Variation in the phase of response to low-frequency pure tones in the guinea pig auditory nerve as functions of stimulus level and frequency

Best frequencies and temporal delays are similar across the low-frequency regions of the guinea pig cochlea

The cochlea maps tones with different frequencies to distinct anatomical locations. For instance, a faint 5000-hertz tone produces brisk responses at a place approximately 8 millimeters into the 18-millimeter-long guinea pig cochlea, but little response elsewhere. This place code pervades the auditory pathways, where neurons have "best frequencies" determined by their connections to the sensory cells in the hearing organ. However, frequency selectivity in cochlear regions encoding low-frequency sounds has not been systematically studied. Here, we show that low-frequency hearing works according to a unique principle that does not involve a place code. Instead, sound-evoked responses and temporal delays are similar across the low-frequency regions of the cochlea. These findings are a break from theories considered proven for 100 years and have broad implications for understanding information processing in the brainstem and cortex and for optimizing the stimulus delivery in auditory implants.

A simplified physiological model of rate-level functions of auditory-nerve fibers

Several approaches have been used to describe the rate-level functions of auditory-nerve fibers (ANFs). One approach uses descriptive models that can be fitted easily to data. Another derives rate-level functions from comprehensive physiological models of auditory peripheral processing. Here, we seek to identify the minimal set of components needed to provide a physiologically plausible account of rate-level functions. Our model consists of a first-order Boltzmann mechanoelectrical transducer function relating the instantaneous stimulus pressure to an instantaneous output, followed by a lowpass filter that eliminates the AC component, followed by an exponential synaptic transfer function relating the DC component to the mean spike rate. This is perhaps the simplest physiologically plausible model capable of accounting for rate-level functions under the assumption that the model parameters for a given ANF and stimulus frequency are level-independent. We find that the model typically accounts well for rate-level functions from cat ANFs for all stimulus frequencies. More complicated model variants having saturating synaptic transfer functions do not perform significantly better, implying the system operates far away from synaptic saturation. Rate saturation in the model is caused by saturation of the DC component of the filter output (e.g., the receptor potential), which in turn is due to the saturation of the transducer function. The maximum mean spike rate is approximately constant across ANFs, such that the slope parameter of the exponential synaptic transfer function decreases with increasing spontaneous rate. If the synaptic parameters for a given ANF are assumed to be constant across stimulus frequencies, then frequency- and level-dependent input nonlinearities are derived that are qualitatively similar to those reported in the literature. Contrary to assumptions in the literature, such nonlinearities are obtained even for ANFs having high spontaneous rates. Finally, spike-rate adaptation is examined and found to be accounted for by a decrease in the slope parameter of the synaptic transfer function over time following stimulus onset.

Encoding sound in the cochlea: from receptor potential to afferent discharge

Ribbon-class synapses in the ear achieve analog to digital transformation of a continuously graded membrane potential to all-or-none spikes. In mammals, several auditory nerve fibres (ANFs) carry information from each inner hair cell (IHC) to the brain in parallel. Heterogeneity of transmission among synapses contributes to the diversity of ANF sound-response properties. In addition to the place code for sound frequency and the rate code for sound level, there is also a temporal code. In series with cochlear amplification and frequency tuning, neural representation of temporal cues over a broad range of sound levels enables auditory comprehension in noisy multi-speaker settings. The IHC membrane time constant introduces a low-pass filter that attenuates fluctuations of the receptor potential above 1-2 kHz. The ANF spike generator adds a high-pass filter via its depolarization-rate threshold that rejects slow changes in the postsynaptic potential and its phasic response property that ensures one spike per depolarization. Synaptic transmission involves several stochastic subcellular processes between IHC depolarization and ANF spike generation, introducing delay and jitter that limits the speed and precision of spike timing. ANFs spike at a preferred phase of periodic sounds in a process called phase-locking that is limited to frequencies below a few kilohertz by both the IHC receptor potential and the jitter in synaptic transmission. During phase-locking to periodic sounds of increasing intensity, faster and facilitated activation of synaptic transmission and spike generation may be offset by presynaptic depletion of synaptic vesicles, resulting in relatively small changes in response phase. Here we review encoding of spike-timing at cochlear ribbon synapses.

Salicylate decreases the spontaneous firing rate of guinea pig auditory nerve fibres

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Spontaneous firing rates were recorded from single auditory fibres in vivo.

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Salicylate was injected at 350 mg/kg by the subcutaneous route.

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Median firing rate decreased by 32 spikes/s in nerve fibres after salicylate injection.

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The high spontaneous rate fibres (type 1A) showed the main reduction.

Tinnitus has similarities to chronic neuropathic pain where there are changes in the firing rate of different types of afferent neurons. We postulated that one possible cause of tinnitus is a change in the distribution of spontaneous firing rates in at least one type of afferent auditory nerve fibre in anaesthetised guinea pigs. In control animals there was a bimodal distribution of spontaneous rates, but the position of the second mode was different depending upon whether the fibres responded best to high (> 4 kHz) or low (≤4 kHz) frequency tonal stimulation. The simplest and most reliable way of inducing tinnitus in experimental animals is to administer a high dose of sodium salicylate. The distribution of the spontaneous firing rates was different when salicylate (350 mg/kg) was administered, even when the sample was matched for the distribution of characteristic frequencies in the control population. The proportion of medium spontaneous rate fibres (MSR, 1≤ spikes/s ≤20) increased while the proportion of the highest, high spontaneous firing rate fibres (HSR, > 80 spikes/s) decreased following salicylate. The median rate fell from 64.7 spikes/s (control) to 35.4 spikes/s (salicylate); a highly significant change (Kruskal-Wallis test p < 0.001). When the changes were compared with various models of statistical probability, the most accurate model was one where most HSR fibres decreased their firing rate by 32 spikes/s. Thus, we have shown a reduction in the firing rate of HSR fibres that may be related to tinnitus.

The cochlear ear horn: geometric origin of tonotopic variations in auditory signal processing

While separating sounds into frequency components and subsequently converting them into patterns of neural firing, the mammalian cochlea processes signal components in ways that depend strongly on frequency. Indeed, both the temporal structure of the response to transient stimuli and the sharpness of frequency tuning differ dramatically between the apical and basal (i.e., the low- and high-frequency) regions of the cochlea. Although the mechanisms that give rise to these pronounced differences remain incompletely understood, they are generally attributed to tonotopic variations in the constituent hair cells or cytoarchitecture of the organ of Corti. As counterpoint to this view, we present a general acoustic treatment of the horn-like geometry of the cochlea, accompanied by a simple 3-D model to elucidate the theoretical predictions. We show that the main apical/basal functional differences can be accounted for by the known spatial gradients of cochlear dimensions, without the need to invoke mechanical specializations of the sensory tissue. Furthermore, our analysis demonstrates that through its functional resemblance to an ear horn (aka ear trumpet), the geometry of the cochlear duct manifests tapering symmetry, a felicitous design principle that may have evolved not only to aid the analysis of natural sounds but to enhance the sensitivity of hearing.

Nitric oxide increases gain in the ventral cochlear nucleus of guinea pigs with tinnitus

Previous work has led to the hypothesis that, during the production of noise-induced tinnitus, higher levels of nitric oxide (NO), in the ventral cochlear nucleus (VCN), increase the gain applied to a reduced input from the cochlea. To test this hypothesis, we noise-exposed 26 guinea pigs, identified evidence of tinnitus in 12 of them and then compared the effects of an iontophoretically applied NO donor or production inhibitor on VCN single unit activity. We confirmed that the mean driven firing rate for the tinnitus and control groups was the same while it had fallen in the non-tinnitus group. By contrast, the mean spontaneous rate had increased for the tinnitus group relative to the control group, while it remained the same for the non-tinnitus group. A greater proportion of units responded to exogenously applied NO in the tinnitus (56%) and non-tinnitus groups (71%) than a control population (24%). In the tinnitus group, endogenous NO facilitated the driven firing rate in 37% (7/19) of neurons and appeared to bring the mean driven rate back up to control levels by a mechanism involving N-methyl-D-aspartic acid (NMDA) receptors. By contrast, in the non-tinnitus group, endogenous NO only facilitated the driven firing rate in 5% (1/22) of neurons and there was no facilitation of driven rate in the control group. The effects of endogenous NO on spontaneous activity were unclear. These results suggest that NO is involved in increasing the gain applied to driven activity, but other factors are also involved in the increase in spontaneous activity.

Nitric oxide regulates the firing rate of neuronal subtypes in the guinea pig ventral cochlear nucleus

The gaseous free radical, nitric oxide (NO) acts as a ubiquitous neuromodulator, contributing to synaptic plasticity in a complex way that can involve either long term potentiation or depression. It is produced by neuronal nitric oxide synthase (nNOS) which is presynaptically expressed and also located postsynaptically in the membrane and cytoplasm of a subpopulation of each major neuronal type in the ventral cochlear nucleus (VCN). We have used iontophoresis in vivo to study the effect of the NOS inhibitor L-NAME (L-NG-Nitroarginine methyl ester) and the NO donors SIN-1 (3-Morpholinosydnonimine hydrochloride) and SNOG (S-Nitrosoglutathione) on VCN units under urethane anaesthesia. Collectively, both donors produced increases and decreases in driven and spontaneous firing rates of some neurones. Inhibition of endogenous NO production with L-NAME evoked a consistent increase in driven firing rates in 18% of units without much effect on spontaneous rate. This reduction of gain produced by endogenous NO was mirrored when studying the effect of L-NAME on NMDA(N-Methyl-D-aspartic acid)-evoked excitation, with 30% of units showing enhanced NMDA-evoked excitation during L-NAME application (reduced NO levels). Approximately 25% of neurones contain nNOS and the NO produced can modulate the firing rate of the main principal cells: medium stellates (choppers), large stellates (onset responses) and bushy cells (primary-like responses). The main endogenous role of NO seems to be to partly suppress driven firing rates associated with NMDA channel activity but there is scope for it to increase neural gain if there were a pathological increase in its production following hearing loss.

A review of mechanical and synaptic processes in otolith transduction of sound and vibration for clinical VEMP testing

Older studies of mammalian otolith physiology have focused mainly on sustained responses to low-frequency (<50 Hz) or maintained linear acceleration. So the otoliths have been regarded as accelerometers. Thus evidence of otolithic activation and high-precision phase locking to high-frequency sound and vibration appears to be very unusual. However, those results are exactly in accord with a substantial body of knowledge of otolith function in fish and frogs. It is likely that phase locking of otolith afferents to vibration is a general property of all vertebrates. This review examines the literature about the activation and phase locking of single otolithic neurons to air-conducted sound and bone-conducted vibration, in particular the high precision of phase locking shown by mammalian irregular afferents that synapse on striolar type I hair cells by calyx endings. Potassium in the synaptic cleft between the type I hair cell receptor and the calyx afferent ending may be responsible for the tight phase locking of these afferents even at very high discharge rates. Since frogs and fish do not possess full calyx endings, it is unlikely that they show phase locking with such high precision and to such high frequencies as has been found in mammals. The high-frequency responses have been modeled as the otoliths operating in a seismometer mode rather than an accelerometer mode. These high-frequency otolithic responses constitute the neural basis for clinical vestibular-evoked myogenic potential tests of otolith function.

Phase Locking of Auditory-Nerve Fibers Reveals Stereotyped Distortions and an Exponential Transfer Function with a Level-Dependent Slope

Phase locking of auditory-nerve-fiber (ANF) responses to the fine structure of acoustic stimuli is a hallmark of the auditory system's temporal precision and is important for many aspects of hearing. Period histograms from phase-locked ANF responses to low-frequency tones exhibit spike-rate and temporal asymmetries, but otherwise retain an approximately sinusoidal shape as stimulus level increases, even beyond the level at which the mean spike rate saturates. This is intriguing because apical cochlear mechanical vibrations show little compression, and mechanoelectrical transduction in the receptor cells is thought to obey a static sigmoidal nonlinearity, which might be expected to produce peak clipping at moderate and high stimulus levels. Here we analyze phase-locked responses of ANFs from cats of both sexes. We show that the lack of peak clipping is due neither to ANF refractoriness nor to spike-rate adaptation on time scales longer than the stimulus period. We demonstrate that the relationship between instantaneous pressure and instantaneous rate is well described by an exponential function whose slope decreases with increasing stimulus level. Relatively stereotyped harmonic distortions in the input to the exponential can account for the temporal asymmetry of the period histograms, including peak splitting. We show that the model accounts for published membrane-potential waveforms when assuming a power-of-three, but not a power-of-one, relationship to exocytosis. Finally, we demonstrate the relationship between the exponential transfer functions and the sigmoidal pseudotransducer functions obtained in the literature by plotting the maxima and minima of the voltage responses against the maxima and minima of the stimuli.SIGNIFICANCE STATEMENT Phase locking of auditory-nerve-fiber responses to the temporal fine structure of acoustic stimuli is important for many aspects of hearing, but the mechanisms underlying phase locking are not fully understood. Intriguingly, period histograms retain an approximately sinusoidal shape across sound levels, even when the mean rate has saturated. We find that neither refractoriness nor spike-rate adaptation is responsible for this behavior. Instead, the peripheral auditory system operates as though it contains an exponential transfer function whose slope changes with stimulus level. The underlying mechanism is distinct from the comparatively weak cochlear mechanical compression in the cochlear apex, and likely resides in the receptor cells.

A Temporal Filter for Binaural Hearing Is Dynamically Adjusted by Sound Pressure Level

In natural environments our auditory system is exposed to multiple and diverse signals of fluctuating amplitudes. Therefore, to detect, localize, and single out individual sounds the auditory system has to process and filter spectral and temporal information from both ears. It is known that the overall sound pressure level affects sensory signal transduction and therefore the temporal response pattern of auditory neurons. We hypothesize that the mammalian binaural system utilizes a dynamic mechanism to adjust the temporal filters in neuronal circuits to different overall sound pressure levels. Previous studies proposed an inhibitory mechanism generated by the reciprocally coupled dorsal nuclei of the lateral lemniscus (DNLL) as a temporal neuronal-network filter that suppresses rapid binaural fluctuations. Here we investigated the consequence of different sound levels on this filter during binaural processing. Our in vivo and in vitro electrophysiology in Mongolian gerbils shows that the integration of ascending excitation and contralateral inhibition defines the temporal properties of this inhibitory filter. The time course of this filter depends on the synaptic drive, which is modulated by the overall sound pressure level and N-methyl-D-aspartate receptor (NMDAR) signaling. In psychophysical experiments we tested the temporal perception of humans and show that detection and localization of two subsequent tones changes with the sound pressure level consistent with our physiological results. Together our data support the hypothesis that mammals dynamically adjust their time window for sound detection and localization within the binaural system in a sound level dependent manner.

Unusual mechanical processing of sounds at the apex of the Guinea pig cochlea

One of the tenets of mammalian auditory physiology is that the frequency selectivity at the cochlear base decreases as a function of stimulus level. Changes in frequency selectivity have been shown to be accompanied by changes in response phases as a function of stimulus level. The existence of such nonlinear properties has been revealed by the analysis of either direct or indirect recordings of mechanical vibrations of the cochlea. Direct measurements of cochlear mechanical vibrations, however, have been carried out with success primarily in cochlear regions that are tuned to frequencies >7 kHz, but not in regions sensitive to lower frequencies. In this paper we continue to analyze recently published data from measurements of sound-induced vibrations at four locations near the apex of the intact guinea pig cochlea, in a region encompassing approximately 25% of its total length. Analysis of the responses at all locations reveal level-dependent phase properties that are rather different from those usually reported at the base of the cochlea of laboratory animals such as the chinchilla. Cochlear group delays, for example, increase or remain constant with increasing stimulus. Similarly, frequency selectivity at all the regions increases as a function of stimulus level.

The Auditory Nerve Overlapped Waveform (ANOW) Detects Small Endolymphatic Manipulations That May Go Undetected by Conventional Measurements

Electrocochleography (ECochG) has been used to assess Ménière's disease, a pathology associated with endolymphatic hydrops and low-frequency sensorineural hearing loss. However, the current ECochG techniques are limited for use at high-frequencies only (≥1 kHz) and cannot be used to assess and understand the low-frequency sensorineural hearing loss in ears with Ménière's disease. In the current study, we use a relatively new ECochG technique to make measurements that originate from afferent auditory nerve fibers in the apical half of the cochlear spiral to assess effects of endolymphatic hydrops in guinea pig ears. These measurements are made from the Auditory Nerve Overlapped Waveform (ANOW). Hydrops was induced with artificial endolymph injections, iontophoretically applied Ca²⁺ to endolymph, and exposure to 200 Hz tones. The manipulations used in this study were far smaller than those used in previous investigations on hydrops. In response to all hydropic manipulations, ANOW amplitude to moderate level stimuli was markedly reduced but conventional ECochG measurements of compound action potential thresholds were unaffected (i.e., a less than 2 dB threshold shift). Given the origin of the ANOW, changes in ANOW amplitude likely reflect acute volume disturbances accumulate in the distensible cochlear apex. These results suggest that the ANOW could be used to advance our ability to identify initial stages of dysfunction in ears with Ménière's disease before the pathology progresses to an extent that can be detected with conventional measures.

Enhancement of phase-locking in rodents. I. An axonal recording study in gerbil

The trapezoid body (TB) contains axons of neurons in the anteroventral cochlear nucleus projecting to monaural and binaural nuclei in the superior olivary complex (SOC). Characterization of these monaural inputs is important for the interpretation of response properties of SOC neurons. In particular, understanding of the sensitivity to interaural time differences (ITDs) in neurons of the medial and lateral superior olive requires knowledge of the temporal firing properties of the monaural excitatory and inhibitory inputs to these neurons. In recent years, studies of ITD sensitivity of SOC neurons have made increasing use of small animal models with good low-frequency hearing, particularly the gerbil. We presented stimuli as used in binaural studies to monaural neurons in the TB and studied their temporal coding. We found that general trends as have been described in the cat are present in gerbil, but with some important differences. Phase-locking to pure tones tends to be higher in TB axons and in neurons of the medial nucleus of the TB (MNTB) than in the auditory nerve for neurons with characteristic frequencies (CFs) below 1 kHz, but this enhancement is quantitatively more modest than in cat. Stronger enhancement is common when TB neurons are stimulated at low frequencies below CF. It is rare for TB neurons in gerbil to entrain to low-frequency stimuli, i.e., to discharge a well-timed spike on every stimulus cycle. Also, complex phase-locking behavior, with multiple modes of increased firing probability per stimulus cycle, is common in response to low frequencies below CF.NEW & NOTEWORTHY Phase-locking is an important property of neurons in the early auditory pathway: it is critical for the sensitivity to time differences between the two ears enabling spatial hearing. Studies in cat have shown an improvement in phase-locking from the peripheral to the central auditory nervous system. We recorded from axons in an output tract of the cochlear nucleus and show that a similar but more limited form of temporal enhancement is present in gerbil.

A Test of the Stereausis Hypothesis for Sound Localization in Mammals

The relative arrival times of sounds at both ears constitute an important cue for localization of low-frequency sounds in the horizontal plane. The binaural neurons of the medial superior olive (MSO) act as coincidence detectors that fire when inputs from both ears arrive near simultaneously. Each principal neuron in the MSO is tuned to its own best interaural time difference (ITD), indicating the presence of an internal delay, a difference in the travel times from either ear to the MSO. According to the stereausis hypothesis, differences in wave propagation along the cochlea could provide the delays necessary for coincidence detection if the ipsilateral and contralateral inputs originated from different cochlear positions, with different frequency tuning. We therefore investigated the relation between interaural mismatches in frequency tuning and ITD tuning during in vivo loose-patch (juxtacellular) recordings from principal neurons of the MSO of anesthetized female gerbils. Cochlear delays can be bypassed by directly stimulating the auditory nerve; in agreement with the stereausis hypothesis, tuning for timing differences during bilateral electrical stimulation of the round windows differed markedly from ITD tuning in the same cells. Moreover, some neurons showed a frequency tuning mismatch that was sufficiently large to have a potential impact on ITD tuning. However, we did not find a correlation between frequency tuning mismatches and best ITDs. Our data thus suggest that axonal delays dominate ITD tuning.SIGNIFICANCE STATEMENT Neurons in the medial superior olive (MSO) play a unique role in sound localization because of their ability to compare the relative arrival time of low-frequency sounds at both ears. They fire maximally when the difference in sound arrival time exactly compensates for the internal delay: the difference in travel time from either ear to the MSO neuron. We tested whether differences in cochlear delay systematically contribute to the total travel time by comparing for individual MSO neurons the best difference in arrival times, as predicted from the frequency tuning for either ear, and the actual best difference. No systematic relation was observed, emphasizing the dominant contribution of axonal delays to the internal delay.

Rhythm judgments reveal a frequency asymmetry in the perception and neural coding of sound synchrony

In modern Western music, melody is commonly conveyed by pitch changes in the highest-register voice, whereas meter or rhythm is often carried by instruments with lower pitches. An intriguing and recently suggested possibility is that the custom of assigning rhythmic functions to lower-pitch instruments may have emerged because of fundamental properties of the auditory system that result in superior time encoding for low pitches. Here we compare rhythm and synchrony perception between low- and high-frequency tones, using both behavioral and EEG techniques. Both methods were consistent in showing no superiority in time encoding for low over high frequencies. However, listeners were consistently more sensitive to timing differences between two nearly synchronous tones when the high-frequency tone followed the low-frequency tone than vice versa. The results demonstrate no superiority of low frequencies in timing judgments but reveal a robust asymmetry in the perception and neural coding of synchrony that reflects greater tolerance for delays of low- relative to high-frequency sounds than vice versa. We propose that this asymmetry exists to compensate for inherent and variable time delays in cochlear processing, as well as the acoustical properties of sound sources in the natural environment, thereby providing veridical perceptual experiences of simultaneity.

Stimulus Frequency Otoacoustic Emission Delays and Generating Mechanisms in Guinea Pigs, Chinchillas, and Simulations

According to coherent reflection theory (CRT), stimulus frequency otoacoustic emissions (SFOAEs) arise from cochlear irregularities coherently reflecting energy from basilar membrane motion within the traveling-wave peak. This reflected energy arrives in the ear canal predominantly with a single delay at each frequency. However, data from humans and animals indicate that (1) SFOAEs can have multiple delay components, (2) low-frequency SFOAE delays are too short to be accounted for by CRT, and (3) "SFOAEs" obtained with a 2nd ("suppressor") tone ≥2 octaves above the probe tone have been interpreted as arising from the area basal to the region of cochlear amplification. To explore these issues, we collected SFOAEs by the suppression method in guinea pigs and time-frequency analyzed these data, simulated SFOAEs, and published chinchilla SFOAEs. Time-frequency analysis revealed that most frequencies showed only one SFOAE delay component while other frequencies had multiple components including some with short delays. We found no systematic patterns in the occurrence of multiple delay components. Using a cochlear model that had significant basilar membrane motion only in the peak region of the traveling wave, simulated SFOAEs had single and multiple delay components similar to the animal SFOAEs. This result indicates that multiple components (including ones with short delays) can originate from cochlear mechanical irregularities in the SFOAE peak region and are not necessarily indicative of SFOAE sources in regions ≥2 octaves basal of the SFOAE peak region. We conclude that SFOAEs obtained with suppressors close to the probe frequency provide information primarily about the mechanical response in the region that receives amplification, and we attribute the too-short SFOAE delays at low frequencies to distortion-source SFOAEs and coherent reflection from multiple cochlear motions. Our findings suggest that CRT needs revision to include reflections from multiple motions in the cochlear apex.

Energy Flux in the Cochlea: Evidence Against Power Amplification of the Traveling Wave

Traveling waves in the inner ear exhibit an amplitude peak that shifts with frequency. The peaking is commonly believed to rely on motile processes that amplify the wave by inserting energy. We recorded the vibrations at adjacent positions on the basilar membrane in sensitive gerbil cochleae and tested the putative power amplification in two ways. First, we determined the energy flux of the traveling wave at its peak and compared it to the acoustic power entering the ear, thereby obtaining the net cochlear power gain. For soft sounds, the energy flux at the peak was 1 ± 0.6 dB less than the middle ear input power. For more intense sounds, increasingly smaller fractions of the acoustic power actually reached the peak region. Thus, we found no net power amplification of soft sounds and a strong net attenuation of intense sounds. Second, we analyzed local wave propagation on the basilar membrane. We found that the waves slowed down abruptly when approaching their peak, causing an energy densification that quantitatively matched the amplitude peaking, similar to the growth of sea waves approaching the beach. Thus, we found no local power amplification of soft sounds and strong local attenuation of intense sounds. The most parsimonious interpretation of these findings is that cochlear sensitivity is not realized by amplifying acoustic energy, but by spatially focusing it, and that dynamic compression is realized by adjusting the amount of dissipation to sound intensity.

Basic response properties of auditory nerve fibers: a review

All acoustic information from the periphery is encoded in the timing and rates of spikes in the population of spiral ganglion neurons projecting to the central auditory system. Considerable progress has been made in characterizing the physiological properties of type-I and type-II primary auditory afferents and understanding the basic properties of type-I afferents in response to sounds. Here, we review some of these properties, with emphasis placed on issues such as the stochastic nature of spike timing during spontaneous and driven activity, frequency tuning curves, spike-rate-versus-level functions, dynamic-range and spike-rate adaptation, and phase locking to stimulus fine structure and temporal envelope. We also review effects of acoustic trauma on some of these response properties.

Assessment of the limits of neural phase-locking using mass potentials

In the diverse mechanosensory systems that animals evolved, the waveform of stimuli can be encoded by phase locking in spike trains of primary afferents. Coding of the fine structure of sounds via phase locking is thought to be critical for hearing. The upper frequency limit of phase locking varies across species and is unknown in humans. We applied a method developed previously, which is based on neural adaptation evoked by forward masking, to analyze mass potentials recorded on the cochlea and auditory nerve in the cat. The method allows us to separate neural phase locking from receptor potentials. We find that the frequency limit of neural phase locking obtained from mass potentials was very similar to that reported for individual auditory nerve fibers. The results suggest that this is a promising approach to examine neural phase locking in humans with normal or impaired hearing or in other species for which direct recordings from primary afferents are not feasible.

A Structural Theory of Pitch(1,2,3)

Musical notes can be ordered from low to high along a perceptual dimension called "pitch". A characteristic property of these sounds is their periodic waveform, and periodicity generally correlates with pitch. Thus, pitch is often described as the perceptual correlate of the periodicity of the sound's waveform. However, the existence and salience of pitch also depends in a complex way on other factors, in particular harmonic content. For example, periodic sounds made of high-order harmonics tend to have a weaker pitch than those made of low-order harmonics. Here we examine the theoretical proposition that pitch is the perceptual correlate of the regularity structure of the vibration pattern of the basilar membrane, across place and time-a generalization of the traditional view on pitch. While this proposition also attributes pitch to periodic sounds, we show that it predicts differences between resolved and unresolved harmonic complexes and a complex domain of existence of pitch, in agreement with psychophysical experiments. We also present a possible neural mechanism for pitch estimation based on coincidence detection, which does not require long delays, in contrast with standard temporal models of pitch.

Reverse correlation analysis of auditory-nerve fiber responses to broadband noise in a bird, the barn owl

While the barn owl has been extensively used as a model for sound localization and temporal coding, less is known about the mechanisms at its sensory organ, the basilar papilla (homologous to the mammalian cochlea). In this paper, we characterize, for the first time in the avian system, the auditory nerve fiber responses to broadband noise using reverse correlation. We use the derived impulse responses to study the processing of sounds in the cochlea of the barn owl. We characterize the frequency tuning, phase, instantaneous frequency, and relationship to input level of impulse responses. We show that, even features as complex as the phase dependence on input level, can still be consistent with simple linear filtering. Where possible, we compare our results with mammalian data. We identify salient differences between the barn owl and mammals, e.g., a much smaller frequency glide slope and a bimodal impulse response for the barn owl, and discuss what they might indicate about cochlear mechanics. While important for research on the avian auditory system, the results from this paper also allow us to examine hypotheses put forward for the mammalian cochlea.

Phase-locking precision is enhanced by multiquantal release at an auditory hair cell ribbon synapse

Sound-evoked spikes in the auditory nerve can phase-lock with submillisecond precision for prolonged periods of time. However, the synaptic mechanisms that enable this accurate spike firing remain poorly understood. Using paired recordings from adult frog hair cells and their afferent fibers, we show here that during sine-wave stimuli, synaptic failures occur even during strong stimuli. However, exclusion of these failures leads to mean excitatory postsynaptic current (EPSC) amplitudes that are independent of Ca(2+) current. Given the intrinsic jitter in spike triggering, evoked synaptic potentials and spikes had surprisingly similar degrees of synchronization to a sine-wave stimulus. This similarity was explained by an unexpected finding: large-amplitude evoked EPSCs have a significantly larger synchronization index than smaller evoked EPSCs. Large EPSCs therefore enhance the precision of spike timing. The hair cells' unique capacity for continuous, large-amplitude, and highly synchronous multiquantal release thus underlies its ability to trigger phase-locked spikes in afferent fibers.

The spatial buildup of compression and suppression in the mammalian cochlea

We recorded responses of the gerbil basilar membrane (BM) to wideband tone complexes. The intensity of one component was varied and the effects on the amplitude and phase of the others were assessed. This suppression paradigm enabled us to vary probe frequency and suppressor frequency independently, allowing the use of simple scaling arguments to analyze the spatial buildup of the nonlinear interaction between traveling waves. Most suppressors had the same effects on probe amplitude and phase as did wideband intensity increments. The main exception were suppressors above the characteristic frequency (CF) of the recording location, for which the frequency range of most affected probes was not constant, but shifted upward with suppressor frequency. BM displacement reliably predicted the effectiveness of low-side suppressors, but not high-side suppressors. We found “anti-suppression” of probes well below CF, i.e., suppressor-induced enhancement of probe response amplitude. Large (>1 cycle) phase effects occurred for above-CF probes. Phase shifts varied nonmonotonically, but systematically, with suppressor level, probe frequency, and suppressor frequency, reconciling apparent discrepancies in the literature. The analysis of spatial buildup revealed an accumulation of local effects on the propagation of the traveling wave, with larger BM displacement reducing the local forward gain. The propagation speed of the wave was also affected. With larger BM displacement, the basal portion of the wave slowed down, while the apical part sped up. This framework of spatial buildup of local effects unifies the widely different effects of overall intensity, low-side suppressors, and high-side suppressors on BM responses.

Effects of temporal stimulus properties on the perception of across-frequency asynchrony

The role of temporal stimulus parameters in the perception of across-frequency synchrony and asynchrony was investigated using pairs of 500-ms tones consisting of a 250-Hz tone and a tone with a higher frequency of 1, 2, 4, or 6 kHz. Subjective judgments suggested veridical perception of across-frequency synchrony but with greater sensitivity to changes in asynchrony for pairs in which the lower-frequency tone was leading than for pairs in which it was lagging. Consistent with the subjective judgments, thresholds for the detection of asynchrony measured in a three-alternative forced-choice task were lower when the signal interval contained a pair with the low-frequency tone leading than a pair with a high-frequency tone leading. A similar asymmetry was observed for asynchrony discrimination when the standard asynchrony was relatively small (≤20 ms) but not for larger standard asynchronies. Independent manipulation of onset and offset ramp durations indicated a dominant role of onsets in the perception of across-frequency asynchrony. A physiologically inspired model, involving broadly tuned monaural coincidence detectors that receive inputs from frequency-selective onset detectors, was able to accurately reproduce the asymmetric distributions of synchrony judgments. The model provides testable predictions for future physiological investigations of responses to broadband stimuli with across-frequency delays.

Ongoing temporal coding of a stochastic stimulus as a function of intensity: time-intensity trading

Stimulus-locked temporal codes are increasingly seen as relevant to perception. The timing of action potentials typically varies with stimulus intensity, and the invariance of temporal representations with intensity is therefore an issue. We examine the timing of action potentials in cat auditory nerve to broadband noise presented at different intensities, using an analysis inspired by coincidence detection and by the binaural "latency hypothesis." It is known that the two cues for azimuthal sound localization, interaural intensity or level differences and interaural time differences (ITDs), interact perceptually. According to the latency hypothesis, the increase in intensity for the ear nearest to a sound source off the midline causes a decrease in response latency in that ear relative to the other ear. We found that changes in intensity cause small but systematic shifts in the ongoing timing of responses in the auditory nerve, generally but not always resulting in shorter delays between stimulus onset and neural response for increasing intensity. The size of the temporal shifts depends on characteristic frequency with a pattern indicating a fine-structure and an envelope response regime. Overall, the results show that ongoing timing is remarkably stable with intensity at the most peripheral neural level. The results are not consistent in a simple way with the latency hypothesis, but because of the acute sensitivity to ITDs, the subtle effects of intensity on timing may nevertheless have perceptual consequences.

Sensitivity of cochlear nucleus neurons to spatio-temporal changes in auditory nerve activity

The spatio-temporal pattern of auditory nerve (AN) activity, representing the relative timing of spikes across the tonotopic axis, contains cues to perceptual features of sounds such as pitch, loudness, timbre, and spatial location. These spatio-temporal cues may be extracted by neurons in the cochlear nucleus (CN) that are sensitive to relative timing of inputs from AN fibers innervating different cochlear regions. One possible mechanism for this extraction is "cross-frequency" coincidence detection (CD), in which a central neuron converts the degree of coincidence across the tonotopic axis into a rate code by preferentially firing when its AN inputs discharge in synchrony. We used Huffman stimuli (Carney LH. J Neurophysiol 64: 437-456, 1990), which have a flat power spectrum but differ in their phase spectra, to systematically manipulate relative timing of spikes across tonotopically neighboring AN fibers without changing overall firing rates. We compared responses of CN units to Huffman stimuli with responses of model CD cells operating on spatio-temporal patterns of AN activity derived from measured responses of AN fibers with the principle of cochlear scaling invariance. We used the maximum likelihood method to determine the CD model cell parameters most likely to produce the measured CN unit responses, and thereby could distinguish units behaving like cross-frequency CD cells from those consistent with same-frequency CD (in which all inputs would originate from the same tonotopic location). We find that certain CN unit types, especially those associated with globular bushy cells, have responses consistent with cross-frequency CD cells. A possible functional role of a cross-frequency CD mechanism in these CN units is to increase the dynamic range of binaural neurons that process cues for sound localization.

Perception of across-frequency asynchrony and the role of cochlear delays

Cochlear filtering results in earlier responses to high than to low frequencies. This study examined potential perceptual correlates of cochlear delays by measuring the perception of relative timing between tones of different frequencies. A brief 250-Hz tone was combined with a brief 1-, 2-, 4-, or 6-kHz tone. Two experiments were performed, one involving subjective judgments of perceived synchrony, the other involving asynchrony detection and discrimination. The functions relating the proportion of "synchronous" responses to the delay between the tones were similar for all tone pairs. Perceived synchrony was maximal when the tones in a pair were gated synchronously. The perceived-synchrony function slopes were asymmetric, being steeper on the low-frequency-leading side. In the second experiment, asynchrony-detection thresholds were lower for low-frequency rather than for high-frequency leading pairs. In contrast with previous studies, but consistent with the first experiment, thresholds did not depend on frequency separation between the tones, perhaps because of the elimination of within-channel cues. The results of the two experiments were related quantitatively using a decision-theoretic model, and were found to be highly correlated. Overall the results suggest that frequency-dependent cochlear group delays are compensated for at higher processing stages, resulting in veridical perception of timing relationships across frequency.

Across-channel timing differences as a potential code for the frequency of pure tones

When a pure tone or low-numbered harmonic is presented to a listener, the resulting travelling wave in the cochlea slows down at the portion of the basilar membrane (BM) tuned to the input frequency due to the filtering properties of the BM. This slowing is reflected in the phase of the response of neurons across the auditory nerve (AN) array. It has been suggested that the auditory system exploits these across-channel timing differences to encode the pitch of both pure tones and resolved harmonics in complex tones. Here, we report a quantitative analysis of previously published data on the response of guinea pig AN fibres, of a range of characteristic frequencies, to pure tones of different frequencies and levels. We conclude that although the use of across-channel timing cues provides an a priori attractive and plausible means of encoding pitch, many of the most obvious metrics for using that cue produce pitch estimates that are strongly influenced by the overall level and therefore are unlikely to provide a straightforward means for encoding the pitch of pure tones.

Response characteristics in the apex of the gerbil cochlea studied through auditory nerve recordings

In this study, we analyze the processing of low-frequency sounds in the cochlear apex through responses of auditory nerve fibers (ANFs) that innervate the apex. Single tones and irregularly spaced tone complexes were used to evoke ANF responses in Mongolian gerbil. The spike arrival times were analyzed in terms of phase locking, peripheral frequency selectivity, group delays, and the nonlinear effects of sound pressure level (SPL). Phase locking to single tones was similar to that in cat. Vector strength was maximal for stimulus frequencies around 500 Hz, decreased above 1 kHz, and became insignificant above 4 to 5 kHz. We used the responses to tone complexes to determine amplitude and phase curves of ANFs having a characteristic frequency (CF) below 5 kHz. With increasing CF, amplitude curves gradually changed from broadly tuned and asymmetric with a steep low-frequency flank to more sharply tuned and asymmetric with a steep high-frequency flank. Over the same CF range, phase curves gradually changed from a concave-upward shape to a concave-downward shape. Phase curves consisted of two or three approximately straight segments. Group delay was analyzed separately for these segments. Generally, the largest group delay was observed near CF. With increasing SPL, most amplitude curves broadened, sometimes accompanied by a downward shift of best frequency, and group delay changed along the entire range of stimulus frequencies. We observed considerable across-ANF variation in the effects of SPL on both amplitude and phase. Overall, our data suggest that mechanical responses in the apex of the cochlea are considerably nonlinear and that these nonlinearities are of a different character than those known from the base of the cochlea.

Spatiotemporal representation of the pitch of harmonic complex tones in the auditory nerve

The pitch of harmonic complex tones plays an important role in speech and music perception and the analysis of auditory scenes, yet traditional rate-place and temporal models for pitch processing provide only an incomplete description of the psychophysical data. To test physiologically a model based on spatiotemporal pitch cues created by the cochlear traveling wave (Shamma, 1985), we recorded from single fibers in the auditory nerve of anesthetized cat in response to harmonic complex tones with missing fundamentals and equal-amplitude harmonics. We used the principle of scaling invariance in cochlear mechanics to infer the spatiotemporal response pattern to a given stimulus from a series of measurements made in a single fiber as a function of fundamental frequency F0. We found that spatiotemporal cues to resolved harmonics are available for F0 values between 350 and 1100 Hz and that these cues are more robust than traditional rate-place cues at high stimulus levels. The lower F0 limit is determined by the limited frequency selectivity of the cochlea, whereas the upper limit is caused by the degradation of phase locking to the stimulus fine structure at high frequencies. The spatiotemporal representation is consistent with the upper F0 limit to the perception of the pitch of complex tones with a missing fundamental, and its effectiveness does not depend on the relative phase between resolved harmonics. The spatiotemporal representation is thus consistent with key trends in human psychophysics.

Level-dependent latency shifts quantified through binaural processing

The mammalian binaural system compares the timing of monaural inputs with microsecond precision. This temporal precision is required for localizing sounds in azimuth. However, temporal features of the monaural inputs, in particular their latencies, highly depend on the overall sound level. In a combined psychophysical, electrophysiological, and modeling approach, we investigate how level-dependent latency shifts of the monaural responses are reflected in the perception and neural representation of interaural time differences. We exploit the sensitivity of the binaural system to the timing of high-frequency stimuli with binaurally incongruent envelopes. Using these novel stimuli, both the perceptually adjusted interaural time differences and the time differences extracted from electrophysiological recordings systematically depend on overall sound pressure level. The perceptual and electrophysiological time differences of the envelopes can be explained in an existing model of temporal integration only if a level-dependent firing threshold is added. Such an adjustment of firing threshold provides a temporally accurate neural code of the temporal structure of a stimulus and its binaural disparities independent of overall sound level.

Round window delivery of dexamethasone ameliorates local and remote hearing loss produced by cochlear implantation into the second turn of the guinea pig cochlea

Application of dexamethasone to the round window has been shown to ameliorate high frequency hearing loss resulting from the trauma of cochlear implantation in experimental animals, but elucidation of the factors influencing protection of the high frequencies has been confounded by the local trauma from electrode array insertion. In this experiment, a second turn cochleostomy and implantation was performed on guinea pigs, to examine protection in the basal turn without the confounding effect of local trauma, as well as to test the efficacy of hearing protection in the second cochlear turn. The implantation resulted in an increase in hearing thresholds across all frequencies examined (2-32 kHz). Local delivery of dexamethasone to the round window prior to implantation protected hearing across frequencies from 2 to 32 kHz. Auditory thresholds improved over the first week after surgery, and then remained stable for the month of the experiment. The protection of hearing in the basal turn increased with longer periods of drug application prior to implantation. The level of hearing protection in the second turn was similar irrespective of the time that the drug was applied, but was greater when a higher steroid concentration was used. It was concluded that steroids protect hearing in the basal turn of the cochlea even when there was no local trauma. The level of hearing protection in the second turn exceeded that expected from models of steroid diffusion through the cochlea, suggesting that inner ear surgery alters the distribution of dexamethasone within the cochlea.

Phase-locked responses to tones of chinchilla auditory nerve fibers: implications for apical cochlear mechanics

Responses to tones with frequency < or = 5 kHz were recorded from auditory nerve fibers (ANFs) of anesthetized chinchillas. With increasing stimulus level, discharge rate-frequency functions shift toward higher and lower frequencies, respectively, for ANFs with characteristic frequencies (CFs) lower and higher than approximately 0.9 kHz. With increasing frequency separation from CF, rate-level functions are less steep and/or saturate at lower rates than at CF, indicating a CF-specific nonlinearity. The strength of phase locking has lower high-frequency cutoffs for CFs >4 kHz than for CFs < 3 kHz. Phase-frequency functions of ANFs with CFs lower and higher than approximately 0.9 kHz have inflections, respectively, at frequencies higher and lower than CF. For CFs >2 kHz, the inflections coincide with the tip-tail transitions of threshold tuning curves. ANF responses to CF tones exhibit cumulative phase lags of 1.5 periods for CFs 0.7-3 kHz and lesser amounts for lower CFs. With increases of stimulus level, responses increasingly lag (lead) lower-level responses at frequencies lower (higher) than CF, so that group delays are maximal at, or slightly above, CF. The CF-specific magnitude and phase nonlinearities of ANFs with CFs < 2.5 kHz span their entire response bandwidths. Several properties of ANFs undergo sharp transitions in the cochlear region with CFs 2-5 kHz. Overall, the responses of chinchilla ANFs resemble those in other mammalian species but contrast with available measurements of apical cochlear vibrations in chinchilla, implying that either the latter are flawed or that a nonlinear "second filter" is interposed between vibrations and ANF excitation.