Phase-locked response to low-frequency tones in single auditory nerve fibers of the squirrel monkey

Electrophysiological measurement of binaural beats: effects of primary tone frequency and observer age

OBJECTIVE

The purpose of this study was to determine the reliability of the electrophysiological binaural beat steady state response as a gauge of temporal fine structure coding, particularly as it relates to the aging auditory system. The hypothesis was that the response would be more robust in a lower, than in a higher, frequency region and in younger, than in older, adults.

DESIGN

Two experiments were undertaken. The first measured the 40 Hz binaural beat steady state response elicited by tone pairs in two frequency regions: lower (390 and 430 Hz tone pair) and higher (810 and 850 Hz tone pair). Frequency following responses (FFRs) evoked by the tones were also recorded. Ten young adults with normal hearing participated. The second experiment measured the binaural beat and FFRs in older adults but only in the lower frequency region. Fourteen older adults with relatively normal hearing participated. Response metrics in both experiments included response component signal-to-noise ratio (F statistic) and magnitude-squared coherence.

RESULTS

Experiment 1 showed that FFRs were elicited in both frequency regions but were more robust in the lower frequency region. Binaural beat responses elicited by the lower frequency pair of tones showed greater amplitude fluctuation within a participant than the respective FFRs. Experiment 2 showed that older adults exhibited similar FFRs to younger adults, but proportionally fewer older participants showed binaural beat responses. Age differences in onset responses were also observed.

CONCLUSIONS

The lower prevalence of the binaural beat response in older adults, despite the presence of FFRs, provides tentative support for the sensitivity of this measure to age-related deficits in temporal processing. However, the lability of the binaural beat response advocates caution in its use as an objective measure of fine structure coding.

Identifying type I excitability using dynamics of stochastic neural firing patterns

The stochastic firing patterns are simulated near saddle-node bifurcation on an invariant cycle corresponding to type I excitability in stochastic Morris-Lecar model. In absence of external periodic signal, the stochastic firing manifests continuous distribution in ISI histogram (ISIH), whose amplitude at first increases sharply and then decreases exponentially. In presence of the external periodic signal, stochastic firing patterns appear as two cases of integer multiple firing with multiple discrete peaks in ISIH. One manifests perfect exponential decay in all peaks and the other imperfect exponential decay except a lower first peak. These stochastic firing patterns simulated with or without external periodic signal can be demonstrated in the experiments on rat hippocampal CA1 pyramidal neurons. The exponential decay laws in the multiple peaks are also acquired using probability analysis method. The perfect decay law is determined by the independent characteristic within the firing while the imperfect decay law is from the inhibitory effect. In addition, the stochastic firing patterns corresponding to type I excitability are compared to those of type II excitability. The results not only reveal the dynamics of stochastic firing patterns with or without external signal corresponding to type I excitability, but also provide practical indicators to availably identify type I excitability.

Auditory nerve fibre responses in the ferret

The ferret (Mustela putorius) is a medium-sized, carnivorous mammal with good low-frequency hearing; it is relatively easy to train, and there is therefore a good body of behavioural data detailing its detection thresholds and localization abilities. However, despite extensive studies of the physiology of the central nervous system of the ferret, even extending to the prefrontal cortex, little is known of the functioning of the auditory periphery. Here, we provide an insight into this peripheral function by detailing responses of single auditory nerve fibres. Our expectation was that the ferret auditory nerve responsiveness would be similar that of its near relative, the cat. However, by comparing a range of variables (the frequency tuning, the variation of rate-level functions with spontaneous rate, and the high-frequency cut-off of phase locking) across several species, we show that the auditory nerve (and hence cochlea) in the ferret is more similar to that of the guinea-pig and chinchilla than to that of the cat. Animal models of hearing are often chosen on the basis of the similarity of their audiogram to that of the human, particularly in the low-frequency region. We show here that whereas the ferret hears well at low frequencies, this is likely to occur via fibres with higher characteristic frequencies. These qualitative differences in response characteristics in auditory nerve fibres are important in interpreting data across all of auditory science, as it has been argued recently that tuning in animals is broader than in humans.

The role of time and place cues in the detection of frequency modulation by hearing-impaired listeners

Frequency modulation detection limens (FMDLs) were measured for five hearing-impaired (HI) subjects for carrier frequencies f(c) = 1000, 4000, and 6000 Hz, using modulation frequencies f(m) = 2 and 10 Hz and levels of 20 dB sensation level and 90 dB SPL. FMDLs were smaller for f(m) = 10 than for f(m) = 2 Hz for the two higher f(c), but not for f(c) = 1000 Hz. FMDLs were also determined with additional random amplitude modulation (AM), to disrupt excitation-pattern cues. The disruptive effect was larger for f(m) = 10 than for f(m) = 2 Hz. The smallest disruption occurred for f(m) = 2 Hz and f(c) = 1000 Hz. AM detection thresholds for normal-hearing and HI subjects were measured for the same f(c) and f(m) values. Performance was better for the HI subjects for both f(m). AM detection was much better for f(m)  = 10 than for f(m) = 2 Hz. Additional tests showed that most HI subjects could discriminate temporal fine structure (TFS) at 800 Hz. The results are consistent with the idea that, for f(m) = 2 Hz and f(c) = 1000 Hz, frequency modulation (FM) detection was partly based on the use of TFS information. For higher carrier frequencies and for all carrier frequencies with f(m) = 10 Hz, FM detection was probably based on place cues.

Spike encoding of neurotransmitter release timing by spiral ganglion neurons of the cochlea

Mammalian cochlear spiral ganglion neurons (SGNs) encode sound with microsecond precision. Spike triggering relies upon input from a single ribbon-type active zone of a presynaptic inner hair cell (IHC). Using patch-clamp recordings of rat SGN postsynaptic boutons innervating the modiolar face of IHCs from the cochlear apex, at room temperature, we studied how spike generation contributes to spike timing relative to synaptic input. SGNs were phasic, firing a single short-latency spike for sustained currents of sufficient onset slope. Almost every EPSP elicited a spike, but latency (300-1500 μs) varied with EPSP size and kinetics. When current-clamp stimuli approximated the mean physiological EPSC (≈300 pA), several times larger than threshold current (rheobase, ≈50 pA), spikes were triggered rapidly (latency, ≈500 μs) and precisely (SD, <50 μs). This demonstrated the significance of strong synaptic input. However, increasing EPSC size beyond the physiological mean resulted in less-potent reduction of latency and jitter. Differences in EPSC charge and SGN baseline potential influenced spike timing less as EPSC onset slope and peak amplitude increased. Moreover, the effect of baseline potential on relative threshold was small due to compensatory shift of absolute threshold potential. Experimental first-spike latencies in response to a broad range of stimuli were predicted by a two-compartment exponential integrate-and-fire model, with latency prediction error of <100 μs. In conclusion, the close anatomical coupling between a strong synapse and spike generator along with the phasic firing property lock SGN spikes to IHC exocytosis timing to generate the auditory temporal code with high fidelity.

Effects of pulsing of a target tone on the ability to hear it out in different types of complex sounds

Judgments of whether a sinusoidal probe is higher or lower in frequency than the closest partial ("target") in a multi-partial complex are improved when the target is pulsed on and off. These experiments explored the contribution of reduction in perceptual confusion and recovery from adaptation to this effect. In experiment 1, all partials except the target were replaced by noise to reduce perceptual confusion. Performance was much better than when the background was composed of multiple partials. When the level of the target was reduced to avoid ceiling effects, no effect of pulsing the target occurred. In experiment 2, the target and background partials were irregularly and independently amplitude modulated. This gave a large effect of pulsing the target, suggesting that if recovery from adaptation contributes to the effect, amplitude fluctuations do not prevent this. In experiment 3, the background was composed of multiple steady partials, but the target was irregularly amplitude modulated. This gave better performance than when the target was unmodulated and a moderate effect of pulsing the target. It is argued that when the target and background are steady tones, pulsing the target may result both in reduction of perceptual confusion and recovery from adaptation.

Exponential decay characteristics of the stochastic integer multiple neural firing patterns

Integer multiple neural firing patterns exhibit multi-peaks in inter-spike interval (ISI) histogram (ISIH) and exponential decay in amplitude of peaks, which results from their stochastic mechanisms. But in previous experimental observation that the decay in ISIH frequently shows obvious bias from exponential law. This paper studied three typical cases of the decay, by transforming ISI series of the firing to discrete binary chain and calculating the probabilities or frequencies of symbols over the whole chain. The first case is the exponential decay without bias. An example of this case was discovered on hippocampal CA1 pyramidal neuron stimulated by external signal. Probability calculation shows that this decay without bias results from a stochastic renewal process, in which the successive spikes are independent. The second case is the exponential decay with a higher first peak, while the third case is that with a lower first peak. An example of the second case was discovered in experiment on a neural pacemaker. Simulation and calculation of the second and third cases indicate that the dependency in successive spikes of the firing leads to the bias seen in decay of ISIH peaks. The quantitative expression of the decay slope of three cases of firing patterns, as well as the excitatory effect in the second case of firing pattern and the inhibitory effect in the third case of firing pattern are identified. The results clearly reveal the mechanism of the exponential decay in ISIH peaks of a number of important neural firing patterns and provide new understanding for typical bias from the exponential decay law.

Temporal and spectral masking release in low- and mid-frequency regions for normal-hearing and hearing-impaired listeners

"Masking release" (MR), the improvement of speech intelligibility in modulated compared with unmodulated maskers, is typically smaller than normal for hearing-impaired listeners. The extent to which this is due to reduced audibility or to suprathreshold processing deficits is unclear. Here, the effects of audibility were controlled by using stimuli restricted to the low- (≤1.5 kHz) or mid-frequency (1-3 kHz) region for normal-hearing listeners and hearing-impaired listeners with near-normal hearing in the tested region. Previous work suggests that the latter may have suprathreshold deficits. Both spectral and temporal MR were measured. Consonant identification was measured in quiet and in the presence of unmodulated, amplitude-modulated, and spectrally modulated noise at three signal-to-noise ratios (the same ratios for the two groups). For both frequency regions, consonant identification was poorer for the hearing-impaired than for the normal-hearing listeners in all conditions. The results suggest the presence of suprathreshold deficits for the hearing-impaired listeners, despite near-normal audiometric thresholds over the tested frequency regions. However, spectral MR and temporal MR were similar for the two groups. Thus, the suprathreshold deficits for the hearing-impaired group did not lead to reduced MR.

An improved model for the rate-level functions of auditory-nerve fibers

Acoustic information is conveyed to the brain by the spike patterns in auditory-nerve fibers (ANFs). In mammals, each ANF is excited via a single ribbon synapse in a single inner hair cell (IHC), and the spike patterns therefore also provide valuable information about those intriguing synapses. Here we reexamine and model a key property of ANFs, the dependence of their spike rates on the sound pressure level of acoustic stimuli (rate-level functions). We build upon the seminal model of Sachs and Abbas (1974), which provides good fits to experimental data but has limited utility for defining physiological mechanisms. We present an improved, physiologically plausible model according to which the spike rate follows a Hill equation and spontaneous activity and its experimentally observed tight correlation with ANF sensitivity are emergent properties. We apply it to 156 cat ANF rate-level functions using frequencies where the mechanics are linear and find that a single Hill coefficient of 3 can account for the population of functions. We also demonstrate a tight correspondence between ANF rate-level functions and the Ca(2+) dependence of exocytosis from IHCs, and derive estimates of the effective intracellular Ca(2+) concentrations at the individual active zones of IHCs. We argue that the Hill coefficient might reflect the intrinsic, biochemical Ca(2+) cooperativity of the Ca(2+) sensor involved in exocytosis from the IHC. The model also links ANF properties with properties of psychophysical absolute thresholds.

Stability of central binaural sound localization mechanisms in mammals, and the Heffner hypothesis

Heffner (2004) provided an overview of data on the evolutionary pressures on sound localization acuity in mammals. Her most important finding was that sound localization acuity was most strongly correlated with width of field of best vision. This correlation leaves unexplained the mechanism through which evolutionary pressures affect localization acuity in different mammals. A review of the neurophysiology of binaural sound localization cue coding, and the behavioural performance it supports, led us to two hypotheses. First, there is little or no evidence that the neural mechanisms for coding binaural sound location cues, or the dynamic range of the code, vary across mammals. Rather, the neural coding mechanism is remarkably constant both across species, and within species across frequency. Second, there is no need to postulate that evolutionary pressures are exerted on the cue coding mechanism itself. We hypothesize instead that the evolutionary pressure may be on the organism's ability to exploit a 'lower envelope principle' (after Barlow, 1972).

Representation of temporal sound features in the human auditory cortex

Temporal information in acoustic signals is important for the perception of environmental sounds, including speech. This review focuses on several aspects of temporal processing within human auditory cortex and its relevance for the processing of speech sounds. Periodic non-speech sounds, such as trains of acoustic clicks and bursts of amplitude-modulated noise or tones, can elicit different percepts depending on the pulse repetition rate or modulation frequency. Such sounds provide convenient methodological tools to study representation of timing information in the auditory system. At low repetition rates of up to 8-10 Hz, each individual stimulus (a single click or a sinusoidal amplitude modulation cycle) within the sequence is perceived as a separate event. As repetition rates increase up to and above approximately 40 Hz, these events blend together, giving rise first to the percept of flutter and then to pitch. The extent to which neural responses of human auditory cortex encode temporal features of acoustic stimuli is discussed within the context of these perceptual classes of periodic stimuli and their relationship to speech sounds. Evidence for neural coding of temporal information at the level of the core auditory cortex in humans suggests possible physiological counterparts to perceptual categorical boundaries for periodic acoustic stimuli. Temporal coding is less evident in auditory cortical fields beyond the core. Finally, data suggest hemispheric asymmetry in temporal cortical processing.

Spread of cochlear excitation during stimulation with pulsed infrared radiation: inferior colliculus measurements

Infrared neural stimulation (INS) has received considerable attention over the last few years. It provides an alternative method to artificially stimulate neurons without electrical current or the introduction of exogenous chromophores. One of the primary benefits of INS could be the improved spatial selectivity when compared with electrical stimulation. In the present study, we have evaluated the spatial selectivity of INS in the acutely damaged cochlea of guinea pigs and compared it to stimulation with acoustic tone pips in normal-hearing animals. The radiation was delivered via a 200 µm diameter optical fiber, which was inserted through a cochleostomy into the scala tympani of the basal cochlear turn. The stimulated section along the cochlear spiral ganglion was estimated from the neural responses recorded from the central nucleus of the inferior colliculus (ICC). ICC responses were recorded in response to cochlear INS using a multichannel penetrating electrode array. Spatial tuning curves (STCs) were constructed from the responses. For INS, approximately 55% of the activation profiles showed a single maximum, ∼22% had two maxima and ∼13% had multiple maxima. The remaining 10% of the profiles occurred at the limits of the electrode array and could not be classified. The majority of ICC STCs indicated that the spread of activation evoked by optical stimuli is comparable to that produced by acoustic tone pips.

Psychometric functions for pure-tone frequency discrimination

The form of the psychometric function (PF) for auditory frequency discrimination is of theoretical interest and practical importance. In this study, PFs for pure-tone frequency discrimination were measured for several standard frequencies (200-8000 Hz) and levels [35-85 dB sound pressure level (SPL)] in normal-hearing listeners. The proportion-correct data were fitted using a cumulative-Gaussian function of the sensitivity index, d', computed as a power transformation of the frequency difference, Δf. The exponent of the power function corresponded to the slope of the PF on log(d')-log(Δf) coordinates. The influence of attentional lapses on PF-slope estimates was investigated. When attentional lapses were not taken into account, the estimated PF slopes on log(d')-log(Δf) coordinates were found to be significantly lower than 1, suggesting a nonlinear relationship between d' and Δf. However, when lapse rate was included as a free parameter in the fits, PF slopes were found not to differ significantly from 1, consistent with a linear relationship between d' and Δf. This was the case across the wide ranges of frequencies and levels tested in this study. Therefore, spectral and temporal models of frequency discrimination must account for a linear relationship between d' and Δf across a wide range of frequencies and levels.

Calcium-dependent synaptic vesicle trafficking underlies indefatigable release at the hair cell afferent fiber synapse

Sensory hair cell ribbon synapses respond to graded stimulation in a linear, indefatigable manner, requiring that vesicle trafficking to synapses be rapid and nonrate-limiting. Real-time monitoring of vesicle fusion identified two release components. The first was saturable with both release rate and magnitude varying linearly with Ca(2+), however the magnitude was too small to account for sustained afferent firing rates. A second superlinear release component required recruitment, in a Ca(2+)-dependent manner, of vesicles not in the immediate vicinity of the synapse. The superlinear component had a constant rate with its onset varying with Ca(2+) load. High-speed Ca(2+) imaging revealed a nonlinear increase in internal Ca(2+) correlating with the superlinear capacitance change, implicating release of stored Ca(2+) in driving vesicle recruitment. These data, supported by a mass action model, suggest sustained release at hair cell afferent fiber synapse is dictated by Ca(2+)-dependent vesicle recruitment from a reserve pool.

In vivo conditions induce faithful encoding of stimuli by reducing nonlinear synchronization in vestibular sensory neurons

Previous studies have shown that neurons within the vestibular nuclei (VN) can faithfully encode the time course of sensory input through changes in firing rate in vivo. However, studies performed in vitro have shown that these same VN neurons often display nonlinear synchronization (i.e. phase locking) in their spiking activity to the local maxima of sensory input, thereby severely limiting their capacity for faithful encoding of said input through changes in firing rate. We investigated this apparent discrepancy by studying the effects of in vivo conditions on VN neuron activity in vitro using a simple, physiologically based, model of cellular dynamics. We found that membrane potential oscillations were evoked both in response to step and zap current injection for a wide range of channel conductance values. These oscillations gave rise to a resonance in the spiking activity that causes synchronization to sinusoidal current injection at frequencies below 25 Hz. We hypothesized that the apparent discrepancy between VN response dynamics measured in in vitro conditions (i.e., consistent with our modeling results) and the dynamics measured in vivo conditions could be explained by an increase in trial-to-trial variability under in vivo vs. in vitro conditions. Accordingly, we mimicked more physiologically realistic conditions in our model by introducing a noise current to match the levels of resting discharge variability seen in vivo as quantified by the coefficient of variation (CV). While low noise intensities corresponding to CV values in the range 0.04-0.24 only eliminated synchronization for low (<8 Hz) frequency stimulation but not high (>12 Hz) frequency stimulation, higher noise intensities corresponding to CV values in the range 0.5-0.7 almost completely eliminated synchronization for all frequencies. Our results thus predict that, under natural (i.e. in vivo) conditions, the vestibular system uses increased variability to promote fidelity of encoding by single neurons. This prediction can be tested experimentally in vitro.

The effects of age and cochlear hearing loss on temporal fine structure sensitivity, frequency selectivity, and speech reception in noise

Temporal fine structure (TFS) sensitivity, frequency selectivity, and speech reception in noise were measured for young normal-hearing (NHY), old normal-hearing (NHO), and hearing-impaired (HI) subjects. Two measures of TFS sensitivity were used: the "TFS-LF test" (interaural phase difference discrimination) and the "TFS2 test" (discrimination of harmonic and frequency-shifted tones). These measures were not significantly correlated with frequency selectivity (after partialing out the effect of audiometric threshold), suggesting that insensitivity to TFS cannot be wholly explained by a broadening of auditory filters. The results of the two tests of TFS sensitivity were significantly but modestly correlated, suggesting that performance of the tests may be partly influenced by different factors. The NHO group performed significantly more poorly than the NHY group for both measures of TFS sensitivity, but not frequency selectivity, suggesting that TFS sensitivity declines with age in the absence of elevated audiometric thresholds or broadened auditory filters. When the effect of mean audiometric threshold was partialed out, speech reception thresholds in modulated noise were correlated with TFS2 scores, but not measures of frequency selectivity or TFS-LF test scores, suggesting that a reduction in sensitivity to TFS can partly account for the speech perception difficulties experienced by hearing-impaired subjects.

Detection of virtual pitch up to 5kHz by mice

Natural sounds consist of a component at the fundamental frequency (f0) and its overtones. Pitch is perceived at f0, even when spectral energy at f0 is missing. This missing f0, or 'virtual pitch', is thought to be detected in the auditory cortex and related cortical areas, but the precise neural mechanisms are unknown. One possibility is that virtual pitch can be retrieved from the periodicity of sound waveforms. However, this mechanism requires the temporal accuracy in periodicity detection, and so far the detection of virtual pitch has only been demonstrated at frequencies lower than 1kHz. We investigated the ability of mice to detect virtual pitch up to 5kHz using a two-step sound discrimination test. In the first step of this test, mice were trained to discriminate between tone bursts at 2.5 and 5kHz. In the second step, we tested the ability of mice to discriminate between virtual pitches at 2.5kHz and at 5kHz. It was demonstrated that the performance of mice to discriminate between virtual pitches at 2.5 and 5kHz was significantly affected by previous discrimination learning between tone bursts, indicating that mice can detect virtual pitch up to 5kHz.

Optimizing periodicity and polymodality in noise-induced genetic oscillators

Many cellular functions are based on the rhythmic organization of biological processes into self-repeating cascades of events. Some of these periodic processes, such as the cell cycles of several species, exhibit conspicuous irregularities in the form of period skippings, which lead to polymodal distributions of cycle lengths. A recently proposed mechanism that accounts for this quantized behavior is the stabilization of a Hopf-unstable state by molecular noise. Here we investigate the effect of varying noise in a model system, namely an excitable activator-repressor genetic circuit, that displays this noise-induced stabilization effect. Our results show that an optimal noise level enhances the regularity (coherence) of the cycles, in a form of coherence resonance. Similar noise levels also optimize the multimodal nature of the cycle lengths. Together, these results illustrate how molecular noise within a minimal gene regulatory motif confers robust generation of polymodal patterns of periodicity.

Auditory frequency-following response: a neurophysiological measure for studying the "cocktail-party problem"

How do we recognize what one person is saying when others are speaking at the same time? The "cocktail-party problem" proposed by Cherry (1953) has puzzled scientific societies for half a century. This puzzle will not be solved without using appropriate neurophysiological investigation that should satisfy the following four essential requirements: (1) certain critical speech characteristics related to speech intelligibility are recorded; (2) neural responses to different speech sources are differentiated; (3) neural correlates of bottom-up binaural unmasking of responses to target speech are measurable; (4) neural correlates of attentional top-down unmasking of target speech are measurable. Before speech signals reach the cerebral cortex, some critical acoustic features are represented in subcortical structures by the frequency-following responses (FFRs), which are sustained evoked potentials based on precisely phase-locked responses of neuron populations to low-to-middle-frequency periodical acoustical stimuli. This review summarizes previous studies on FFRs associated with each of the four requirements and suggests that FFRs are useful for studying the "cocktail-party problem".

Bats and frogs and animals in between: evidence for a common central timing mechanism to extract periodicity pitch

Widely divergent vertebrates share a common central temporal mechanism for representing periodicities of acoustic waveform events. In the auditory nerve, periodicities corresponding to frequencies or rates from about 10 Hz to over 1,000 Hz are extracted from pure tones, from low-frequency complex sounds (e.g., 1st harmonic in bullfrog calls), from mid-frequency sounds with low-frequency modulations (e.g., amplitude modulation rates in cat vocalizations), and from time intervals between high-frequency transients (e.g., pulse-echo delay in bat sonar). Time locking of neuronal responses to periodicities from about 50 ms down to 4 ms or less (about 20-300 Hz) is preserved in the auditory midbrain, where responses are dispersed across many neurons with different onset latencies from 4-5 to 20-50 ms. Midbrain latency distributions are wide enough to encompass two or more repetitions of successive acoustic events, so that responses to multiple, successive periods are ongoing simultaneously in different midbrain neurons. These latencies have a previously unnoticed periodic temporal pattern that determines the specific times for the dispersed on-responses.

Processing of temporal fine structure as a function of age

OBJECTIVES

The purpose of this study was to determine whether the processing of temporal fine structure diminishes with age, even in the presence of relatively normal audiometric hearing. Temporal fine structure processing was assessed by measuring the discrimination of interaural phase differences (IPDs). The hypothesis was that IPD discrimination is more acute in middle-aged observers than in older observers but that acuity in middle-aged observers is nevertheless poorer than in young adults.

DESIGN

Two experiments were undertaken. The first measured discrimination of 0- and π-radian interaural phases as a function of carrier frequency. The stimulus was a 5-Hz sinusoidally amplitude-modulated tone in which, in the signal waveform, the interaural phase of the carrier was inverted during alternate modulation periods. The second experiment measured IPD discrimination at fixed frequencies. The stimulus was a pair of tone pulses in which, in the signal, the trailing pulse contained an IPD. A total of 39 adults with normal audiograms ≤2000 Hz participated in this study, of which 15 were younger, 12 middle aged, and 12 older.

RESULTS

Experiment 1 showed that the highest carrier frequency at which a π-radian IPD could be discriminated from the diotic, 0-radian standard was significantly lower in middle-aged listeners than young adults, and still lower in older listeners. Experiment 2 indicated that middle-aged listeners were less sensitive to IPDs than young adults at all but the lowest frequencies tested. Older listeners, as a group, had the poorest thresholds.

CONCLUSIONS

These results suggest that deficits in temporal fine structure processing are evident in the presenescent auditory system. This adds to the accumulating evidence that deficiencies in some aspects of auditory temporal processing emerge relatively early in the aging process. It is possible that early-emerging temporal processing deficits manifest themselves in challenging speech in noise environments.

The role of temporal fine structure information for the low pitch of high-frequency complex tones

The fused low pitch evoked by complex tones containing only unresolved high-frequency components demonstrates the ability of the human auditory system to extract pitch using a temporal mechanism in the absence of spectral cues. However, the temporal features used by such a mechanism have been a matter of debate. For stimuli with components lying exclusively in high-frequency spectral regions, the slowly varying temporal envelope of sounds is often assumed to be the only information contained in auditory temporal representations, and it has remained controversial to what extent the fast amplitude fluctuations, or temporal fine structure (TFS), of the conveyed signal can be processed. Using a pitch matching paradigm, the present study found that the low pitch of inharmonic transposed tones with unresolved components was consistent with the timing between the most prominent TFS maxima in their waveforms, rather than envelope maxima. Moreover, envelope cues did not take over as the absolute frequency or rank of the lowest component was raised and TFS cues thus became less effective. Instead, the low pitch became less salient. This suggests that complex pitch perception does not rely on envelope coding as such, and that TFS representation might persist at higher frequencies than previously thought.

Phase Coupling Estimation from Multivariate Phase Statistics

Coupled oscillators are prevalent throughout the physical world. Dynamical system formulations of weakly coupled oscillator systems have proven effective at capturing the properties of real-world systems and are compelling models of neural systems. However, these formulations usually deal with the forward problem: simulating a system from known coupling parameters. Here we provide a solution to the inverse problem: determining the coupling parameters from measurements. Starting from the dynamic equations of a system of symmetrically coupled phase oscillators, given by a nonlinear Langevin equation, we derive the corresponding equilibrium distribution. This formulation leads us to the maximum entropy distribution that captures pairwise phase relationships. To solve the inverse problem for this distribution, we derive a closed-form solution for estimating the phase coupling parameters from observed phase statistics. Through simulations, we show that the algorithm performs well in high dimensions (d = 100) and in cases with limited data (as few as 100 samples per dimension). In addition, we derive a regularized solution to the estimation and show that the resulting procedure improves performance when only a limited amount of data is available. Because the distribution serves as the unique maximum entropy solution for pairwise phase statistics, phase coupling estimation can be broadly applied in any situation where phase measurements are made. Under the physical interpretation, the model may be used for inferring coupling relationships within cortical networks.

Otoferlin is a calcium sensor that directly regulates SNARE-mediated membrane fusion

Otoferlin is a large multi-C2 domain protein proposed to act as a calcium sensor that regulates synaptic vesicle exocytosis in cochlear hair cells. Although mutations in otoferlin have been associated with deafness, its contribution to neurotransmitter release is unresolved. Using recombinant proteins, we demonstrate that five of the six C2 domains of otoferlin sense calcium with apparent dissociation constants that ranged from 13-25 µM; in the presence of membranes, these apparent affinities increase by up to sevenfold. Using a reconstituted membrane fusion assay, we found that five of the six C2 domains of otoferlin stimulate membrane fusion in a calcium-dependent manner. We also demonstrate that a calcium binding-deficient form of the C2C domain is incapable of stimulating membrane fusion, further underscoring the importance of calcium for the protein's function. These results demonstrate for the first time that otoferlin is a calcium sensor that can directly regulate soluble N-ethyl-maleimide sensitive fusion protein attachment protein receptor-mediated membrane fusion reactions.

The diverse roles of ribbon synapses in sensory neurotransmission

Sensory synapses of the visual and auditory systems must faithfully encode a wide dynamic range of graded signals, and must be capable of sustained transmitter release over long periods of time. Functionally and morphologically, these sensory synapses are unique: their active zones are specialized in several ways for sustained, rapid vesicle exocytosis, but their most striking feature is an organelle called the synaptic ribbon, which is a proteinaceous structure that extends into the cytoplasm at the active zone and tethers a large pool of releasable vesicles. But precisely how does the ribbon function to support tonic release at these synapses? Recent genetic and biophysical advances have begun to open the 'black box' of the synaptic ribbon with some surprising findings and promise to resolve its function in vision and hearing.

Laser stimulation of single auditory nerve fibers

OBJECTIVES/HYPOTHESIS

One limitation with cochlear implants is the difficulty stimulating spatially discrete spiral ganglion cell groups because of electrode interactions. Multipolar electrodes have improved on this some, but also at the cost of much higher device power consumption. Recently, it has been shown that spatially selective stimulation of the auditory nerve is possible with a mid-infrared laser aimed at the spiral ganglion via the round window. However, these neurons must be driven at adequate rates for optical radiation to be useful in cochlear implants. We herein use single-fiber recordings to characterize the responses of auditory neurons to optical radiation.

STUDY DESIGN

In vivo study using normal-hearing adult gerbils.

METHODS

Two diode lasers were used for stimulation of the auditory nerve. They operated between 1.844 μm and 1.873 μm, with pulse durations of 35 μs to 1,000 μs, and at repetition rates up to 1,000 pulses per second (pps). The laser outputs were coupled to a 200-μm-diameter optical fiber placed against the round window membrane and oriented toward the spiral ganglion. The auditory nerve was exposed through a craniotomy, and recordings were taken from single fibers during acoustic and laser stimulation.

RESULTS

Action potentials occurred 2.5 ms to 4.0 ms after the laser pulse. The latency jitter was up to 3 ms. Maximum rates of discharge averaged 97 ± 52.5 action potentials per second. The neurons did not strictly respond to the laser at stimulation rates over 100 pps.

CONCLUSIONS

Auditory neurons can be stimulated by a laser beam passing through the round window membrane and driven at rates sufficient for useful auditory information. Optical stimulation and electrical stimulation have different characteristics; which could be selectively exploited in future cochlear implants.

Auditory cortical N100 in pre- and post-synaptic auditory neuropathy to frequency or intensity changes of continuous tones

OBJECTIVES

Auditory cortical N100s were examined in ten auditory neuropathy (AN) subjects as objective measures of impaired hearing.

METHODS

Latencies and amplitudes of N100 in AN to increases of frequency (4-50%) or intensity (4-8 dB) of low (250 Hz) or high (4000 Hz) frequency tones were compared with results from normal-hearing controls. The sites of auditory nerve dysfunction were pre-synaptic (n=3) due to otoferlin mutations causing temperature sensitive deafness, post-synaptic (n=4) affecting other cranial and/or peripheral neuropathies, and undefined (n=3).

RESULTS

AN consistently had N100s only to the largest changes of frequency or intensity whereas controls consistently had N100s to all but the smallest frequency and intensity changes. N100 latency in AN was significantly delayed compared to controls, more so for 250 than for 4000 Hz and more so for changes of intensity compared to frequency. N100 amplitudes to frequency change were significantly reduced in ANs compared to controls, except for pre-synaptic AN in whom amplitudes were greater than controls. N100 latency to frequency change of 250 but not of 4000 Hz was significantly related to speech perception scores.

CONCLUSIONS

As a group, AN subjects' N100 potentials were abnormally delayed and smaller, particularly for low frequency. The extent of these abnormalities differed between pre- and post-synaptic forms of the disorder.

SIGNIFICANCE

Abnormalities of auditory cortical N100 in AN reflect disorders of both temporal processing (low frequency) and neural adaptation (high frequency). Auditory N100 latency to the low frequency provides an objective measure of the degree of impaired speech perception in AN.

Optical cochlear implants: evaluation of surgical approach and laser parameters in cats

Previous research has shown that neural stimulation with infrared radiation (IR) is spatially selective and illustrated the potential of IR in stimulating auditory neurons. The present work demonstrates the application of a miniaturized pulsed IR stimulator for chronic implantation in cats, quantifies its efficacy, and short-term safety in stimulating auditory neurons. IR stimulation of the neurons was achieved using an optical fiber inserted through a cochleostomy drilled in the basal turn of the cat cochlea and was characterized by measuring compound action potentials (CAPs). Neurons were stimulated with IR at various pulse durations, radiant exposures, and pulse repetition rates. Pulse durations as short as 50 mus were successful in evoking CAPs in normal as well as deafened cochleae. Continual stimulation was provided at 200 pulses per second, at 200 mW per pulse, and 100 mus pulse duration. Stable CAP amplitudes were observed for up to 10 h of continual IR stimulation. Combined with histological data, the results suggest that pulsed IR stimulation does not lead to detectable acute tissue damage and validate the stimulation parameters that can be used in future chronic implants based on pulsed IR.

Mechanisms of Sound Localization in Mammals

The ability to determine the location of a sound source is fundamental to hearing. However, auditory space is not represented in any systematic manner on the basilar membrane of the cochlea, the sensory surface of the receptor organ for hearing. Understanding the means by which sensitivity to spatial cues is computed in central neurons can therefore contribute to our understanding of the basic nature of complex neural representations. We review recent evidence concerning the nature of the neural representation of auditory space in the mammalian brain and elaborate on recent advances in the understanding of mammalian subcortical processing of auditory spatial cues that challenge the “textbook” version of sound localization, in particular brain mechanisms contributing to binaural hearing.

Onset coding is degraded in auditory nerve fibers from mutant mice lacking synaptic ribbons

Synaptic ribbons, found at the presynaptic membrane of sensory cells in both ear and eye, have been implicated in the vesicle-pool dynamics of synaptic transmission. To elucidate ribbon function, we characterized the response properties of single auditory nerve fibers in mice lacking Bassoon, a scaffolding protein involved in anchoring ribbons to the membrane. In bassoon mutants, immunohistochemistry showed that fewer than 3% of the hair cells' afferent synapses retained anchored ribbons. Auditory nerve fibers from mutants had normal threshold, dynamic range, and postonset adaptation in response to tone bursts, and they were able to phase lock with normal precision to amplitude-modulated tones. However, spontaneous and sound-evoked discharge rates were reduced, and the reliability of spikes, particularly at stimulus onset, was significantly degraded as shown by an increased variance of first-spike latencies. Modeling based on in vitro studies of normal and mutant hair cells links these findings to reduced release rates at the synapse. The degradation of response reliability in these mutants suggests that the ribbon and/or Bassoon normally facilitate high rates of exocytosis and that its absence significantly compromises the temporal resolving power of the auditory system.

Phase locking below rate threshold in noisy model neurons

The property of a neuron to phase-lock to an oscillatory stimulus before adapting its spike rate to the stimulus frequency plays an important role for the auditory system. We investigate under which conditions neurons exhibit this phase locking below rate threshold. To this end, we simulate neurons employing the widely used leaky integrate-and-fire (LIF) model. Tuning parameters, we can arrange either an irregular spontaneous or a tonic spiking mode. When the neuron is stimulated in both modes, a significant rise of vector strength prior to a noticeable change of the spike rate can be observed. Combining analytic reasoning with numerical simulations, we trace this observation back to a modulation of interspike intervals, which itself requires spikes to be only loosely coupled. We test the limits of this conception by simulating an LIF model with threshold fatigue, which generates pronounced anticorrelations between subsequent interspike intervals. In addition we evaluate the LIF response for harmonic stimuli of various frequencies and discuss the extension to more complex stimuli. It seems that phase locking below rate threshold occurs generically for all zero mean stimuli. Finally, we discuss our findings in the context of stimulus detection.

A comparison of auditory brainstem responses and behavioral estimates of hearing sensitivity in Lemur catta and Nycticebus coucang

Primates depend on acoustic signals and cues to avoid predators, locate food, and share information. Accordingly, the structure and function of acoustic stimuli have long been emphasized in studies of primate behavioral and cognitive ecology. Yet, few studies have addressed how well primates hear such stimuli; indeed, the auditory thresholds of most primate species are unknown. This empirical void is due in part to the logistic and economic challenges attendant on traditional behavioral testing methods. Technological advances have produced a safe and cost-effective alternative-the auditory brainstem response (ABR) method, which can be utilized in field conditions, on virtually any animal species, and without subject training. Here we used the ABR and four methods of threshold determination to construct audiograms for two strepsirrhine primates: the ring-tailed lemur (Lemur catta) and slow loris (Nycticebus coucang). Next, to verify the general efficacy of the ABR method, we compared our results to published behaviorally-derived audiograms. We found that the four ABR threshold detection methods produced similar results, including relatively elevated thresholds but similarly shaped audiograms compared to those derived behaviorally. The ABR and behavioral absolute thresholds were significantly correlated, and the frequencies of best sensitivity and high-frequency limits were comparable. However, at frequencies < or =2 kHz, ABR thresholds were especially elevated, resulting in decreased agreement with behavioral thresholds and, in Lemur, the ABR 10-dB range starting points were more than 2 octaves higher than the behavioral points. Finally, a comparison of ABR- and behaviorally-derived audiograms from various animal taxa demonstrates the widespread efficacy of the ABR for estimating frequency of best sensitivity, but otherwise suggests caution; factors such as stimulus properties and threshold definition affect results. We conclude that the ABR method is a promising technique for estimating primate hearing sensitivity, but that additional data are required to explore its efficacy for estimating low-frequency thresholds.

Précis of Sensory Analysis

Sensory analysis is that initial, preconscious stage of perception at which primitive features (edges, temporal discontinuities, and periodicities) are picked out from the random fluctuations that characterize the physical stimulation of sensory receptors. Sensory analysis may be studied by means of signal-detection, psychometric-function, and threshold experiments, and Sensory Analysis presents a succinct, quasi-quantitative account of the phenomena revealed thereby. This account covers all five sensory modalities, emphasising the similarities between them.

A succinct account depends on identifying simple principles of wide generality, of which the most fundamental are that (a) sensory discriminations are differentially coupled to the physical stimuli and that (b) small stimuli are subject to a square-law transform which makes them less detectable than they would otherwise be. These two principles are established by comparisons between different configurations of two stimulus levels to be discriminated; they are realized within a simple physical-analogue model which affords certain low-level comparisons with neurophysiological observation. That physical-analogue model consists of a sequence of elementary operations on the stimulus constituting a stage of sensory processing. The concatenation of two or three stages in cascade accommodates an increased range of experimental phenomena, especially the detection of sinusoidal gratings.

This précis is organized in three parts: Part I surveys Sensory Analysis as economically as may be, beginning from the simplest, most fundamental ideas and working toward phenomena of increasing complexity. A rather shorter Part II reviews the most important alternative models addressed to some part or other of the phenomena surveyed. Finally, a very short Part III contributes some metatheoretic remarks on the function of a theory of sensory discrimination.

Influence of inhibitory synaptic kinetics on the interaural time difference sensitivity in a linear model of binaural coincidence detection

Temporal correlations between the sound waves arriving at the two ears are used to extract the azimuthal position of sound sources. Nerve cells in the mammalian medial superior olive (MSO) that extract these binaural correlations are sensitive to interaural time differences (ITDs) in the range of about 10 micros. These neurons receive inputs from the two ears via four pathways, two excitatory and two inhibitory ones. In this paper, a simple linear model is fitted to the frequency dependence of ITD sensitivity of MSO neurons, which is quantified by the two parameters, characteristic phase and characteristic delay. The fit parameters are the relative delays and the relative strengths of the two inhibitory pathways and thus specify the underlying ITD-detecting circuit assuming a non-Jeffress-like situation, i.e., no excitatory delay lines but phase-locked inhibition. The fitting procedure finds the parameters of these inhibitory pathways such that they account for a desired frequency dependence of ITD sensitivity. It is found that positive characteristic delays require a finite amount of ipsilateral inhibition that arrives at roughly the same time as ipsilateral excitation as well as contralateral inhibition that lags contralateral excitation so much that it effectively leads excitation of the next cycle.

Temporal pitch percepts elicited by dual-channel stimulation of a cochlear implant

McKay and McDermott [J. Acoust. Soc. Am. 100, 1081-1092 (1996)] found that when two different amplitude-modulated pulse trains are presented to two channels separated by <1.5 mm, some cochlear implant (CI) listeners perceive the aggregate temporal pattern. The present study attempted to extend this general finding and to test whether dual-electrode stimulation would increase the upper limit of temporal pitch perception in CIs. Six subjects were asked to rank 12 dual-channel stimuli differing in their rate [ranging from 92 to 516 pps (pulses per second) on each individual channel] and in their inter-channel delay (pulses on the two channels being either nearly simultaneous or delayed by half the period). The data showed that, for an electrode separation of 0.75 or 1.1 mm, (a) the perceived pitch was on average slightly higher for the long-delay than for the short-delay stimuli but never matched the pitch corresponding to the aggregate temporal pattern, (b) the upper limit of temporal pitch did not increase using long-delay stimuli, and (c) the pitch differences between short- and long-delay stimuli were largely insensitive to channel order and to electrode configuration. These results suggest that there may be more independence between CI channels than previously thought.

Mode-locked spike trains in responses of ventral cochlear nucleus chopper and onset neurons to periodic stimuli

We report evidence of mode-locking to the envelope of a periodic stimulus in chopper units of the ventral cochlear nucleus (VCN). Mode-locking is a generalized description of how responses in periodically forced nonlinear systems can be closely linked to the input envelope, while showing temporal patterns of higher order than seen during pure phase-locking. Re-analyzing a previously unpublished dataset in response to amplitude modulated tones, we find that of 55% of cells (6/11) demonstrated stochastic mode-locking in response to sinusoidally amplitude modulated (SAM) pure tones at 50% modulation depth. At 100% modulation depth SAM, most units (3/4) showed mode-locking. We use interspike interval (ISI) scattergrams to unravel the temporal structure present in chopper mode-locked responses. These responses compared well to a leaky integrate-and-fire model (LIF) model of chopper units. Thus the timing of spikes in chopper unit responses to periodic stimuli can be understood in terms of the complex dynamics of periodically forced nonlinear systems. A larger set of onset (33) and chopper units (24) of the VCN also shows mode-locked responses to steady-state vowels and cosine-phase harmonic complexes. However, while 80% of chopper responses to complex stimuli meet our criterion for the presence of mode-locking, only 40% of onset cells show similar complex-modes of spike patterns. We found a correlation between a unit's regularity and its tendency to display mode-locked spike trains as well as a correlation in the number of spikes per cycle and the presence of complex-modes of spike patterns. These spiking patterns are sensitive to the envelope as well as the fundamental frequency of complex sounds, suggesting that complex cell dynamics may play a role in encoding periodic stimuli and envelopes in the VCN.

Effects of lowpass and highpass filtering on the intelligibility of speech based on temporal fine structure or envelope cues

This study aimed to assess whether or not temporal envelope (E) and fine structure (TFS) cues in speech convey distinct phonetic information. Syllables uttered by a male and female speaker were (i) processed to retain either E or TFS within 16 frequency bands, (ii) lowpass or highpass filtered at different cut-off frequencies, and (iii) presented for identification to seven listeners. Psychometric functions were fitted using a sigmoid function, and used to determine crossover frequencies (cut-off frequencies at which lowpass and highpass filtering yielded equivalent performance), and gradients at each point of the psychometric functions (change in performance with respect to cut-off frequency). Crossover frequencies and gradients were not significantly different across speakers. Crossover frequencies were not significantly different between E and TFS speech ( approximately 1.5kHz). Gradients were significantly different between E and TFS speech in various filtering conditions. When stimuli were highpass filtered above 2.5kHz, performance was significantly above chance level and gradients were significantly different from 0 for E speech only. These findings suggest that E and TFS convey important but distinct phonetic cues between 1 and 2kHz. Unlike TFS, E conveys information up to 6kHz, consistent with the characteristics of neural phase locking to E and TFS.

Response of a Hodgkin-Huxley neuron to a high-frequency input

We study the response of a Hodgkin-Huxley neuron stimulated by a periodic sequence of conductance pulses arriving through the synapse in the high-frequency regime. In addition to the usual excitation threshold there is a smooth crossover from the firing to the silent regime for increasing pulse amplitude gsyn. The amplitude of the voltage spikes decreases approximately linearly with gsyn. In some regions of parameter space the response is irregular, probably chaotic. In the chaotic regime between the mode-locked regions 3:1 and 2:1 near the lower excitation threshold, the output interspike interval histogram (ISIH) undergoes a sharp transition. If the driving period is below the critical value, T<T*, the output histogram contains only odd multiples of Ti. For Ti>T* even multiples of Ti also appear in the histogram, starting from the largest values. Near T* the ISIH scales logarithmically on both sides of the transition. The coefficient of variation of ISIH has a cusp singularity at T*. The average response period has a maximum slightly above T*. Near the excitation threshold in the chaotic regime the average firing rate rises sublinearly from frequencies of order 1 Hz.

Selective filtering to spurious localization cues in the mammalian auditory brainstem

The cues used by mammals to localize sound can become corrupted when multiple sound sources are present due to the interference of sound waves. Under such circumstances these localization cues become spurious and often fluctuate rapidly (>100 Hz). By contrast, rapid fluctuations in sound pressure level do not indicate a corrupted signal, but rather may convey important information about the sound source. It is proposed that filtering in the auditory brainstem acts to selectively attenuate signals associated with the presence of rapidly fluctuating (spurious) localization cues, but not those associated with slowly varying cues. Further it is proposed that specific inhibitory circuitry in the auditory brainstem, centered on the dorsal nucleus of the lateral lemniscus (DNLL), contributes to this selective filtering. Data from extra-cellular recordings in anesthetized Mongolian gerbils are presented to support these hypotheses for a subpopulation of DNLL neurons. These results provide new insights into how the mammalian auditory system processes information about multiple sound sources.

Neural synchrony in ventral cochlear nucleus neuron populations is not mediated by intrinsic processes but is stimulus induced: implications for auditory brainstem implants

The aim of this investigation was to elucidate if neural synchrony forms part of the spike time-based theory for coding of sound information in the ventral cochlear nucleus (VCN) of the auditory brainstem. Previous research attempts to quantify the degree of neural synchrony at higher levels of the central auditory system have indicated that synchronized firing of neurons during presentation of an acoustic stimulus could play an important role in coding complex sound features. However, it is unknown whether this synchrony could in fact arise from the VCN as it is the first station in the central auditory pathway. Cross-correlation analysis was conducted on 499 pairs of multiunit clusters recorded in the urethane-anesthetized rat VCN in response to pure tones and combinations of two tones to determine the presence of neural synchrony. The shift predictor correlogram was used as a measure for determining the synchrony owing to the effects of the stimulus. Without subtraction of the shift predictor, over 65% of the pairs of multiunit clusters exhibited significant correlation in neural firing when the frequencies of the tones presented matched their characteristic frequencies (CFs). In addition, this stimulus-evoked neural synchrony was dependent on the physical distance between electrode sites, and the CF difference between multiunit clusters as the number of correlated pairs dropped significantly for electrode sites greater than 800 microm apart and for multiunit cluster pairs with a CF difference greater than 0.5 octaves. However, subtraction of the shift predictor correlograms from the raw correlograms resulted in no remaining correlation between all VCN pairs. These results suggest that while neural synchrony may be a feature of sound coding in the VCN, it is stimulus induced and not due to intrinsic neural interactions within the nucleus. These data provide important implications for stimulation strategies for the auditory brainstem implant, which is used to provide functional hearing to the profoundly deaf through electrical stimulation of the VCN.

Noise-gated encoding of slow inputs by auditory brain stem neurons with a low-threshold K+ current

Phasic neurons, which do not fire repetitively to steady depolarization, are found at various stages of the auditory system. Phasic neurons are commonly described as band-pass filters because they do not respond to low-frequency inputs even when the amplitude is large. However, we show that phasic neurons can encode low-frequency inputs when noise is present. With a low-threshold potassium current (I(KLT)), a phasic neuron model responds to rising and falling phases of a subthreshold low-frequency signal with white noise. When the white noise was low-pass filtered, the phasic model also responded to the signal's trough but still not to the peak. In contrast, a tonic neuron model fired mostly to the signal's peak. To test the model predictions, whole cell slice recordings were obtained in the medial (MSO) and lateral (LSO) superior olivary neurons in gerbil from postnatal day 10 (P10) to 22. The phasic MSO neurons with strong I(KLT), mostly from gerbils aged P17 or older, showed firing patterns consistent with the preceding predictions. Moreover, injecting a virtual I(KLT) into weak-phasic MSO and tonic LSO neurons with putative weak or no I(KLT) (from gerbils younger than P17) shifted the neural response from the signal's peak to the rising phase. These findings advance our knowledge about how noise gates the signal pathway and how phasic neurons encode slow envelopes of sounds with high-frequency carriers.

Stochastic properties of coincidence-detector neural cells

Neural information is characterized by sets of spiking events that travel within the brain through neuron junctions that receive, transmit, and process streams of spikes. Coincidence detection is one of the ways to describe the functionality of a single neural cell. This letter presents an analytical derivation of the output stochastic behavior of a coincidence detector (CD) cell whose stochastic inputs behave as a nonhomogeneous Poisson process (NHPP) with both excitatory and inhibitory inputs. The derivation, which is based on an efficient breakdown of the cell into basic functional elements, results in an output process whose behavior can be approximated as an NHPP as long as the coincidence interval is much smaller than the refractory period of the cell's inputs. Intuitively, the approximation is valid as long as the processing rate is much faster than the incoming information rate. This type of modeling is a simplified but very useful description of neurons since it enables analytical derivations. The statistical properties of single CD cell's output make it possible to integrate and analyze complex neural cells in a feedforward network using the methodology presented here. Accordingly, basic biological characteristics of neural activity are demonstrated, such as a decrease in the spontaneous rate at higher brain levels and improved signal-to-noise ratio for harmonic input signals.