摘要:

Distortion‐product otoacoustic emission (DPE) input/output (I/O) curves were measured at 2f1−f2frequencies with spectral‐averaging and time‐averaging procedures from ten normal‐hearing ears. Stimuli were equal‐levelf1andf2primary tones withf2/f1ratios of 1.2 andf2frequencies at 1200, 2400, and 4800 Hz. Time‐averaging procedures lowered noise‐floor (NF) levels, compared to spectral averaging, so DPEs could be elicited by stimuli as low as 18 dB SPL in some ears. DPEs were corrected for power summation with NF levels and the resulting DPE I/O curves were fit with sixth‐order polynomials. Slopes of fitted I/O curves were specified by the first derivative as a function of the level of the primary tones. Slopes of uncorrected spectrally averaged DPE I/O curves were strongly influenced by the NF level. Slopes of NF‐corrected spectrally averaged DPE I/O curves were more representative of the true slope, specified as the slope of the corresponding time‐averaged DPE I/O curve well above its NF. True DPE I/O slopes decreased with level from a mean slope near 1.0 dB/dB at 30 dB SPL and below for all threef2frequencies. Nonmonotonicities in I/O curves werenotseen at very low stimulus levels. At moderate stimulus levels and above, nonmonotonicities in individual DPE I/O curves reduced the slopes of individual I/O curves and reduced the average slope across ears.

ISSN:0001-4966    

DOI:10.1121/1.414558

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Sound envelope temporal fluctuations are important for effective processing of biologically relevant acoustic information including speech, animal vocalizations, sound‐source location, and pitch. Amplitude modulation (AM) of sound envelopes can be encoded in quiet with high fidelity by many auditory neurons including those of the auditory nerve (AN) and cochlear nucleus. From both neurophysiological and clinical perspectives, it is critical to understand the effects of background masking noise on the processing of AM. To further this goal, single‐unit recordings were made from AN fibers in anesthetized chinchillas. Units were classified according to spontaneous firing rate (SR) and threshold. Best frequency (BF) pure‐tone bursts and AM (10–500 Hz) tone bursts were employed as stimuli at several sound levels, both in quiet and in the presence of a continuous wideband noise. It was found that (1) in quiet, low SR AN fibers show the strongest AM coding, followed in order by medium SR and high SR fibers, respectively. (2) AN units of all three classes generally preserve their AM coding even in the presence of loud (0 or +6 dB S/N) background noise and at high sound levels (over 75 dB SPL). (3) This preservation is usually achieved by lowering the average firing rate proportionately to decreases in the synchronous (fundamental frequency) response. (4) For a few AN fibers, the AM coding increases or is reduced in the presence of the background noise. These findings suggest that AN preservation of AM coding in the presence of a continuous masking noise results from shifts in the operating ranges and firing rates of AN fibers resulting from cochlear nonlinearities and adaptive mechanisms.

ISSN:0001-4966    

DOI:10.1121/1.414559

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

An objective quantitative approach to the decision of when to stop averaging sweeps in auditory brain‐stem response (ABR) testing is presented. This decision is based on (1) the knowledge of the amplitude distributions of wave V in the ABRs of normal hearing individuals for varying stimulus levels, (2) calculated estimates of the residual background noise in the average, and (3) use of a quantitative statistical detector of an evoked potential. Several reasons for terminating an average are presented along with a specific protocol for each of the reasons. These protocols provide a general but consistent framework to address the issue of when to stop averaging and should improve the efficiency of ABR testing. Furthermore, it is quite possible to automate the procedure and the decision process.

ISSN:0001-4966    

DOI:10.1121/1.414560

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The strength of the olivocochlear reflex has been assayed by comparing ipsilateral cochlear responses with and without contralateral sound. In humans, ipsilateral cochlear responses have usually been inferred by measuring otoacoustic emissions (OAEs), whereas, in animal work, they have been assessed by measuring compound action potentials (CAPs). Thus reports that the reflex strength is smaller in humans than in animals cannot be interpreted until the differences between the two tests are better understood. The present study directly compares reflex assays using distortion‐product (DP) OAE and CAP measures in the same animals. For ipsilateral frequencies of 2–8 kHz and levels from 25 to 80 dB SPL, efferent reflex strength was computed from the CAP or DPOAE amplitude‐versus‐level curves measured with and without contralateral noise. The ‘‘effective attenuation’’ produced by efferent activation was, with few exceptions, greater when measured with the CAP than with the DPOAE assay. Differences between the two measures increased as frequency increased, with differences as large as 10 dB observed. These results, coupled with previous measurements on humans and animals, suggest that the efferent reflex is at least as strong in humans as has been shown in animal experiments.

ISSN:0001-4966    

DOI:10.1121/1.414508

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Two‐interval forced‐choice tracking was used to measure amplitude discrimination for 20‐Hz vibrotactile test stimuli presented to the thenar eminence of three human observers. For all observers, relative difference threshold could be decreased by adaptation to a 20‐ or 100‐Hz stimulus. Maximal enhancement of discrimination occurred when the amplitude of the adapting stimulus was such that it excited the NP I system to approximately the same degree that the test stimuli did. A signal detection analysis determined that shifts in the observers’ criteria could not consistently account for the enhancement of amplitude discrimination. A more likely explanation, in view of recent physiological discoveries, is that under optimal conditions of adaptation test stimuli differing slightly in amplitude become more distinctive because CNS events underlying the resultant sensory experiences become more refined and stimulus specific.

ISSN:0001-4966    

DOI:10.1121/1.414509

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

A series of experiments investigated listeners’ ability to encode the fundamental frequency (F0) of a group of harmonics (the ‘‘target’’) in the presence of a second, spectrally overlapping, group (the ‘‘masker’’). Experiment 1a was a sequentialF0 discrimination task between two targets, whoseF0s were geometrically centered on 210 Hz, in the presence of a 210‐Hz masker. The target and the masker were bandpass filtered identically, either from 20 to 1420 Hz (‘‘low‐frequency’’ condition) or from 3900 to 5400 Hz (‘‘high‐frequency’’ condition). In the low‐frequency condition the masker affected performance only moderately, regardless of whether it was gated synchronously with, or was turned on 150 ms before and off 150 ms after, each 200‐ms target. In the high‐frequency condition, the synchronous masker also had a moderate effect, but the asynchronous masker reduced performance dramatically. Whatever the masker gating, listeners did not hear the combination of the masker and target in this region as a mixture of two complex tones, but experienced a unitary noiselike or ‘‘crackle’’ percept. Experiment 1b showed that the large deterioration seen in the high‐frequency condition of experiment 1a could be obtained in the low‐frequency condition by reducing theF0 to 62.5 Hz, suggesting that the resolvability of adjacent harmonics was important for the effect.Experiment 2 required listeners to detect a difference inF0 (‘‘ΔF0’’) between two simultaneous groups of components, one filtered in the high region and the other in the low region. Performance was only slightly degraded by a continuous masker filtered in the low region, but was reduced to chance by a masker in the high region. Experiment 3 showed that, as the ΔF0 between a masker and a target in the low region increased from 1% to 8%, listeners identified the mixture as sounding progressively ‘‘less fused,’’ but this was not the case in the high region. It is concluded that listeners are poor at extracting theF0s of two groups of unresolved harmonics in the same frequency region. The experiments provide no evidence that listeners can use the leading part of an asynchronous masker to identify itsF0 and thereby help extract the target’sF0 from the mixture.

ISSN:0001-4966    

DOI:10.1121/1.414510

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

A series of experiments investigated the finding that the fundamental frequency (F0) discrimination of a group of unresolved harmonics (the ‘‘target’’) is impaired more by a masker which starts before and ends after it than when the masker and target are gated on and off together [Carlyon, J. Acoust. Soc. Am.99, 517–524 (1996)]. Generally, the masker was a group of unresolved harmonics with anF0 of 210 Hz, the targetF0s were geometrically centered on 210 Hz, both the targets and the masker were filtered between 3900 and 5400 Hz, and the target duration was 200 ms. The additional deterioration produced by the portion of the masker occurring before the target (the ‘‘forward fringe’’) was greater than that produced by the portion after the target (‘‘backward fringe’’), but both had some effect. The forward and backward fringes reduced sensitivity even when the portion of the masker synchronous with the targets was absent, although the reduction was greatest when it was present. The deterioration was markedly reduced by filtering the fringes into a frequency region remote from the target and the synchronous portion of the masker, by attenuating the fringes by 10 dB, or by presenting them either contralaterally to the target or diotically. It could not be reduced by allowing theF0 of the fringes and of the synchronous portion of the masker to differ greatly from that of the target, or by adding a low‐frequency portion to the fringe, thereby providing an additional cue to the time of transition between fringe and target. Explanations based on peripheral adaptation and on seemingly similar effects previously observed with brief targets [Massaro, J. Acoust. Soc. Am.58, 1059–1065 (1975); Kelly and Watson, J. Acoust. Soc. Am.79, 1934–1938 (1986); Divenyi and Hirsh, Percept. Psychophys.17, 246–252 (1975)] were rejected. The data are discussed in terms of a central mechanism which includes parts of the fringes in its estimate of the pitch of the target/masker mixture.

ISSN:0001-4966    

DOI:10.1121/1.414511

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

This study sought to determine the degree to which the auditory system can perform simultaneous across‐frequency analyses on concurrent, but independent, modulation patterns. This was tested by measuring detection of a multicomponent signal presented against a background comprised of multiple modulation patterns. The results indicated that independent comodulation masking releases (CMRs) could occur simultaneously, but only if the independent modulation patterns were restricted to relatively discrete frequency regions. Moreover, the analysis accomplished in each region did not appear to be optimal. This suggests that there are limits to the independence with which simultaneous across‐frequency analyses can be performed.

ISSN:0001-4966    

DOI:10.1121/1.415228

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

In the probe‐signal method, subjects are required to detect a signal in noise that is presented on the majority of trials at an ‘‘expected’’ (target) frequency but on a minority of trials at an ‘‘unexpected’’ probe frequency. Detection of the probes worsens with increasing separation between the target and probe frequencies. This result has often been interpreted as indicating that subjects monitor the output of a single auditory filter centered at the target frequency. To test this idea, a two‐stage experiment was conducted. In the first stage, auditory‐filter shapes were estimated using the notched‐noise method at center frequencies of 1000, 1259, 1585, and 2000 Hz. These were the frequencies that were used for the targets in the second stage of the experiment. In the second stage, low‐pass filtered white noise was presented continuously. On each trial, a cue tone was presented at one of the four possible target frequencies. The specific frequency was selected randomly on each trial. This was followed by two observation intervals during one of which a further sinusoidal tone was presented. This tone was either a target (the same as the cue frequency) (on 60% of trials), or had one of four possible probe frequencies corresponding to that target. The four probe frequencies were chosen to correspond to specific points on the estimated response curve of the auditory filter centered at the target frequency. The percentage of correct detections of a given probe was compared with that obtained in a separate condition where the frequency of the tone was fixed throughout, the cue frequency always equaled the target frequency, and the target was attenuated by an amount corresponding to the attenuation of the auditory filter at the probe frequency. Two subjects with normal hearing and two subjects with unilateral cochlear hearing loss were used. Comparison of the results for the normal and impaired ears suggests that the detectability of the probes is governed more by the selectivity of the auditory filters than by the ratios of the expected and probe frequencies. However, detection of the probes was generally better than would occur if subjects monitored the output of a single auditory filter centered at the target frequency.

ISSN:0001-4966    

DOI:10.1121/1.414512

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The ability to detect frequency changes in transposed sequences of tones was examined in a series of seven experiments. Listeners were asked to judge which of two transposed (i.e., frequency‐shifted) comparison patterns preserved the sequence of relative frequencies presented in a preceding standard pattern. The task was performed with five‐tone and two‐tone patterns under conditions of high and minimal pattern uncertainty. Regardless of pattern length or level of uncertainty, frequency discrimination thresholds for a change in the relative frequency of a single tone were considerably higher when patterns were transposed than when they were not. There was a tendency for performance to worsen with increasing degrees of transposition (primarily under high uncertainty) but most of the detrimental effects of transposition occurred within the first two semitones of transposition. Minimal uncertainty testing resulted in large improvements with five‐tone patterns (as much as one order of magnitude), but there was no effect of level of uncertainty on performance with two‐tone patterns. Thresholds for changes in two‐tone patterns were similar to (although slightly higher than) those for five‐tone patterns under minimal‐uncertainty testing. This pattern of results reveals that the effects of stimulus complexity (sequence length) and pattern familiarity (level of uncertainty) on relative‐frequency discrimination are quite similar to the effects of these variables on absolute‐frequency discrimination.

ISSN:0001-4966    

DOI:10.1121/1.414513

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP