Spike Discharge Patterns of Spontaneous and Continuously Stimulated Activity in the Cochlear Nucleus of Anesthetized Cats

Ultrastructural maturation of the endbulb of Held active zones comparing wild-type and otoferlin-deficient mice

Endbulbs of Held are located in the anteroventral cochlear nucleus and present the first central synapses of the auditory pathway. During development, endbulbs mature functionally to enable rapid and powerful synaptic transmission with high temporal precision. This process is accompanied by morphological changes of endbulb terminals. Loss of the hair cell-specific protein otoferlin (Otof) abolishes neurotransmission in the cochlea and results in the smaller endbulb of Held terminals. Thus, peripheral hearing impairment likely also leads to alterations in the morphological synaptic vesicle (SV) pool size at individual endbulb of Held active zones (AZs). Here, we investigated endbulb AZs in pre-hearing, young, and adult wild-type and Otof^−/− mice. During maturation, SV numbers at endbulb AZs increased in wild-type mice but were found to be reduced in Otof^−/− mice. The SV population at a distance of 0–15 nm was most strongly affected. Finally, overall SV diameters decreased in Otof^−/− animals during maturation.

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Maturation of wt endbulb of Held active zones leads to more synaptic vesicles

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At endbulbs of otoferlin knockout mice, synaptic vesicles decline with age

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Mainly two distinct synaptic vesicle populations are affected

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Synaptic vesicles sizes are reduced in six-month-old otoferlin knockout animals

Inferring phenomenological models of first passage processes

Biochemical processes in cells are governed by complex networks of many chemical species interacting stochastically in diverse ways and on different time scales. Constructing microscopically accurate models of such networks is often infeasible. Instead, here we propose a systematic framework for building phenomenological models of such networks from experimental data, focusing on accurately approximating the time it takes to complete the process, the First Passage (FP) time. Our phenomenological models are mixtures of Gamma distributions, which have a natural biophysical interpretation. The complexity of the models is adapted automatically to account for the amount of available data and its temporal resolution. The framework can be used for predicting behavior of FP systems under varying external conditions. To demonstrate the utility of the approach, we build models for the distribution of inter-spike intervals of a morphologically complex neuron, a Purkinje cell, from experimental and simulated data. We demonstrate that the developed models can not only fit the data, but also make nontrivial predictions. We demonstrate that our coarse-grained models provide constraints on more mechanistically accurate models of the involved phenomena.

Single-Cell Sequencing Applications in the Inner Ear

Genomics studies face specific challenges in the inner ear due to the multiple types and limited amounts of inner ear cells that are arranged in a very delicate structure. However, advances in single-cell sequencing (SCS) technology have made it possible to analyze gene expression variations across different cell types as well as within specific cell groups that were previously considered to be homogeneous. In this review, we summarize recent advances in inner ear research brought about by the use of SCS that have delineated tissue heterogeneity, identified unknown cell subtypes, discovered novel cell markers, and revealed dynamic signaling pathways during development. SCS opens up new avenues for inner ear research, and the potential of the technology is only beginning to be explored.

Mechanisms and Functional Consequences of Presynaptic Homeostatic Plasticity at Auditory Nerve Synapses

Multiple forms of homeostasis influence synaptic function under diverse activity conditions. Both presynaptic and postsynaptic forms of homeostasis are important, but their relative impact on fidelity is unknown. To address this issue, we studied auditory nerve synapses onto bushy cells in the cochlear nucleus of mice of both sexes. These synapses undergo bidirectional presynaptic and postsynaptic homeostatic changes with increased and decreased acoustic stimulation. We found that both young and mature synapses exhibit similar activity-dependent changes in short-term depression. Experiments using chelators and imaging both indicated that presynaptic Ca²⁺ influx decreased after noise exposure, and increased after ligating the ear canal. By contrast, Ca²⁺ cooperativity was unaffected. Experiments using specific antagonists suggest that occlusion leads to changes in the Ca²⁺ channel subtypes driving neurotransmitter release. Furthermore, dynamic-clamp experiments revealed that spike fidelity primarily depended on changes in presynaptic depression, with some contribution from changes in postsynaptic intrinsic properties. These experiments indicate that presynaptic Ca²⁺ influx is homeostatically regulated in vivo to enhance synaptic fidelity.SIGNIFICANCE STATEMENT Homeostatic mechanisms in synapses maintain stable function in the face of different levels of activity. Both juvenile and mature auditory nerve synapses onto bushy cells modify short-term depression in different acoustic environments, which raises the question of what the underlying presynaptic mechanisms are and the relative importance of presynaptic and postsynaptic contributions to the faithful transfer of information. Changes in short-term depression under different acoustic conditions were a result of changes in presynaptic Ca²⁺ influx. Spike fidelity was affected by both presynaptic and postsynaptic changes after ear occlusion and was only affected by presynaptic changes after noise-rearing. These findings are important for understanding regulation of auditory synapses under normal conditions and also in disorders following noise exposure or conductive hearing loss.

Functional Development of Principal Neurons in the Anteroventral Cochlear Nucleus Extends Beyond Hearing Onset

Sound information is transduced into graded receptor potential by cochlear hair cells and encoded as discrete action potentials of auditory nerve fibers. In the cochlear nucleus, auditory nerve fibers convey this information through morphologically distinct synaptic terminals onto bushy cells (BCs) and stellate cells (SCs) for processing of different sound features. With expanding use of transgenic mouse models, it is increasingly important to understand the in vivo functional development of these neurons in mice. We characterized the maturation of spontaneous and acoustically evoked activity in BCs and SCs by acquiring single-unit juxtacellular recordings between hearing onset (P12) and young adulthood (P30) of anesthetized CBA/J mice. In both cell types, hearing sensitivity and characteristic frequency (CF) range are mostly adult-like by P14, consistent with rapid maturation of the auditory periphery. In BCs, however, some physiological features like maximal firing rate, dynamic range, temporal response properties, recovery from post-stimulus depression, first spike latency (FSL) and encoding of sinusoid amplitude modulation undergo further maturation up to P18. In SCs, the development of excitatory responses is even more prolonged, indicated by a gradual increase in spontaneous and maximum firing rates up to P30. In the same cell type, broadly tuned acoustically evoked inhibition is immediately effective at hearing onset, covering the low- and high-frequency flanks of the excitatory response area. Together, these data suggest that maturation of auditory processing in the parallel ascending BC and SC streams engages distinct mechanisms at the first central synapses that may differently depend on the early auditory experience.

Neuronal variability and tuning are balanced to optimize naturalistic self-motion coding in primate vestibular pathways

It is commonly assumed that the brain’s neural coding strategies are adapted to the statistics of natural stimuli. Specifically, to maximize information transmission, a sensory neuron’s tuning function should effectively oppose the decaying stimulus spectral power, such that the neural response is temporally decorrelated (i.e. ‘whitened’). However, theory predicts that the structure of neuronal variability also plays an essential role in determining how coding is optimized. Here, we provide experimental evidence supporting this view by recording from neurons in early vestibular pathways during naturalistic self-motion. We found that central vestibular neurons displayed temporally whitened responses that could not be explained by their tuning alone. Rather, computational modeling and analysis revealed that neuronal variability and tuning were matched to effectively complement natural stimulus statistics, thereby achieving temporal decorrelation and optimizing information transmission. Taken together, our findings reveal a novel strategy by which neural variability contributes to optimized processing of naturalistic stimuli.

Auditory gap detection: psychometric functions and insights into the underlying neural activity

The detection of a silent interval or gap provides important insight into temporal processing by the auditory system. Previous research has uncovered a multitude of empirical findings leaving the mechanism of gap detection poorly understood and key issues unresolved. Here, we expand the findings by measuring psychometric functions for a number of conditions including both across-frequency and across-intensity gap detection as a first study of its kind. A model is presented which not only accounts for our findings in a quantitative manner, but also helps frame the body of work on auditory gap research. The model is based on the peripheral response and postulates that the identification of gap requires the detection of activity associated with silence.

Transmission Disrupted: Modeling Auditory Synaptopathy in Zebrafish

Sensorineural hearing loss is the most common form of hearing loss in humans, and results from either dysfunction in hair cells, the sensory receptors of sound, or the neurons that innervate hair cells. A specific type of sensorineural hearing loss, referred to as auditory synaptopathy, occurs when hair cells are able to detect sound but fail to transmit sound stimuli at the hair-cell synapse. Auditory synaptopathy can originate from genetic alterations that specifically disrupt hair-cell synapse function. Additionally, environmental factors such as noise exposure can leave hair cells intact but result in loss of hair-cell synapses, and represent an acquired form of auditory synaptopathy. The zebrafish model has emerged as a valuable system for studies of hair-cell function, and specifically hair-cell synaptopathy. In this review, we describe the experimental tools that have been developed to study hair-cell synapses in zebrafish. We discuss how zebrafish genetics has helped identify and define the roles of hair-cell synaptic proteins crucial for hearing in humans, and highlight how studies in zebrafish have contributed to our understanding of hair-cell synapse formation and function. In addition, we also discuss work that has used noise exposure or pharmacological mimic of noise-induced excitotoxicity in zebrafish to define cellular mechanisms underlying noise-induced hair-cell damage and synapse loss. Lastly, we highlight how future studies in zebrafish could enhance our understanding of the pathological processes underlying synapse loss in both genetic and acquired auditory synaptopathy. This knowledge is critical in order to develop therapies that protect or repair auditory synaptic contacts.

On a unified point process approach for the characterization of bioelectric discrete phenomena

This paper discusses a unified method based on the theory of point processes to characterize various types of bioelectric discrete signals such as heart beat timing, myoelectric activity, discharge of primary sensory neurons or neurons in the central nervous systems. The doubly stochastic point processes, in which the discrete event occurring intensity is stochastic, forms the most general class to characterize the discrete phenomena. In this paper the self-exciting process has been shown to be useful to characterize wide range of discrete biosignals. The modeling of conditional intensity function is the essential part of the characterization. When the intensity has a parametric model, the maximum likelihood parameter estimation will be the useful way to characterize the phenomena. The effectiveness of the method is demonstrated by a specific modeling of the spontaneous neuronal burst discharges recorded from the brain thalamus during the neuro surgery. The first approximation model has four parameters obtained by the instantaneous nonlinearly transformed sinusoidal function. An extended model allows arbitrary periodic intensity with refractory period. Predicted interval histograms show good agreement with the observed ones indicating the validity of the proposed method.

Background firing in the auditory midbrain of the frog

Statistical characteristics of background firing in the midbrain auditory units of grass frog (Rana t. temporaria) located in torus semicircular (TS) were investigated. Only about 5% of the cells demonstrated prominent spontaneous firing. For such units the following characteristics were obtained: the distribution of interpulse intervals, the autocorrelation functions (ACF) for the real firing process and for the process with shuffled intervals, the hazard function (HF) and the joint distribution of adjacent interpulse intervals. The burstiness of firing was also estimated. In the absolute majority of the cells, the background firing demonstrated considerable deviation from the renewal process. There was weak but significant positive correlation between adjacent interpulse intervals. The burstiness of firing in the midbrain auditory units was moderate but higher than reported for medullary auditory neurons. The value of burstiness did not decrease after interval shuffling. Along with the reduction in excitability (generalized refractoriness) in many neurons observed post-spike facilitation effects were observed. Comparing background activity in medullary and midbrain nucleus suggests that there is an increase in complexity of the information processing along the auditory pathway.

Enlargement of Ribbons in Zebrafish Hair Cells Increases Calcium Currents But Disrupts Afferent Spontaneous Activity and Timing of Stimulus Onset

In sensory hair cells of auditory and vestibular organs, the ribbon synapse is required for the precise encoding of a wide range of complex stimuli. Hair cells have a unique presynaptic structure, the synaptic ribbon, which organizes both synaptic vesicles and calcium channels at the active zone. Previous work has shown that hair-cell ribbon size is correlated with differences in postsynaptic activity. However, additional variability in postsynapse size presents a challenge to determining the specific role of ribbon size in sensory encoding. To selectively assess the impact of ribbon size on synapse function, we examined hair cells in transgenic zebrafish that have enlarged ribbons, without postsynaptic alterations. Morphologically, we found that enlarged ribbons had more associated vesicles and reduced presynaptic calcium-channel clustering. Functionally, hair cells with enlarged ribbons had larger global and ribbon-localized calcium currents. Afferent neuron recordings revealed that hair cells with enlarged ribbons resulted in reduced spontaneous spike rates. Additionally, despite larger presynaptic calcium signals, we observed fewer evoked spikes with longer latencies from stimulus onset. Together, our work indicates that hair-cell ribbon size influences the spontaneous spiking and the precise encoding of stimulus onset in afferent neurons.

SIGNIFICANCE STATEMENT Numerous studies support that hair-cell ribbon size corresponds with functional sensitivity differences in afferent neurons and, in the case of inner hair cells of the cochlea, vulnerability to damage from noise trauma. Yet it is unclear whether ribbon size directly influences sensory encoding. Our study reveals that ribbon enlargement results in increased ribbon-localized calcium signals, yet reduces afferent spontaneous activity and disrupts the timing of stimulus onset, a distinct aspect of auditory and vestibular encoding. These observations suggest that varying ribbon size alone can influence sensory encoding, and give further insight into how hair cells transduce signals that cover a wide dynamic range of stimuli.

Intensity-dependent timing and precision of startle response latency in larval zebrafish

KEY POINTS

Using high-speed videos time-locked with whole-animal electrical recordings, simultaneous measurement of behavioural kinematics and field potential parameters of C-start startle responses allowed for discrimination between short-latency and long-latency C-starts (SLCs vs. LLCs) in larval zebrafish. Apart from their latencies, SLC kinematics and SLC field potential parameters were intensity independent. Increasing stimulus intensity increased the probability of evoking an SLC and decreased mean SLC latencies while increasing their precision; subtraction of field potential latencies from SLC latencies revealed a fixed time delay between the two measurements that was intensity independent. The latency and the precision in the latency of the SLC field potentials were linearly correlated to the latencies and precision of the first evoked action potentials (spikes) in hair-cell afferent neurons of the lateral line. Together, these findings indicate that first spike latency (FSL) is a fast encoding mechanism that can serve to precisely initiate startle responses when speed is critical for survival.

ABSTRACT

Vertebrates rely on fast sensory encoding for rapid and precise initiation of startle responses. In afferent sensory neurons, trains of action potentials (spikes) encode stimulus intensity within the onset time of the first evoked spike (first spike latency; FSL) and the number of evoked spikes. For speed of initiation of startle responses, FSL would be the more advantageous mechanism to encode the intensity of a threat. However, the intensity dependence of FSL and spike number and whether either determines the precision of startle response initiation is not known. Here, we examined short-latency startle responses (SLCs) in larval zebrafish and tested the hypothesis that first spike latencies and their precision (jitter) determine the onset time and precision of SLCs. We evoked startle responses via activation of Channelrhodopsin (ChR2) expressed in ear and lateral line hair cells and acquired high-speed videos of head-fixed larvae while simultaneously recording underlying field potentials. This method allowed for discrimination between primary SLCs and less frequent, long-latency startle responses (LLCs). Quantification of SLC kinematics and field potential parameters revealed that, apart from their latencies, they were intensity independent. We found that increasing stimulus intensity decreased SLC latencies while increasing their precision, which was significantly correlated with corresponding changes in field potential latencies and their precision. Single afferent neuron recordings from the lateral line revealed a similar intensity-dependent decrease in first spike latencies and their jitter, which could account for the intensity-dependent changes in timing and precision of startle response latencies.

Physiological recordings from the zebrafish lateral line

During sensory transduction, external physical stimuli are translated into an internal biological signal. In vertebrates, hair cells are specialized mechanosensory receptors that transduce sound, gravitational forces, and head movements into electrical signals that are transmitted with remarkable precision and efficiency to afferent neurons. Hair cells have a conserved structure between species and are also found in the lateral line system of fish, including zebrafish, which serve as an ideal animal model to study sensory transmission in vivo. In this chapter, we describe the methods required to investigate the biophysical properties underlying mechanosensation in the lateral line of the zebrafish in vivo from microphonic potentials and single hair cell patch-clamp recordings to single afferent neuron recordings. These techniques provide real-time measurements of hair-cell transduction and transmission following delivery of controlled and defined stimuli and their combined use on the intact zebrafish provides a powerful platform to investigate sensory encoding in vivo.

Quantitative analysis of ribbons, vesicles, and cisterns at the cat inner hair cell synapse: correlations with spontaneous rate

Cochlear hair cells form ribbon synapses with terminals of the cochlear nerve. To test the hypothesis that one function of the ribbon is to create synaptic vesicles from the cisternal structures that are abundant at the base of hair cells, we analyzed the distribution of vesicles and cisterns around ribbons from serial sections of inner hair cells in the cat, and compared data from low and high spontaneous rate (SR) synapses. Consistent with the hypothesis, we identified a "sphere of influence" of 350 nm around the ribbon, with fewer cisterns and many more synaptic vesicles. Although high- and low-SR ribbons tended to be longer and thinner than high-SR ribbons, the total volume of the two ribbon types was similar. There were almost as many vesicles docked at the active zone as attached to the ribbon. The major SR-related difference was that low-SR ribbons had more synaptic vesicles intimately associated with them. Our data suggest a trend in which low-SR synapses had more vesicles attached to the ribbon (51.3 vs. 42.8), more docked between the ribbon and the membrane (12 vs. 8.2), more docked at the active zone (56.9 vs. 44.2), and more vesicles within the "sphere of influence" (218 vs. 166). These data suggest that the structural differences between high- and low-SR synapses may be more a consequence, than a determinant, of the physiological differences.

Characterizing context-dependent differential firing activity in the hippocampus and entorhinal cortex

The rat hippocampus and entorhinal cortex have been shown to possess neurons with place fields that modulate their firing properties under different behavioral contexts. Such context-dependent changes in neural activity are commonly studied through electrophysiological experiments in which a rat performs a continuous spatial alternation task on a T-maze. Previous research has analyzed context-based differential firing during this task by describing differences in the mean firing activity between left-turn and right-turn experimental trials. In this article, we develop qualitative and quantitative methods to characterize and compare changes in trial-to-trial firing rate variability for sets of experimental contexts. We apply these methods to cells in the CA1 region of hippocampus and in the dorsocaudal medial entorhinal cortex (dcMEC), characterizing the context-dependent differences in spiking activity during spatial alternation. We identify a subset of cells with context-dependent changes in firing rate variability. Additionally, we show that dcMEC populations encode turn direction uniformly throughout the T-maze stem, whereas CA1 populations encode context at major waypoints in the spatial trajectory. Our results suggest scenarios in which individual cells that sparsely provide information on turn direction might combine in the aggregate to produce a robust population encoding.

Effects of the rates of pseudo-spontaneous spikes generated by electric stimuli on information transmission in an auditory nerve fiber model

In this study, the effects of the rate of pseudo-spontaneous spikes on information transmission of the spike trains in response to the electric pulsatile stimulus currents in an auditory nerve fiber (ANF) model is investigated through computer simulation. The pseudo-spontaneous spikes can be generated by high rate pulsatile electric stimuli, making it possible to efficiently encode sound stimuli into the spike trains of the ANF in cochlear prostheses. In this investigation, the information rate of the spike trains in response to sinusoidally modulated pulsatile electric stimuli was estimated as the amplitude of the pulsatile electric stimuli (the rate of pseudo-spontaneous spikes) was varied. The results show that the information rates increased, reached a maximum, and then decreased, in several different values of modulation depth, as the rate of pseudo-spontaneous spikes increased. This may imply a resonance phenomenon dependent on the rate of pseudo-spontaneous spikes generated by electric stimuli in the ANF model. These findings may play a key role in the design of better cochlear prostheses.

A general likelihood framework for characterizing the time course of neural activity

We develop a general likelihood-based framework for use in the estimation of neural firing rates, which is designed to choose the temporal smoothing parameters that maximize the likelihood of missing data. This general framework is algorithm-independent and thus can be applied to a multitude of established methods for firing rate or conditional intensity estimation. As a simple example of the use of the general framework, we apply it to the peristimulus time histogram and kernel smoother, the methods most widely used for firing rate estimation in the electrophysiological literature and practice. In doing so, we illustrate how the use of the framework can employ the general point process likelihood as a principled cost function and can provide substantial improvements in estimation accuracy for even the most basic of rate estimation algorithms. In particular, the resultant kernel smoother is simple to implement, efficient to compute, and can accurately determine the bandwidth of a given rate process from individual spike trains. We perform a simulation study to illustrate how the likelihood framework enables the kernel smoother to pick the bandwidth parameter that best predicts missing data, and we show applications to real experimental spike train data. Additionally, we discuss how the general likelihood framework may be used in conjunction with more sophisticated methods for firing rate and conditional intensity estimation and suggest possible applications.

A stochastic model of the repetitive activity of neurons

A recurrent model of the repetitive firing of neurons responding to stimuli of long duration is given. The model assumes a deterministic threshold potential and a membrane potential which is composed of both deterministic and random components. The model accurately reproduces interval statistics obtained from different neurons discharging repetitively over a wide range of discharge rates. It is shown that the model has three important parameters; the time course of threshold recovery following a discharge, the variance of the random component, and the level of excitatory drive. The model is extended, by the use of hyperpolarizing afterpotentials, to include negative correlation between successive interspike intervals.

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.

Three-dimensional tonotopic organization of the C57 mouse cochlear nucleus

The cochlear nucleus (CN) is the first sound processing center in the central auditory system that receives the almost unprocessed auditory information from the auditory periphery. The functional organization of the CN has been studied to a great extent in many mammals, including the cat, rat and bat. Yet, despite the general usefulness of the mouse, including the availability of various inbred strains and gene-manipulated lines, our current understanding of the mouse CN remains limited. The purpose of this study was to illustrate the functional organization of the CN in C57 mice, using an electrophysiological approach. Our results showed that the auditory response properties of CN neurons were similar in all three of the CN subdivisions. Sound frequency was systematically represented in each of the three CN subdivisions, i.e., the anteroventral, posteroventral and the dorsal divisions. The best frequency of CN neurons decreased along the dorsomedial-to-ventrolateral axis in an orderly progression whereas the tonotopic axes were relatively indistinct in the rostrocaudal plane. There was no disruption of the tonotopic map within each subdivision of the CN. The findings indicate that the CN tonotopic organization in the C57 mouse is similar to that in the cat and other mammals.

Beyond Poisson: increased spike-time regularity across primate parietal cortex

Cortical areas differ in their patterns of connectivity, cellular composition, and functional architecture. Spike trains, on the other hand, are commonly assumed to follow similarly irregular dynamics across neocortex. We examined spike-time statistics in four parietal areas using a method that accounts for nonstationarities in firing rate. We found that, whereas neurons in visual areas fire irregularly, many cells in association and motor-like parietal regions show increasingly regular spike trains by comparison. Regularity was evident both in the shape of interspike interval distributions and in spike-count variability across trials. Thus, Poisson-like randomness is not a universal feature of neocortex. Rather, many parietal cells have reduced trial-to-trial variability in spike counts that could provide for more reliable firing-rate signals. These results suggest that spiking dynamics may play different roles in different cortical areas and should not be assumed to arise from fundamentally irreducible noise sources.

Interspike interval statistics in the stochastic Hodgkin-Huxley model: coexistence of gamma frequency bursts and highly irregular firing

When the classical Hodgkin-Huxley equations are simulated with Na- and K-channel noise and constant applied current, the distribution of interspike intervals is bimodal: one part is an exponential tail, as often assumed, while the other is a narrow gaussian peak centered at a short interspike interval value. The gaussian arises from bursts of spikes in the gamma-frequency range, the tail from the interburst intervals, giving overall an extraordinarily high coefficient of variation--up to 2.5 for 180,000 Na channels when I approximately 7 microA/cm(2). Since neurons with a bimodal ISI distribution are common, it may be a useful model for any neuron with class 2 firing. The underlying mechanism is due to a subcritical Hopf bifurcation, together with a switching region in phase-space where a fixed point is very close to a system limit cycle. This mechanism may be present in many different classes of neurons and may contribute to widely observed highly irregular neural spiking.

Contributions of Aage Møller in the study of the cochlear nucleus

At a time when little was known about processing in the auditory system, Aage Møller undertook an extensive investigation of the response properties of cochlear nucleus (CN) neurons. With an excellent background in physiological acoustics and a command of computational techniques he systematically explored neural tuning, rate-level functions, and receptive fields of CN neurons using microelectrode recordings. He chose to employ more natural stimuli than just pure tones and employed a variety of stimuli consisting of tones, clicks, noise, amplitude- and frequency-modulated signals to document both intensity and temporal response characteristics. The response to noise stimuli was quantified using linear systems analysis which was very innovative at that time. By choosing to perform the studies in the white rat rather than cat, he provided important comparative data on this first center of the central auditory system. Over a span of ten years he provided a significant body of observations of CN units properties that has rarely been equaled.

Single-unit responses in the auditory cortex of monkeys performing a conditional acousticomotor task

The general goal of the present study was to assess the response properties to tones of single neurons in the auditory cortex (primary auditory area, A1, and middle lateral auditory belt, ML) of two macaque monkeys while performing an acousticomotor discrimination task requiring a controlled level of attention and motivation. For each neuron, an approximation of the frequency receptive field (FRF) was first established. Second, based on the FRF, sets of paired tone frequencies were defined in which two different tone frequencies had to be associated by the monkey, following a trial and error strategy, to a left or a right key-press with the left arm. After acquisition of the association, the two tones of the pair were presented randomly ("instruction stimulus") and, if the monkey touched the correct key, the stimulus was repeated ("confirmation stimulus") and a reward was delivered. The majority of units (63%) had a FRF formed by multiple peaks, whereas 25% and 12% of units exhibited a simple U-shaped FRF and a "mosaic" FRF, composed of several separated zones of response, respectively. Five principal response patterns were observed: On, Off, On-Off, Sustained, and Inhibition. In relation to the acousticomotor association task, some auditory cortical neurons (33%) exhibited a different response to the same stimulus when presented, in the same trials, as instruction or as confirmation. It was also observed that the response to the same instruction stimulus could differ when comparing correct trials with erroneous trials (wrong motor response). In conclusion, the response properties of auditory cortical neurons in behaving monkeys are strongly dependent on the physical parameters of sounds (frequency, intensity, etc.) as indicated by FRF characteristics, but a substantial influence of the behavioral context and performance may also play an important role.

Parallel auditory pathways: projection patterns of the different neuronal populations in the dorsal and ventral cochlear nuclei

The cochlear nuclear complex gives rise to widespread projections to nuclei throughout the brainstem. The projections arise from separate, well-defined populations of cells. None of the cell populations in the cochlear nucleus projects to all brainstem targets, and none of the targets receives inputs from all cell types. The projections of nine distinguishable cell types in the cochlear nucleus-seven in the ventral cochlear nucleus and two in the dorsal cochlear nucleus-are described in this review. Globular bushy cells and two types of spherical bushy cells project to nuclei in the superior olivary complex that play roles in sound localization based on binaural cues. Octopus cells convey precisely timed information to nuclei in the superior olivary complex and lateral lemniscus that, in turn, send inhibitory input to the inferior colliculus. Cochlear root neurons send widespread projections to areas of the reticular formation involved in startle reflexes and autonomic functions. Type I multipolar cells may encode complex features of natural stimuli and send excitatory projections directly to the inferior colliculus. Type II multipolar cells send inhibitory projections to the contralateral cochlear nuclei. Fusiform cells in the dorsal cochlear nucleus appear to be important for the localization of sounds based on spectral cues and send direct excitatory projections to the inferior colliculus. Giant cells in the dorsal cochlear nucleus also project directly to the inferior colliculus; some of them may convey inhibitory inputs to the contralateral cochlear nucleus as well.

Construction and analysis of non-Poisson stimulus-response models of neural spiking activity

A paradigm for constructing and analyzing non-Poisson stimulus-response models of neural spike train activity is presented. Inhomogeneous gamma (IG) and inverse Gaussian (IIG) probability models are constructed by generalizing the derivation of the inhomogeneous Poisson (IP) model from the exponential probability density. The resultant spike train models have Markov dependence. Quantile-quantile (Q-Q) plots and Kolmogorov-Smirnov (K-S) plots are developed based on the rate-rescaling theorem to assess model goodness-of-fit. The analysis also expresses the spike rate function of the neuron directly in terms of its interspike interval (ISI) distribution. The methods are illustrated with an analysis of 34 spike trains from rat CA1 hippocampal pyramidal neurons recorded while the animal executed a behavioral task. The stimulus in these experiments is the animal's position in its environment and the response is the neural spiking activity. For all 34 pyramidal cells, the IG and IIG models gave better fits to the spike trains than the IP. The IG model more accurately described the frequency of longer ISIs, whereas the IIG model gave the best description of the burst frequency, i.e. ISIs < or = 20 ms. The findings suggest that bursts are a significant component of place cell spiking activity even when position and the background variable, theta phase, are taken into account. Unlike the Poisson model, the spatial and temporal rate maps of the IG and IIG models depend directly on the spiking history of the neurons. These rate maps are more physiologically plausible since the interaction between space and time determines local spiking propensity. While this statistical paradigm is being developed to study information encoding by rat hippocampal neurons, the framework should be applicable to stimulus-response experiments performed in other neural systems.

Sodium pentobarbital abolishes bursting spontaneous activity of dorsal cochlear nucleus in rat brain slices

There is evidence that pentobarbital, a commonly used anesthetic, can affect neuronal activity, but its effects on particular neurons of the dorsal cochlear nucleus (DCN) are not well known. Bursting (complex spiking) spontaneous activity has been observed in the DCN in brain slice preparations and in recordings from unanesthetized decerebrate animals, but seldom in experiments with anesthetized animals. This study investigated the effects of pentobarbital on spontaneous activity in the DCN in brain slices. Most extracellularly recorded bursting neurons decreased firing rates and reversibly changed their firing to simple spiking with irregular intervals during pentobarbital. Some reversibly stopped firing after the change to an irregular pattern. Most neurons with regular spontaneous activity (simple spiking) showed decreased firing rates and more irregular intervals during pentobarbital. The results also suggest some involvement of gamma-aminobutyric acid type A receptors in the pentobarbital effects.

Intracellular response properties of units in the dorsal cochlear nucleus of unanesthetized decerebrate gerbil

Intracellular recording experiments on the dorsal cochlear nuclei of unanesthetized decerebrate gerbils were conducted. Acceptable recordings were those in which resting potentials were -50 mV or less and action potentials (APs) were > or = 40 mV. Responses to short-duration tones and noise, and to current pulses delivered via recording electrodes, were acquired. Units were classified according to the response map scheme (types I-IV). Ninety-two acceptable recordings were made. Most units had simple APs (simple-spiking units); nine units had both simple and complex APs, which are bursts of spikes embedded on slow, transient depolarizations (complex-spiking units). Of 83 simple-spiking units, 46 were classified as follows: type I/III (9 units), type II (9 units), type III (25 units), type IV (2 units), and type IV-T (1 unit). One complex-spiking unit was classifiable (a type III unit); six were unclassifiable because of weak acoustic responses. Classifying 39 other simple-spiking units and 2 complex-spiking units was impossible, because they were either injured or lost before sufficient data were acquired. Many simple-spiking units showed depolarization or hyperpolarization (approximately 5-10 mV) during acoustic stimulation; some were hyperpolarized during the stimulus-off period. Type I/III units were not hyperpolarized during off-best-frequency (off-BF) stimulation. In contrast, many type II units were hyperpolarized by off-BF frequencies, suggesting that they received strong inhibitory sideband inputs. When inhibited, some type III units were hyperpolarized. Type IV units were hyperpolarized during inhibition even at low levels (<60 dB SPL); sustained depolarizations occurred only at higher levels, suggesting that they receive strong inhibitory and weak excitatory inputs. Several intracellular response properties were statistically different from those of extracellularly recorded units. Intracellularly recorded type II units had higher thresholds and lower maximum BF-driven and noise-driven rates than their extracellularly recorded counterparts. Type I/III units recorded intracellularly had lower maximum BF-driven rates. Type III units recorded intracellularly had higher maximum noise rates compared with those recorded extracellularly. Weaker acoustic responses most likely result from membrane disruption, but heightened responses may be related to weakened chloride-channel-dependent inhibition due to altered driving forces resulting from KCl leakage. Firing rates of simple-spiking units increased monotonically with increasing levels of depolarizing current pulses. In contrast, many complex-spiking units responded nonmonotonically to depolarizing current injection. The monotonic rate-versus-current curves and the nonmonotonic rate-versus-sound level curves of type IV and III units suggest that the acoustic behavior is the result of extrinsic inhibitory inputs and not due solely to intrinsic membrane properties.

Input from the medial nucleus of trapezoid body to an interaural level detector

The medial nucleus of the trapezoid body (MNTB) contains components of a neural network that functions as an interaural level difference (ILD) detector. In the cat, lateral superior olivary (LSO) neurons compare the contralateral inhibitory input from the MNTB with an excitatory input form the ipsilateral anteroventral cochlear nucleus to extract information about binaural stimuli. To better specify the inhibitory inputs to the LSO and gain a better understanding of the inhibitory component of the LSO network, the response characteristics of MNTB neurons were examined in cats under stimulus conditions similar to those used to study LSO inhibitory responses. The inhibitory tuning curves of LSO units were wider than the tuning curves of MNTB units. Hence, MNTB neurons with similar, but not identical, characteristic frequencies converge to provide inhibitory input to single LSO neurons. Variations in the number of converging MNTB inputs produced a range of LSO excitatory-inhibitory threshold differences, thus creating a coding mechanism for representing the ILD. Convergence of MNTB inputs also increased the dynamic range over which contralateral stimulus level effects LSO binaural responses beyond the dynamic ranges of individual MNTB units, thus expanding the ILD range encoded by the LSO network. The differences between the first-spike latencies of MNTB and LSO tone burst responses were small and the precision of the LSO first-spike discharges was significantly greater than that of MNTB units. As tone bursts delivered simultaneously to the two ears can consistently inhibit LSO first-spike discharges, the inhibitory input must match the LSO precision by converging a number of the more variably timed MNTB discharges. Because of their precision LSO first-spike discharges may be used to encode interaural time-of-arrival differences of mid- to high-frequency transients. These findings add to the foundation for a comprehensive network model that describes the inputs to the LSO as point processes, delimits the biophysical mechanisms underlying excitatory and inhibitory interactions at the single neuron level, and reveals how these inputs determine the response to different binaural stimulus conditions.

Periodicity coding in the auditory system

Periodic envelope fluctuations are a common feature of acoustic communication signals, and as a result of physical constraints, many natural, nonliving sound sources also produce periodic waveforms. In human speech and music, for example, periodic sounds are abundant and reach a high degree of complexity. Under noisy conditions these amplitude fluctuations may be reliable indicators of a common sound source responsible for the activation of different frequency channels of the basilar membrane. To make use of this information, a central periodicity analysis is necessary in addition to the peripheral frequency analysis. The present review summarizes our present knowledge about representation and processing of periodic signals, from the cochlea to the cortex in mammals, and in homologous or analogous anatomical structures as far as these exist and have been investigated in other animals. The first sections describe important physical and perceptual attributes of periodic signals, and the last sections address some theoretical issues.

Separate mechanisms control spike numbers and inter-spike intervals in transient responses of cat auditory cortex neurons

In the anesthetized cat, some cortical auditory neurons discharge a train of up to 5 spikes in response to the onset of a characteristic frequency tone pulse. This report provides the first description of the inter-spike intervals (ISIs) in these responses. The ISIs were typically close to 2.0 ms in length, and, as indexed by the standard deviation of the interval length, were very regular. Except at threshold levels of stimulation, mean ISIs were relatively insensitive to both tone amplitude and repetition rate. This was true even over ranges of those variables that exerted dramatic effects on spike numbers and first spike latency. These data suggest that the relative timing of discharges within the spike burst is controlled by a mechanism which is separable from that which determines the number of spikes in them. The brevity of the ISIs suggest that they may be a means of enhancing the salience of the transient response against a background of spontaneous discharges.

Time structure and stimulus dependence of precisely replicating patterns present in monkey cortical neuronal spike trains

Evidence is presented on the parameters that affect the occurrence of precisely replicating patterns of neural discharge present as 'hidden' patterns in individual neuronal discharge trains of the visual cortical cells of the rhesus monkey in response to precisely controlled stimuli described in our previous publication. Using the All-Interval analytical paradigm we demonstrate: (1) that precisely replicating patterns are present in numbers that cannot be generated through continuous, smoothly varying probability distributions of interspike intervals; (2) that the records contain very large numbers of precisely replicating patterns--doublets, triplets, quadruplets, quintuplets and hextuplets of pulses; (3) that triplet-antitriplet pairs and symmetrical quadruplets are also present in improbable numbers; (4) that different stimuli generate different triplets; (5) and that the first order decay constant of capacity to generate specific precise patterns is a direct function of the number of events making up the patterns and thus that a temporary memory of the occurrence of a pattern exists following the presentation of a stimulus. It is concluded that such patterns of pulses are almost certainly coded symbols related to visual information; that such symbols are sufficiently precise in their replication to permit them to be decoded through spatial summation mechanisms and finally that the ability to generate and the capacity to store such symbols are probably present in the brain as related and coordinated complexes of specific facilitated synapses. Some properties of a proposed model for the production and decoding of such patterns are presented and discussed as are specific mechanisms through which neural networks may implement such functions. Finally, existing and further experimental tests of the mechanisms proposed are outlined.

Intracellular marking of physiologically characterized cells in the ventral cochlear nucleus of the cat

In the cat ventral cochlear nucleus, separate neuronal classes have been defined based on morphological characteristics; physiologically defined unit types have also been described based on the shape of post-stimulus-time-histograms in response to tone bursts at characteristic frequency. The aim of the present study was to address directly the issue of how morphological cell types relate to physiological unit types. We used intracellular injections of horseradish peroxidase to stain individual neurons after their response characteristics were determined by intracellular recordings. The maintenance of a continuous negative resting potential, the correspondence of the calculated position of the electrode tip at the time of injection to the location of the stained neuron, and the similarity of response properties collected before and after the injection provide evidence that the injected, stained, and recovered neuron corresponds to the functionally defined unit. In the region around the nerve root in the anteroventral cochlear nucleus, two " primarylike " and one " primarylike with notch" units were "bushy" cells. "Bushy" cells are characterized by primary dendrites arising from one hemisphere of the soma and ramifying repeatedly to produce their bushy dendritic arbor. In this same region, the "chopper" and two "on" units were also "bushy" cells. In the posteroventral cochlear nucleus, the "chopper" unit was a "stellate" cell and the "on" unit was an "octopus" cell. These results are partially consistent with previous conclusions based on correlations established between the regional distribution of physiological unit types and morphological cell types. More importantly, they confirm and extend recent intracellular marking data (Rhode et al., ' 83b ). If our classification schemes have functional significance, we are left with the conclusion that the distinction between "bushy" and "stellate" cells in the auditory nerve root region of the ventral cochlear nucleus does not correspond in any simple way to distinctions between physiological unit types. More than one morphological cell type can exhibit the same particular response patter, and the same morphological cell type can exhibit several different response patterns.

The interspike interval of a cable model neuron with white noise input

The firing time of a cable model neuron in response to white noise current injection is investigated with various methods. The Fourier decomposition of the depolarization leads to partial differential equations for the moments of the firing time. These are solved by perturbation and numerical methods, and the results obtained are in excellent agreement with those obtained by Monte Carlo simulation. The convergence of the random Fourier series is found to be very slow for small times so that when the firing time is small it is more efficient to simulate the solution of the stochastic cable equation directly using the two different representations of the Green's function, one which converges rapidly for small times and the other which converges rapidly for large times. The shape of the interspike interval density is found to depend strongly on input position. The various shapes obtained for different input positions resemble those for real neurons. The coefficient of variation of the interspike interval decreases monotonically as the distance between the input and trigger zone increases. A diffusion approximation for a nerve cell receiving Poisson input is considered and input/output frequency relations obtained for different input sites. The cases of multiple trigger zones and multiple input sites are briefly discussed.

A theoretical basis for large coefficient of variation and bimodality in neuronal interspike interval distributions

We consider the classic Stein (1965) model for stochastic neuronal firing under random synaptic input. Our treatment includes the additional effect of synaptic reversal potentials. We develop and employ two numerical methods (in addition to Monte Carlo simulations) to study the relation of the various parameters of the model to the shape of the theoretical interspike interval distribution. Contrary to the results of Tuckwell (1979) we are unable to account, on the basis of substantial synaptic inhibition and with parameter settings in the known physiologic range, for experimental interspike interval distributions which exhibit large coefficients of variation or bimodality. We therefore introduce a time varying threshold into the model, which readily allows for such distributions and which has physiological justification.

Single unit analysis of the posteroventral cochlear nucleus of the decerebrate cat

Single unit recordings were obtained in the cochlear nuclear complex of the unanesthetized, decerebrate cat. Sixty-six of 282 units were localized to the posteroventral cochlear nucleus, 17 from the multipolar cell area and 49 from the octopus cell area. Spontaneous rates ranged from less than 1 to 75 spikes per second in the multipolar cell area and from less than 1 to 135 spikes per second in the octopus cell area. Poststimulus time histograms revealed four response types, at the best frequency, in the posteroventral cochlear nucleus. These responses were: (1) primary-like (maximum response shortly after the stimulus onset, followed by a reduction in activity to a steady state); (2) chopper (similar to primary-like but with multiple peaks in the first 10-15 milliseconds); (3) onset-ex (onset response followed by a low level of excitation); and (4) onset-in (onset response followed by inhibition). The onset-in responses represented the first observations of inhibition, at best frequency, for onset units in the mammalian cochlear nuclear complex. Analysis of interspike interval distributions showed that both spontaneous and driven activity consisted of irregular intervals for all four response types. Activity-intensity functions for primary-like, chopper and onset-ex units showed monotonic increases with increases in stimulus intensity. Activity-intensity functions for onset-in units were non-monotonic. Latency-intensity functions for primary-like, chopper and onset-ex units exhibited monotonic decreases with increases in intensity. Latency-intensity functions for onset-in units were non-monotonic. In contrast to primary-like, chopper and onset-ex units, onset-in units do not retain the intensity and temporal information coded in the eighth nerve, as least for stimuli above 2 kilohertz. It is hypothesized that a depolarization block, caused by the massive eighth nerve input to octopus cells, is responsible for the inhibition observed from onset-in units.

Origin of ascending projections to inferior colliculus in the mustache bat, Pteronotus parnellii

The origins of pathways to the inferior colliculus of the mustache bat were identified by retrograde transport of horseradish peroxidase (HRP). A specific goal of this study was to obtain evidence that would help determine whether the nuclei, shown in the previous paper to have unusual cytoarchitectural features, are unique to bats, or whether they are homologous to areas that are not well differentiated in other mammals. The auditory pathways in the lower brain stem of Pteronotus appear to conform to the same basic organization as in other mammals: After injection of HRP into one inferior colliculus, labeled cells are located contralaterally in the cochlear nucleus, ipsilaterally in the medical superior olive, bilaterally in the lateral superior olive, ipsilaterally in the ventral and intermediate nuclei of the lateral lemniscus, and bilaterally in the dorsal nucleus of the lateral lemniscus. These patterns of labeling provide a basis for understanding how the specialized auditory areas of the bat may be organized within a basic plan of mammalian auditory systems. In the anteroventral cochlear nucleus the unusually small spherical cells seem to be homologous to stellate cells in the anteroventral cochlear nucleus of the cat. In the superior olive, differences in patterns of labeled cells distinguish the medial from the lateral superior olive. In the lateral lemniscus the pattern of labeled cells shows clear differences between the two special parts, intermediate and ventral nuclei, as well as between these and the dorsal nucleus of the lateral lemniscus. The results are consistent with the hypothesis that the unusual auditory nuclei of the bat have homologues in mammals whose auditory systems are not specialized for echolocation.

Tonotopic organization of the anteroventral cochlear nucleus of the cat

A quantitatively accurate map of the tonotopic organization of the anteroventral cochlear nucleus (AVCN) was derived from single unit recordings. Histologically localized single unit recordings from many animals were mapped onto a computerized atlas of the cochlear nucleus, and surfaces of constant characteristic frequency (CF) estimated with the aid of computer graphics. In anterior AVCN the surfaces of constant CF were found to be parallel planes, whereas in posterior AVCN they progressively deviated from this simple description. A further complication was noted in the most posterior portion of the AVCN where units with very different CF was found in close proximity. Comparison of the tonotopic map with descriptions of cellular organization shows conclusively that different CF ranges are dominant in the various cytoarchitectonic regions of the AVCN.

Glutamate and aspartate: alteration of thresholds and response patterns of auditory neurons

Iontophoretic application of the excitant amino acids glutamate and aspartate onto neurons in the chinchilla cochlear nucleus results in a lowering of the threshold of response to auditory stimuli. Neurons that display 'on'-type phasic responses to toneburst stimuli may become tonic, sustained responders with iontophoretic application of glutamate or aspartate. The ability of either glutamate or aspartate to effect changes in thresholds and response patterns of cochlear nucleus neurons is further evidence that one of these amino acids may be the afferent transmitter of the auditory nerve. The effects seen with these excitant amino acids may also provide insight into the underlying synaptic events involved in the generation of a particular response pattern.

Frequency-following responses in primary auditory and reticular formation structures

The responses of the cat brain to tonal stimuli were recorded from the inferior colliculus, medial geniculate, reticular formation and the far field. The response consisted of an onset component and a frequency-following response (FFR) component in the inferior colliculus (IC) and the far field. In contrast to previous work, the FFR was also observed in the reticular formation. The response in the reticular formation was abolished at lower doses of pentobarbital and at lower relative intensities of masking than that in the IC and far field. The amplitude of the FFR increased and the latency decreased with progressive ventral movement of the electrode through the IC. The onset component of the response in IC was more easily masked than the FFR component, while the FFR component was depressed to a somewhat greater extent by pentobarbital administration. These findings suggest that the different components of the response to tonal stimuli are generated by different mechanisms.

A model for the variability of interspike intervals during sustained firing of a retinal neuron

The statistics of the variability of interspike intervals of ganglion cells in the retina of goldfish are modeled by assuming the noise in an integrate-and-fire mechanism is proportional to the reciprocal of a normally distributed variable. This model meets the constraint that the coefficient of variation of the interspike. This does not change when the mean firing rate of the neuron changes. Alternative sources of variability of interspike intervals are discussed.

Single unit activity in the posteroventral cochlear nucleus of the cat

Single unit activity in the posteroventral cochlear nucleus (PVCN) was recorded for a variety of stimulus conditions. The units were classified according to their response characteristics. The locations of units were plotted onto a three-dimensional block model of the cochlear nucleus. Certain types of units that responded best to the onsets of stimuli were located predominantly in the octopus cell region of the PVCN. The remainder of the PVCN, which contains a rather heterogeneous collection of small and multipolar cells, was found to contain several types of units with the dominant type being "chopper" units.

A block model of the cat cochlear nucleus

A three-dimensional block model of the cochlear nucleus of the cat was constructed from histologic sections. Boundaries of various subdivisions, based on cytoarchitectonic criteria, were included in the model. Usage of the block model in correlating physiological and anatomical data is illustrated by localizing characteristic waveforms of gross evoked responses and characteristic frequencies of single units.

Single unit activity in the dorsal cochlear nucleus of the cat

Single unit activity was examined in three component layers of the dorsal cochlear nucleus (DCN): the molecular layer, the fusiform cell layer, and the polymorphic layer (deep DCN). Electrophysiological units were classified into types on the basis of their activity under a variety of stimulus conditions. In the molecular layer spike activity was small and difficult to isolate. Almost all units in the fusiform cell layer could be classified as either "pauser" or "buildup" units. Classification of units in the deep DCN was sometimes difficult, but "pauser," "chopper," and some "on" units were found. The "on" types of units tended to be located in the more superficial part of the deep DCN. Unit locations were referred to a three-dimensional block model of the cochlear nucleus.