Comodulation masking release for various monaural and binaural combinations of the signal, on-frequency, and flanking bands

The threshold for a signal masked by a narrow band of noise centered at the signal frequency (the on-frequency band) may be reduced by adding to the masker a second band of noise (the flanking band) whose envelope is correlated with that of the first band. This effect is called comodulation masking release (CMR). These experiments examine two questions. (1) How does the CMR vary with the number and ear of presentation of the flanking band(s)? (2) Is it possible to obtain a CMR when a binaural masking level difference (BMLD) is already present, and vice versa? Thresholds were measured for a 400-ms signal in a continuous 25-Hz-wide noise centered at signal frequencies (fs) of 250, 1000, and 4000 Hz. This masker was presented either alone or with one or more continuous flanking bands whose envelopes were either correlated or uncorrelated with that of the on-frequency band; their frequencies ranged from 0.5fs to 1.5fs. CMRs were measured for six conditions in which the signal, the on-frequency band, and the flanking band(s) were presented in various monaural and binaural combinations. When a single flanking band was used, the CMR was typically around 2-3 dB. The CMR increased to 5-6 dB if an additional flanking band was added. The effect of the additional band was similar whether it was in the same ear as the original band or in the opposite ear. At the lowest signal frequency, a large CMR was observed in addition to a BMLD and vice versa. At the highest signal frequency, the extra release from masking was small. The results are interpreted in terms of the cues producing the CMR and the BMLD.     

Modulation cues influence binaural masking-level difference in masking-pattern experiments

Binaural masking patterns show a steep decrease in the binaural masking-level difference (BMLD) when masker and signal have no frequency component in common. Experimental threshold data are presented together with model simulations for a diotic masker centered at 250 or 500 Hz and a bandwidth of 10 or 100 Hz masking a sinusoid interaurally in phase (S(0)) or in antiphase (S(π)). Simulations with a binaural model, including a modulation filterbank for the monaural analysis, indicate that a large portion of the decrease in the BMLD in remote-masking conditions may be due to an additional modulation cue available for monaural detection.     

Masker fringe and binaural detection

Yost [J. Acoust. Soc. Am. 78,901-907 (1985)] found that the detectability of a 30-ms dichotic signal (S pi) in a 30-ms diotic noise (No) was not affected by the presence of a 500-ms dichotic forward fringe (N pi). Kollmeier and Gilkey [J. Acoust. Soc. Am. 87, 1709-1719, (1990)] performed a somewhat different experiment and varied the onset time of a 25-ms S pi signal in a 750-ms noise that switched, after 375-ms, from N pi to No. In contrast to Yost, they found that the N pi segment of the noise reduced the detectability of the signal even when the signal was temporally delayed well into the No segment of the noise and suggested that the N pi segment of noise acted as a forward masker. To resolve this apparent conflict, the present study investigated the detectability of a brief S pi signal in the presence of an No masker of the same duration as the signal. The masker was preceded by quiet or an N pi forward fringe and followed by quiet, an No, or N pi backward fringe. The present study differs from most previous studies of the effects of the masker fringe in that the onset time of the signal was systematically varied to examine how masking changes during the time course of the complex fringe-masker-fringe stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)     

The role of monaural frequency selectivity in binaural analysis

The relation between the monaural critical band and binaural analysis was examined using an NoSm MLD paradigm, in order to resolve ambiguities about the width of the masking spectrum important for binaural detection. A 500-Hz pure-tone signal was presented with a 600-Hz-wide band of masking noise to the signal ear. Bands of noise ranging in width from 25 to 600 Hz, or noise notches (imposed on a 600-Hz-wide band centered on the signal frequency) ranging in width from 0 to 600 Hz were presented to the nonsignal ear. All noise bands and notches were centered on 500 Hz, the frequency of the signal. The effects of varying bandwidth were radically different from those of varying notchwidth: the MLD changed from zero to approximately 8 dB over a bandwidth range of 400 Hz; for notchwidths, however, the MLD changed 8 dB over a range of only 50 Hz. The results support an interpretation that the fine frequency selectivity of monaural analysis is preserved in peripheral binaural interaction, but that a relatively wide frequency range of critical bands is scanned at a later stage of binaural processing. It was suggested that the wide spectral range of binaural analysis may provide a background against which binaural differences due to the signal are detected.     

Measurement of the binaural auditory filter using a detection task

The spectral resolution of the binaural system was measured using a tone-detection task in a binaural analog of the notched-noise technique. Three listeners performed 2-interval, 2-alternative, forced choice tasks with a 500-ms out-of-phase signal within 500 ms of broadband masking noise consisting of an "outer" band of either interaurally uncorrelated or anticorrelated noise, and an "inner" band of interaurally correlated noise. Three signal frequencies were tested (250, 500, and 750 Hz), and the asymmetry of the filter was measured by keeping the signal at a constant frequency and moving the correlated noise band relative to the signal. Thresholds were taken for bandwidths of correlated noise ranging from 0 to 400 Hz. The equivalent rectangular bandwidth of the binaural filter was found to increase with signal frequency, and estimates tended to be larger than monaural bandwidths measured for the same listeners using equivalent techniques.     

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 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.     

The effect of diotic and dichotic level-randomization on the binaural masking-level difference

Detection thresholds for tones in narrow-band noise were measured for two binaural configurations: N(o)S(o) and N(o)S(pi). The 30-Hz noise band had a mean overall level of 65 dB SPL and was centered on 250, 500, or 5000 Hz. Signals and noise were simultaneously gated for 500, 110, or 20 ms. Three conditions of level randomization were tested: (1) no randomization; (2) diotic randomization--the stimulus level (common to both ears) was randomly chosen from an uniformly distributed 40-dB range every presentation interval; and (3) dichotic randomization--the stimulus levels for each ear were each independently and randomly chosen from the 40-dB range. Regardless of binaural configuration, level randomization had small effects on thresholds at 500 and 110 ms, implying that binaural masking-level differences (BMLDs) do not depend on interaural level differences for individual stimuli. For 20-ms stimuli, both diotic and dichotic randomization led to markedly poorer performance than at 500- and 110-ms durations; BMLDs diminished with no randomization and dichotic randomization but not with diotic randomization. The loss of BMLDs at 20 ms, with degrees-of-freedom (2WT) approximately 1, implies that changes in intracranial parameters occurring during the course of the observation interval are necessary for BMLDs when mean-level and mean-intracranial-position cues have been made unhelpful.     

Effects of non-simultaneous masking on the binaural masking level difference

The present study sought to clarify the role of non-simultaneous masking in the binaural masking level difference for maskers that fluctuate in level. In the first experiment the signal was a brief 500-Hz tone, and the masker was a bandpass noise (100-2000 Hz), with the initial and final 200-ms bursts presented at 40-dB spectrum level and the inter-burst gap presented at 20-dB spectrum level. Temporal windows were fitted to thresholds measured for a range of gap durations and signal positions within the gap. In the second experiment, individual differences in out of phase (NoSπ) thresholds were compared for a brief signal in a gapped bandpass masker, a brief signal in a steady bandpass masker, and a long signal in a narrowband (50-Hz-wide) noise masker. The third experiment measured brief tone detection thresholds in forward, simultaneous, and backward masking conditions for a 50- and for a 1900-Hz-wide noise masker centered on the 500-Hz signal frequency. Results are consistent with comparable temporal resolution in the in phase (NoSo) and NoSπ conditions and no effect of temporal resolution on individual observers' ability to utilize binaural cues in narrowband noise. The large masking release observed for a narrowband noise masker may be due to binaural masking release from non-simultaneous, informational masking.     

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.     

Detection of time- and bandlimited increments and decrements in a random-level noise.

The purpose of this study was to compare detection of increments and decrements occurring over limited regions of time and frequency within a 500-ms broadband (0-6000 Hz) noise. Three listeners tracked detection thresholds adaptively in a two-interval, two-alternative forced-choice task. Thresholds were measured for both increments and decrements in level [delta L = 10 log10(1 + delta N0/N0) dB, where N0 is the spectral power density of the noise] as a function of signal duration (T = 30-500 ms) for a range of signal bandwidths (W = 62-6000 Hz) that were logarithmically centered around 2500 Hz. Listeners were forced to rely on temporal- and spectral-profile cues for detection due to randomization of overall presentation level from interval to interval, which rendered overall energy an inconsistent cue. Increments were detectable for all combinations of W and T, whereas decrements were not consistently detectable for W < 500 Hz. Narrow-band decrements were not detectable due to spread of excitation from the spectral edges of the noise into the decrements. Increment and decrement thresholds were similar for W > or = 1000 Hz. Temporal- and spectral-integration effects were observed for both increments and decrements. The exceptions were for random-level conditions in which the signal matched the bandwidth or duration of the standard. A multicue decision process is described qualitatively to explain how the combination of temporal- and spectral-profile cues can produce temporal- and spectral-integration effects in the absence of overall-energy cues.     

Spectral loudness summation for sequences of short noise bursts

Recent loudness data of single noise bursts indicate that spectral loudness summation depends on signal duration. To gain insight into the mechanisms underlying this duration effect, loudness was measured as a function of signal bandwidth centered around 2 kHz for sequences of 10-ms noise bursts at various repetition rates and, for comparison, for single noise bursts of either 10- or 1000-ms duration. The test-signal bandwidth was varied from 200 to 6400 Hz. For the repeated noise bursts, the reference signal had a bandwidth of 400 Hz. For the single noise bursts, data were obtained for two reference bandwidths: 400 and 3200 Hz. In agreement with previous results, the magnitude of spectral loudness summation was larger for the 10-ms than for the 1000-ms noise bursts. The reference bandwidth had no significant effect on the results for the single noise bursts. Up to repetition rates of 50 Hz, the magnitude of spectral loudness summation for the sequences of noise bursts was the same as for the single short noise burst. The data indicate that the mechanism underlying the duration effect in spectral loudness is considerably faster than the time constant of about 100 ms commonly associated with the temporal integration of loudness.     

Exploring the additivity of binaural and monaural masking release

Experiment 1 examined comodulation masking release (CMR) for a 700-Hz tonal signal under conditions of N(o)S(o) (noise and signal interaurally in phase) and N(o)S(π) (noise in phase, signal out of phase) stimulation. The baseline stimulus for CMR was either a single 24-Hz wide narrowband noise centered on the signal frequency [on-signal band (OSB)] or the OSB plus, a set of flanking noise bands having random envelopes. Masking noise was either gated or continuous. The CMR, defined with respect to either the OSB or the random noise baseline, was smaller for N(o)S(π) than N(o)S(o) stimulation, particularly when the masker was continuous. Experiment 2 examined whether the same pattern of results would be obtained for a 2000-Hz signal frequency; the number of flanking bands was also manipulated (two versus eight). Results again showed smaller CMR for N(o)S(π) than N(o)S(o) stimulation for both continuous and gated masking noise. The CMR was larger with eight than with two flanking bands, and this difference was greater for N(o)S(o) than N(o)S(π). The results of this study are compatible with serial mechanisms of binaural and monaural masking release, but they indicate that the combined masking release (binaural masking-level difference and CMR) falls short of being additive.     

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.     

Spectral and level effects in noise-on-tone suppression

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

Comodulation masking release (CMR) as a function of masker bandwidth, modulator bandwidth, and signal duration

These experiments examine how comodulation masking release (CMR) varies with masker bandwidth, modulator bandwidth, and signal duration. In experiment 1, thresholds were measured for a 400-ms, 2000-Hz signal masked by continuous noise varying in bandwidth from 50-3200 Hz in 1-oct steps. In one condition, using random noise maskers, thresholds increased with increasing bandwidth up to 400 Hz and then remained approximately constant. In another set of conditions, the masker was multiplied (amplitude modulated) by a low-pass noise (bandwidth varied from 12.5-400 Hz in 1-oct steps). This produced correlated envelope fluctuations across frequency. Thresholds were generally lower than for random noise maskers with the same bandwidth. For maskers less than one critical band wide, the release from masking was largest (about 5 dB) for maskers with low rates of modulation (12.5-Hz-wide low-pass modulator). It is argued that this release from masking is not a "true" CMR but results from a within-channel cue. For broadband maskers (greater than 400 Hz), the release from masking increased with increasing masker bandwidth and decreasing modulator bandwidth, reaching an asymptote of 12 dB for a masker bandwidth of 800 Hz and a modulator bandwidth of 50 Hz. Most of this release from masking can be attributed to a CMR. In experiment 2, the modulator bandwidth was fixed at 12.5 Hz and the signal duration was varied. For masker bandwidths greater than 400 Hz, the CMR decreased from 12 to 5 dB as the signal duration was decreased from 400 to 25 ms.(ABSTRACT TRUNCATED AT 250 WORDS)     

Binaural unmasking with multiple adjacent masking electrodes in bilateral cochlear implant users

Bilateral cochlear implant (BiCI) users gain an advantage in noisy situations from a second implant, but their bilateral performance falls short of normal hearing listeners. Channel interactions due to overlapping electrical fields between electrodes can impair speech perception, but its role in limiting binaural hearing performance has not been well characterized. To address the issue, binaural masking level differences (BMLD) for a 125 Hz tone in narrowband noise were measured using a pair of pitch-matched electrodes while simultaneously presenting the same masking noise to adjacent electrodes, representing a more realistic stimulation condition compared to prior studies that used only a single electrode pair. For five subjects, BMLDs averaged 8.9 ± 1.0 dB (mean ± s.e.) in single electrode pairs but dropped to 2.1 ± 0.4 dB when presenting noise on adjacent masking electrodes, demonstrating a negative impact of the additional maskers. Removing the masking noise from only the pitch-matched electrode pair not only lowered thresholds but also resulted in smaller BMLDs. The degree of channel interaction estimated from auditory nerve evoked potentials in three subjects was significantly and negatively correlated with BMLD. The data suggest that if the amount of channel interactions can be reduced, BiCI users may experience some performance improvements related to binaural hearing.     

Detectability of a pulsed tone in the presence of a masker with time-varying interaural correlation.

Detectability of a filtered probe tone (250, 500, or 1000 Hz) was measured in the presence of a narrow-band Gaussian masker centered at the signal frequency. The signal was interaurally phase-reversed (Spi), and the masker's interaural correlation varied sinusoidally between +1.00 (NO) and -1.00 (Npi) at a varaible rate (fm = 0--4 Hz). The signal was presented at various points on the masker's modulation cycle. For 0-Hz modulation (fixed interaural correlation) signal threshold decreased monotonically as the masker's interaural correlation was changed from -1.00 to +1.00 (by a total of about 20, 16, and 8 dB, respectively, for 250-, 500-, and 1000-Hz signals). For fm greater than 0 the function relating signal threshold to the masker's interaural correlation at the moment of signal presentation became progressively flatter with increasing fm for all signal frequencies. For fm = 4 Hz the function was flat; there was no measurable effect of masker interaural correlation on signal detectability. Estimates of minimum binaural integration time based on these data ranged from 44--243 ms, supporting previous studies which have noted the binaural system's relative insensitivity to dynamic stimulation. Additionally, the estimated time constants were approximately twice as large at 250 Hz as at 500 Hz, indicating observers could follow binaural fluctuations better at 500 Hz. The time-constant estimates at 1000 Hz were not suggiciently reliable to permit comparisons with the lower-frequency data.     

Thresholds for segregating a narrow-band from a broadband noise based on interaural phase and level differences

Either an interaural phase shift or level difference was introduced to a narrow section of broadband noise in order to measure the acuity of the binaural system to segregate a narrowband from a broadband stimulus. Listeners were asked to indicate whether this dichotic noise or a totally diotic noise was presented in a single-interval procedure. Thresholds for interaural phase and level differences were estimated from four point psychometric functions. These thresholds were determined for three bandwidths of interaurally altered noise (2, 10, and 100 Hz) centered at four center frequencies (200, 500, 1000, and 1600 Hz). Thresholds were lowest when the interaurally altered band of noise was centered at 500 Hz, and thresholds increased as the bandwidth of the interaurally altered noise decreased. Performance did not exceed 75% correct when either an interaural phase shift (180 degrees) or interaural level difference (50 dB) was introduced to a 100 Hz band of noise centered at frequencies higher than 1600 Hz. In a second set of conditions, performance was measured when both an interaural phase shift and level difference were presented in a 10-Hz-wide band of noise centered at 500 Hz. A version of the Durlach E-C model was able to account for a great deal of the data. The results are discussed in terms of the Huggins dichotic pitch.     

Effect of masker-fringe onset asynchrony on overshoot (L)

The study examines how overshoot is influenced by masker-signal onset asynchrony when the masker contains frequencies above or below the signal frequency. Masked thresholds were measured for a 2-ms tone at 5 kHz. The measurements were made in a reference condition with a narrow center-band (CB) noise masker (4590-5464 Hz), and in conditions with either a low-fringe (1900-4590 Hz) or a high-fringe noise band (5500-11 000 Hz) added to the CB. The signal always came on 2 ms after the onset of the CB. The time interval, between fringe and signal onsets varied from -98 ms (signal onset before fringe onset) to +502 ms (signal onset after fringe onset). Results show that overshoot is largest, 8-11 dB, for a high fringe with onset occurring between 8 ms before the signal onset and 12 ms after it. Thus, pronounced overshoot is observed even when the high fringe is gated on after the signal's offset. Low fringes produce no more than 4 dB of overshoot, much less than high fringes produce. The finding of pronounced overshoot elicited by a high fringe presented shortly after the end of the signal suggests that overshoot is governed, at least in part, by central mechanisms.