Auditory nerve representation of a complex communication sound in background noise.

A population study of auditory nerve responses in the bullfrog, Rana catesbeiana, analyzed the relative contributions of spectral and temporal coding in representing a complex, species-specific communication signal at different stimulus intensities and in the presence of background noise. At stimulus levels of 70 and 80 dB SPL, levels which approximate that received during communication in the natural environment, average rate profiles plotted over fiber characteristic frequency do not reflect the detailed spectral fine structure of the synthetic call. Rate profiles do not change significantly in the presence of background noise. In ambient (no noise) and low noise conditions, both amphibian papilla and basilar papilla fibers phase lock strongly to the waveform periodicity (fundamental frequency) of the synthetic advertisement call. The higher harmonic spectral fine structure of the synthetic call is not accurately reflected in the timing of fiber firing, because firing is "captured" by the fundamental frequency. Only a small number of fibers synchronize preferentially to any harmonic in the call other than the first, and none synchronize to any higher than the third, even when fiber characteristic frequency is close to one of these higher harmonics. Background noise affects fiber temporal responses in two ways: It can reduce synchronization to the fundamental frequency, until fiber responses are masked; or it can shift synchronization from the fundamental to the second or third harmonic of the call. This second effect results in a preservation of temporal coding at high noise levels. These data suggest that bullfrog eighth nerve fibers extract the waveform periodicity of multiple-harmonic stimuli primarily by a temporal code.     

Auditory nerve fiber responses to electric stimulation: modulated and unmodulated pulse trains

Many modern cochlear implants use sound processing strategies that stimulate the cochlea with modulated pulse trains. Rubinstein et al. [Hear. Res. 127, 108 (1999)] suggested that representation of the modulator in auditory nerve responses might be improved by the addition of a sustained, high-rate, desynchronizing pulse train (DPT). In addition, activity in response to the DPT may mimic the spontaneous activity (SA) in a healthy ear. The goals of this study were to compare responses of auditory nerve fibers in acutely deafened, anesthetized cats elicited by high-rate electric pulse trains delivered through an intracochlear electrode with SA, and to measure responses of these fibers to amplitude-modulated pulse trains superimposed upon a DPT. Responses to pulse trains showed variability from presentation to presentation, but differed from SA in the shape of the envelope of the interval histogram (IH) for pulse rates above 4.8 kpps (kilo pulses per second). These IHs had a prominent mode near 5 ms that was followed by a long tail. Responses to modulated biphasic pulse trains resembled responses to tones in intact ears for small (<10%) modulation depths, suggesting that acousticlike responses to sinusoidal stimuli might be obtained with a DPT. However, realistic responses were only observed over a narrow range of levels and modulation depths. Improved coding of complex stimulus waveforms may be achieved by signal processing strategies for cochlear implants that properly incorporate a DPT.     

Temporal resolution and temporal integration of short pulses at the auditory periphery of echolocating animals

To explain the temporal integration and temporal resolution abilities revealed in echolocating animals by behavioral and electrophysiological experiments, the peripheral coding of sounds in the high-frequency auditory system of these animals is modeled. The stimuli are paired pulses similar to the echolocating signals of the animals. Their duration is comparable with or smaller than the time constants of the following processes: formation of the firing rate of the basilar membrane, formation of the receptor potentials of internal hair cells, and recovery of the excitability of spiral ganglion neurons. The models of auditory nerve fibers differ in spontaneous firing rate, response thresholds, and abilities to reproduce small variations of the stimulus level. The formation of the response to the second pulse of a pair of pulses in the multitude of synchronously excited high-frequency auditory nerve fibers may occur in only two ways. The first way defined as the stochastic mechanism implies the formation of the response to the second pulse as a result of the responses of the fibers that did not respond to the first pulse. This mechanism is based on the stochastic nature of the responses of auditory nerve fibers associated with the spontaneous firing rate. The second way, defined as the repeatition mechanism, implies the appearance of repeated responses in fibers that already responded to the first pulse but suffered a decrease in their response threshold after the first spike generation. This mechanism is based on the deterministic nature of the responses of fibers associated with refractoriness. The temporal resolution of pairs of short pulses, which, according to the data of behavioral experiments, is about 0.1–0.2 ms, is explained by the formation of the response to the second pulse through the stochastic mechanism. A complete recovery of the response to the second pulse, which, according to the data of electrophysiological studies of short-latency evoked brainstem potentials in dolphins, occurs within 5 ms, is explained by the formation of the response to the second pulse through the repetition mechanism. The time constant of temporal integration, which, according to the behavioral experiments at threshold levels of pulses, is about 0.2–0.3 ms, is explained by the integrating properties of internal hair cells, etc. It is shown that, at the high-frequency auditory periphery, the temporal integration imposes no limitations on the temporal resolution, because both integration and resolution are different characteristics of the same multiple response of synchronously excited fibers.     

The use of neural networks for the determination of the signal modulation frequency from the firing activity pattern in the auditory neurons of the frog

A two-layer backward propagation neural network was used for the determination of the modulation frequency of tonal signals from the firing patterns of single neurons located in the cochlear nuclei and the torus semicircularis of the grass frog (Rana t. temporaria). As an input of the neural network, a sum of several single responses of a neuron to an amplitude-modulated stimulus was used. The number of inputs corresponded to the number of the time readouts of the summarized response (usually 60), and the number of output elements corresponded to the number of modulation frequencies to be distinguished (from 3 to 15). In the case of a good synchronization of the input firing activity with the signal envelope, the classification was successful even if the training and classification were performed with the use of individual responses. An increase in the number of summed responses to 10–20 lead to a simplification of the training procedure. The results of the study were discussed in the context of the problem of the formation of periodicity detectors at the upper levels of the auditory pathway in vertebrates.     

A central spectrum model for the perception of coloration in filtered Gaussian noise

In this paper we describe a monaural auditory signal-processing model for the perception of coloration. The model gives a central spectrum display of a stationary input signal. The central spectrum level for a nerve fiber tuned to a given frequency is computed as a combination of the average firing rate and the firing synchronized to the center frequency of the nerve. The model incorporates a critical-band filter bank, steady-state representations of the average and synchronized firing rates, and temporal integration. The central spectrum model, when used to process simulated data, accurately predicts the perception of coloration in filtered Gaussian noise.     

Rate responses of auditory nerve fibers to tones in noise near masked threshold

The rate responses of auditory nerve fibers were measured for best frequency (BF) tone bursts in the presence of continuous background noise. Rate functions for BF tones were constructed over a 32-dB range of levels, centered on the behavioral masked thresholds of cats. The tone level at which noticeable rate changes are evoked by the tones corresponds closely to behavioral masked threshold at all noise levels used (-10- to 30-dB spectrum level). As the noise level increases, the response rate to the background noise approaches saturation, and the incremental rate response to tones decreases. At high noise levels, the rate responses to tones of low and medium spontaneous rate fibers are larger than those of high spontaneous rate fibers. Empirical statistics of auditory nerve fiber spike counts are reported; these differ from those expected of a Poisson process in that the variance is smaller than the mean. A new measure of discharge rate is described that allows rate changes to be expressed in units of a standard deviation. This measure allows tone-evoked responses to be interpreted in terms of their detectability in a signal detection task. Rate responses of low and medium spontaneous rate fibers are more detectable than those of high spontaneous rate fibers, especially at high noise levels. There appears to be sufficient information in the rate response of a small number of auditory nerve fibers to support behaviorally observed levels of detection performance.     

Neuronal responses to amplitude-modulated and pure-tone stimuli in the guinea pig inferior colliculus, and their modification by broadband noise

Neuronal responses were recorded to pure and to sinusoidally amplitude-modulated (AM) tones at the characteristic frequency (CF) in the central nucleus of the inferior colliculus of anesthetized guinea pigs. Temporal (synchronized) and mean-rate measures were derived from period histograms locked to the stimulus modulation waveform to characterize the modulation response. For stimuli presented in quiet, the modulation gain at low frequencies of modulation (approx less than 50 Hz) was inversely proportional to the neuron's mean firing rate in response to both the modulated stimulus and to a pure tone at an equivalent level. In 43% of units the mean discharge rates in response to the AM stimuli were greatest for those modulation frequencies that generated the largest temporal responses. These discharge-rate maxima occurred at signal intensities corresponding to the steeply sloping part of the neuron's pure-tone rate-intensity function (RIF). The change in mean-rate response to modulated stimuli, as a function of intensity, was qualitatively similar to the pure-tone RIF. Adding broadband noise to the modulated stimulus increased the neuron's temporal response to low modulation frequencies. This increase in modulation gain was correlated with mean firing rate in response to the modulation but did not bear a simple relationship to the noise-induced shift in the RIF measured for a pure tone.     

Sound intensity processing by the goldfish

Capacities of the goldfish for intensity discrimination were studied using classical respiratory conditioning and a staircase psychophysical procedure. Physiological studies on single saccular (auditory) nerve fibers under similar stimulus conditions helped characterize the dimensions of neural activity used in intensity discrimination. Incremental intensity difference limens (IDLs in dB) for 160-ms increments in continuous noise, 500-ms noise bursts, and 500-ms, 800-Hz tone bursts are 2 to 3 dB, are independent of overall level, and vary with signal duration according to a power function with a slope averaging - 0.33. Noise decrements are relatively poorly detected and the silent gap detection threshold is about 35 ms. The IDLs for increments and decrements in an 800-Hz continuous tone are about 0.13 dB, are independent of duration, and are level dependent. Unlike mammalian auditory nerve fibers, some goldfish saccular fibers show variation in recovery time to tonal increments and decrements, and adaptation to a zero rate. Unit responses to tone increments and decrements show rate effects generally in accord with previous observations on intracellular epsp's in goldfish saccular fibers. Neurophysiological correlates of psychophysical intensity discrimination data suggest the following: (1) noise gap detection may be based on spike rate increments which follow gap offset; (2) detection of increments and decrements in continuous tones may be determined by steep low-pass filtering in peripheral neural channels which enhance the effects of spectral "splatter" toward the lower frequencies; (3) IDLs for pulsed signals of different duration can be predicted from the slopes of rate-intensity functions and spike rate variability in individual auditory nerve fibers; and (4) at different sound pressure levels, different populations of peripheral fibers provide the information used in intensity discrimination.     

Responses to amplitude-modulated tones in the auditory nerve of the cat.

Sinusoidally amplitude-modulated (AM) tones are frequently used in psychophysical and physiological studies, yet a comprehensive study on the coding of AM tones in the auditory nerve is lacking. AM responses of single auditory-nerve fibers of the cat are studied, systematically varying modulation depth, frequency, and sound level. Synchrony-level functions were nonmonotonic with maximum values that were inversely correlated with spontaneous rate (SR). In most fibers, envelope phase-locking showed a positive gain. Modulation transfer functions were uniformly low pass. Their corner frequency increased with characteristic frequency (CF), but changed little for CFs above 10 kHz. The highest modulation frequencies to which phase locking occurred were more than 0.8 oct lower than the highest frequencies to which phase locking to pure tones occurs. Cumulative, or unwrapped, phase increased linearly with modulation frequency: The slope was inversely related to CF, and slightly higher than group delays reported for pure tones. High SR, low CF fibers showed the poorest envelope phase locking. In some low CF fibers, phase locking increased at high levels, associated with "peak-splitting" phenomena. Changes in average rate due to modulation were small, and could be enhancement or suppression.     

Across-ear interference from parametrically degraded synthetic speech signals in a dichotic cocktail-party listening task

Recent results have shown that listeners attending to the quieter of two speech signals in one ear (the target ear) are highly susceptible to interference from normal or time-reversed speech signals presented in the unattended ear. However, speech-shaped noise signals have little impact on the segregation of speech in the opposite ear. This suggests that there is a fundamental difference between the across-ear interference effects of speech and nonspeech signals. In this experiment, the intelligibility and contralateral-ear masking characteristics of three synthetic speech signals with parametrically adjustable speech-like properties were examined: (1) a modulated noise-band (MNB) speech signal composed of fixed-frequency bands of envelope-modulated noise; (2) a modulated sine-band (MSB) speech signal composed of fixed-frequency amplitude-modulated sinewaves; and (3) a "sinewave speech" signal composed of sine waves tracking the first four formants of speech. In all three cases, a systematic decrease in performance in the two-talker target-ear listening task was found as the number of bands in the contralateral speech-like masker increased. These results suggest that speech-like fluctuations in the spectral envelope of a signal play an important role in determining the amount of across-ear interference that a signal will produce in a dichotic cocktail-party listening task.     

Temporal resolution in frog auditory-nerve fibers

Sinusoidally amplitude-modulated (SAM) noise was monaurally presented to the neotropical frog, Eleutherodactylus coqui, while recording intracellularly from auditory-nerve fibers. Neuronal phase locking was measured to the SAM noise envelope in the form of a period histogram. The modulation depth was changed (in 10% steps) until the threshold modulation depth was determined. This was repeated for various modulation frequencies (20-1200 Hz) and different levels of SAM noise (34-64 dB/Hz). From these data, temporal modulation transfer functions (TMTFs) were produced and minimum integration time (MIT) for each auditory fiber was calculated. The median MIT was 0.42 ms (lower quartile 0.32, upper quartile 0.68 ms). A noise level-dependent effect was noted on the shape of the TMTF as well as the minimum integration time. The latter results may be explained as a loss in spectral resolution with increasing noise level, which is consistent with the correlation that was found between minimum integration time and bandwidth.     

Using electrical noise to enhance the ability of humans to detect subthreshold mechanical cutaneous stimuli

Stochastic resonance (SR) is a phenomenon wherein the response of a nonlinear system to a weak input signal is optimized by the presence of a particular, nonzero level of noise. Our objective was to demonstrate cross-modality SR in human sensory perception. Specifically, we were interested in testing the hypothesis that the ability of an individual to detect a subthreshold mechanical cutaneous stimulus can be significantly enhanced by introducing a particular level of electrical noise. Psychophysical experiments were performed on 11 healthy subjects. The protocol consisted of the presentation of: (a) a subthreshold mechanical stimulus plus electrical noise, or (b) no mechanical stimulus plus electrical noise. The intensity of the electrical noise was varied between trials. Each subject's ability to identify correctly the presence of the mechanical stimulus was determined as a function of the noise intensity. In 9 of the 11 subjects, the introduction of a particular level of electrical noise significantly enhanced the subject's ability to detect the subthreshold mechanical cutaneous stimulus. In 2 of the 11 subjects, the introduction of electrical noise did not significantly change the subject's ability to detect the mechanical stimulus. These findings indicate that input electrical noise can serve as a negative masker for subthreshold mechanical tactile stimuli, i.e., electrical noise can increase the detectability of weak mechanical signals. Thus, for SR-type effects to be observed in human sensory perception, the noise and stimulus need not be of the same modality. From a bioengineering and clinical standpoint, this work suggests that an electrical noise-based technique could be used to improve tactile sensation in humans when the mechanical stimulus is around or below threshold. (c) 1998 American Institute of Physics.     

Mechanism for propagation of rate signals through a 10-layer feedforward neuronal network

Using numerical simulations, we explore the mechanism for propagation of rate signals through a 10-layer feedforward network composed of Hodgkin--Huxley (HH) neurons with sparse connectivity. When white noise is afferent to the input layer, neuronal firing becomes progressively more synchronous in successive layers and synchrony is well developed in deeper layers owing to the feedforward connections between neighboring layers. The synchrony ensures the successful propagation of rate signals through the network when the synaptic conductance is weak. As the synaptic time constant τ_syn varies, coherence resonance is observed in the network activity due to the intrinsic property of HH neurons. This makes the output firing rate single-peaked as a function of τ_syn, suggesting that the signal propagation can be modulated by the synaptic time constant. These results are consistent with experimental results and advance our understanding of how information is processed in feedforward networks.     

Response of Hodgkin–Huxley stochastic bursting neuron to single-pulse stimulus

We investigate Hodgkin–Huxley neuron model with external Gaussian noise in the range of parameters where it exhibits bistability of silent and firing states, and noise-induced bursts occur. We study the response of the system to brief single pulse of current. When noise amplitude increases, the delay time between the stimulus and the first spike decreases substantially even for subthreshold stimulus. The mean number of spikes in a post-stimulus burst has a maximum in a certain range of noise amplitudes. Therefore, we found that Hodgkin–Huxley neuron in the stochastic bursting regime has more improved sensitivity to single-pulse stimulus than in the silent one.     

Diversity of adaptation patterns in responses of eighth nerve fibers in the bullfrog, Rana catesbeiana

The firing patterns of eighth nerve fibers in the bullfrog, Rana catesbeiana, were analyzed for responses to long duration tone bursts at best excitatory frequency ( BEF ) and at frequencies along the upper and lower boundaries of the excitatory tuning curve of each fiber. These firing patterns were used as an index of the degree of short-term adaptation of each fiber. Amphibian papilla fibers (with BEFs 100-1000 Hz) exhibited marked diversity in their firing patterns to BEF tones, ranging from very flat or tonic (sustained responses throughout the duration of the stimulus) to very peaked or phasic (responding primarily or exclusively to stimulus onset). Moreover, the degree of short-term adaptation shown by an individual fiber varied with stimulating frequency. The firing patterns of amphibian papilla fibers tended to become more tonic as stimulus frequency was lowered below BEF ; conversely, as stimulus frequency was increased above BEF , firing patterns either showed little change from that at BEF , or became more phasic. A similar frequency dependence of adaptation has not been reported in responses of mammalian eighth nerve fibers with comparable BEFs . The firing patterns of basilar papilla fibers ( BEFs greater than 1000 Hz) remained similar in response to both BEF and non- BEF tones. These data reveal that the firing patterns and degrees of short-term adaptation of amphibian papilla fibers vary considerably across the tuning curve, whereas those of basilar papilla fibers remain relatively more constant with changes in stimulating frequency.(ABSTRACT TRUNCATED AT 250 WORDS)     

Correlation of neural responses in the cochlear nucleus with low-frequency noise amplitude modulation of a tonal signal

The responses of single neurons of the cochlear nucleus of a grass frog to long tonal signals amplitude-modulated by repeat intervals of low-frequency noise have been studied. The carrier frequency always corresponded to the characteristic frequency of the studied cell (a range of 0.2 kHz–2 kHz); the modulated signal was noise in the ranges 0–15 Hz, 0–50 Hz, or 0–150 Hz. We obtained the correlation functions of the cyclic histogram reflecting the change in probability of a neuron pulse discharge (spike) during the modulation period with the shape of the signal envelope in the same period. The form of the obtained correlation functions usually does not change qualitatively with a change in carrier level or modulation depth; however, this could essentially depend of the frequency component of the modulating function. In the majority of cases, comparison of the cyclic histogram of the reaction with only the current amplitude value does not adequately reveal the signal’s time features that determine the reaction of a neuron. The response is also determined by the other sound features, primarily by the rate of the change in amplitude. The studied neurons differed among themselves, both in preference toward a certain range of modulated frequencies and in the features of the envelope that caused the cell’s response.     

Auditory analysis of the periodicity of sound and its envelope: A mathematical model

A mathematical model of the auditory analysis of periodicity of sound and its envelope is proposed. The model consists of a sequence of mathematical transformations that describe the signal processing stages. The following parameters and properties of the auditory system are taken into account: the crude analysis of the input acoustic signal accurate to the width of the aural critical band; the frequency dependence of the width of the aural critical band; the spectrum of the input signal analysis by a set of 3500 filters closely spaced in frequency; the absolute audibility thresholds at a given frequency; the time-domain analysis of both the output signals of each filter and the envelope profile with the help of the periodicity function; the pulsed activity of auditory neurons; the ability of the auditory system to memorize the spectral-time images of the signal and its individual parameters; the ability of the auditory system to form the perception of the loudness of sound, to memorize and compare the loudness of sound at different time moments, and to conclude which of them is higher or lower or whether they are equal accurate to a certain threshold; the dependence of the critical modulation bandwidth on the modulation frequency; and the dependence of the audibility thresholds of amplitude modulation on the modulation frequency. By an example of processing amplitude-modulated signals with various carrier-to-envelope frequency ratios, the model is shown to give a satisfactory explanation of their pitch and auditory estimate of the envelope period.     

Signal detection in temporally modulated and spectrally shaped maskers.

The first part of this paper presents several experiments on signal detection in temporally modulated noise, yielding a general approach toward the concept of comodulation masking release (CMR). Measurements were made on masked thresholds of both long- and short-duration, narrow-band signals presented in a 100% sinusoidally amplitude-modulated (SAM) noise masker (modulation frequency 32 Hz), as a function of masker bandwidth from 1/3 oct up to 13/3 octs, while the masker band was geometrically centered at signal frequency. With the short-duration signals placed in the valley of the masker, a substantial CMR (i.e., a decrease of masked threshold with increasing masker bandwidth) was found, whereas for the long-duration signals CMR was smaller. Furthermore, investigations were carried out to determine whether CMR changes when the bandwidth of the signals, consisting of bandpass impulse responses, is increased. The data indicate that substantial CMR remains even when all masker bands contain a signal component, thus minimizing across-channel differences. This finding is not in line with current models accounting for the CMR phenomenon. The second part of this paper concerns signal detection in spectrally shaped noise. Also investigated was whether release from masking occurs for the detection of a pure-tone signal at a valley or a peak of a simultaneously presented masking noise with a sinusoidally rippled power spectrum, when this masker was preceded and followed by a second noise (temporal flanking burst) with an identical spectral shape as the on-signal noise. Similar to CMR effects for temporal modulations, the data indicate that coshaping masking release (CSMR) occurs when the signal is placed in a valley of the spectral envelope of the masker, whereas no release from masking is found when the signal is placed at a peak of the spectral envelope of the masker. The implications of these experiments for measures of spectral and temporal resolution are discussed.     

Response dynamics of goldfish saccular fibers: effects of stimulus frequency and intensity on fibers with different tuning, sensitivity, and spontaneous activity

The effects of stimulus frequency and intensity on response patterns (PST histograms) to tone burst stimulation were examined in differently tuned saccular fibers of the goldfish. In addition, the sensitivity of these fibers to amplitude-modulated (AM) signals of different carrier frequencies was measured. The response patterns evoked by unmodulated signals were a complex function of tuning, spontaneous activity and sensitivity of the fiber, and the frequency and intensity of the signal. Frequency-dependent response patterns were found in low-frequency fibers with best frequencies (BF) below 200 Hz. Responses in these fibers ranged from tonic to phasic in nonspontaneous fibers and included more complex patterns in spontaneously active fibers, such as suppression of evoked activity below spontaneous levels. Midfrequency fibers (BF = 500-600 Hz) showed responses similar to those in low-frequency fibers, but with less dependence on frequency. In contrast, both high-frequency (BF = 800-1000 Hz) and wideband, untuned fibers showed frequency-invariant patterns of adaptation. High-frequency fibers were equally sensitive to AM signals at all frequencies tested. The sensitivity of low-frequency fibers to AM, however, increased as a function of carrier frequency and corresponded to the degree of adaptation in response to unmodulated tones. In general, the AM sensitivity of a fiber could be predicted more by its pattern of response to unmodulated signals than by its tuning characteristics.