relationship with BF was observed. Most units had a selectivity for ITD which was rather independent from frequency and absolute intensity level. In 62% of the units interaural time difference could be traded by interaural intensity difference. In most cases this so-called time-intensity trading could be explained by the intensity-latency characteristics of auditory nerve fibres. About 20% was sensitive to IID only and 5% to ITD only. A binaural model is proposed which is based on the intensity-rate and intensity-latency characteristics of auditory nerve fibres, the linear summation of excitatory and inhibitory post synaptic potentials in second order neurons, and spatiotemporal integration at the level of third order neurons. By variation of only a small number of parameters, namely strengths and time constants of the connectivities, the range of experimentally observed response patterns could be reproduced.

A combined sensitivity for frequency and interaural intensity difference in neurons in the auditory midbrain of the grassfrog

The relation between spectral tuning and sensitivity for interaural intensity difference (IID) was studied for single units in the auditory midbrain of the grassfrog. The stimuli consisted of sequences of pure tones of different frequency and interaural intensity differences presented by means of a closed sound system. At best excitatory frequency, three types of binaural interaction were observed: E0 (one ear excitatory 23%), EE (both ears excitatory 9%) and EI (one ear excitatory, the other inhibitory 67%). For a considerable number of units different types of binaural interaction were observed for different stimulus frequencies. More than 30% of the binaural units had multiple excitatory and inhibitory regions in their spectrotemporal selectivity. E0 and EI units had uniformly distributed best frequencies, EE units generally had best frequencies near 1.0 kHz. The E0 and EE categories had response latencies less than about 70 ms whereas EI units could have longer latencies. Most EE and all EI category units had sigmoidally shaped IID-rate curves. About 40% of the units had a combined sensitivity for sound spectrum and IID which was invariant to overall stimulus intensity. For nearly all EI units the inhibitory influence of the ipsilateral ear was confined to frequencies in the 0.4-1.6 kHz range and was not correlated with a unit's best frequency. By means of a simple additive model we demonstrated that determination of sound source laterality can be achieved by ensemble coding in the auditory midbrain.

Modelling the response of auditory midbrain neurons in the grassfrog to temporally structured monaural stimuli

In a previous paper [Van Stokkum and Gielen, Hear. Res. 41, 71-86, 1989] a model was presented to describe the processing of monaural stimuli by the auditory periphery of the grassfrog. The main components of this model were: a middle ear filter, transduction and tuning of the haircell, short-term adaptation, action potential (event) generation with refractory properties, and spatiotemporal integration of converging inputs. The model is now extended to model auditory midbrain neurons as third order neurons. The mechanisms that generate selectivity for temporal characteristics of sound are adaptation, coincidence detection of second order neurons, temporal integration of third order neurons, and most important, event generation of the first, second and third order model neurons. Variation of the parameters of the model successfully reproduces the range of response patterns which have been obtained from eighth nerve fibres, dorsal medullary nucleus neurons, and torus semicircularis neurons without inhibition. With a single set of parameters the output of the model in response to a set of spectrally and temporally structured stimuli qualitatively resembles the responses of a single neuron to all these stimuli. In this way the responses to the different stimuli are synthesized into a framework, which functionally describes the neuron.

A model for the peripheral auditory nervous system of the grassfrog

A model is presented which incorporates several data from the literature on isolated parts of the peripheral auditory nervous system into a coherent model. The usefulness of the model lies in the fact that it describes the functional properties of eighth nerve fibres and dorsal medullary nucleus neurons in response to monaural stimuli. The components are: a middle ear filter, transduction and tuning of the haircell, short-term adaptation, event generation with refractory properties, and coincidence detection. In a previous paper [Van Stokkum (1987), Hear. Res. 29, 223-235] a class of dorsal medullary nucleus neurons was described, which preferred fast intensity changes. Using a coincidence detection mechanism the proposed model reproduces the same preference. Variation of the parameters of the model successfully reproduces the range of response patterns which have been obtained from eighth nerve fibres and dorsal medullary nucleus neurons. With one set of parameters the output of the model in response to a set of spectrally and temporally structured stimuli qualitatively resembles the response of a single neuron. In this way the responses to the different stimuli are synthesized into a framework, which functionally describes the neuron.

Coding of free field intensity in the auditory midbrain of the leopard frog. I. Results for tonal stimuli

Single unit recordings in the auditory midbrain of the leopard frog were obtained in response to free field, random frequency, tone pip stimulation. Five sequences of 81 logarithmically distributed frequencies from 100 Hz to 3200 Hz inclusive at stimulus levels between 94 dB SPL and the unit's threshold were presented. Sigmoidal response-intensity functions were fit through the data points for monotonic and nearly monotonic units (53 in total) at all frequencies for which the units were responsive. From the fitcurves a generalized slope and inflection point were obtained and converted into a dynamic range and a threshold value. Single unit dynamic ranges at CF frequencies were between 5 and 80 dB, and thresholds were between 20 dB and 90 dB. For the population of units the dynamic ranges were positively correlated with the threshold values. A population response was computed by adding the response at each intensity-frequency combination, and again curve fitting with the sigmoidal function. The population dynamic range was about 70 dB for the basilar papilla range (around 1200 Hz) and somewhat less, about 45 dB, for the amphibian papilla neurons. The amount of synchronization of the firings of the individual units with the stimulus was calculated on basis of the shifted auto-coincidence function. The stimulus synchronization appeared to be largely independent of stimulus intensity. Inter-neuron synchronization was computed on basis of the cross-coincidence function of two neuronal spike trains. Also this synchronization was stimulus independent. It was concluded that synchronization or other forms of multiplicative action cannot on its own code for stimulus intensity. From the application of signal detection theory it is concluded that a rate coding on basis of a modest population of units has an acceptable dynamic range, roughly covering 30-100 dB SPL, produces a more than adequate sensitivity for intensity differences and may be used as a code for intensity.

Biomechanics of vibration reception in the bullfrog, Rana catesbeiana

The opercularis system (OPS) of amphibians consists of an opercularis muscle that connects the shoulder girdle skeleton to the operculum, a movable element in the oval window of the otic capsule. The role of the OPS in reception of vibrations was examined in bullfrogs (Rana catesbeiana) tested in various postures that manipulated differential motion between the shoulder girdle (the origin of the opercularis muscle) and skull (including the inner ear). Amplitude and phase relationship of motions of the suprascapular cartilage of the shoulder girdle and the posterior skull were also measured during these tests. 1. Microphonic responses to vertical vibrations from 25-200 Hz were typically highest when frogs were in a normal, sitting posture with the head held off the vibrating platform. Responses from animals in which the head directly contacted the platform were often less (by up to 10 dB at certain frequencies). Responses from all test positions were highest at lower frequencies, especially between 50-100 Hz. 2. Suprascapular accelerations were typically highest in the normal, sitting posture, and at lower frequencies (50-75 Hz) were often greater than that of the vibrating platform by up to 8 dB. The shoulder girdle skeleton of the bullfrog is therefore readily affected by vertical substrate motion. 3. The amplitude of microphonic responses in the different test postures did not correspond well with head acceleration. Rather, response amplitude corresponded best with the absolute difference between shoulder and head motion. For example, in the normal posture, suprascapular motion was much greater than head motion, and responses were relatively high. If only the head was vibrated, head motion was high and shoulder motion low, and responses also were relatively high. If the head and body were vibrated together, their motions were similar, and responses to the same platform accelerations were often reduced. Phase differences between shoulder and head motions were small at the frequencies examined and may be of little functional significance. The importance of differences in shoulder and head motion suggests that the resulting differential motion of the operculum and inner ear fluids can produce waves that stimulate appropriate end organs (such as the saccule). 4. Removal of the opercularis muscle reduced responses up to 18 dB at certain frequencies in some of the test postures. The most significant reductions were observed in those postures with a significant difference between shoulder and head motion (such as the normal posture).(ABSTRACT TRUNCATED AT 400 WORDS)

Accessory pathway for sound transfer in a neotropical frog

A portion of the lateral body wall overlying the lung cavity of the arboreal frog, Eleutherodactylus coqui, vibrates in response to free-field sound. Peak displacement amplitude of the body wall in response to a natural call note presented at 73 decibels sound pressure level is 1.70 X 10(-9) m, roughly 8 decibels less than that of the ipsilateral eardrum, as measured by laser Doppler vibrometry. We show that the vibration magnitude varies predictably across the body profile and is posture and frequency dependent. Two routes to the inner ear are described for sounds impinging on the body wall; either of these accessory pathways could modify direct input from the peripheral auditory system and enhance sound localization in these small vertebrates.

Tympanic and extratympanic sound transmission in the leopard frog

The inner ear of the leopard frog, Rana pipiens, receives sound via two separate pathways: the tympanic-columellar pathway and an extra-tympanic route. The relative efficiency of the two pathways was investigated. Laser interferometry measurements of tympanic vibration induced by free-field acoustic stimulation reveal a broadly tuned response with maximal vibration at 800 and 1500 Hz. Vibrational amplitude falls off rapidly above and below these frequencies so that above 2 kHz and below 300 Hz tympanic vibration is severely reduced. Electrophysiological measurements of the thresholds of single eighth cranial nerve fibers from both the amphibian and basilar papillae in response to pure tones were made in such a way that the relative efficiency of tympanic and extratympanic transmission could be assessed for each fiber. Thresholds for the two routes are very similar up to 1.0 kHz, above which tympanic transmission eventually becomes more efficient by 15-20 dB. By varying the relative phase of the two modes of stimulation, a reduction of the eighth nerve response can be achieved. When considered together, the measurements of tympanic vibration and the measurements of tympanic and extratympanic transmission thresholds suggest that under normal conditions in this species (1) below 300 Hz extratympanic sound transmission is the main source of inner ear stimulation; (2) for most of the basilar papilla frequency range (i.e., above 1.2 kHz) tympanic transmission is more important; and (3) both routes contribute to the stimulation of amphibian papilla fibers tuned between those points. Thus acoustic excitation of the an uran's inner ear depends on a complex interaction between tympanic and extratympanic sound transmission.