Interaural Time Discrimination Research Articles

Can monaural temporal masking explain the ongoing precedence effect?

The precedence effect for transient sounds has been proposed to be based primarily on monaural processes, manifested by asymmetric temporal masking. This study explored the potential for monaural explanations with longer ("ongoing") sounds exhibiting the precedence effect. Transient stimuli were single lead-lag noise burst pairs; ongoing stimuli were trains of 63 burst pairs. Unlike with transients, monaural masking data for ongoing sounds showed no advantage for the lead, and are inconsistent with asymmetric audibility as an explanation for ongoing precedence. This result, along with supplementary measurements of interaural time discrimination, suggests different explanations for transient and ongoing precedence.

Interaural time discrimination of envelopes carried on high-frequency tones as a function of level and interaural carrier mismatch.

The present study investigated interaural time discrimination for binaurally mismatched carrier frequencies in listeners with normal hearing. One goal of the investigation was to gain insights into binaural hearing in patients with bilateral cochlear implants, where the coding of interaural time differences (ITDs) may be limited by mismatches in the neural populations receiving stimulation on each side. Temporal envelopes were manipulated to present low frequency timing cues to high-frequency auditory channels. Carrier frequencies near 4 kHz were amplitude modulated at 128 Hz via multiplication with a half-wave rectified sinusoid, and that modulation was either in-phase across ears or delayed to one ear. Detection thresholds for nonzero ITDs were measured for a range of stimulus levels and a range of carrier frequency mismatches. Data were also collected under conditions designed to limit cues based on stimulus spectral spread, including masking and truncation of sidebands associated with modulation. Listeners with normal hearing can detect ITDs in the face of substantial mismatches in carrier frequency across ears. The processing of ITDs in listeners with normal hearing is likely based on spread of excitation into binaurally matched auditory channels. Sensitivity to ITDs in listeners with cochlear implants may depend on spread of current that results in the stimulation of neural populations that share common tonotopic space bilaterally.

Interaural Time Discrimination of Envelopes Carried on High-Frequency Tones as a Function of Level and Interaural Carrier Mismatch

Interaural Time Discrimination of Envelopes Carried on High-Frequency Tones as a Function of Level and Interaural Carrier Mismatch

Observer weighting strategies in interaural time-difference discrimination and monaural level discrimination for a multi-tone complex

Two experiments measured listeners' abilities to weight information from different components in a complex of 553, 753, and 953 Hz. The goal was to determine whether or not the ability to adjust perceptual weights generalized across tasks. Weights were measured by binary logistic regression between stimulus values that were sampled from Gaussian distributions and listeners' responses. The first task was interaural time discrimination in which listeners judged the laterality of the target component. The second task was monaural level discrimination in which listeners indicated whether the level of the target component decreased or increased across two intervals. For both experiments, each of the three components served as the target. Ten listeners participated in both experiments. The results showed that those individuals who adjusted perceptual weights in the interaural time experiment could also do so in the monaural level discrimination task. The fact that the same individuals appeared to be analytic in both tasks is an indication that the weights measure the ability to attend to a particular region of the spectrum while ignoring other spectral regions.

Interference and enhancement effects on interaural time discrimination and level discrimination in listeners with normal hearing and those with hearing loss.

It is known that many listeners with sensorineural hearing loss (SNHL) have difficulty performing binaural tasks. In this study, interference and enhancement effects on interaural time discrimination and level discrimination were investigated in 4 listeners with normal hearing (NH) and 7 listeners with SNHL. Just-noticeable differences were measured using 1/3-octave narrowband noises centered at 0.5 and 4 kHz. Noises were presented in isolation and together at equivalent sound pressure level (EqSPL) and equivalent sensation level (EqSL). Each noise served as target and distractor in the dual-band conditions. Congruent conditions included interaural differences in both noises that varied together, and incongruent conditions included an interaural difference in one noise with the second noise diotic. No significant enhancement effects were observed for either group in either task. Interference effects for the NH group were limited to the interaural level discrimination task in the 0.5-kHz target and 4-kHz distractor condition. Performance of participants with SNHL was similar to that of the NH group for interaural time discrimination with noises at EqSL but not EqSPL. In interaural level discrimination, listeners with SNHL demonstrated interference with a 4-kHz target and 0.5-kHz distractor. Results indicated that the relative levels of low- and high-frequency targets and distractors could affect binaural performance of individuals with SNHL but that in some conditions listeners with SNHL performed similarly to those with normal hearing. Implications of these results for binaural clinical tests and hearing aid fitting strategies are discussed.

Assessment of dolphin (Tursiops truncatus) auditory sensitivity and hearing loss using jawphones.

Devices known as jawphones have previously been used to measure interaural time and intensity discrimination in dolphins. This study introduces their use for measuring hearing sensitivity in dolphins. Auditory thresholds were measured behaviorally against natural background noise for two bottlenose dolphins (Tursiops truncatus); a 14-year-old female and a 33-year-old male. Stimuli were delivered to each ear independently by placing jawphones directly over the pan bone of the dolphin's lower jaw, the assumed site of best reception. The shape of the female dolphin's auditory functions, including comparison measurements made in the free field, favorably matches that of the accepted standard audiogram for the species. Thresholds previously measured for the male dolphin at 26 years of age indicated a sensitivity difference between the ears of 2-3 dB between 4-10 kHz, which was considered unremarkable at the time. Thresholds for the male dolphin reported in this study suggest a high-frequency loss compared to the standard audiogram. Both of the male's ears have lost sensitivity to frequencies above 55 kHz and the right ear is 16-33 dB less sensitive than the left ear over the 10-40 kHz range, suggesting that males of the species may lose sensitivity as a function of age. The results of this study support the use of jawphones for the measurement of dolphin auditory sensitivity.

Comparison of Interaural Time and Intensity Difference Discrimination in Patients with Temporal Lobe Lesions

We studied the sound lateralization in 15 patients with left (12 patients) or bilateral (3 patients) temporal lobe cerebrovascular lesions. The lesions included the auditory cortex. Interaural time difference (ITD) and interaural intensity difference (IID) discriminations were separately measured by using a self-recording apparatus. All 12 patients with left temporal lobe lesions could discriminate ITD, and their ITD thresholds were significantly higher than those in healthy volunteers. None of the 3 patients with bilateral temporal lobe lesions could discriminate ITD. On the other hand, all patients with either unilateral or bilateral temporal lobe lesions could discriminate IID and their IID thresholds were significantly higher than those in healthy volunteers. The present findings indicate that the auditory cortex plays an important role in discriminating both cues, but that the role may not be essential for IID.

CETACEAN AUDITORY PSYCHOPHYSICS

ABSTRACT The dolphin continues to capture the imagination of investigators because of its ability to echolocate. Echolocation is essentially a special extension and adaptation of the dolphin's hearing system, coupled with the animal's ability to generate special sounds. Humans have demonstrated the ability to judge room size based on reverberation from a voice, and some of the visually challenged use self-generated sounds to detect large reflective objects. Echolocation represents a highly refined acoustic ability on a broad acoustic sensory continuum. Research on the auditory and echolocation performance of cetaceans has moved forward slowly due to limited animal resources and the general high cost of maintaining these animals in a laboratory environment. This paper reviews some of the more relevant psychoacoustic data on cetaceans, and concentrates on the bottlenose dolphin Tursiops truncatus. The information presented is not at all exhaustive. Early work with dolphins focused mainly on the animal's ability to use its echolocation system. Once echolocation capability was demonstrated using a blindfolded dolphin, the quest to understand dolphin sonar moved from qualifying the dolphin's echolocation skill to quantifying its basic capabilities. Psychophysics, and more precisely psychoacoustics, provides the tools to study dolphin echolocation. The procedures, theories and even the apparatuses from the traditional psychoacoustics laboratory are adapted to the dolphin experimental setting to measure and analyze the sensory phenomenon of dolphin echolocation. Basic auditory phenomena such as the audiogram, the effects of masking, critical ratio and critical band, and interaural time and intensity discrimination capabilities have been explored in the dolphin. Additionally, special experiments investigating the psychoacoustics of the echolocation system in particular have been conducted.

Application of confidence intervals and joint confidence regions to the estimation of psychometric functions

A mathematical description of a psychometric function with two free parameters is fit to fixed‐increment data by a nonlinear gradient search technique that incorporates a weighted least squares algorithm. The statistical confidence in the parameter estimates is considered by comparing results from standard confidence interval analysis with those from joint confidence region analysis. Confidence intervals estimate the variability of each parameter alone, ignoring the interaction between the parameters, whereas the joint confidence region gives the confidence in the joint estimation of both parameters together. In our study, both analyses were applied to fixed‐increment data from a variety of tests (interaural time and intensity discrimination and N0Sπ binaural detection) and subjects for two‐interval forced choice experiments. Results show that the parameter estimates are affected primarily by the data collected near the midpoint which is not surprising since the parameters in the model are the slope and translation at the midpoint. However, the joint confidence region analysis shows that the statistical confidence of the parameter estimates are greatly affected by data collected at levels that give performance near chance and near perfect. [Work supported by NIDCD (Grant DC00100).]

Cross-frequency interaction for interaural time and intensity discrimination in listeners with normal hearing and impaired hearing

The ability to discriminate interaural differences in complex signals was examined in listeners with normal hearing and with bilateral sensorineural hearing impairment. Just noticeable differences (jnds) for interaural time and interaural intensity were obtained using 1/3-octave narrow band noise (NBN) stimuli centered at 500 and 4000 Hz in three conditions. In condition 1 the NBNs were played in isolation; in condition 2 the NBNs were played simultaneously with congruent interaural information across frequency; and in condition 3 the NBNs were played simultaneously with either the 500-Hz band dichotic and the 4000-Hz band diotic (3a) or the 500-Hz band diotic and the 4000-Hz band dichotic (3b). The best interaural time and intensity discrimination for the listeners with impaired hearing is seen when both bands contain congruent interaural information (condition 2). Subjects with impaired hearing and normal hearing show reduced sensitivity to interaural time differences when the 500-Hz band is diotic and discrimination is based on the 4000-Hz band (3b). The listeners with impaired hearing also demonstrate poorest interaural intensity discrimination for this condition (3b). Results suggest that subjects with impaired hearing may benefit from congruent interaural information in more than one frequency region. [Work supported by the Dept. of Veterans Affairs and NIH Grant No. DC00428.]

Multiple sclerosis lesions of the auditory pons are not silent.

To understand the relationship between brainstem lesions and auditory neurology in patients with multiple sclerosis, we compared behavioural, electrophysiological and imaging data in 38 patients with probable or definite multiple sclerosis and normal or near normal hearing. Behavioural measures included (i) general hearing tests (audiogram, speech discrimination) and (ii) hearing tests likely to be critically dependent upon brainstem processing (masking level difference, interaural time and level discrimination). Brainstem auditory evoked potentials provided the electrophysiological data. Multiplanar high-resolution MRI of the brainstem provided the anatomical data. Interaural time discrimination for high-frequency sounds was by far the most sensitive of all tests with abnormalities in 71% of all subjects. Whenever any other test was abnormal this test was always abnormal. Interaural time discrimination for low-frequency sounds and evoked potentials were closely related and next most sensitive with abnormalities in approximately 40% of all subjects. Interaural level discrimination and masking level difference were least sensitive with abnormalities in < 10% of subjects. Speech discrimination scores correlated significantly with the masking level differences, as well as with interaural time discrimination for high-frequency sounds. Pontine lesions were found in five of the 16 patients, in whom an objective method for detecting magnetic resonance lesions could be applied. All four with lesions involving the pontine auditory pathway had marked abnormalities in interaural time discrimination and evoked potentials. None of the other 12 had evoked potentials abnormalities. We conclude that neurological tests requiring precise neural timing can reveal behavioural deficits for multiple sclerosis lesions of the auditory pons that are otherwise 'silent'. Of all neurological systems the auditory system at the level of the pons is probably the most sensitive to multiple sclerosis lesions, because of its exceptional dependence upon neural timing in the microsecond range and the lack of redundancy in the encoding of high-frequency sounds. Precise neural timing may be critical for some aspects of speech processing.

方向感検査による小脳橋角部腫ようの早期診断

The authors report 22 patients with cerebello-pontine angle lesions: 15 with acoustic tumors and 7 with menigiomas, epidermoids or trigeminal neurinomas. Nearly all showed close to normal or only slightly impaired hearing threshold levels on their audiograms.All patients received the full battery of directional hearing tests, speech audiometry, acoustic reflex threshold, and decay test, ENG and caloric test, and ABR test. Finally, they received CT scanning or MRI tests, and cerebello-pontine angle lesions were confirmed.Twenty patients (91%) had very positive findings, especially increased thresholds of interaural time discrimination in the directional hearing tests. This test has good sensitivity for retrocochlear lesions; it is very simple and needs only a few minutes to perform. The authors conclude that this test provides the most useful information in the early diagnosis of cerebello-pontine angle tumors, as a screening test.We present a flow-chart of the battery of tests in which the directional hearing test is performed in the early stage of test procedures.

難聴・耳鳴に対するセレボートの臨床効果

The therapeutic effect of Celeport® was studied in 35 patients with chronic sensorineural hearing loss. Their ages ranged from 41 to 79 years (average 65.2). The oral dose of Celeport® was 150 mg daily. The severity of hearing loss and of tinnitus was recorded before and the end of treatment.The medication was effective in improving hearing or reducing tinnitus in 51.4% of the patients. In 35.4% of 65 ears hearing improved. The average hearing level, maximum discrimination level in speech articulation test and threshold of interaural time discrimination in directional hearing were improved. In the patients whose hearing was improved subjectively, the average hearing level and threshold of interaural time discrimination were improved significantly (p<0.001 and p<0.05). However, there was no significant difference in maximum discrimination level before and after of the medication. These results indicate that this drug can improve neural function in the brainstem.This drug reduced tinnitus in 40.0% of the 25 patients with hearing loss plus tinnitus. It was interesting that the medication caused no reduction of pure tone type tinnitus. There was a significant difference in efficacy between band noise type tinnitus and pure tone type tinnitus (p=0.0035).A side effect, erosion of the oral mucosa was detected in one patient, who kept the drug in his mouth for several minutes.Celeport® is considered to be useful in the treatment of chronic hearing loss and/or tinnitus due to neural dysfunction in the brainstem.

Interaural time discrimination in the bottlenose dolphin

Binaural hearing is the advantage of two ears over one. Localization studies with marine mammals have shown that acoustic localization underwater is mediated by the same binaural mechanisms used by other mammals in air. Dolphin echolocation abilities are enhanced by the use of two separate receivers. Au and Moore [J. Acoust. Soc. Am. 75, 255–262 (1984)] modeled the animal’s hearing system as a two‐element array using receiving beam pattern measurements. The present study sought to measure the interaural phenomena of interaural time difference discrimination (ITD) and interaural intensity difference discrimination (IID). A dolphin was trained to accept contact hydrophones, attached by suction cups, to the outer surface of the lower jaw in the area of the pan‐bone and to perform detection threshold experiments. Audiometric data at several frequencies indicate that the dolphin minimum threshold for pure tones may be 20 dB different when measured with jaw phones. The first behavioral measures of ITD and IID discrimination and for a cetacean have been obtained. ITD and IID for clicks trains of various frequencies, presented with interclick intervals simulating various echolocation target ranges were collected. This method of collecting audiometric data represents a new approach for assessing bearing capabilities in cetaceans.

Effects of multiple sclerosis brainstem lesions on sound lateralization and brainstem auditory evoked potentials

Magnetic resonance (MR) imaging, brainstem auditory evoked potentials (BAEPs), and tests of interaural time and level discrimination were performed on sixteen subjects with multiple sclerosis (MS). Objective criteria were used to define MR lesions. Of the eleven subjects in whom no pontine lesions were detected and the one subject who had pontine lesions that did not encroach upon the auditory pathways, all had normal BAEPs and interaural level discrimination, although a few had abnormal interaural time discrimination. Of four subjects with lesions involving the pontine auditory pathway, all had both abnormal BAEPs and abnormal interaural time discrimination; one also had abnormal interaural level discrimination. Analysis of the data suggest the following: waves I and II are generated peripheral to the middle of the ventral acoustic stria (VAS); wave III is generated ipsilaterally in the region of the rostral VAS, caudal superior olivary complex (SOC) and trapezoid body (TB); and waves V and L are generated contralaterally, rostral to the SOC-TB. The region of the ipsilateral rostral SOC-TB is implicated as part of the pathway involved in the generation of waves V and L. Interaural time discrimination of both high and low frequency stimuli were affected by all brainstem lesions that encroached on auditory pathways. A unilateral lesion in the region of the LL affected interaural time discrimination for low-frequency stimuli less severely than bilateral lesions of the LL or a unilateral lesion of the VAS. The only interaural level discrimination abnormality occurred for a subject with a unilateral lesion involving the entire rostral VAS. It appears that detailed analysis of lesion locations coupled with electrophysiological and psychophysical data holds promise for testing hypotheses concerning the function of various human auditory brainstem structures.

Binaural auditory processing in multiple sclerosis subjects

In order to relate human auditory processing to physiological and anatomical experimental animal data, we have examined the interrelationships between behavioral, electrophysiological and anatomical data obtained from human subjects with focal brainstem lesions. Thirty-eight subjects with multiple sclerosis were studied with tests of interaural time and level discrimination (just noticeable differences or jnds), brainstem auditory evoked potentials and magnetic resonance (MR) imaging. Interaural testing used two types of stimuli, high-pass (> 4000 Hz) and low-pass (< 1000 Hz) noise bursts. Abnormal time jnds (Tjnd) were far more common than abnormal level jnds (70% vs 11%); especially for the high-pass (Hp) noise (70% abnormal vs 40% abnormal for low-pass (Lp) noise). The HpTjnd could be abnormal with no other abnormalities; however, whenever the BAEPs, LpTjnd and/or level jnds were abnormal HpTjnd was always abnormal. Abnormal wave III amplitude was associated with abnormalities in both time jnds, but abnormal wave III latency with only abnormal HpTjnds. Abnormal wave V amplitude, when unilateral, was associated with a major HpTjnd abnormality, and, when bilateral, with both HpTjnd and LpTjnd major abnormalities. Sixteen of the subjects had their MR scans obtained with a uniform protocol and could be analyzed with objective criteria. In all four subjects with lesions involving the pontine auditory pathway, the BAEPs and both time jnds were abnormal. Of the twelve subjects with no lesions involving the pontine auditory pathway, all had normal BAEPs and level jnds. ten had normal LpTjnds, but only five had normal HpTjnds. We conclude that interaural time discrimination is closely related to the BAEPs and is dependent upon the stimulus spectrum. Redundant encoding of low-frequency sounds in the discharge patterns of auditory neurons, may explain why the HpTjnd is a better indicator of neural desynchrony than the LpTjnd. Encroachment of MS lesions upon the pontine auditory pathway always is associated with abnormal BAEPs and abnormal interaural time discrimination but may have normal interaural level discrimination. Our data provide one of the most direct demonstrations in humans of relationships among auditory performance, evoked potentials and anatomy. We present a model showing that many of these interrelationships can be readily interpreted using ideas developed from work on animals, even though these relationships could not have been predicted with confidence beforehand. This work provides a clear advance in our understanding of human auditory processing and should serve as a basis for future studies.

Comparison of adaptive procedures for obtaining psychophysical thresholds using computer simulation

In order to compare the efficacy and efficiency of various adaptive psychophysical procedures, a program that simulates these procedures has been developed and used to compare several adaptive methods for three experiments: binaural detection, interaural intensity discrimination, and interaural time discrimination. The simulation program takes as input a psychometric function, which specifies the probability of a correct response for any value of the adapting variable. A threshold is generated by choosing the next value of the variable according to the rules of the procedure being simulated, including the rules for stopping and calculating the threshold estimate. The software generates distributions of thresholds and statistical analyses are performed, such as bias and variance of the obtained distribution. The program has been applied to psychometric functions appropriate for normal-hearing and hearing-impaired listeners for each of the three experiments noted above, and has compared a number of adaptive procedures including the Transformed Up-Down (such as 2-down-1-up), PEST, BUDTIF, and Weighted Up-Down methods. [Work supported by NIDCD and Dept. of Education.]

Diagnosis of cerebellopontine angle tumors.

Between 1980 and 1990, 9,176 patients suffering from otoneurological disorders were investigated in our clinic. Sixty-six (0.72%) internal auditory canal or other cerebellopontine angle tumors (CPAT) were diagnosed. Brainstem auditory evoked potentials (BAEP), interaural time discrimination, and magnetic resonance imaging (MRI) revealed to be the most sensitive and efficient tests for the detection of CPAT. However, the investigation of otoneurological symptoms cannot be limited to BAEP and MRI on the a priori of a hypothetical CPAT. Electrophysiologic tests such as impedancemetry, and electronystagmographic testing are also needed to elucidate the many causes of otoneurological symptoms.

Sound Localization and Sensitivity to Interaural Time Differences in Human Infants

The role of interaural time differences in infants' sound localization was investigated. One experiment on free-field sound localization corroborated previous findings that the minimum audible angle changes substantially toward the end of the first half year after birth, and 3 experiments explored interaural time discrimination in that age range. The first of these 3 experiments used an adaptive psychophysical procedure, showing that infants responded appropriately in a sound lateralization task that provides a direct measure of interaural time discrimination. The other 2 experiments improved on the psychophysical procedure by taking into account the ceiling level on performance in the task. Infants aged 16, 20, and 28 weeks had thresholds in the range of 50 to 75 microsec, with no apparent age difference. These thresholds were much lower than would be predicted from studies of free-field sound localization, indicating that sensitivity to interaural time differences is not a limiting factor for the precision of sound localization in this age range. Instead, age-related changes in free-field sound localization may reflect the need to integrate across different localization cues and to calibrate the changing values of cues due to head growth.

Speech Intelligibility Enhancement by Interaural Magnification.

Hearing-impaired persons suffer from loss of speech intelligibility in noisy environments as well as from a decrease in binaural performance (e.g., interaural time and intensity discrimination). In order to improve speech intelligibility for spatially separated signal and noise sources by compensating for the reduced binaural performance, we implemented an algorithm increasing binaural dissimilarity on a digital signal processor. This algorithm is a modification of the proposal by Durlach & Pang (J. Acoust. Soc. Am. 1986; 80: 1849-1850). Speech discrimination experiments were performed under simulated spatial conditions with recordings of the speech material and the interfering noise source using a dummy head. With increasing complexity of the spatial masking condition, the binaural advantage as well as the benefits from the algorithm decreased for normal listeners. For four sensorineurally hearing-impaired listeners, only one spatial condition was tested. In addition, a battery of binaural tests (binaural masking level difference, interaural time and intensity discrimination) was performed. The results indicate that the benefits from the signal-processing technique are correlated with the remaining binaural performance.