Conservation of adapting components in auditory-nerve responses

The responses of single auditory-nerve fibers of Mongolian gerbil were studied using tonal stimuli. The peristimulatory adaptation of firing rate in response to tone bursts presented in quiet and during a background stimulus is described quantitatively. The total transient response which can be produced to the onset of a tone burst, whether presented in quiet or as an intensity increment, is limited and appears to demonstrate a form of conservation. Specifically, the total numbers of spikes produced by the rapidly adapting component, and the slower short-term adaptation component, are proportional at all intensities, and are limited for each fiber. Furthermore, when an incremental stimulus is presented on a background, the total transient response to the background and to the increment is limited and depends upon the final intensity, not the background intensity. When the presumed underlying synaptic drive is derived by removing the effects of refractoriness from the spike train, the same conservation of the transient response components, and proportionality between rapid and short-term components, are observed.     

The detection of increments and decrements is not facilitated by abrupt onsets or offsets

Two experiments measured thresholds for the detection of increments and decrements in the intensity of a quasi-continuous broadband-noise (experiment 1) or increments in a 477-Hz pure-tone pedestal (experiment 2). A variety of onset and offset ramps for the intensity change were tested, from instantaneous onsets or offsets to ramps lasting several tens of milliseconds. For increments and decrements with equal duration, the characteristics of the ramps had little effect on performance. Abrupt rise times, which are associated with strong transient responses in auditory neurons, did not facilitate detection in comparison to much slower rise times. The temporal window model of temporal resolution provided a good account of the data when the decision statistic was the maximum magnitude of the change in the output of the window produced by the increment or decrement, but provided a poor account of the data when the decision statistic was the maximum rate of change in the output of the window over time. Overall the results suggest that, in the absence of cues in the audio-frequency domain, rapid changes in envelope contribute little to near-threshold increment or decrement detection.     

Adaptation, saturation, and physiological masking in single auditory-nerve fibers.

Results are reviewed concerning some effects, at a units's characteristic frequency, of a short-term conditioning stimulus on the responses to perstimulatory and poststimulatory test tones. A phenomenological equation is developed from the poststimulatory results and shown to be consistent with the perstimulatory results. According to the results and equation, the response to a test tone equals the unconditioned or unadapted response minus the decrement produced by adaptation to the conditioning tone. Furthermore, the decrement is proportional to the driven response to the conditioning tone and does not depend on sound intensity per se. The equation has a simple interpretation in terms of two processes in cascade--a static saturating nonlinearity followed by additive adaptation. Results are presented to show that this functional model is sufficient to account for the "physiological masking" produced by wide-band backgrounds. According to this interpretation, a sufficiently intense background produces saturation. Consequently, a superimposed test tone cause no change in response. In addition, when the onset of the background precedes the onset of the test tone, the total firing rate is reduced by adaptation. Evidence is reviewed concerning the possible correspondence between the variables in the model and intracellular events in the auditory periphery.     

Detection of intensity decrements by the chinchilla.

Three monaural chinchillas were trained to detect intensity decrements in broadband noise (20 kHz) using a shock-avoidance conditioning procedure. The intensity decrements were presented at one of nine different durations between 2 and 35 ms at noise levels of 25, 45, and 65 dB SPL. At each intensity-duration combination, the level of the decrement was varied to obtain a decrement threshold. The minimal detectable decrement decreased from approximately 20 dB at the shortest duration to an asymptote of roughly 4 dB at approximately 30 ms. The data were modeled by a low-pass filter with an 11-ms time constant. The decrement detection function of the chinchilla is similar to that of humans. However, long-duration decrement thresholds are larger in the chinchilla, as would be predicted from the large intensity difference limen of the chinchilla. In general, there was little change in the decrement function across background intensities except that 2-ms decrements were not detected at the 25-dB SPL background intensity.     

Time course of adaptation and recovery from adaptation in the cat auditory-nerve neurophonic

The auditory-nerve neurophonic (ANN) reflects the ensemble response of phase-locked firing in single auditory-nerve fibers to sustained signals. Consequently, neural response properties such as adaptation and recovery from adaptation can be observed. In this study, ANN responses to 800-Hz, 100-ms tones presented at 10-30-dB SL were recorded using bipolar platinum-iridium electrodes placed on the auditory nerve of the cat. The cat ANN adaptation properties were determined and fit to the equation: A(tp) = Yre(-tp/tau Ar) + Yse(-tp/tau As) + Ass. The rapid time constant of adaptation (tau Ar) was invariant across stimulus level, with a mean value of 4.8 (+/- 2.1) ms. The short-term time constant (tau As) decreased approximately 21 ms for each 10-dB increase in probe amplitude. The mean tau As was 116 ms at 10 dB SL, 83.2 ms at 20 dB SL, and 73.5 ms at 30 dB SL. The ANN recovery from adaptation data was analyzed and fit to the equation: A(delta t) = Amax - Yre(-delta t/tau Rr) - Yse(-delta t/tau Rs). Here, tau Rr, the rapid time constant of recovery, and tau Rs, the short-term time constant, were independent of masker intensity in the studied range, with values of 16.2(+/- 9.8) and 125(+/- 50.1) ms, respectively. The results of this study indicate that ANN time constants are comparable to those measured for single units and that the adaptation behavior of phase-locked and nonphase-locked activity appears to be similar.     

Recovering from long-term and short-term adaptation of the whole nerve action potential

Properties of adaptation and recovery from adaptation are measured in response to adapter tones of short duration (100 ms), intermediate duration (1 m), and long duration (2-15 m). The decrease in N1 potential in response to a probe tone burst was used to assess the magnitude of adaptation. By measuring the response at different times after adapter offset the time course of recovery was assessed. By measuring the response to different probe frequencies, the spread of adaptation across place in the cochlea was assessed. Adapters of different duration showed quite different spread of adaptation across probe frequency. Short-duration adapters resulted in decrements of probe response greatest at and above probe frequency. Intermediate- and long-duration adapters showed progressively greater shifts in peak decrement to higher probe frequencies as adapter level was increased.     

Forward masking of amplitude modulation: basic characteristics

In this study we demonstrate an effect for amplitude modulation (AM) that is analogous to forward making of audio frequencies, i.e., the modulation threshold for detection of AM (signal) is raised by preceding AM (masker). In the study we focused on the basic characteristics of the forward-masking effect. Functions representing recovery from AM forward masking measured with a 150- ms 40- Hz masker AM and a 50- ms signal AM of the same rate imposed on the same broadband-noise carrier, showed an exponential decay of forward masking with increasing delay from masker offset. Thresholds remained elevated by more than 2 dB over an interval of at least 150 ms following the masker. Masked-threshold patterns, measured with a fixed signal rate (20, 40, and 80 Hz) and a variable masker rate, showed tuning of the AM forward-masking effect. The tuning was approximately constant across signal modulation rates used and consistent with the idea of modulation-rate selective channels. Combining two equally effective forward maskers of different frequencies did not lead to an increase in forward masking relative to that produced by either component alone. Overall, the results are consistent with modulation-rate selective neural channels that adapt and recover from the adaptation relatively quickly.     

A high-precision magnetoencephalographic study of human auditory steady-state responses to amplitude-modulated tones

The cerebral magnetic field of the auditory steady-state response (SSR) to sinusoidal amplitude-modulated (SAM) tones was recorded in healthy humans. The waveforms of underlying cortical source activity were calculated at multiples of the modulation frequency using the method of source space projection, which improved the signal-to-noise ratio (SNR) by a factor of 2 to 4. Since the complex amplitudes of the cortical source activity were independent of the sensor position in relation to the subject's head, a comparison of the results across experimental sessions was possible. The effect of modulation frequency on the amplitude and phase of the SSR was investigated at 30 different values between 10 and 98 Hz. At modulation frequencies between 10 and 20 Hz the SNR of harmonics near 40 Hz were predominant over the fundamental SSR. Above 30 Hz the SSR showed an almost sinusoidal waveform with an amplitude maximum at 40 Hz. The amplitude decreased with increasing modulation frequency but was significantly different from the magnetoencephalographic (MEG) background activity up to 98 Hz. Phase response at the fundamental and first harmonic decreased monotonically with increasing modulation frequency. The group delay (apparent latency) showed peaks of 72 ms at 20 Hz, 48 ms at 40 Hz, and 26 ms at 80 Hz. The effects of stimulus intensity, modulation depth, and carrier frequency on amplitude and phase of the SSR were also investigated. The SSR amplitude decreased linearly when stimulus intensity or the modulation depth were decreased in logarithmic steps. SSR amplitude decreased by a factor of 3 when carrier frequency increased from 250 to 4000 Hz. From the phase characteristics, time delays were found in the range of 0 to 6 ms for stimulus intensity, modulation depth, and carrier frequency, which were maximal at low frequencies, low intensities, or maximal modulation depth.     

Multiple reservoir model of neurotransmitter release by a cochlear inner hair cell

A probabilistic model is described for transmitter release from hair cells, auditory neuron EPSP's, and discharge patterns. The present model assumes that several reservoirs of neurotransmitter exist, having individual probability-of-release functions centered at successively higher intensities. The model accurately mimics the adaptation of successive EPSP amplitudes of the afferent neuron of the goldfish sacculus and, for mammalian auditory-nerve fibers, the adaptation of neural discharge rate, the saturation of onset and steady-state neural rate versus intensity, and the change in neural rate in response to incremental stimuli. The model also produces realistic interval and period histograms. The data shown support the hypothesis that multiple populations of neurotransmitter are involved in the afferent hair-cell synapses.     

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.     

The effect of broadband noise on the human brain-stem auditory evoked response. IV. Additivity of forward-masking and rate-induced wave V latency shifts

The additivity of forward masking and repetitive stimulation effects on wave V of the brain-stem auditory evoked response (BAER) was investigated. The effects of repetitive stimulation were evaluated for a stimulus train (called the adaptation series), with a 12.5-ms within-train interclick interval. The forward masker was a 100-ms, 80-dB SPL broadband noise with forward-masker intervals ranging from 12.5-87.5 ms. Forward masking and repetitive stimulation increased the latency of wave V of the BAER. The combined forward masking/adaptation series produced less wave V latency shift than the summed individual effects. Forward masking reduced wave V amplitude at brief forward masker intervals, while repetitive stimulation did not affect wave V amplitude. Wave V amplitude was decreased for the combined forward masking/adaptation series, and the time course of amplitude recovery of the combination was prolonged compared to the forward masking alone condition. The nonadditivity of forward masking and rate effects on wave V latency is similar to that found for repetitive stimulation and simultaneous masking [Burkard and Hecox, J. Acoust. Soc. Am. 74, 1204-1213 (1983)]. These findings are consistent with the position that forward masking and rate effects on wave V latency are produced by overlapping mechanisms.     

Effect of masker level on overshoot

Overshoot refers to the phenomenon where signal detectability improves for a short-duration signal as the onset of that signal is delayed relative to the onset of a longer duration masker. A popular explanation for overshoot is that it reflects short-term adaptation in auditory-nerve fibers. In this study, overshoot was measured for a 10-ms, 4-kHz signal masked by a broadband noise. In the first experiment, masker duration was 400 ms and signal onset delay was 1 or 195 ms; masker spectrum level ranged from - 10-50 dB SPL. Overshoot was negligible at the lowest masker levels, grew to about 10-15 dB at the moderate masker levels, but declined and approached 0 dB at the highest masker levels. In the second experiment, the masker duration was reduced to 100 ms, and the signal was presented with a delay of 1 or 70 ms; masker spectrum level was 10, 30, or 50 dB SPL. Overshoot was about 10 dB for the two lower masker levels, but about 0 dB at the highest masker level. The results from the second experiment suggest that the decline in overshoot at high masker levels is probably not due to auditory fatigue. It is suggested, instead, that the decline may be attributable to the neural response at high levels being dominated by those auditory-nerve fibers that do not exhibit short-term adaptation (i.e., those with low spontaneous rates and high thresholds).     

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.     

基于磁光调制的多光谱目标识别系统的研究

基于特征光谱的目标识别技术具有检出能力强,可分辨目标种类等优点,但也存在一定的问题,即需要事先获取背景光谱作为先验知识且要求背景光谱随时间的变化较小。由此限制了其在新环境、复杂环境中实时目标识别方面的应用。设计了一种采用磁光调制配合特征光谱分析的技术手段,使目标识别过程中无需事先获取背景谱,从而实现了一次采集获取被测目标信息的功能,相比传统的目标检测方法而言,对战场的适应能力更强,具有较好的实用意义。同时,磁光调制技术有效地抑制了背景杂散光的干扰,从而提高了目标识别概率。由于磁光调制提供了目标光谱的累加迭代信息,故即使未知背景光谱或者背景光谱变化较大时,也可以通过目标光谱的迭代信息大幅提高目标识别率。针对不同被测目标的回波光强与背景光强值进行实验分析,结果显示,三种目标对调制线偏振光的反射能力明显强于背景。采用伪装色的被测目标对可见光成像目标识别影响很大,而调制偏振型系统仍能很好地识别目标。在此基础上,对0.5～2 km范围内的目标进行多特征波长目标种类识别。采用三个特征波长时,目标识别概率在2 km左右明显降低,采用四个或五个特征波长位置时,可以实现95.0%以上的目标识别概率,同时为了降低运算量提高系统的实时检测能力,最终采用四个特征波长。     

Detection of partially filled gaps in noise and the temporal modulation transfer function

Results of experiments on the detection of silent intervals, or gaps, in broadband noise are reported for normal-hearing listeners. In some preliminary experiments, a gap threshold of about 2 ms was measured. This value was independent of the duration of the noise burst, variation of the noise level on each presentation, or the temporal position of the gap within the noise burst. In the main experiments, the thresholds for partial decrements in the noise waveform as well as brief increments were determined. As predicted by a model that assumes a single fixed peak-to-valley detection ratio, thresholds for increments are slightly higher than thresholds for decrements when the signal is measured as the change in rms noise level. A first-order model describes the temporal properties of the auditory system as a low-pass filter with a 7- to 8-ms time constant. Temporal modulation transfer functions were determined for the same subjects, and the estimated temporal parameters agreed well with those estimated from the gap detection data. More detailed modeling was carried out by simulating Viemeister's three-stage temporal model. Simulations, using an initial stage bandwidth of 4000 Hz and a 3-ms time constant for the low-pass filter, generate data that are very similar to those obtained from human subjects in both modulation and gap detection.     

Absence of overshoot in a dichotic masking condition

Brief tonal signals presented soon after the onset of a masking noise are known to be less detectable than signals delayed by several hundred milliseconds. This difference in detectability is known as the "overshoot." Signals of two sorts were studied here--either interaurally in phase (S o) or interaurally out of phase by 180 degrees (S pi). When S omicron signals of 750 Hz and about 14 ms in duration were presented 4 ms after the onset of a diotic, broadband masking noise (N o), detectability was about 6 dB worse than when the signal was presented 325 ms after onset. By contrast, there was no such overshoot when S pi signals were presented at varying times after masker onset; detectability was about the same for all values of signal delay. Accordingly, the difference in performance between N o S o and N o S pi--the masking-level difference or MLD--was large (about 16 dB) with the shortest delays used and diminished (to about 9 dB) as the delay was increased. This absence of overshoot with the S pi signals is in accord with the well-established view that detectability in the dichotic masking conditions is based upon different stimulus information from that used in the diotic masking conditions. Specifically, the evidence confirms the common view that detectability in the diotic conditions is based more or less directly on neural firing rate, whereas, in the dichotic conditions, it is based upon interaural time differences encoded in the periodicity of neural firings.     

Simulation of auditory-neural transduction: further studies

A computational model of mechanical to neural transduction at the hair cell-auditory-nerve synapse is presented. It produces a stream of events (spikes) that are precisely located in time in response to an arbitrary stimulus and is intended for use as an input to automatic speech recognition systems as well as a contribution to the theory of the origin of auditory-nerve spike activity. The behavior of the model is compared to data from animal studies in the following tests: (a) rate-intensity functions for adapted and unadapted responding; (b) two-component short-term adaptation; (c) frequency-limited phase locking of events; (d) additivity of responding following stimulus-intensity increases and decreases; (e) recovery of spontaneous activity following stimulus offset; and (f) recovery of ability to respond to a second stimulus following offset of a first stimulus. The behavior of the model compares well with empirical data but discrepancies in tests (d) and (f) point to the need for further development. Additional functions that have been successfully simulated in previous tests include realistic interspike-interval histograms for silence and intense sinusoidal stimuli, realistic poststimulus period histograms at various intensities and nonmonotonic functions relating incremental and decremental responses to background stimulus intensity. The model is computationally convenient and well suited to use in automatic recognition devices that use models of the peripheral auditory system as input devices. It is particularly well suited to devices that require stimulus phase information to be preserved at low frequencies.     

The shape of the ear's temporal window

This article examines the idea that the temporal resolution of the auditory system can be modeled using a temporal window (an intensity weighting function) analogous to the auditory filter measured in the frequency domain. To estimate the shape of the hypothetical temporal window, threshold was measured for a brief sinusoidal signal presented in a temporal gap between two bursts of noise. The duration of the gap was systematically varied and the signal was placed both symmetrically and asymmetrically within the gap. The data were analyzed by assuming that the temporal window had the form of a simple mathematical expression with a small number of free parameters. The values of the parameters were adjusted to give the best fit to the data. The analysis assumed that, for each condition, the temporal window was centered at the time giving the highest signal-to-masker ratio, and that threshold corresponded to a fixed ratio of signal energy to masker energy at the output of the window. The data were fitted well by modeling each side of the window as the sum of two rounded-exponential functions. The window was highly asymmetric, having a shallower slope for times before the center than for times after. The equivalent rectangular duration (ERD) of the window was typically about 8 ms. The ERD increased slightly when the masker level was decreased, but did not differ significantly for signal frequencies of 500 and 2000 Hz. The temporal-window model successfully accounts for the data from a variety of experiments measuring temporal resolution. However, it fails to predict certain aspects of forward masking and of the detection of amplitude modulation at high rates.     

Frequency and intensity discrimination in human infants and adults

Frequency and intensity DLs were measured in 26 human infants (ages 7-9 months) and six young adults using a repeating standard "yes-no" operant headturning technique and an adaptive staircase (tracking) psychophysical procedure. Subjects were visually reinforced for responding to frequency increments, frequency decrements, intensity increments, or intensity decrements in an ongoing train of 1.0-kHz tone bursts, and stimulus control was monitored using randomly interleaved probe and catch trials. Infants were easily conditioned to respond to both increments and decrements in frequency, and DLs ranged from 11-29 Hz, while adult DLs ranged from 3-5 Hz. Infants also easily discriminated intensity increments, and DLs ranged from 3-12 dB, while adult DLs ranged from 1-2 dB. No infants successfully discriminated intensity decrements, although adults experienced no difficulty with this task and produced DLs similar to those for increments. The apparent inability of infants to discriminate intensity decrements suggests that the infant CNS may not be well adapted to monitor rate decreases in populations of peripheral auditory neurons.     

Group delay measurement from spiral ganglion cells in the basal turn of the guinea pig cochlea

Measurements of group delay were made extracellularly from spiral ganglion cells in the 3.7 to 5.0-mm region of the guinea pig cochlea, using sinusoidally amplitude modulated tones with constant modulating frequency (100 Hz) and depth of modulation (0.19). Threshold cochlear tuning was accompanied by frequency-dependent group delays. The group delay on the low-frequency tail was independent of carrier frequency; the interunit variation was 0.28-1.28 ms. The difference in group delay between CF and the low-frequency tail decreased as the CF threshold increased (-0.09 +/- 0.02 ms per 10 dB, beginning at 0.62 +/- 0.07 ms at 0 dB SPL). The group delay decreased above CF; at the units' maximum frequency it was less than the low-frequency tail value, and was sometimes negative. Following arterial injections of furosemide the CF threshold increased and the group delay peak decreased; the low-frequency tail was unaffected. The group delay decreased with increasing intensity; the reduction near and above CF was not only larger than that on the low-frequency tail, but also the change at 5-10 dB above threshold was far greater than expected from the Q10dB of the suprathreshold iso-rate tuning curves. A minimum-phase analysis suggested that the group delay response above CF, together with its nonlinear behavior, can be accounted for by a high-frequency, level-independent, amplitude plateau, in combination with the single unit, amplitude nonlinearity which is known to exist above CF.