Phase Locking of Auditory Nerve Fibers: The Role of Lowpass Filtering by Hair Cells

KV1.8 (Kcna10) potassium channels enhance fast, linear signaling in vestibular hair cells and facilitate vestibulomotor reflexes and balance

Vestibular hair cells (HCs) faithfully and rapidly detect head motions and gravity, driving motor reflexes that stabilize balance and gaze during locomotion. With the transition from water to land, the amniote vestibular inner ear added type I HCs, which differ from amniote type II HCs and anamniote HCs by their large calyx afferent synapse, non-quantal afferent transmission, and a large, low-voltage-activated K+ conductance (gK,L). We recently showed that both gK,L and the major type II K+ conductances (A-type and delayed rectifier) require KV1.8 (Kcna10) subunits. Here we compared KV1.8-null (Kcna10 -/-) and control animals to see how KV1.8 affects function as measured by receptor potentials and nonquantal postsynaptic potentials evoked by direct hair bundle motions, and by vestibulomotor behaviors. Recordings were taken from extrastriolar zones of the utricle. In both HC types, KV1.8 affected receptor potentials by reducing response time and gain, increasing dampening, and expanding the frequency bandwidth toward high frequencies. Effects are most prominent in type I HCs: lowpass corner frequencies of receptor potentials in Kcna10 -/- HCs of both types were ~20 Hz, vs. ~400 Hz in control type I and ~70 Hz in control type II. We recorded nonquantal postsynaptic potentials from extrastriolar calyces, and found that the synaptic transfer function had lower gain and greater phase lag in Kcna10 -/- mice. In behavioral tests, Kcna10 -/- mice had vestibular-ocular reflexes with different response dynamics at low frequencies, impaired performance on a narrow balance beam, abnormal body posture and abnormal head motions in water and on land, and also rarely assumed bipedal stances. These vestibulomotor deficits in Kcna10 -/- mice likely reflect the changes noted in HCs, where KV1.8 expression is concentrated; that is, slower signaling of high-frequency head motions by Kcna10 -/- HCs fails to fully stabilize body and head position during locomotion. Thus, gK,L (KV1.8) contributes to fast signal transmission in the amniote vestibular inner ear and supports improved performance on challenging vestibulomotor tasks.

Vestibular Testing-New Physiological Results for the Optimization of Clinical VEMP Stimuli

Both auditory and vestibular primary afferent neurons can be activated by sound and vibration. This review relates the differences between them to the different receptor/synaptic mechanisms of the two systems, as shown by indicators of peripheral function-cochlear and vestibular compound action potentials (cCAPs and vCAPs)-to click stimulation as recorded in animal studies. Sound- and vibration-sensitive type 1 receptors at the striola of the utricular macula are enveloped by the unique calyx afferent ending, which has three modes of synaptic transmission. Glutamate is the transmitter for both cochlear and vestibular primary afferents; however, blocking glutamate transmission has very little effect on vCAPs but greatly reduces cCAPs. We suggest that the ultrafast non-quantal synaptic mechanism called resistive coupling is the cause of the short latency vestibular afferent responses and related results-failure of transmitter blockade, masking, and temporal precision. This "ultrafast" non-quantal transmission is effectively electrical coupling that is dependent on the membrane potentials of the calyx and the type 1 receptor. The major clinical implication is that decreasing stimulus rise time increases vCAP response, corresponding to the increased VEMP response in human subjects. Short rise times are optimal in human clinical VEMP testing, whereas long rise times are mandatory for audiometric threshold testing.

A Fundamental Inequality Governing the Rate Coding Response of Sensory Neurons

A fundamental inequality governing the spike activity of peripheral neurons is derived and tested against auditory data. This inequality states that the steady-state firing rate must lie between the arithmetic and geometric means of the spontaneous and peak activities during adaptation. Implications towards the development of auditory mechanistic models are explored.

Reverse effect of home-use binaural beats brain stimulation

Binaural beats brain stimulation is a popular strategy for supporting home-use cognitive tasks. However, such home-use brain stimulation may be neutral to cognitive processes, and any intellectual improvement may be only a placebo effect. Thus, without belief in it, it may bring no benefits. Here we test 1000 individuals at their homes as they perform a two-part fluid intelligence test. Some took the second part listening to binaural beats, while others took it in silence or listening to other sounds. The binaural beats group was divided into three subgroups. The first one was informed that they would listen to sounds that improve the brain's work, the second that neutral sounds, and the third that some sounds the nature of which was not defined. We found that listening to binaural beats was not neutral, as it dramatically deteriorated the score irrespective of the condition. Silence or other sounds had no effect. Thus, home-use binaural beats brain stimulation brings reverse effects to those assumed: instead of supporting the effectiveness of cognitive activities, it may weaken them.

Characterization of the decline in auditory nerve phase locking at high frequencies

The frequency dependence of phase locking in the auditory nerve influences various auditory coding mechanisms. The decline of phase locking with increasing frequency is commonly described by a low-pass filter. This study compares fitted low-pass filter parameters with the actual rate of phase locking decline. The decline is similar across studies and only 40 dB per decade, corresponding to the asymptotic decline of a second order filter.

The Long Outer-Hair-Cell RC Time Constant: A Feature, Not a Bug, of the Mammalian Cochlea

The cochlea of the mammalian inner ear includes an active, hydromechanical amplifier thought to arise via the piezoelectric action of the outer hair cells (OHCs). A classic problem of cochlear biophysics is that the RC (resistance-capacitance) time constant of the hair-cell membrane appears inconveniently long, producing an effective cut-off frequency much lower than that of most audible sounds. The long RC time constant implies that the OHC receptor potential-and hence its electromotile response-decreases by roughly two orders of magnitude over the frequency range of mammalian hearing, casting doubt on the hypothesized role of cycle-by-cycle OHC-based amplification in mammalian hearing. Here, we review published data and basic physics to show that the "RC problem" has been magnified by viewing it through the wrong lens. Our analysis finds no appreciable mismatch between the expected magnitude of high-frequency electromotility and the sound-evoked displacements of the organ of Corti. Rather than precluding significant OHC-based boosts to auditory sensitivity, the long RC time constant appears beneficial for hearing, reducing the effects of internal noise and distortion while increasing the fidelity of cochlear amplification.

Intrinsic mechanical sensitivity of mammalian auditory neurons as a contributor to sound-driven neural activity

Mechanosensation – by which mechanical stimuli are converted into a neuronal signal – is the basis for the sensory systems of hearing, balance, and touch. Mechanosensation is unmatched in speed and its diverse range of sensitivities, reaching its highest temporal limits with the sense of hearing; however, hair cells (HCs) and the auditory nerve (AN) serve as obligatory bottlenecks for sounds to engage the brain. Like other sensory neurons, auditory neurons use the canonical pathway for neurotransmission and millisecond-duration action potentials (APs). How the auditory system utilizes the relatively slow transmission mechanisms to achieve ultrafast speed, and high audio-frequency hearing remains an enigma. Here, we address this paradox and report that the mouse, and chinchilla, AN are mechanically sensitive, and minute mechanical displacement profoundly affects its response properties. Sound-mimicking sinusoidal mechanical and electrical current stimuli affect phase-locked responses. In a phase-dependent manner, the two stimuli can also evoke suppressive responses. We propose that mechanical sensitivity interacts with synaptic responses to shape responses in the AN, including frequency tuning and temporal phase locking. Combining neurotransmission and mechanical sensation to control spike patterns gives the mammalian AN a secondary receptor role, an emerging theme in primary neuronal functions.

Theoretical Relationship Between Two Measures of Spike Synchrony: Correlation Index and Vector Strength

Information processing in the nervous system critically relies on temporally precise spiking activity. In the auditory system, various degrees of phase-locking can be observed from the auditory nerve to cortical neurons. The classical metric for quantifying phase-locking is the vector strength (VS), which captures the periodicity in neuronal spiking. More recently, another metric, called the correlation index (CI), was proposed to quantify the temporally reproducible response characteristics of a neuron. The CI is defined as the peak value of a normalized shuffled autocorrelogram (SAC). Both VS and CI have been used to investigate how temporal information is processed and propagated along the auditory pathways. While previous analyses of physiological data in cats suggested covariation of these two metrics, general characterization of their connection has never been performed. In the present study, we derive a rigorous relationship between VS and CI. To model phase-locking, we assume Poissonian spike trains with a temporally changing intensity function following a von Mises distribution. We demonstrate that VS and CI are mutually related via the so-called concentration parameter that determines the degree of phase-locking. We confirm that these theoretical results are largely consistent with physiological data recorded in the auditory brainstem of various animals. In addition, we generate artificial phase-locked spike sequences, for which recording and analysis parameters can be systematically manipulated. Our analysis results suggest that mismatches between empirical data and the theoretical prediction can often be explained with deviations from the von Mises distribution, including skewed or multimodal period histograms. Furthermore, temporal relations of spike trains across trials can contribute to higher CI values than predicted mathematically based on the VS. We find that, for most applications, a SAC bin width of 50 ms seems to be a favorable choice, leading to an estimated error below 2.5% for physiologically plausible conditions. Overall, our results provide general relations between the two measures of phase-locking and will aid future analyses of different physiological datasets that are characterized with these metrics.

Towards a unifying basis of auditory thresholds: Thresholds for multicomponent stimuli

Sounds consisting of multiple simultaneous or consecutive components can be detected by listeners when the stimulus levels of the components are lower than those needed to detect the individual components alone. The mechanisms underlying such spectral, spectrotemporal, temporal, or across-ear integration are not completely understood. Here, we report threshold measurements from human subjects for multicomponent stimuli (tone complexes, tone sequences, diotic or dichotic tones) and for their individual sinusoidal components in quiet. We examine whether the data are compatible with the detection model developed by Heil, Matysiak, and Neubauer (HMN model) to account for temporal integration (Heil et al. 2017), and we compare its performance to that of the statistical summation model (Green 1958), the model commonly used to account for spectral and spectrotemporal integration. In addition, we compare the performance of both models with respect to previously published thresholds for sequences of identical tones and for diotic tones. The HMN model is similar to the statistical summation model but is based on the assumption that the decision variable is a number of sensory events generated by the components via independent Poisson point processes. The rate of events is low without stimulation and increases with stimulation. The increase is proportional to the time-varying amplitude envelope of the bandpass-filtered component(s) raised to an exponent of 3. For an ideal observer, the decision variable is the sum of the events from all channels carrying information, for as long as they carry information. We find that the HMN model provides a better account of the thresholds for multicomponent stimuli than the statistical summation model, and it offers a unifying account of spectral, spectrotemporal, temporal, and across-ear integration at threshold.

Similarities and Differences Between Vestibular and Cochlear Systems - A Review of Clinical and Physiological Evidence

The evoked response to repeated brief stimuli, such as clicks or short tone bursts, is used for clinical evaluation of the function of both the auditory and vestibular systems. One auditory response is a neural potential - the Auditory Brainstem Response (ABR) - recorded by surface electrodes on the head. The clinical analogue for testing the otolithic response to abrupt sounds and vibration is the myogenic potential recorded from tensed muscles - the vestibular evoked myogenic potential (VEMP). VEMPs have provided clinicians with a long sought-after tool - a simple, clinically realistic indicator of the function of each of the 4 otolithic sensory regions. We review the basic neural evidence for VEMPs and discuss the similarities and differences between otolithic and cochlear receptors and afferents. VEMPs are probably initiated by sound or vibration selectively activating afferent neurons with irregular resting discharge originating from the unique type I receptors at a specialized region of the otolithic maculae (the striola). We review how changes in VEMP responses indicate the functional state of peripheral vestibular function and the likely transduction mechanisms allowing otolithic receptors and afferents to trigger such very short latency responses. In section "ELECTROPHYSIOLOGY" we show how cochlear and vestibular receptors and afferents have many similar electrophysiological characteristics [e.g., both generate microphonics, summating potentials, and compound action potentials (the vestibular evoked potential, VsEP)]. Recent electrophysiological evidence shows that the hydrodynamic changes in the labyrinth caused by increased fluid volume (endolymphatic hydrops), change the responses of utricular receptors and afferents in a way which mimics the changes in vestibular function attributed to endolymphatic hydrops in human patients. In section "MECHANICS OF OTOLITHS IN VEMPS TESTING" we show how the major VEMP results (latency and frequency response) follow from modeling the physical characteristics of the macula (dimensions, stiffness etc.). In particular, the structure and mechanical operation of the utricular macula explains the very fast response of the type I receptors and irregular afferents which is the very basis of VEMPs and these structural changes of the macula in Menière's Disease (MD) predict the upward shift of VEMP tuning in these patients.

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

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

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

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

A comprehensive computational model of animal biosonar signal processing

Computational models of animal biosonar seek to identify critical aspects of echo processing responsible for the superior, real-time performance of echolocating bats and dolphins in target tracking and clutter rejection. The Spectrogram Correlation and Transformation (SCAT) model replicates aspects of biosonar imaging in both species by processing wideband biosonar sounds and echoes with auditory mechanisms identified from experiments with bats. The model acquires broadband biosonar broadcasts and echoes, represents them as time-frequency spectrograms using parallel bandpass filters, translates the filtered signals into ten parallel amplitude threshold levels, and then operates on the resulting time-of-occurrence values at each frequency to estimate overall echo range delay. It uses the structure of the echo spectrum by depicting it as a series of local frequency nulls arranged regularly along the frequency axis of the spectrograms after dechirping them relative to the broadcast. Computations take place entirely on the timing of threshold-crossing events for each echo relative to threshold-events for the broadcast. Threshold-crossing times take into account amplitude-latency trading, a physiological feature absent from conventional digital signal processing. Amplitude-latency trading transposes the profile of amplitudes across frequencies into a profile of time-registrations across frequencies. Target shape is extracted from the spacing of the object's individual acoustic reflecting points, or glints, using the mutual interference pattern of peaks and nulls in the echo spectrum. These are merged with the overall range-delay estimate to produce a delay-based reconstruction of the object's distance as well as its glints. Clutter echoes indiscriminately activate multiple parts in the null-detecting system, which then produces the equivalent glint-delay spacings in images, thus blurring the overall echo-delay estimates by adding spurious glint delays to the image. Blurring acts as an anticorrelation process that rejects clutter intrusion into perceptions.