Comodulation detection difference and binaural unmasking.

The present study investigated the combined effect of binaural cues and comodulation for a narrowband target noise masked by a narrowband noise. The threshold difference between a diotic condition (same stimuli in both ears) and a dichotic condition (target interaural phase difference of π and diotic masker) decreased with spectral distance between masker and target, irrespective of across-frequency envelope correlation. The threshold difference between a condition with comodulated target and masker and a corresponding uncorrelated condition, i.e., the comodulation detection difference, did not depend on target frequency and interaural correlation, indicating that these two stimulus properties are processed independently.

Modeling off-frequency binaural masking for short- and long-duration signals.

Experimental binaural masking-pattern data are presented together with model simulations for 12- and 600-ms signals. The masker was a diotic 11-Hz wide noise centered on 500 Hz. The tonal signal was presented either diotically or dichotically (180° interaural phase difference) with frequencies ranging from 400 to 600 Hz. The results and the modeling agree with previous data and hypotheses; simulations with a binaural model sensitive to monaural modulation cues show that the effect of duration on off-frequency binaural masking-level differences is mainly a result of modulation cues which are only available in the monaural detection of long signals.

Binaural spectral resolution as a function of interaural masker correlation.

Thresholds for an antiphasic 500-Hz sinusoid (Sπ) were measured in the presence of a notched-noise masker for three different interaural masker correlations: -1 (Nπ), 1 (N0), and 0.87. The difference between thresholds for the three masker correlations was largest for a notch width of zero and decreased continuously with increasing notch width. The bandwidth of a gammatone filter fitted to the data was 85 Hz for the interaural masker correlation of -1 (NπSπ) and 182 Hz for the interaural masker correlation of 1 (N0Sπ). For the intermediate correlation (0.87) the effective filter width was 134 Hz. This result is at odds with corresponding literature results of a bandwidening experiment where an effectively larger binaural bandwidth is only found with extreme interaural disparities of signal and masker, such as N0Sπ. The notched-noise thresholds were predicted if a detrimental across-channel process was included in the model. This approach failed to predict the effect of masker correlation in the bandwidening experiments. A beneficial across-channel process successfully used in the literature to simulate bandwidening data with extreme binaural parameters also failed to predict this effect. The effect may be due to interaural parameter fluctuations currently not used in the model framework.

Interaural-phase discrimination in notched noise.

Discrimination thresholds for the interaural-phase difference (IPD) of a 500-Hz sinusoid were measured in the presence of a diotic notched-noise masker as a function of notch width. The reference sinusoid had zero IPD. The level was set to 3 dB above the individual masked threshold for the diotic sinusoid at the respective notch width. Just-noticeable IPDs increased from about 20° for notch widths up to 100 Hz to 100° at the largest notch width of 800 Hz. Model simulations show that the data are consistent with the equalization-cancellation theory, if effectively wider binaural filters are assumed.

Binaural notched-noise masking and auditory-filter shape.

Thresholds for sinusoids interaurally in phase (S0) and antiphase (Sπ) were measured in the presence of a diotic notched-noise masker (N0) as a function of notch width. The signal frequency was 250, 500, 1000, or 2000 Hz. For all signal frequencies, the difference between N0S0 and N0Sπ thresholds (binaural masking-level difference, BMLD) decreased continuously as the notch width increased. Model simulations showed that this result cannot be accounted for by a model that only processes the output of the auditory filter centered at the signal frequency, even if the nonlinear behavior of the monaural frequency selectivity or interaural differences in the filter shape are considered. The data were predicted well if a detrimental across-channel process was included, either by an addition of portions of the output of adjacent filters to the output of the on-frequency filter or by a notch-width dependent adverse shift in interaural phase in the binaural stage. The strength of this detrimental across-channel process tends to decrease with increasing signal frequencies.

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.

Monaural and binaural frequency selectivity in hearing-impaired subjects.

Sensorineurally hearing-impaired (HI) subjects often report difficulties in complex acoustical environments. To investigate whether these problems arise from specific deficits in the frequency selectivity in binaural listening conditions, thresholds were measured for a 500-Hz sinusoid in phase (So) or antiphase (Spi) masked by a diotic notched noise (No). The equivalent rectangular bandwidth (ERB) for filters derived from diotic (NoSo) and dichotic (NoSpi) threshold curves is larger for the HI subjects than for the normal-hearing (NH) subjects. However, the ratio of binaural to monaural ERB is the same. The data indicate that there is no additional retrocochlear impairment reducing the binaural frequency selectivity of HI subjects. A specific binaural impairment was also tested by measuring the perception of binaural pitch (Huggins' pitch). Two out of eight HI subjects failed to perceive this pitch, although in the masking experiment they obtained a binaural masking-level difference of up to 10 dB. The current data therefore provide no clear evidence for a specific binaural impairment factor in hearing impairment that deteriorates several aspects of binaural processing in a similar way.

The role of across-frequency processes in dichotic listening conditions.

In the bandwidening experiment with a diotic noise masker, an apparently wider critical bandwidth has often been reported when a dichotic signal (Spi) is used instead of a diotic signal (So). Two competing across-channel processes were proposed to account for this apparently wider critical bandwidth: (i) A detrimental across-channel effect reducing the binaural masking-level difference (BMLD) for broadband maskers and (ii) a beneficial integration of information across channels for narrowband maskers. The two hypotheses result in different predictions of the BMLD in the notched-noise experiment: According to the first hypothesis, the change in BMLD with notch width is determined by the level-dependence of the BMLD for a narrowband masker centered at the signal frequency, whereas the second hypothesis predicts that it is determined by the level-dependence of the BMLD for a broadband masker. To test the hypotheses, masked thresholds of a diotic or dichotic 500-Hz signal were measured for a diotic notched-noise masker as a function of notch width. In addition, thresholds were measured for a diotic broadband and narrowband masker as a function of masker level. The data indicate that neither of the two hypotheses is able to predict the continuous decrease in the BMLD as the notch width increases.