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PubMed1992

Manifestations of intense noise stimulation on spontaneous otoacoustic emission and threshold microstructure: experiment and model.

Comparison between changes that occur simultaneously on spontaneous otoacoustic emissions (SOAEs) and on other cochlear origin phenomena can contribute to the understanding of cochlear micromechanical activity. The temporary changes that arise after short noise exposure are investigated in the following paper. The effects of noise exposure on the threshold microstructure near an SOAE and on the amplitude and frequency of the SOAE were measured. These experimental results indicate the following: (1) exposure to wideband noise for a short time causes a temporary reduction in the SOAE frequency and amplitude, and alters reversibly the threshold microstructure in the vicinity of the SOAE. The difference between the minimum and maximum in the threshold microstructure is reduced, and the frequency that yields the minimum threshold decreases; (2) the threshold at the SOAE frequency is most sensitive to noise exposure; (3) intense stimulation causes a relatively small increase, or even a decrease, in threshold at frequencies near the SOAE. The experimental results are interpreted in terms of a nonlinear transmission line model which includes nonlinear amplifiers. The effect of the noise exposure is modeled by reduction in the cochlear partition amplification term. Most of the experimental results are predicted by this model.

Auditory Fatigue↗
PubMed1992

Effect of signal frequency and masker level on the frequency regions responsible for the overshoot effect.

Two experiments investigated the relative influence of components close to and remote from the signal frequency (fs) on the overshoot effect. Overshoot was defined as the difference in threshold between a signal presented 4 ms after, and that for one presented 300 ms after, the onset of a 350-ms masker. Experiment 1 measured the overshoot effect using both wideband and narrow-band maskers (centered on fs), at two signal frequencies and three masker levels. Experiment 2 used a masker consisting of a "middle band" (MB, centered on fs) and two flanking bands (FBs, which, when combined with the MB, produced a flat wideband spectrum). The masker was continuous except for a 300-ms interval just prior to the signal, during which either all three bands, the MB alone, or the FBs alone were turned off. The results of both experiments showed that the overshoot effect was usually determined by off-frequency components. However, the on-frequency components played a substantial role when signal-to-noise ratios (SNRs) at threshold were unusually high, such as at fs = 6500 Hz and intermediate masker levels. It is suggested that two different mechanisms contribute to the overshoot effect: an off-frequency mechanism which operates at all fs and masker levels, and an on-frequency mechanism which contributes to the overshoot effect only at high-threshold SNRs.

Attention↗
PubMed1992

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.

Animals↗
PubMed1992

The detection of temporal gaps as a function of frequency region and absolute noise bandwidth.

Temporal gap detection was measured as a function of absolute signal bandwidth at a low-, a mid-, and a high-frequency region in six listeners with normal hearing sensitivity. Gap detection threshold decreased monotonically with increasing stimulus bandwidth at each of the three frequency regions. Given conditions of equivalent absolute bandwidth, gap detection thresholds were not significantly different for upper cutoff frequencies ranging from 600 to 4400 Hz. A second experiment investigated gap detection thresholds at two pressure-spectrum levels, conditions typically resulting in substantially different estimates of frequency selectivity. Estimates of frequency selectivity were collected at the two levels using a notched-noise masker technique. The gap threshold-signal bandwidth functions were almost identical at pressure-spectrum levels of 70 dB and 40 dB for the two subjects in experiment II, while estimates of frequency selectivity showed poorer frequency selectivity at the 70-dB level than at 40 dB. Data from both experiments indicated that gap detection in bandlimited noise was inversely related to signal bandwidth and that gap detection did not vary significantly with changes in signal frequency over the range of 600 to 4400 Hz. Over the range of frequencies investigated, the results indicated no clear relation between gap detection for noise stimuli and peripheral auditory filtering.

Adult↗
PubMed1992

Stabilization of perceived echo amplitudes in echolocating bats. I. Echo detection and automatic gain control in the big brown bat, Eptesicus fuscus, and the fishing bat, Noctilio leporinus.

Previous research on echo detection in bats has suggested that the effective threshold is a function of the acoustic clutter in the experimental environment, as might be expected given the low ambient noise levels typical of such psychophysical research. This paper demonstrates that theory of signal detectability (TSD) methodology is applicable to bats and uses it to show that an important element of clutter limiting in Eptesicus fuscus and Noctilio leporinus is backward masking of phantom targets by the real echo from the loudspeakers used to generate them. This information suggests that a previous estimate of the magnitude of automatic gain control (AGC) is too high, due to variable backward masking inherent in the experimental method used. A re-examination of gain control using a masking-free method shows that it reduces auditory sensitivity by 6 to 7 dB per halving of target range, rather than 11 dB as previously thought.

Animals↗
PubMed1992

Sonar gain control and echo detection thresholds in the echolocating bat, Eptesicus fuscus.

The echolocating bat, Eptesicus fuscus, detects sonar echoes with a sensitivity that changes according to the time elapsed between broadcasting of each sonar signal and reception of echoes. When tested in an electronic target simulator on a two-choice echo-detection task, the bat's threshold improved by 11.5 dB as echo delay changed from 2.3 to 4.6 ms (target ranges of 40 and 80 cm). Earlier experiments measured the change in detection threshold for delays from 1 to 6.4 ms (target ranges from about 17 to 110 cm) and obtained about 11 dB of improvement per doubling of delay. The new experiments used electronic delay lines to simulate echo delay, thus avoiding movement of loudspeakers to different distances and the possible creation of delay-dependent backward masking between stimulus echoes and cluttering echoes from the loudspeaker surfaces. The slope of the threshold shift defines an echo gain control that keeps echoes from point targets at a fixed sensation level--reducing sensitivity by 11 to 12 dB as echo amplitude increases by 12 dB per halving of range during the bat's approach to the target. A recent experiment using loudness discrimination of echoes at 70 to 80 dB SPL (roughly 50 dB above threshold) found a slope of about 6 dB per halving of range, so the gain-control effect may be level dependent. The observed effect is operationally equivalent to forward masking of echoes by the transmission, but any events correlated with vocalization which impair hearing sensitivity for a short interval following transmissions could cause a decline in sensitivity to echoes. Contractions of the bat's middle-ear muscles synchronized to transmissions may account for the observed threshold shift, at least for a span of echo delays associated with the most critical portion of the approach stage of pursuit. Forward masking by the sonar transmissions may contribute to the threshold shift, too, but middle-ear muscle contractions do occur and must be a significant part of the cause.

Animals↗
PubMed1992

Frequency dependence of binaural performance in listeners with impaired binaural hearing.

Binaural performance was measured as a function of stimulus frequency for four impaired listeners, each with bilaterally symmetric audiograms. The subjects had various degrees and configurations of audiometric losses: two had high-frequency, sensorineural losses; one had a flat sensorineural loss; and one had multiple sclerosis with normal audiometric thresholds. Just noticeable differences (jnd's) in interaural time, interaural intensity, and interaural correlation as well as detection thresholds for NoSo and NoS pi conditions were obtained for narrow-band noise stimuli at octave frequencies from 250-4000 Hz. Performance of the impaired listeners was generally poorer than that of normal-hearing listeners, although it was comparable to normal in a few instances. The patterns of binaural performance showed no apparent relation to the audiometric patterns; even the two subjects with similar degree and configuration of hearing loss have very different binaural performance, both in the level and frequency dependence of their performance. The frequency dependence of performance on individual tests is irregular enough that one cannot confidently interpolate between octaves. In addition, it appears that no subset of the measurements is adequate to characterize the performance in the rest of the measurements with the exception that, within limits, interaural correlation discrimination and NoS pi detection performance are related.

Acoustic Stimulation↗
PubMed1992

Fringe effects in modulation masking.

Modulation detection thresholds (20 log ms) for a sinusoidally amplitude-modulated (SAM) noise were measured in the presence of a SAM noise masker with a modulation depth (mm) of 1.0 and a modulation frequency of 16 or 64 Hz. The signal and masker carriers were presented continuously, and the signal was modulated during one of the two 500-ms observation intervals. The masker was modulated during both observation intervals and, in some conditions, for a certain amount of time before and after signal modulation. The duration of this "fringe" ranged from 62.5 ms to continuous (masker modulated throughout the thresholds estimate). The first experiment showed that a 500-ms fringe could reduce masked thresholds by 4-6 dB, but only at low signal modulation frequencies (2-8 Hz). In the second and third experiments, it was found that the fringe had to have a duration of 500 ms and a depth of about 0.75 to be maximally effective. A final, supplementary experiment indicated that the fringe effect is not due solely to the fringe that occurs prior to the observation intervals. The results are discussed in terms of both peripheral and central auditory processing.

Adult↗
PubMed1992

Differences in auditory performance between monaural and dichotic conditions. I: masking thresholds in frozen noise.

Thresholds of a 5-ms, 1-kHz signal were determined in the presence of a frozen-noise masker. The noise had a flat power spectrum between 20 Hz and 5 kHz and was presented with a duration of 300 ms. The following interaural conditions were tested with four listeners: Noise and signal monaural at the same ear (monaural condition, NmSm), noise and signal identical at both ears (diotic condition, NoSo), noise identical at both ears and signal monaural (dichotic condition, NoSm) and uncorrelated noise at the two ears and signal monaural (NuSm). The signal was presented at a fixed temporal position with respect to the frozen noise in all measurements and thresholds were determined for different starting phases of the carrier frequency of the signal. Variation of the carrier phase strongly influenced the detection in the diotic condition and the masked thresholds varied by more than 10 dB. The pattern of thresholds for the monaural condition was less variable and the thresholds were generally higher than for the diotic condition. The monaural-diotic difference for specific starting phases amounted to as much as 8 dB. Comparison measurements using running noise maskers revealed no such difference. This relation between monaural and diotic thresholds was further investigated with eight additional subjects. Again, monaural and diotic thresholds in running noise were identical, while in frozen noise, diotic thresholds were consistently lower than monaural thresholds, even when the ear with the lower NmSm threshold was compared. For the starting phase showing the largest monaural-diotic difference, the thresholds for NoSm lay between the monaural and the diotic values. At other starting phases, the NoSm threshold was clearly lower than both the NmSm and the NoSo threshold. One possible explanation of the observed monaural-diotic differences relates to contralateral efferent interaction between the right and the left hearing pathway. A prediction based on this explanation was verified in a final experiment, where frozen-noise performance for NmSm was improved by simultaneously presenting an uncorrelated running noise to the opposite ear.

Acoustic Stimulation↗
PubMed1992

On using coherence to measure distortion in hearing aids.

Coherence is a frequency-domain measure of the degree to which the output of a system is linearly related to the system input. The signal-to-distortion ratio (SDR), where the distortion term includes all nonlinear effects and noise in the system, can be computed from the coherence. The coherence estimate, however, is subject to sources of variance and bias that reduce the accuracy of the measured SDR. The origins of the variance and bias and their effects on distortion measurements are presented. New procedures for reducing the variance and bias effects are described, and the processing effectiveness is demonstrated for a simulated hearing-aid response.

Fourier Analysis↗
PubMed1992

The effect of interaural delay of the masker on masking-level differences in young and old adults.

Diotic (SoNo) thresholds and dichotic (S pi N pi tau) thresholds were measured for young and old adults using a 500-Hz pure-tone signal and broadband burst masking noise at 37 dB SPL/Hz. In the dichotic condition both the signal and the masker were phase reversed and the masker was presented with an interaural delay of 0.25, 0.5, 0.75, 1, 1.25, 1.5, 1.75, 2, 3, or 5 ms. Masking-level differences (MLDs) were determined by subtracting dichotic thresholds from diotic thresholds. The SoNo thresholds for the old subjects did not differ significantly from those for the young subjects; however, when MLDs were plotted as a function of delay, the pattern of results differed significantly between young and old subjects. This difference in pattern was completely accounted for in terms of a delay-line version of Durlach's equalization and cancellation (EC) model [N. I. Durlach, in Foundations of Modern Auditory Theory, edited by J. V. Tobias (Academic, New York, 1972); B. A. Schneider and P. M. Zurek, J. Acoust. Soc. Am. 86, 1756-1763 (1989)] by assuming that temporal jitter increases with internal delay in young subjects but that it does not vary with the amount of internal delay in old subjects.

Adult↗
PubMed1992

Comparison of frequency selectivity and consonant recognition among hearing-impaired and masked normal-hearing listeners.

Frequency selectivity and consonant recognition were determined for normal-hearing and hearing-impaired listeners using techniques that facilitate comparisons of performance among listeners whose absolute thresholds vary in magnitude and configuration. First, for each of six subjects with cochlear hearing loss, masked thresholds in notched noise and narrow-band-noise maskers were obtained and compared to results for three normal-hearing listeners whose thresholds were precisely matched to the impaired listeners' by masking with spectrally shaped broadband noise. Second, for hearing-impaired listeners and their masked normal-hearing controls, measurements of consonant recognition were obtained at several speech-presentation levels selected on the basis of articulation-index predictions to assure equal speech-spectrum audibility across listeners. The results suggest that frequency selectivity is poorer for hearing-impaired listeners than for masked normal-hearing listeners, even when thresholds among subjects are equated, but the deviation from normal frequency selectivity is smaller than estimated from comparisons with normal-hearing listeners in quiet. Critical ratios for hearing-impaired listeners are equivalent to normal. Although frequency selectivity is reduced, there is no consistent difference in consonant recognition between hearing-impaired subjects and masked normal-hearing subjects, when performance is assessed under conditions that assure equal speech-spectrum audibility across subjects.

Adolescent↗
PubMed1992

The role of envelope fluctuations in an apparent demonstration of suppression in simultaneous masking.

Fastl and Bechly [J. Acoust. Soc. Am. 74, 754-757 (1983)] reported that the threshold of a brief 900-Hz signal simultaneously masked by a band of noise. 100 Hz wide, centered at 1000 Hz, was reduced by approximately 8 dB by the addition of an 1150-Hz tone having a level of 20 dB above that of the narrow-band masker. They concluded that this decrease in threshold was a demonstration of suppression in simultaneous masking. Here it is argued that Fastl and Bechly's results simply reflect the poorer detectability of signals masked by higher-frequency fluctuating maskers (their narrow-band masker) than by relatively flat-envelope maskers (their composite narrow-band plus tonal masker). The results of three experiments support the masker-envelope explanation. In the first experiment, as in the report of Fastl and Bechly, the masker centered at 1000 Hz (M1) was a narrow-band noise and the masker centered at 1150 Hz (M2) was a tone. Fastl and Bechly's result was replicated. However, thresholds obtained when M1 was presented alone (the M1-only condition) were more affected by the starting level of the signal within each adaptive track than were thresholds obtained when M1 and M2 were presented together (the M1+M2 condition). This result paralleled a previous report that starting level influenced performance more with fluctuating than with flat-envelope maskers. For the four of seven subjects wh showed learning, there was also more improvement in the M1-only than in the M1 + M2 condition. In the second experiment, M1 was a tone and M2 was a narrow-band noise.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗
PubMed1992

Temporal modulation transfer functions in patients with cochlear implants.

Thresholds for the detection of amplitude modulation were measured in cochlear implant patients as a function of modulation frequency. Three types of threshold measures were taken: detection of amplitude modulation, detection of low-frequency sinusoidal current waveforms, and detection of beats in two-tone complexes. The temporal modulation transfer function (TMTF), defined as the plot of modulation detection thresholds as a function of modulation frequency, show low-pass filter characteristics with similar cutoff frequencies for all three tasks. The similarity of these three measures suggests a common temporal mechanism. While modulation detection differs somewhat in normal-hearing and implanted listeners, both exhibit the same general characteristics. The TMTFs are low pass with a cutoff frequency near 70 Hz for normal-hearing listeners and near 140 Hz for implanted listeners. Patients with cochlear implants can best detect temporal modulation at modulation frequencies below 300 Hz, and are most sensitive to 80- to 100-Hz modulation. At high carrier levels many implant patients could detect smaller modulation amplitudes than normal-hearing listeners, a finding that is consistent with the smaller intensity DLs for some implanted listeners at high levels. These results demonstrate that, while implant listeners cannot discriminate steady-state, high-frequency stimuli, speech information might be conveyed by the envelope of the high-frequency components of speech.

Auditory Threshold↗
PubMed1992

Intensity discrimination in forward masking.

A nonmonotonic intensity discrimination function was recently reported in which a midlevel hump occurred for 25-ms sinusoidal standards ranging from 20 to 100 dB SPL and presented 100 ms after an intense narrow-band noise forward masker [F.-G. Zeng et al., Hear. Res. 55, 223-230 (1991)]. This paper provides additional data on how the midlevel hump is affected by three factors of forward masking: signal delay, masker level, and frequency. Specifically, just-noticeable differences (jnd's) in intensity were obtained at signal delays of 50, 200, and 400 ms. Results show that at the midlevels the forward-masked intensity jnd's did not recover to their unmasked values, even at the 400-ms signal delay. The longer the signal delay, the smaller this midlevel hump. This slow recovery of the midlevel jnd's is consistent with the finding that low-spontaneous rate (SR) neurons have a slow recovery from forward masking [E. M. Relkin and J. R. Doucet, Hear. Res. 55, 215-222 (1991)]. The large midlevel effect decreased sharply as masker level was reduced from 90 to 60 dB SPL, and disappeared for masker levels less than 40 dB SPL. A frequency selectivity effect for the large midlevel jnd effect was also observed, as maskers with frequency components 2 to 3 oct away from the signal frequency did not affect the jnd's. Overall, the present data are consistent with the hypothesis of Zeng et al. (1991) that low-SR neurons are involved in the midlevel hump of intensity discrimination in forward masking.

Adult↗
PubMed1992

Use of sound-pressure level in auditory distance discrimination by 6-month-old infants and adults.

Six-month-old infants have been found to respond differentially to sounding objects placed within reach and beyond reach when no visual cues were available. The goal of the present study was to investigate whether sound-pressure level (SPL) serves as an auditory cue in distance discrimination. Thirty-two 6-month-olds were presented with recordings of sounding objects first in the light at midline position, then in the dark at 45 degrees to the right and left. On half of the dark trials the object was near (15 cm), and on half it was far (100 cm). For the control group the near sound was naturally 7 dB louder than the far. The experimental group had SPL counterbalanced across near and far locations to provide an inconsistent cue. Measures of infant reaching were scored from videotape. Two groups of adults were run under similar conditions; adults were tested on reaching as well as verbal reports of distance judgment. All infants reached more for the near object, regardless of the sound's SPL, suggesting that infants did not rely on this as a major distance cue. In contrast, adults' verbal judgments of distance were based heavily on SPL, a strategy that produced higher error rates in the group with SPL counterbalanced across distance. A followup study in which adults were instructed to move their heads before judging the sound's distance did not support the hypothesis that infants' head movements were responsible for their overcoming the misleading SPL information.

Adult↗
PubMed1992

A cognitive influence on the loudness of tones that change continuously in level.

The loudness of tones that change continuously in level over time was studied using magnitude estimation and a delayed loudness-balance procedure. In the magnitude estimation task, subjects estimated the loudness of continuous and intermittent 1000-Hz tones that either increased or decreased continuously in level over roughly 3 min. Two stimulus ranges were used: 30 to 70 and 50 to 90 dB SPL. For the low-level conditions, the results are essentially identical to those of Canévet and Scharf [J. Acoust. Soc. Am. 88, 2136-2142 (1990)], who ran similar conditions. Tones that increased in level changed in loudness as a function of SPL at a slower rate than tones that decreased in level. For the high-level conditions, the same result was obtained, but the magnitude of the difference among conditions is smaller. A delayed loudness-matching procedure was used to measure the physical magnitude corresponding to loudness differences among a subset of the conditions. Judged equal-loudness levels showed a sharp decline in loudness for conditions with tones that decreased continuously in level, but the magnitude of the reduction was less than that interpolated from magnitude estimates for identical stimuli. The source of the difference is unclear. To explore the role of cognitive influences, subjects' attention was diverted by a video word-identification task during the period of adaptation. For this task, the loudness decline was reduced for loudness balances and magnitude estimations compared to conditions where attention was concentrated on the adapting stimulus. In another magnitude-estimation task, the loudness decline for monaural stimulation was found to occur only in the "adapted" ear.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1992

Characterization of external ear impulse responses using Golay codes.

This report explains the use of a complementary series, Golay codes, for probing the impulse response of the external ear. The codes are used to measure both the resonance of the human ear canal, using a sealed sound-delivery system, and to measure the transfer function of the pinna, using a free-field source. With two series of 512 binary numbers, the improvement in signal-to-noise ratio over a single impulse approaches the theoretical value of 30.1 dB [10 log(2.512)]. This technique has many of the same properties as maximal-length sequences [M. R. Schroeder, J. Acoust. Soc. Am. 66, 497-500 (1979)], but it has the added advantage that the sequence length is an integer power of two and is, therefore, particularly convenient to use with modern Fourier transform techniques.

Ear Canal↗