Masking Level Difference Research Articles

Comodulation masking release for suprathreshold signals.

Previous studies of comodulation masking release (CMR) have shown that the threshold of a signal masked by a narrow band of noise centered on the signal can be improved by the presence of energy remote from the signal frequency, provided that the amplitude envelope of the remote noise is correlated with the envelope of the noise centered on the signal frequency. The extent to which an advantage in comodulated noise may apply also to suprathreshold signals is currently being investigated. The present study deals specifically with the ability to detect changes in the intensity of a pure tone presented in a narrow band of noise, both as a function of the level of the standard, and whether comodulated flanking bands are present or not. Preliminary data indicate that the amount of CMR with increase in the level of the standard depends upon the basis of comparison between comodulated and uncorrelated noise conditions. Comparing equal SPL standards, advantage for comodulated noise persists until the standard in the comodulated case is about 10 to 15 dB above masked threshold; comparing equal SL standards, performance is generally better for the uncorrelated envelope case. Data on similar conditions will be presented for the masking-level difference paradigm. [Work supported by AFOSR.]

Free-field release from masking

Free-field release from masking was studied as a function of the spatial separation of a signal and masker in a two-interval, forced-choice (2IFC) adaptive paradigm. The signal was a 250-ms train of clicks (100/s) generated by filtering 50-microseconds pulses with a TDH-49 speaker (0.9 to 9.0 kHz). The masker was continuous broadband (0.7 to 11 kHz) white noise presented at a level of 44 dBA measured at the position of the subject's head. In experiment I, masked and absolute thresholds were measured for 36 signal source locations (10 degree increments) along the horizontal plane as a function of seven masking source locations (30 degree increments). In experiment II, both absolute and masked thresholds were measured for seven signal locations along three vertical planes located at azimuthal rotations of 0 degrees (median vertical plane), 45 degrees, and 90 degrees. In experiment III, monaural absolute and masked thresholds were measured for various signal-masker configurations. Masking-level differences (MLDs) were computed relative to the condition where the signal and mask were in front of the subjects after using absolute thresholds to account for differences in the signal's sound-pressure level (SPL) due to direction. Maximum MLDs were 15 dB along the horizontal plane, 8 dB along the vertical, and 9 dB under monaural conditions.

Binaural masking level differences in children with a history of otitis media : Moore D.R.; Hutchings M.E.; Meyer S.E.AUDIOLOGY (1991) 30/2 (91–101)

Binaural masking level differences in children with a history of otitis media : Moore D.R.; Hutchings M.E.; Meyer S.E.AUDIOLOGY (1991) 30/2 (91–101)

Otitis Media With Effusion in Children: Binaural Hearing Before and After Corrective Surgery

The masking-level difference (MLD) was investigated in a group of children having no known history of ear disease, and a group of children having a history of otitis media with effusion and hearing loss. The MLD is a psychoacoustic measure of the sensitivity of the auditory system to subtle interaural difference cues of time and amplitude and relates to the ability of the listener to detect and to recognize signals in noisy backgrounds. In the otitis media with effusion group, MLDs were measured both before and 1 and 3 months after the placement of pressure equalization tubes. The MLDs were often abnormally small in the otitis media with effusion group before surgery, when hearing loss was present. Significantly, MLDs sometimes remained abnormally small after surgery (after normal hearing had returned). The postsurgery MLD was particularly likely to be abnormally reduced in subjects who had experienced asymmetric losses of hearing.

Comodulation masking release and the masking-level difference

An experiment was performed to determine if the mechanism that mediates comodulation masking release (CMR) is associated with that used to improve detection by the masking-level difference (MLD). The experiment consisted of first improving detectability of a masked diotic tone burst by adding a synchronous noise band at another frequency region (CMR), and then measuring an MLD in the usual manner, by inverting the tone-burst signal to one ear. Results indicate that a substantial MLD can be measured for a signal whose detectability has already been improved by CMR. However, that MLD (9 dB) is smaller than that measured in random noise (14 dB). Put another way, a small CMR (4 dB) can be produced even when the detectability of a stimulus has already been improved due to the MLD. These data are in general agreement with those of Hall et al. [J. Acoust. Soc. Am. 83, 1839-1845 (1988)] and Schooneveldt and Moore [J. Acoust. Soc. Am. 85, 262-272 (1989)].

Binaural modulation masking

Modulation thresholds were measured in three subjects for a sinusoidally amplitude-modulated (SAM) wideband noise (the signal) in the presence of a second amplitude-modulated wideband noise (the masker). In monaural conditions (Mm-Sm) masker and signal were presented to only one ear; in binaural conditions (M0-S pi) the masker was presented diotically while the phase of modulation of the SAM noise signal was inverted in one ear relative to the other. In experiment 1 masker modulation frequency (fm) was fixed at 16 Hz, and signal modulation frequency (fs) was varied from 2-512 Hz. For monaural presentation, masking generally decreased as fs diverged from fm, although there was a secondary increase in masking for very low signal modulation frequencies, as reported previously [Bacon and Grantham, J. Acoust. Soc. Am. 85, 2575-2580 (1989)]. The binaural masking patterns did not show this low-frequency upturn: binaural thresholds continued to improve as fs decreased from 16 to 2 Hz. Thus, comparing masked monaural and masked binaural thresholds, there was an average binaural advantage, or masking-level difference (MLD) of 9.4 dB at fs = 2 Hz and 5.3 dB at fs = 4 Hz. In addition, there were positive MLDs for the on-frequency condition (fm = fs = 16 Hz: average MLD = 4.4 dB) and for the highest signal frequency tested (fs = 512 Hz: average MLD = 7.3 dB). In experiment 2 the signal was a SAM noise (fs = 16 Hz), and the masker was a wideband noise, amplitude-modulated by a narrow band of noise centered at fs. There was no effect on monaural or binaural thresholds as masker modulator bandwidth was varied from 4 to 20 Hz (the average MLD remained constant at 8.0 dB), which suggests that the observed "tuning" for modulation may be based on temporal pattern discrimination and not on a critical-band-like filtering mechanism. In a final condition the masker modulator was a 10-Hz-wide band of noise centered at the 64-Hz signal modulation frequency. The average MLD in this case was 7.4 dB. The results are discussed in terms of various binaural capacities that probably play a role in binaural release from modulation masking, including detection of varying interaural intensity differences (IIDs) and discrimination of interaural correlation.

Binaural Masking Level Differences in Children with a History of Otitis Media

The binaural hearing of children with and without a history of otitis media (OM) was assessed by determining their binaural masking level differences (BMLDs). The test was also administered to a group of adults. BMLDs for the non-OM children were not significantly different from those of adults. However the mean BMLD of the OM children was significantly lower than that of non-OM children. Some children in the OM group had small (15-25 dB HL) sensitivity deficits in one or both ears. However, no correlation was found between BMLD and hearing level in cases of either symmetric or asymmetric loss. Exclusion of OM children with residual middle ear abnormalities did not abolish the significant difference between the OM and non-OM groups. We suggest that the small BMLDs in the OM group may be associated with these children having difficulties detecting and attending to signals in noisy environments.

Subclinical changes of auditory function in the aged

Elderly normal hearing subjects were studied using tests for cochlear functions (remote masking, brief tone audiometry, critical ratio, evoked otoacoustic emission), for neural auditory function (tone decay) and for central auditory functions (ipsilateral vs. contralateral acoustic reflex, tonal masking level difference). The findings demonstrated that ageing can cause subclinical abnormalities, almost consistently localized to the inner ear vibrating structures and only occasionally to the sensory and/or neural elements.

Psychophysical Measures of Central Auditory Dysfunction in Multiple Sclerosis

Central auditory function was assessed in 15 patients with multiple sclerosis (MS) to determine whether the demyelinating lesions resulted in disruption of temporal processing. Auditory evoked potential (AEP) recordings included all three latency regions: Auditory brain stem responses (ABRs), midlatency responses (MLRs), and long-latency responses (LLRs). Two psychophysical tasks thought to involve temporal processing were used: a monaural-processing task (gap-detection) and a binaural-processing task (masking level difference; MLD). Further, AEP abnormalities and psychophysical performance deficits were related to lesion location, based on magnetic resonance imaging (MRI) scans. Reduced MLDs were seen in six MS subjects. Abnormal MLDs were always accompanied by abnormal ABRs and MLRs, and compared to subjects with normal MLDs, the subjects with abnormal MLDs were more likely to have bilateral abnormalities in the AEPs. Further, MLR indices of abnormal binaural interaction appeared to be specifically related to the psychophysical measure of binaural processing. The MRI data of these patients indicated widespread involvement of the auditory pathway. MS subjects with abnormal MRI signals restricted to levels caudal to the lateral lemniscus did not have abnormal MLDs. Gap-detection thresholds were more resistant to the effects of the demyelinating lesions; only two subjects had abnormal gap-detection thresholds. These subjects had extensive AEP abnormalities (bilaterally, in all three latency regions). The gap-detection thresholds were most specifically related to abnormalities of the LLRs. In addition, the subjects with elevated gap-detection thresholds were the only ones with a prolonged interval between the ABRs and MLRs. Thus, efficient neural conduction between the upper brain stem and auditory cortex appears to be crucial for normal monaural temporal processing. The results indicate that demyelinating lesions can cause deficits in temporal processing in the central auditory pathway. However, auditory temporal processing is not a unitary phenomenon since abnormalities at different levels of the auditory system disrupt different types of temporal processing. Finally, abnormal psychophysical performance was not seen in all subjects with AEP and MRI evidence of involvement of the auditory pathway; rather, these psychophysical measures appeared to be sensitive to disruption only in specific portions of the auditory system.

Loudness and the binaural masking level difference

The large threshold difference found between a low-frequency antiphasic signal, Sπ, and in-phase signal, SO, in the presence of a homophasic noise masker (NO) is referred to as the binaural masking level difference (BMLD). The magnitude of the BMLD diminishes when the antiphasic signal is increased to suprathreshold levels. The decrease in effectiveness of a 180° phase-shifted signal, at suprathreshold levels, produces a concomitant decrease in perceived loudness of the signal. This decrease in effectiveness at the higher signal levels was examined in a loudness-matching paradigm. A change in neural processing was hypothesized to account for the variation in loudness, detectability, discrimination, and identification when the signal level approached 20 dB SL.

Localization and binaural detection with monaural and binaural amplification

As part of a study comparing the effects of monaural and binaural amplification, localization and binaural detection was measured for two groups of listeners with bilateral, sensorineural hearing losses. The subjects in one group have moderate‐to‐severe, flat losses; the subjects in the other group have moderate‐to‐severe, high‐frequency losses. Masking level differences (MLDs) were obtained using speech‐spectrum noise maskers and octave‐band speech‐spectrum noise targets. Localization in the horizontal plane (nine locations from − 90 to + 90 deg) was measured using three‐word phrases in an anechoic and a reverberant environment. Both binaural detection and localization performance vary between subject groups and within subject groups, and all subjects generally have smaller MLDs and poorer localization than normal‐hearing listeners. The subjects with flat hearing losses usually have better localization and detection with binaural amplification than monaural amplification, and the high‐frequency hearing‐loss subjects tend to have better overall performance than the flat hearing‐loss listeners. The data for all the subjects combined show a tendency for RMS errors in localization to decrease as MLDs increase. [Work supported by NIH.]

Binaural masking experiments using noise maskers with frequency‐dependent interaural phase differences. I: Influence of signal and masker duration

In this paper previous experiments on auditory filter shapes in binaural masking experiments [A. Kohlrausch, J. Acoust. Soc. Am. 84, 573-583 (1988)] are extended to a wider range of masker and signal durations. The masker was a dichotic broadband noise with frequency-dependent interaural parameters. The interaural phase difference of the masker was 0 below 500 Hz and pi above 500 Hz. Signal frequency varied between 200 and 800 Hz, and the signal was presented either monaurally (Sm) or binaurally in antiphase (S pi). In the first experiment, the masker duration was fixed at 500 ms and signals of 250 and 20 ms were used. In the second experiment, the signal duration was fixed at 20 ms, and the masker duration was reduced to 25 ms. The results from both experiments are consistent with studies using No or N pi maskers: The binaural masking level difference (BMLD) increases slightly for shorter test signals and decreases strongly for short maskers. The BMLD patterns of the first experiment are well described by the auditory-filter model derived for stationary test signals, if the additional influence of "off-frequency listening" for the short test signal is taken into account. The BMLDs resulting from the second experiment (25-ms masker), however, are much lower than predicted by this filter model This outcome supports previous observations that binaural unmasking becomes less effective for very short masker durations and indicates that this effect is even stronger for maskers with a complex structure of interaural parameters.

Binaural masking experiments using noise maskers with frequency-dependent interaural phase differences. II: Influence of frequency and interaural-phase uncertainty

This study investigates whether binaural signal detection is improved by the listener's previous knowledge about the interaural phase relations of masker and test signal. Binaural masked thresholds were measured for a 500-ms dichotic noise masker that had an interaural phase difference of 0 below 500 Hz and of pi above 500 Hz. The thresholds for two difference 20-ms test signals were determined within the same measurement using an interleaved adaptive 3-interval forced-choice (3IFC) procedure. In each 3IFC trial, both signals could occur with equal probability (uncertainty). The two signals differed in frequency and interaural phase in such a way that one signal always had a frequency above the masker edge frequency (500 Hz) and no interaural phase difference (So), whereas the other signal frequency was below 500 Hz and the interaural phase difference was pi (S pi). The frequencies of a signal pair remained fixed during the whole 3IFC track. These two signals thus lead to two different binaural conditions, i.e., NoS pi for the low-frequency signal and N pi So for the high-frequency signal. For comparison, binaural masked thresholds were measured with the same masker for fixed signal frequency and phase. The binaural masking level differences (BMLDs) resulting from the two experimental conditions show no significant difference. This indicates that the binaural system is able to apply different internal transformations or processing strategies simultaneously in different critical bands and even within the same critical band.

Patterns of Auditory Abnormality in Multiple Sclerosis

This paper stresses the relatively high prevalence of auditory abnormality in multiple sclerosis. An auditory test battery consisting of the acoustic reflex (AR), the auditory brainstem response (ABR), the masking level difference (MLD) and speech audiometry (SA) was administered to 62 patients with diagnosed 'definite' multiple sclerosis. The AR showed the highest identification rate (71%). SA was next (55%), followed by the ABR (52%) and the MLD (45%). The combination of an abnormality on either AR, ABR or SA yielded a 90% identification rate. Interestingly, the combination of AR or SA or MLD yielded an 87% identification rate without any contribution from ABR.

Predicting Binaural Hearing After Stapedectomy From Presurgery Results

Past research has indicated that binaural hearing abilities are not always normal in patients having otosclerosis who have received bilaterally normal pure-tone thresholds through stapedectomy. This study investigated whether poststapedectomy performance on the binaural masking-level difference test could be predicted from presurgery tests in which the two ears were stimulated at an equal sound pressure level and at an equal sensation level. Results indicated that (1) binaural hearing usually improved following stapedectomy, though some patients maintained abnormally poor binaural hearing even though postsurgery pure-tone thresholds were normal; and (2) prediction of postsurgery results was poor, with postsurgery results often being underestimated from presurgery results. Results are interpreted in terms of possible forms of auditory deprivation and acoustic crossover.

Auditory backward recognition masking: Effect of interaural phase on masker efficacy

The effect of interaural phase on pitch and lateralization recognition was examined. Tonal signals were followed by a variable interstimulus interval and an interference tone. Stimuli were presented in a binaural masking-level difference paradigm as a means of manipulating the perceptual location of the signal within the head. In separate tasks, subjects were required to recognize the pitch or location of the signal. The pitch-recognition task resulted in the expected increase in performance as the interstimulus interval increased. There was no effect of interaural phase on pitch-recognition performance. There was no significant effect of interstimulus interval on performance in location recognition. Subjects were proficient at recognizing location of the signal regardless of the interstimulus interval. The data suggest that a strict single-channel interpretation of the preperceptual storage model of backward auditory recognition is inadequate.

Binaural forward and backward masking: Evidence for sluggishness in binaural detection

The threshold of a short interaurally phase-inverted probe tone (20 ms, 500 Hz, S pi) was obtained in the presence of a 750-ms noise masker that was switched after 375 ms from interaurally phase-inverted (N pi) to interaurally in-phase (No). As the delay between probe-tone offset and noise phase transition is increased, the threshold decays from the N pi S pi threshold (masking level difference = 0 dB) to the No S pi threshold (masking level difference = 15 dB). The decay in this "binaural" situation is substantially slower than in a comparable "monaural" situation, where the interaural phase of the masker is held constant (N pi), but the level of the masker is reduced by 15 dB. The prolonged decay provides evidence for additional binaural sluggishness associated with "binaural forward masking." In a second experiment, "binaural backward masking" is studied by time reversing the maskers described above. Again, the situation where the phase is switched from No to N pi exhibits a slower transition than the situation with constant interaural phase (N pi) and a 15-dB increase in the level of the masker. The data for the binaural situations are compatible with the results of a related experiment, previously reported by Grantham and Wightman [J. Acoust. Soc. Am. 65, 1509-1517 (1979)] and are well fit by a model that incorporates a double-sided exponential temporal integration window.

NoSo and NoSπ detection as a function of masker bandwidth in normal-hearing and cochlear-impaired listeners

NoSo and NoS pi detection thresholds for a 500-Hz pure-tone signal were measured as a function of masking noise bandwidth in normal-hearing and cochlear hearing-impaired subjects. NoSo and NoS pi critical bands were derived from the bandlimited noise functions. A notched noise measure of the monaural critical band was also obtained for each ear. One hypothesis tested was that an asymmetrical monaural critical band would result in a relatively steep improvement of the NoS pi detection threshold as a function of decreasing masker bandwidth and would, therefore, be associated with a wider binaural critical band. This was hypothesized because the outputs of the left and right auditory filters would be more decorrelated the greater the interaural difference in the monaural critical band. However, as the noise bandwidth was narrowed, the decorrelation would lessen, resulting in a relatively steep improvement in NoS pi detection. Results indicated that the masking level difference (MLD) was smaller and that the monaural critical bands were generally wider in cochlear-impaired listeners. NoSo and NoS pi critical bands were somewhat larger in the cochlear hearing-impaired listeners having relatively wide monaural critical bands. There was a significant correlation between monaural critical band asymmetry and the NoS pi critical band; however, this correlation was insignificant when a control was employed for the critical band in the worse ear. Therefore, the present results did not support a strong association between monaural critical band asymmetry and the width of the NoS pi critical band.

Auditory image movement in evoked potentials

Long-latency auditory evoked potentials (AEPs) were tested in subjects following binaural stimulation with click trains with gradually changing interaural time delays (ΔTs). With appropriate change of the ΔTs this sound signal could produce the sensation of a moving fused auditory image (FI). It was found that the N1-P2 complex of the AEPs rose in amplitude with the increase of the click rate above 15 Hz, and to a greater extent for the moving than for the unmoved FIs. Binaural release from masking (as measured by the binaural masking level difference (BMLD) for the AEPs) amounted to 6 dB for the unmoved FI. For the moving FI the BMLD for the AEPs amounted to 2.8 and 3.5 dB in men and 13.4 and 11.3 dB in women, for the left and right hemispheres respectively. The amplitude of the N1-P2 complex following stimulation with the ‘moving’ and ‘unmoved’ sounds was larger in women than in men.

Masking level difference before and after surgery in unilateral otosclerosis

The object of this paper was to evaluate the binaural hearing in 20 patients suffering from unilateral otosclerosis. Binaural performance was assessed at a test frequency of 500 Hz using masking level difference (MLD) before and after surgery. The postsurgical results showed abnormal MLDs in patients in whom stapedotomy had successfully corrected the threshold asymmetry. Three possible hypotheses regarding this phenomenon were investigated: (1) cochlear damage, (2) auditory deprivation and (3) a mechanical middle ear anomaly due to the surgical prosthesis or healing processes. The analyses suggest that the conductive hearing loss may effect central auditory processing resulting in poor binaural hearing (auditory deprivation theory).