Frequency Threshold Curves Research Articles

Effect of octopamine on the frequency tuning of the auditory system in <i>Culex pipiens pipiens</i> mosquito (Diptera, Culicidae)

We aimed at demonstrating the direct physiological effects of octopamine on the auditory responses of the Johnston’s organ sensory neurons in Culex pipiens pipiens mosquitoes. Responses to acoustic stimulation were measured as the frequency-threshold curves and as an instantly recorded autoexcitation frequency before and after the injection of octopamine (or chlordimeform, an agonist of octopamine receptors). The autoexcitation frequency was assessed by including the auditory neurons into a positive feedback loop, when the amplified neuronal response was used to drive the stimulating loudspeaker. Our results indicate that: octopamine affects the properties of the auditory system both in male and female mosquitoes; in female mosquitoes, octopamine significantly reduces the sensitivity of auditory neurons below 90 Hz and produces little effect in the high-frequency range (100 Hz and above); in male mosquitoes, octopamine significantly raises the tuning frequency of the auditory system (ratio of tuning frequencies before and after the injection 1.32–1.55). There is a strong sexual dimorphism in the octopaminergic modulation of the mosquito auditory system. The observed physiological effects of octopamine, both in male and female mosquitoes, cannot be fully explained by the modulation of mechanical stiffness of the antenna and must include the changes in frequency tuning of the auditory neurons.

Suppression tuning of spontaneous otoacoustic emissions in the barn owl (Tyto alba)

Spontaneous otoacoustic emissions (SOAEs) have been observed in a variety of different vertebrates, including humans and barn owls (Tyto alba). The underlying mechanisms producing the SOAEs and the meaning of their characteristics regarding the frequency selectivity of an individual and species are, however, still under debate. In the present study, we measured SOAE spectra in lightly anesthetized barn owls and suppressed their amplitudes by presenting pure tones at different frequencies and sound levels. Suppression effects were quantified by deriving suppression tuning curves (STCs) with a criterion of 2 dB suppression. SOAEs were found in 100% of ears (n = 14), with an average of 12.7 SOAEs per ear. Across the whole SOAE frequency range of 3.4–10.2 kHz, the distances between neighboring SOAEs were relatively uniform, with a median distance of 430 Hz. The majority (87.6%) of SOAEs were recorded at frequencies that fall within the barn owl’s auditory fovea (5–10 kHz). The STCs were V-shaped and sharply tuned, similar to STCs from humans and other species. Between 5 and 10 kHz, the median Q10dB value of STC was 4.87 and was thus lower than that of owl single-unit neural data. There was no evidence for secondary STC side lobes, as seen in humans. The best thresholds of the STCs varied from 7.0 to 57.5 dB SPL and correlated with SOAE level, such that smaller SOAEs tended to require a higher sound level to be suppressed. While similar, the frequency-threshold curves of auditory-nerve fibers and STCs of SOAEs differ in some respects in their tuning characteristics indicating that SOAE suppression tuning in the barn owl may not directly reflect neural tuning in primary auditory nerve fibers.

Frequency threshold curves of auditory interneurons of male mosquitoes Culex pipiens pipiens L. (Diptera, Culicidae)

Frequency threshold curves of auditory interneurons of male mosquitoes Culex pipiens pipiens L. (Diptera, Culicidae)

Comparative cat studies: Are tigers auditory specialists?

Although representatives of the 41 extant cat species inhabit nearly every biome on the planet and may have faced a highly diverse set of selection pressures during their evolution, relatively little effort has been made to compare commonly measured features of peripheral auditory function among species representing the family. Given their extensive geographic range, it is reasonable to suggest that auditory system adaptations may have occurred, leading to functional specialization in a subset of species. In that light, frequency-threshold curves and response latency-level and latency-frequency relationships will be compared in cats of widely varying body mass inhabiting a variety of habitats. The body mass of felids spans a range of more than two orders of magnitude, with small cats like the desert sandcat, Felis margarita, weighing as little as 2 kg and the Amur tiger, Panthera tigris altaica, weighing as much as 300 kg. While most cats studied thus far appear to satisfy the conditions necessary to be labeled auditory generalists, the tiger, and perhaps members of the Panthera genus generally, may be exceptions. We will consider the possibility that big cats are auditory generalists with regard to acoustic sensitivity, but exhibit a peripheral specialization affecting low-frequency neural latencies. [Funding provided by NSF Grant No. 0823417].

Low‐voltage activated Kv1.1 subunits are crucial for the processing of sound source location in the lateral superior olive in mice

Voltage-gated potassium (Kv) channels containing Kv1.1 subunits are strongly expressed in neurons that fire temporally precise action potentials (APs). In the auditory system, AP timing is used to localize sound sources by integrating interaural differences in time (ITD) and intensity (IID) using sound arriving at both cochleae. In mammals, the first nucleus to encode IIDs is the lateral superior olive (LSO), which integrates excitation from the ipsilateral ventral cochlear nucleus and contralateral inhibition mediated via the medial nucleus of the trapezoid body. Previously we reported that neurons in this pathway show reduced firing rates, longer latencies and increased jitter in Kv1.1 knockout (Kcna1−/−) mice. Here, we investigate whether these differences have direct impact on IID processing by LSO neurons. Single-unit recordings were made from LSO neurons of wild-type (Kcna1+/+) and from Kcna1−/− mice. IID functions were measured to evaluate genotype-specific changes in integrating excitatory and inhibitory inputs. In Kcna1+/+ mice, IID sensitivity ranged from +27 dB (excitatory ear more intense) to −20 dB (inhibitory ear more intense), thus covering the physiologically relevant range of IIDs. However, the distribution of IID functions in Kcna1−/− mice was skewed towards positive IIDs, favouring ipsilateral sound positions. Our computational model revealed that the reduced performance of IID encoding in the LSO of Kcna1−/− mice is mainly caused by a decrease in temporal fidelity along the inhibitory pathway. These results imply a fundamental role for Kv1.1 in temporal integration of excitation and inhibition during sound source localization.

Effect of temperature on auditory receptor functions in crickets (Orthoptera, Tettigoniodea)

By electrophysiological methods, effect of temperature on bushcricket tympanal organ functions was studied. Activity of auditory receptors was recorded intracellularly in the 5th nerve of I thoracic ganglion in Tettigonia cantans, Metrioptera roeselii, M. bicolor, Platycleis albopunctata, Pholidoptera griseoaptera, and Phaneroptera falcata. The temperature was changed in the range from 17 to 34 degrees C. Heating of the tympanal organ to 30-32 degrees C led to a decrease of impulse amplitude, shortening of their duration, an increase of sensitivity, of the burst instantaneous frequency, and of the number of impulses in responses as well as to a decrease of latent periods (LP) of receptor reaction. The optimal frequency in all studied cells did not change, although range of perceived frequencies was enlarged. The frequency threshold curve of receptors either was shifted down along the ordinate scale without changes of its shape or the thresholds at various frequencies decreased non-uniformly. Thus, the obtained data indicate the absence of changes in the frequency tuning of the auditory receptors with changes of temperature.

Possible involvement of the spiral limbus in chinchilla cochlear mechanics

Differences between cochlear mechanical tuning curves and those of auditory nerve fibers (ANFs) exist. In particular, mechanical transfer functions exhibit a high-frequency plateau; ANFs frequency threshold curves (FTCs) do not. ANF-FTCs may have a low-frequency slope due to a velocity forcing function operating on inner hair cells at low frequencies. Neither basilar membrane velocity nor displacement adequately explain the entire ANF tuning curve. A displacement sensitive interferometer was used to study basilar membrane and spiral limbus mechanics in the 6-kHz region of the chinchilla cochlea. The spiral limbus vibrates at the same phase as the basilar membrane nearly up to the location’s characteristic frequency. In the plateau region, the limbus appears to vibrate 0 to 20 dB less than the basilar membrane. The basilar membrane/limbus amplitude transfer function has a low-frequency slope of ∼3 dB/oct at low frequencies and is ∼10 dB lower than the basilar membrane amplitude at 1 kHz. It appears that spiral limbus vibration may contribute to the excitation of the cilia of the inner hair cells. [Work supported by NIDCD grant R01 DC001910.]

Electrophysiological characteristics of inferior colliculus neurons in mutant mice with hereditary absence of cochlear outer hair cells

One strain of homozygous Kit W-v mice (formerly known as W v/W v) lack 98% of the cochlear outer hair cells (OHCs) from birth. Inner hair cells (IHCs) and supporting cells develop normally. Thus, this strain is an attractive model to study the effect of complete OHC absence on central frequency representation. Frequency threshold curves were recorded along the tonotopic axis of inferior colliculus (IC) in mutant and control mice of the genetic background strain (C57BL/6J) and a different outbred strain (NMRI/wild mouse hybrids) known to be free of any cochlear pathology. The average threshold level of neurons in the mutants was 100 dB sound pressure level, 60 dB higher than in C57BL/6J and NMRI mice. Their tuning curves lacked the sharply tuned tip. In the C57BL/6J mice, although younger than four months, abnormal tuning curves were found for about 30% of the neurons, especially in the high frequency range. No abnormal tuning curves were found in the NMRI mice. The bandwidth of the tuning curves, measured at 10 dB above threshold, was on average 1.27 octaves in mutants, 0.62 octaves in C57BL/6J mice, and 0.34 octaves in NMRI mice. The range for the high cut-off frequency of the tuning curves at 10 dB above threshold was 6.4–61.1 kHz in the NMRI and 7–59.5 kHz in C57BL/6J. In the mutants, the range was limited to 11.1–41.7 kHz. The tonotopic gradient based on the cut-off frequency was less steep in the IC of the mutants than in both control groups.

Topography of acoustic response characteristics in the auditory cortex of the Kunming mouse

Topography of acoustic response characteristics in the auditory cortex (AC) of the Kunming (KM) mouse has been examined by using microelectrode recording techniques. Based on best-frequency (BF) maps, both the primary auditory field (AI) and the anterior auditory field (AAF) are tonotopically organized with a counter running frequency gradient. Within an isofrequency stripe, the width of the frequency-threshold curves of single neurons increases, and minimum threshold (MT) decreases towards more ventral locations. BFs in AI and AAF range from 4 to 38 kHz. Auditory neurons with BFs above 40 kHz are located at the rostrodorsal part of the AC. The findings suggest that the KM mouse is a good model suitable for auditory research.

Sound direction modifies the inhibitory as well as the excitatory frequency tuning characteristics of single neurons in the frog torus semicircularis (inferior colliculus).

Single-unit recordings were made from the frog inferior colliculus to determine whether or not the direction-dependent sharpening of a unit's free-field excitatory frequency-threshold curve (FTCe) was accompanied by a broadening of its inhibitory frequency-threshold curve (FTCi). To determine the FTCi, a two-tone-suppression paradigm was employed. The unit's FTCis and FTCes were collected at three azimuths: contralateral to the recording site, ipsilateral to the recording site, and frontal midline. The result showed that: (1) most inferior colliculus neurons (95%) displayed two-tone suppression, (2) the majority (54%) of neurons displayed stronger two-tone-suppression leading to broader FTCis when the sound was presented from the ipsilateral side than from the contralateral side, (3) for some neurons, the borders of the FTCes and FTCis were closely aligned, and this juxtaposition persisted at all sound azimuths (namely, when a change in sound direction produced a narrowing of a unit's FTCe, its FTCi was broadened concomitantly). For the remaining neurons, however, direction-dependent sharpening of the FTCe was not accompanied by an increase in two-tone-suppression. The neural mechanisms that underlie the direction-dependent changes in the FTCes and FTCis are discussed.

Long-Term Effects of Sectioning the Olivocochlear Bundle in Neonatal Cats

The olivocochlear bundle (OCB) was cut in neonatal cats to evaluate its role in the development of normal cochlear function. Approximately 1 year after deefferentation, acute auditory nerve fiber (ANF) recordings were made from lesioned animals, lesion shams, and normal controls. The degree of deefferentation was quantified via light microscopic evaluation of the density of OCB fascicles in the tunnel of Corti, and selected cases were analyzed via electron microscopy. In the most successful cases, the deefferentation was virtually complete. ANFs from successfully lesioned animals exhibited significant pathophysiology compared with normals and with other animals in which the surgery failed to interrupt the OCB. Thresholds at the characteristic frequency (CF), the frequency at which ANFs are most sensitive, were elevated across the CF range, with maximal effects for CFs in the 10 kHz region. Frequency threshold or tuning curves displayed reduction of tip-to-tail ratios (the difference between CF and low-frequency "tail" thresholds) and decreased sharpness of tuning. These pathological changes are generally associated with outer hair cell (OHC) damage. However, light microscopic histological analysis showed minimal hair cell loss and no significant differences between normal and deefferented groups. Spontaneous discharge rates (SRs) were lower than normal; however, those fibers with the highest SRs remained more sensitive than those with lower SRs. Findings suggest that the interaction between OC efferents and OHCs early in development may be critical for full expression of active mechanical processes.

Auditory-nerve fiber responses to clicks in guinea pigs with a damaged cochlea.

This paper describes auditory-nerve single-fiber responses to clicks in noise-damaged cochleas. Poststimulus time histograms (PSTHs) were recorded for various click intensities and for the two click polarities. The PSTHs found in fibers with elevated thresholds are discussed in relation to the frequency threshold curves (FTCs) measured in these fibers. Five types of abnormal FTCs are distinguished. Type I is elevated as a whole, type II has an elevated (and often broadened) tip and a tail at normal level, type III has low thresholds in the tail (often hypersensitive), type IV represents a flat tuning, and type V has no tip but shows a clear appearance of the tail (often hypersensitive). The click PSTHs of abnormal fibers were compared to normal PSTHs at equal sound-pressure levels, and various abnormal trends were found corresponding to the type of FTC. PSTHs for type I have longer dominant-peak latencies and smaller amplitudes; PSTHs for type II were normal well above the fiber's threshold; PSTHs for type III revealed remarkable patterns with multiple peaks, part of them with a latency strongly varying with polarity; PSTHs for type IV showed narrow peaks and steep amplitude/intensity curves; PSTHs for type V showed a multiple peaked pattern and large amplitudes and steep amplitude/intensity curves to rarefaction polarity. The various features in the click responses were in most cases consistent with the type of FTC. The results can be used to explain deviations in whole-nerve recordings in abnormal cochleas.

Cytoarchitecture and Sound Activated Responses in the Auditory Cortex of the Big Brown Bat, Eptesicus fuscus

Under free field and closed-system stimulation conditions, we studied the frequency threshold curves, auditory spatial sensitivity and binaurality of neurons in the primary auditory cortex (AC) of the big brown bat, Eptesicus fuscus. All 298 recorded AC neurons discharged phasically. They were recorded at depths less than 1,000 μm with response latencies of 7–25 ms, best frequencies (BFs) of 28–97 kHz and minimum thresholds (MTs) of 8–90 dB SPL. They received excitatory inputs from the contralateral ear and either excitatory (EE) or inhibitory (EI) inputs from the ipsilateral ear. These cortical neurons were tonotopically organized along the anteroposterior axis of the AC. High best frequency neurons were located anteriorly and low best frequency neurons posteriorly. They were most sensitive to sounds delivered from a restricted region of the contralateral frontal auditory space (0°-50° in azimuth and 2° up, 15° down in elevation). Frontal auditory space representation appears to be systematically arranged according to the tonotopic axis such that the lateral space is represented posteriorly and the middle space anteriorly. Cortical neurons sequentially isolated from an orthogonally penetrated electrode had similar frequency threshold curves, BFs, MTs, points of maximal auditory spatial sensitivity and binaurality. The EE and EI columns are organized concentrically such that the small number of centrally located EE columns were surrounded by an overwhelming number of El columns. Using Nissl and Golgi stains as well as c-fos immunocytochemistry, we studied the Cytoarchitecture, cell types and sound elicited Fos-like immunoreactivity in the primary AC of this bat species. The primary AC of this bat species can be described into molecular (137 μm), external granular (55 μm), external pyramidal (95 μm), internal granular (102 μm), internal pyramidal (191 μm) and multiform (120 μm) layers. The main type of cells distributed among these six layers are the small, medium and large pyramidal cells. Others include the stellate, horizontal, granular, fusiform, basket, and Martinotii cells. When stimulated with 30 kHz and 79 dB SPL sounds under natural conditions, bilaterally and symmetrically distributed Fos-like immunoreactive neurons were observed in about 20% of neurons in each AC. When stimulated under monaurally plugged conditions, 39–48% more of Fos-like immunoreactive neurons were observed in the ipsilateral AC. This finding supports the fact that the primary AC receives auditory inputs mainly from the contralateral ear.

Frequency tuning and spontaneous activity in the auditory nerve and cochlear nucleus magnocellularis of the barn owl Tyto alba.

Single-unit recordings were obtained from the brain stem of the barn owl at the level of entrance of the auditory nerve. Auditory nerve and nucleus magnocellularis units were distinguished by physiological criteria, with the use of the response latency to clicks, the spontaneous discharge rate, and the pattern of characteristic frequencies encountered along an electrode track. The response latency to click stimulation decreased in a logarithmic fashion with increasing characteristic frequency for both auditory nerve and nucleus magnocellularis units. The average difference between these populations was 0.4-0.55 ms. The average most sensitive thresholds were approximately 0 dB SPL and varied little between 0.5 and 9 kHz. Frequency-threshold curves showed the simple V shape that is typical for birds, with no indication of a low-frequency tail. Frequency selectivity increased in a gradual, power-law fashion with increasing characteristic frequency. There was no reflection of the unusual and greatly expanded mapping of higher frequencies on the basilar papilla of the owl. This observation is contrary to the equal-distance hypothesis that relates frequency selectivity to the spatial representation in the cochlea. On the basis of spontaneous rates and/or sensitivity there was no evidence for distinct subpopulations of auditory nerve fibers, such as the well-known type I afferent response classes in mammals. On the whole, barn owl auditory nerve physiology conformed entirely to the typical patterns seen in other bird species. The only exception was a remarkably small spread of thresholds at any one frequency, this being only 10-15 dB in individual owls. Average spontaneous rate was 72.2 spikes/s in the auditory nerve and 219.4 spikes/s for nucleus magnocellularis. This large difference, together with the known properties of endbulb-of-Held synapses, suggests a convergence of approximately 2-4 auditory nerve fibers onto one nucleus magnocellularis neuron. Some auditory nerve fibers as well as nucleus magnocellularis units showed a quasiperiodic spontaneous discharge with preferred intervals in the time-interval histogram. This phenomenon was observed at frequencies as high as 4.7 kHz.

Binaural inhibition is important in shaping the free-field frequency selectivity of single neurons in the inferior colliculus.

1. We have shown previously that under free-field stimulation in the frontal field, frequency selectivity of the majority of inferior colliculus (IC) neurons became sharper when the loudspeaker was shifted to ipsilateral azimuths. These results indicated that binaural inhibition may be responsible for the direction-dependent sharpening of frequency selectivity. To test the above hypothesis directly, we investigated the frequency selectivity of IC neurons under several conditions: monaural stimulation using a semiclosed acoustical stimulation system, binaural stimulation dichotically also using a semiclosed system, free-field stimulation from different azimuths, and free-field stimulation when the ipsilateral ear was occluded monaurally (coated with a thick layer of petroleum jelly, which effectively attenuated acoustic input to this ear). 2. The binaural interaction pattern of 98 IC neurons of northern leopard frogs (Rana pipiens pipiens) were evaluated; of these neurons, there were 34 EE and 64 EO neurons. The majority of IC neurons (92 of 98) showed some degree of binaural inhibition (i.e., showing diminished response when the ipsilateral and contralateral ears were stimulated simultaneously) whether they were designated as EE or EO; these IC neurons thus were classified as EE-I or EO-I. Neurons were classified as exhibiting strong inhibition if the ILD function showed a pronounced response decrement, i.e., a decrease of > or = 50% of the response to monaural stimulation of the contralateral ear. Those neurons that showed smaller response decrements (decrease was > or = 25% but < 50%) were designated as showing weak inhibition. Most of these EE-I and EO-I neurons (n = 68) showed strong binaural inhibition. 3. In agreement with results from our earlier studies, frequency threshold curves (FTCs) of IC neurons were altered by sound azimuth. Independent of binaural interaction pattern, most IC neurons (59 of 98) showed a narrowing of the FTC as sound direction was changed from contralateral 90 deg (c90 degrees) to ipsilateral 90 deg (i90 degrees). IC neurons that exhibited the largest direction-dependent changes in frequency selectivity were typically those that displayed stronger binaural inhibition. Occlusion of the ipsilateral ear, which reduced the strength of binaural inhibition by this ear, abolished direction-dependent frequency selectivity. 4. FTCs of IC neurons that exhibited little to moderate direction-dependent effects on frequency selectivity were associated typically with neurons that displayed weak binaural inhibition. Associated with this weak binaural inhibition, central neural responses under monaural occlusion also displayed only small effects; the FTCs were only slightly broader than those derived in the intact condition, and as before, the experimental manipulation resulted in abolishment of direction-dependent frequency selectivity. 5. In contrast to most IC neurons, which showed direction-dependent narrowing of the FTC, about one-third (34 of 98) of IC neurons studied showed a broadening of the FTC when sound direction was shifted to ipsilateral azimuths. Interestingly, for 90% of these 34 neurons, monaural occlusion resulted in narrowing of the bandwidth at each azimuth instead of broadening of the FTC bandwidth. We have evidence to suggest that this direction-dependent broadening is actually a consequence of a truncation or loss of the tip of the FTC derived at c90 degrees, which results from strong binaural inhibition. 6. To compare the frequency threshold tuning in response to monaural stimulation of each ear with free-field FTCs, we measured FTCs for each of the 34 EE neurons to independent contralateral and ipsilateral stimulation. FTCs derived from ipsilateral monaural stimulation were significantly narrower than those resulting from contralateral monaural stimulation, independent of a neuron's direction-dependent changes in frequency selectivity.

Onset and development of auditory brainstem responses in the Mongolian gerbil ( Meriones unguiculatus)

The ontogeny of auditory function in the Mongolian gerbil (Meriones unguiculatus) was examined by measuring auditory brainstem response (ABR) thresholds for clicks and tone bursts at frequencies between 0.125 and 32 kHz. ABRs were elicited as early as 10 postnatal days (PND) in a small proportion of animals (28.6%). Three phases of development were identified based on the appearance of frequency-threshold curves: (a) an onset period (10–12 PND) during which ABRs could be elicited only by intense stimuli (95–110 dB SPL) within a restricted range of frequencies (0.25–8 kHz); (b) a transition period (13–14 PND) during which thresholds improved but frequency-threshold curves remained relatively flat; and (c) a refinement period (15–30 PND) during which frequency-threshold curves exhibited an adult-like (‘V’) shape and thresholds improved steadily to adult-like values. Opening of the external auditory meatus was associated with threshold improvements but not with changes in the frequency region of greatest sensitivity. Between the ages of 13 and 21 PND, low-frequency (0.125–1 kHz) and high-frequency (10–32 kHz) thresholds improved at statistically equivalent rates (4.9 and 5.6 dB/day, respectively), while thresholds for mid-frequency stimuli (2–8 kHz) improved at a significantly faster rate of 9.2 dB/day.

Effects of streptomycin and amiloride on the function of the pigeon inner ear

In vitro, streptomycin and amiloride block the mechano-electrical transduction channels in isolated auditory hair cells of mammals and birds. The present in vivo experiments investigate their action in the intact pigeon inner ear. Streptomycin dissolved in artificial endolymph at concentrations between 1.0 and 6.2 mmol/l was applied into the scala media. After a bolus injection (0.09 to 0.36 μl) the endocochlear potential (EP) was elevated from 6.8 to 7.9 mV. Similarly applied boli of 0.09 to 0.55 μl of amiloride in concentrations of 1–2 mmol/l did not significantly elevate EP (8.7 versus 9.4 mV). Compound action potential (CAP) frequency threshold curves (FTCs) were elevated by both streptomycin and amiloride. The threshold elevation increased with frequency. With streptomycin injection, a complete loss of CAPs was found at frequencies above 1.2 kHz. With amiloride, the loss in sensitivity was 40 dB for the same frequency range. Single fibre recordings showed elevated thresholds and decreased sharpness of tuning with both drugs. With streptomycin, the evoked activity was affected regard-less of stimulation frequencies (above or below the fibre's characteristic frequency). Amiloride, however, preferentially elevated thresholds at frequencies above characteristic frequency. Thus amiloride seems to interfere with hair cell tuning mechanisms. Spontaneous activity decreased by about 30%, following the application of streptomycin. In contrast, injections of amiloride into the scala media were followed by an increase in spontaneous discharge rates.

Hearing in the Neotropical Frog, Atelopus chiriquiensis

Few studies have measured the hearing of frequencies above 1 kHz in anurans lacking middle ear structures. Auditory frequency threshold curves were determined under similar laboratory conditions for Atelopus chiriquiensis, which lacks a tympanum, middle ear cavity, and stapes, and Hyla regilla, a similarly sized species which has these middle ear structures characteristic of most frogs. Both species were found to have similar hearing capabilities, with bimodal threshold curves and peak sensitivities at about 0.2 kHz and 2-3 kHz. This record of hearing sensitivity in a frog lacking a tympanic ear corroborates a similar observation made by Loftus-Hills in 1973 for Pseudophryne semimarmorata, which also lacks most middle ear structures and is in a different family.

Frequency extent of two-tone facilitation in onset units in the ventral cochlear nucleus.

1. The frequency threshold curves (FTCs) of 91 single units in the cochlear nucleus of the anesthetized guinea pig were measured using a conventional single-tone paradigm and a two-tone paradigm designed to elucidate the frequency extent of two-tone facilitation in onset units (On). Units were classified according to existing classification schemes into primary-like (n = 3), chopper (n = 23), and three onset groups: OnI (n = 12), OnC (n = 29), and OnL (n = 24). Histological reconstructions show onset units to be widely distributed within the ventral cochlear nucleus in a manner generally consistent with its tonotopic organization. 2. The FTCs of onset units differed in their minimum thresholds, the steepness of their high- and low-frequency cutoffs, and their sharpness of tuning as quantified by the quality factor at 10 dB (Q10dB) above best frequency (BF) threshold values. There was considerable overlap in the sharpness of tuning between onset units and auditory nerve fibers, as indicated by the distribution of Q10dB values in the octave around 10 kHz: onset units had Q10dB values of 3.56 +/- 1.38 (SD), compared with 6.3 +/- 2.48 for auditory nerve fibers. The tuning of chopper units was similar to that of auditory nerve fibers (5.52 +/- 1.46). 3. Seventy-five percent of onset units showed some degree of facilitation (a threshold reduction) when their FTCs were measured in the presence of BF tones 4 dB below BF threshold. The frequency extent of such facilitation was variable, with a maximum of 6 octaves around the BF. In extreme cases facilitation could be measured when the BF tone was as low as 30 dB below BF threshold. 4. In 17% of onset units, suppressive effects were evident, as shown by noncontiguous frequency regions of facilitation. These suppressive effects might be a reflection either of suppression in the auditory nerve input or of a direct inhibitory input to the onset units. The strength of this effect suggests that inhibition is a likely explanation, consistent with the finding in previous morphological studies of profuse synapses with pleomorphic vesicles on multipolar cells. 5. FTCs of chopper and primary-like units measured in the presence of BF tones showed little facilitation. The facilitation that was observed in chopper units was confined to a narrow region around BF and disappeared when the facilitatory tone was lowered to 4 dB below BF threshold. 6. These data support the hypothesis that onset units, but not chopper or primary-like units, receive excitatory inputs from auditory nerve fibers with a wide range of BFs. However, the frequency range of facilitation and the magnitude of the threshold facilitation varied from unit to unit, suggesting that the off-BF inputs from auditory nerve fibers are not evenly distributed or equally effective in all units.

Effects of furosemide on distortion product otoacoustic emissions and on neuronal responses in the anteroventral cochlear nucleus.

1. The objective of this study was to precisely evaluate the relationship between the threshold of neurons in the anteroventral cochlear nucleus (AVCN) and the properties of distortion product otoacoustic emissions (DPOAEs). Response areas of multiunit clusters in the AVCN and DPOAEs in the ear canal were measured alternately in the adult gerbil during furosemide-induced changes of the endocochlear potential. Stimulus frequencies of the probe tones for DPOAE measurement were in the range of f1 = 1.7-7.6 kHz and f2 = 2.0-9.0 kHz; the ratio f2:f1 was always 1.18. Stimulus amplitudes were varied in 5-dB steps from 30 to 80 dB SPL, with either equal amplitudes (L1 = L2) or unequal, with L1 set 10 dB above L2. Multiunit response areas were determined from cluster responses to a series of 100-ms tone bursts presented with a pseudo-random sequence in frequency and intensity. 2. Changes in the multiunit discharge properties after 50-75 mg/kg furosemide injection were as follows: the best frequency (BF) threshold increased from initial values in the range of 20-30 dB SPL to 50-80 dB SPL at 10-20 min postinjection and then recovered fully by 60-90 min. The spontaneous discharge activity decreased to zero before any changes in the frequency threshold curve were observed and did not return to initial values for several hours. Likewise, total discharge rates of stimulus elicited responses were reduced and tended to stay reduced even after BF threshold had fully recovered. 3. From the DPOAE measurements, the changes observed in the cubic distortion tone (CDT, 2f1-f2) emission after furosemide injection were as follows: at high levels of the probe tones, changes in the emission intensities generally stayed within a 10-dB range. The CDT amplitudes for low stimulus levels, however, were typically reduced by up to 40 dB, but recovered (depending on the furosemide dosage) by approximately 60-90 min. 4. At low to moderate stimulus levels of 40-60 dB SPL, there was a near perfect, minute-by-minute covariation of the ear canal CDT amplitude and the BF threshold measured in the AVCN. A 10-dB increase in threshold was associated with a 5- to 7-dB decrease in the CDT emission. 5. The optimum stimulus parameter set for the noninvasive estimation of cochlear performance from the CDT response was for stimulus amplitudes L1 = 50, L2 = 40 dB SPL. 6. This experiment demonstrates that CDT emissions at low stimulus levels are very good predictors of the thresholds of cochlear afferents, but this validity is lost for BF thresholds greater than approximately 60-70 dB SPL. 7. The ear canal CDT amplitude is better correlated with the BF threshold sensitivity of neuronal response areas in the AVCN than with the spontaneous discharge rate or absolute above-threshold discharge rates.