Distinguishing cochlear pathophysiology in 4-aminopyridine and furosemide treated ears using a nonlinear systems identification technique

To test the adequacy of physiologic indices derived from a third-order polynomial model quantifying cochlear mechano-electric transduction (MET), 24 Mongolian gerbils were exposed to either 250-mM glucose (control), 150-mM 4-aminopyridine (4-AP), or 30-mM furosemide solutions applied to the round window (RW) membrane. The cochlear microphonic (CM) was recorded from the RW in response to 68- and 88-dB SPL Gaussian noise. A nonlinear systems identification technique (NLID) provided the frequency-domain parameters and physiologic indices of the polynomial model of MET. The control group showed no change in both compound action potential (CAP) thresholds and CM. Exposure to 4-AP and furosemide resulted in a similar elevation in CAP thresholds and a reduction in CM. However, the polynomial model of MET showed different changes. The operating point, slope, and symmetry of the MET function, the polynomial model parameters, and related nonlinear coherences differed between the experimental groups. It is concluded that the NLID technique is sensitive and specific to alterations in the cochlear physiology.     

Characterizing cochlear mechano-electric transduction with a nonlinear system identification technique: the influence of the middle ear

Previously a third-order polynomial equation characterizing mechano-electric transduction was obtained from a nonlinear system identification procedure applied to an ear canal acoustic signal and cochlear microphonic (CM/AC). In this paper, we examine the influence of the linearity and frequency response of the intervening middle ear on the nonlinearity, frequency response, and coherence of the third-order polynomial model of mechano-electric transduction (MET). Ear canal sound pressure (AC), cochlear microphonics (CM), and stapes velocity (SV) were simultaneously recorded from Mongolian gerbils. Linear and nonlinear transfer and coherence functions relating stapes velocity to the acoustic signal (SV/AC), CM to the acoustic signal (CM/AC), and CM to the stapes velocity (CM/SV) were computed. The results showed that SV/AC was linear while CM/AC and CM/SV were not, indicating that the nonlinearity of CM/AC was not due to nonlinearity of the middle ear. The frequency response of the linear term of CM/AC was similar to that of ST/AC but differed from that of CM/SV while the cubic term of CM/AC was similar to that of CM/SV. This indicates that the frequency dependence of CM/AC was due to both the middle ear and frequency dependence of the inner ear. Finally the fit of the polynomial model of MET without the middle ear (CM/SV) did not improve from the fit including the middle ear (CM/AC). A cochlear model of the CM indicated that the lack of improvement was due to the limitations of a third-order polynomial equation characterizing the hair cell transducer function.     

Effects of peripheral nonlinearity on psychometric functions for forward-masked tones

Psychometric functions (PFs) for forward-masked tones were obtained for conditions in which signal level was varied to estimate threshold at several masker levels (variable-signal condition), and in which masker level was varied to estimate threshold at several signal levels (variable-masker condition). The changes in PF slope across combinations of masker frequency, masker level, and signal delay were explored in three experiments. In experiment 1, a 2-kHz, 10-ms tone was masked by a 50, 70 or 90 dB SPL, 20-ms on-frequency forward masker, with signal delays of 2, 20, or 40 ms, in a variable-signal condition. PF slopes decreased in conditions where signal threshold was high. In experiments 2 and 3, the signal was a 4-kHz, 10-ms tone, and the masker was either a 4- or 2.4-kHz, 200-ms tone. In experiment 2, on-frequency maskers were presented at 30 to 90 dB SPL in 10-dB steps and off-frequency maskers were presented at 60 to 90 dB SPL in 10-dB steps, with signal delays of 0, 10, or 30 ms, in a variable-signal condition. PF slopes decreased as signal level increased, and this trend was similar for on- and off-frequency maskers. In experiment 3, variable-masker conditions with on- and off-frequency maskers and 0-ms signal delay were presented. In general, the results were consistent with the hypothesis that peripheral nonlinearity is reflected in the PF slopes. The data also indicate that masker level plays a role independent of signal level, an effect that could be accounted for by assuming greater internal noise at higher stimulus levels.     

The effect of a precursor on growth of forward masking

This study examined the effect of an on-frequency precursor on growth-of-masking (GOM) functions measured using an off-frequency masker. The signal was a 6-ms, 4-kHz tone. A GOM function was measured using a 40-ms, 2.8-kHz tone (the off-frequency masker). GOM functions were then measured with an on-frequency, fixed level precursor presented before the off-frequency masker. The precursor was 50 or 60 dB SPL, and 160 ms in duration. For the 60-dB SPL precursor, a 40-ms duration was also used. Two-line functions were fit to the GOM data to estimate the basilar membrane input-output function. The precursors reduced the gain of the input-output function, and this decrease was graded with precursor level. Both precursor durations had the same effect on gain. Changes in masking following a precursor were larger than would be predicted by additivity of masking. The observed decrease in gain may be consistent with activation of the medial olivocochlear reflex by the precursor.     

Spectral and level effects in noise-on-tone suppression

The effective internal level of a 1-kHz tone at 50 dB SPL was estimated by measuring the forward masking produced on a 10-ms signal tone of the same frequency. Noise containing a spectral notch was then added to the masker tone, and its influence on the effective level of the tone was measured with a variety of noise levels, notch widths, and notch shapes. In experiment 1, the masker tone was centered in the spectral notch, itself centered in a 2-kHz band of noise. As the spectrum level in the noise passbands increased from 6 dB/Hz to 36 dB/Hz, signal threshold decreased, indicating a decrease in masking by the masker tone. This "unmasking" effect of the noise was attributed to suppression of the masker tone by the components in the noise. Unmasking was greatest with the narrowest spectral notch (250 Hz), and decreased to zero as the notch widened to 1500 Hz. Compared to its level when presented alone, the effective internal level of the masker tone could be reduced by up to 30 dB (250-Hz notch, 36 dB/Hz). The relative suppressive strength of individual noise components was estimated in experiment 2, in which the 1-kHz masker tone was located at one edge of a spectral notch, rather than in the center. Noise spectrum level was fixed at 16 dB/Hz. As notch width decreased to zero, on either the high-frequency or low-frequency side of the masker tone, its effective internal level was again reduced by approximately 30 dB. In a tentative analysis, the first derivative of the smoothed threshold function was taken, to provide an estimate of the relative contributions to suppression at 1 kHz of noise components between 250 and 1740 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)     

Recovery of distortion-product otoacoustic emissions after a 2-kHz monaural sound-exposure in humans: effects on fine structures

A better understanding of the vulnerability of the fine structures of distortion-product otoacoustic emissions (DPOAEs) after acoustic overexposure may improve the knowledge about DPOAE generation, cochlear damage, and lead to more efficient diagnostic tools. It is studied whether the DPOAE fine structures of 16 normal-hearing human subjects are systematically affected after a moderate monaural sound-exposure of 10 min to a 2-kHz tone normalized to an exposure level L(EX,8h) of 80 dBA. DPOAEs were measured before and in the following 70 min after the exposure. The experimental protocol allowed measurements with high time and frequency resolution in a 1/3-octave band centered at 3 kHz. On average, DPOAE levels were reduced approximately 5 dB in the entire measured frequency-range. Statistically significant differences in pre- and post-exposure DPOAE levels were observed up to 70 min after the end of the sound exposure. The results show that the effects on fine structures are highly individual and no systematic change was observed.     

The influence of middle-ear muscle contraction on auditory threshold for selected pure tones

The influence of middle-ear muscle (MEM) contraction on auditory threshold has been measured for pure tones of 0.25, 0.5, and 1.5 kHz. The reflex-activating signal was a 3-kHz pure tone. Signal paradigms were chosen to reduce or eliminate the effects of binaural loudness summation, contralateral direct masking, and contralateral remote and backward masking effects, and to maximize the influence of MEM contraction. Results indicate that under no condition was behavioral threshold affected by the MEM contraction induced using a pure-tone stimulus of 3 kHz, 105 dB SPL.     

Comparing behavioral and physiological measures of combination tones: Sex and race differences

Both distortion-product otoacoustic emissions (DPOAEs) and performance in an auditory-masking task involving combination tones were measured in the same frequency region in the same ears. In the behavioral task, a signal of 3.6?kHz (duration 300?ms, rise/fall time 20?ms) was masked by a 3.0-kHz tone (62?dB SPL, continuously presented). These two frequencies can produce a combination tone at 2.4?kHz. When a narrowband noise (2.0-2.8?kHz, 17?dB spectrum level) was added as a second masker, detection of the 3.6-kHz signal worsened by 6-9?dB (the Greenwood effect), revealing that listeners had been using the combination tone at 2.4?kHz as a cue for detection at 3.6?kHz. Several outcomes differed markedly by sex and racial background. The Greenwood effect was substantially larger in females than in males, but only for the White group. When the magnitude of the Greenwood effect was compared with the magnitude of the DPOAE measured in the 2.4?kHz region, the correlations typically were modest, but were high for Non-White males. For many subjects, then, most of the DPOAE measured in the ear canal apparently is not related to the combination-tone cue that is masked by the narrowband noise.     

The effect of narrow-band noise maskers on increment detection

It is often assumed that listeners detect an increment in the intensity of a pure tone by detecting an increase in the energy falling within the critical band centered on the signal frequency. A noise masker can be used to limit the use of signal energy falling outside of the critical band, but facets of the noise may impact increment detection beyond this intended purpose. The current study evaluated the impact of envelope fluctuation in a noise masker on thresholds for detection of an increment. Thresholds were obtained for detection of an increment in the intensity of a 0.25- or 4-kHz pedestal in quiet and in the presence of noise of varying bandwidth. Results indicate that thresholds for detection of an increment in the intensity of a pure tone increase with increasing bandwidth for an on-frequency noise masker, but are unchanged by an off-frequency noise masker. Neither a model that includes a modulation-filter-bank analysis of envelope modulation nor a model based on discrimination of spectral patterns can account for all aspects of the observed data.     

Suprathreshold effects of adaptation produced by amplitude modulation

This work extends the study of adaptation to amplitude modulation (AM) to the perception of highly detectable modulation. A fixed-level matching procedure was used to find perceptually equivalent modulation depths for 16-Hz modulation imposed on a 1-kHz standard and a 4-kHz comparison. The modulation depths in the two stimuli were compared before and after a 10-min exposure to a 1-kHz tone (adaptor) 100% modulated in amplitude at different rates. For modulation depths of 63% (20 log m = -4) and smaller, the perceived modulation depth was reduced after exposure to the adaptor that was modulated at the same rate as the standard. The size of this reduction expressed as a difference between the post- and pre-exposure AM depths was similar to the increase in AM-detection threshold observed after adaptation. Postexposure suprathreshold modulation depth was not appreciably reduced when the modulation depth of the standard was large (approached 100%). A much smaller or no reduction in the perceived modulation depth was also observed when the modulation rates of the adaptor and the standard tone were different. The tuning of the observed effect of the adaptor appears to be much sharper than the tuning shown by modulation-masking results.     

音码混合测距在深空探测中的应用研究

深空探测具有目标距离远、信号往返时延大等特点。提出了一种适用于深空探测的音码混合测距方法,详细分析了测距信号发送、接收时序,并阐述了距离捕获、解模糊和跟踪的过程。最后进行了实验研究,与纯侧音测距相比,音码混合测距精度更高,测距值更稳定。     

Dead regions and pitch perception

The perception of pitch for pure tones with frequencies falling inside low- or high-frequency dead regions (DRs) was examined. Subjects adjusted a variable-frequency tone to match the pitch of a fixed tone. Matches within one ear were often erratic for tones falling in a DR, indicating unclear pitch percepts. Matches across ears of subjects with asymmetric hearing loss, and octave matches within ears, indicated that tones falling within a DR were perceived with an unclear pitch and/or a pitch different from "normal" whenever the tones fell more than 0.5 octave within a low- or high-frequency DR. One unilaterally impaired subject, with only a small surviving region between 3 and 4 kHz, matched a fixed 0.5-kHz tone in his impaired ear with, on average, a 3.75-kHz tone in his better ear. When asked to match the 0.5-kHz tone with an amplitude-modulated tone, he adjusted the carrier and modulation frequencies to about 3.8 and 0.5 kHz, respectively, suggesting that some temporal information was still available. Overall, the results indicate that the pitch of low-frequency tones is not conveyed solely by a temporal code. Possibly, there needs to be a correspondence between place and temporal information for a normal pitch to be perceived.     

The relationship between precursor level and the temporal effect

Previous studies have suggested that temporal effects in masking may be consistent with a decrease in cochlear gain. One paradigm used to show this is to measure the level of a long-duration masker required to just mask a short-duration tone that occurs near masker onset. The temporal effect is revealed when the signal is detected at a lower signal-to-noise ratio following preceding stimulation (either an extension of the masker or a separate precursor). The present study examined whether this effect depends on precursor level. The signal was a 10-ms, 4-kHz tone. The masker was 200 ms. A fixed-level precursor had the same frequency characteristics as the masker, and was 205 ms. The masker and precursor had either no notch or a wide notch about the signal frequency. For a given precursor level, the growth of masker level with signal level was determined. These data were used to estimate input-output functions. The results are consistent with a graded decrease in gain at the signal frequency when there is no notch in the masker and precursor, and a graded decrease in suppression when there is a large notch. These results could be consistent with the action of the medial olivocochlear reflex.     

Temporal integration in zebra finches (Poephila guttata)

Three zebra finches were trained with operant techniques to respond to pure tones. Absolute thresholds were obtained for nine durations of a 3-kHz tone and five durations of a 1-kHz tone. The temporal integration functions were described using the negative exponential function proposed by Plomp and Bouman [J. Acoust. Soc. Am. 31, 749-758 (1959)]. The time constants obtained for zebra finches are about 250 ms, which are similar to those reported for a number of species, including humans and other bird species.     

Characterization of an EPSP-like potential recorded remotely from the round window

The whole-nerve cochlear action potential (CAP), to tone burst stimulation, was recorded before and after application of tetrodotoxin (TTX) to the intact round window (RW) membrane. TTX abolished the CAP leaving a residual negative potential without altering the summating potential (SP) or the cochlear microphonic (CM). The residual potential retained its polarity when recorded from scala vestibuli. The peak latency, amplitude, and tuning properties of the residual potential showed features similar to the CAP. Application of kainic acid to the RW membrane eliminated the residual potential, leaving the SP and CM unaltered. It is hypothesized that the sources of the residual potential are the excitatory post-synaptic potentials from the peripheral processes of afferent dendrites under the inner hair cells.     

Detecting a repeated tone burst in repeated noise

Samples of wideband noise 0.05, 0.1, 0.2, or 0.4 s in duration were digitized and then replayed cyclically to produce repeated-noise maskers. The signal was a repeating tone burst (0.4 or 1.6 kHz). It was half the duration of the noise sample, centered in the noise temporally, and it was repeated at the same point in each repetition of the noise. In the antiphasic conditions of the experiment, either the noise sample or the tone burst was inverted in alternate repetitions of the masker; in the homophasic conditions both the tone burst and noise, or neither, were inverted in alternative repetitions. If the auditory system were capable of storing detailed waveforms of sufficient length, alternate repetitions could be added or subtracted and we might expect a release from masking in the antiphasic conditions. The results show a small but significant advantage for the antiphasic conditions when the signal frequency was 0.4 kHz, but no difference with the 1.6-kHz signal.     

Strategies for the representation of a tone in background noise in the temporal aspects of the discharge patterns of auditory-nerve fibers

The responses of populations of auditory-nerve fibers to both a 1.0-kHz tone, and 1.0-kHz tone in broadband noise, have been measured. Period histograms were generated from fiber spike trains and discrete Fourier transforms (DFTs) with a resolution of 125 Hz were computed from each histogram. Sample mean and sample variance statistics were generated for period histograms of response and for temporal response measures derived from discrete Fourier transforms. It is demonstrated how the statistical properties of auditory-nerve fiber response determine the strategy for the estimation and discrimination of particular stimulus components. When the tone is presented alone, the entire population of auditory-nerve fibers provides statistically reliable estimates of the 1.0-kHz tone. Upon addition of the broadband noise stimulus only those units with characteristic frequencies which are close in frequency to the 1.0-kHz stimulus provide spectral estimates which have high signal-to-noise ratios (mean-squared-to-variance ratios). Estimates of the 1.0-kHz-tone stimulus derived from auditory-nerve fibers with characteristic frequencies which are far from the 1.0-kHz stimulus are statistically unreliable. Based on the responses of the population of auditory-nerve fibers, the strategy for estimating the 1.0-kHz-tone stimulus is to derive estimates of the 1.0-kHz stimulus from the subpopulation of neurons with characteristic frequencies close to the 1.0-kHz stimulus. It is concluded that neurons which are tuned close to 1.0-kHz provide the central nervous system (CNS) with the most salient information about the 1.0-kHz stimulus in the presence of the broadband background.     

The "overshoot" effect and sensory hearing impairment

The threshold for the detection of a brief tone masked by a longer-duration noise burst is higher when the tone is presented shortly after the onset of the noise than at longer delay times. This finding has been termed the "overshoot" effect [E. Zwicker, J. Acoust. Soc. Am. 37, 653-663 (1965)]. The present letter compared the size of the effect in the better and more impaired ear of six subjects with high-frequency unilateral or asymmetric hearing losses of sensory origin. Thresholds were measured for 5-ms 4-kHz tones presented 10, 200, and 390 ms after the onset of a 400-ms, 2- to 8-kHz noise burst. The better ear of each subject was tested using two noise levels, one equal in sound-pressure level and one equal in sensation level to that used for the impaired ear. Thresholds for all subjects and all ears decreased monotonically with increasing delay time, with the size of the effect typically 5 dB. Thus a small overshoot effect was observed regardless of hearing impairment.     

Basilar membrane mechanics at the base of the chinchilla cochlea. II. Responses to low-frequency tones and relationship to microphonics and spike initiation in the VIII nerve

Low-frequency stimuli (40- to 1000-Hz tones) have been used to correlate the motion of the 8-to 9-kHz place of the chinchilla basilar membrane with the cochlear microphonics recorded at the round window and with the responses of auditory nerve fibers with appropriate characteristic frequency. At the lowest stimulus frequencies, maximum displacement of the basilar membrane toward scala tympani occurs in near synchrony with maximum rarefaction at the eardrum and maximum negativity at the round window; at higher frequencies, the mechanical and microphonic response phases progressively lag rarefaction, reaching - 240 deg at 1000 Hz. At most frequencies (40-1000 Hz) near-threshold neural responses, once corrected for neural travel-time and synaptic delays, somewhat lead (by some 40 deg) maximal scala tympani displacement and maximal negativity of the round window microphonics. The variation of sensitivity with frequency is similar for basilar membrane displacement and microphonic responses: Under open-bulla conditions, sensitivity is constant for frequencies between 100 and 1000 Hz; below 100 Hz, sensitivity decreases at rates close to 12 dB/oct toward lower frequencies. Neural response sensitivity matches BM displacement more closely than BM velocity.     

Two- versus four-tone masking, revisited

Canahl [J. Acoust. Soc. Am. 50, 471-474 (1971)] measured thresholds for a 1.0-kHz sinusoid masked either by two or by four surrounding tones. He reported four-tone masked thresholds that exceeded, by 5-7.5 dB, the energy sum of the masking produced by the individual tone pairs. The present paper reports on a series of experiments investigating the effects of several factors on this 5-7.5 dB "excess" masking. In each experiment, thresholds for a 1.0-kHz 250-ms sinusoid were measured as a function of the overall level of two or four equal amplitude sinusoids with frequencies arithmetically centered around 1.0 kHz. For conditions similar to those of the Canahl experiment, 5-6 dB of excess masking was obtained independent of the level of the masking tones. Randomly varying overall level across presentations had no effect on the excess masking. The excess masking was reduced or eliminated when the masking tones were generated using an amplitude modulation technique, when they were gated on and off with the signal, or when their waveshapes were fixed across trials. Canahl's result may reflect listeners' ability to detect the signal as a change in the waveshape of the multitone masker.