Binaural processing of noisy stimuli: internal/external noise ratios for diotic and dichotic stimuli

A set of ten digitized statistically similar Gaussian maskers was used in one-internal tone-in-noise detection experiments under diotic (NoSo) and dichotic (NoS pi) interaural conditions. Stimulus/response matrices were generated for each masker in the presence or absence of a target 500-Hz tone. For both NoSo and NoS pi, nonparametric analyses show that response probabilities and sensitivities vary significantly across noise waveforms, indicating a considerable external noise component in subject response variability. A parametric model is developed that maps individual stimulus waveforms onto a decision axis, facilitating evaluation of internal/external noise variance ratios. For both NoSo and NoS pi, internal and external noise variance are of similar magnitude.     

The effects of signal duration on NoSo and NoS pi thresholds at 500 Hz and 4 kHz

A two-interval, two-alternative temporal forced-choice procedure was used to measure NoSo and NoS pi masked thresholds with 500-Hz and 4-kHz tonal signals. The duration of the signal was either 10, 20, 40, or 320 ms. The maskers were 200-Hz-wide bands of Gaussian noise centered at the frequency of the signal and presented continuously. Decreasing the duration of the 500-Hz tonal signal resulted in a modest increase (1.5 dB or so) in the masking-level difference (MLD) measured between NoSo and NoS pi conditions. In contrast, decreasing the duration of the 4-kHz tonal signal resulted in a substantial decrease (4.5 dB or so) in the MLD. Comparisons of the data with thresholds predicted from analyses based on "windows of temporal integration" provided quantitatively acceptable accounts of the data. The data obtained in the NoS pi condition at 4 kHz, which are novel and were of primary interest, were well-accounted for in a statistical sense. However, there were small, but systematic, discrepancies between the predictions and the data. Those discrepancies, although small in magnitude, suggest that binaural temporal integration at high frequencies, where the envelopes of the stimuli convey the information, may be inherently different from both monaural temporal integration and binaural temporal integration at low frequencies.     

NoSo and NoS pi 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.     

Masking effects in binaural detection and interaural time discrimination

This study was designed to investigate the effects of masker level and frequency on binaural detection and interaural time discrimination. Detection and interaural time discrimination of a 700-Hz sinusoidal signal were measured as a function of the center frequency and level of a narrow-band masking noise. The masker was a continuous, diotic, 80-Hz-wide noise that varied in center frequency from 250 to 1370 Hz. In the detection experiment, the signal was presented either diotically (NoSo) or interaurally phase reversed (NoS pi). In the interaural time discrimination experiment, the signal level needed to discriminate a 30-microseconds interaural delay was measured. As would be expected, the presence of the masker has a greater effect on NoSo detection than NoS pi detection, and for masker frequencies at or near the signal frequency. In contrast, interaural time discrimination can be improved by the presence of a low-level masker. Also, performance improves more rapidly as the signal/masker frequency separation increases for NoSo detection than for interaural time discrimination and NoS pi detection. For all three tasks, significant upward spread of masking occurs only at the highest masker level; at low masker levels, there is a tendency toward downward spread of masking.     

Discrimination of interaural envelope correlation and its relation to binaural unmasking at high frequencies.

Listeners' sensitivity to interaural correlation of the envelope of high-frequency waveforms and whether such sensitivity might account for detectability in a masking-level difference paradigm were assessed. Thresholds of interaural envelope decorrelation (from a reference correlation of 1.0) were measured for bands of noise centered at 4 kHz and bandwidths ranging from 50-1600 Hz. Decorrelation of the envelope was achieved by "mixing" two independent narrow-band noises. Separately, with the same listeners, NoSo and NoS pi detection thresholds were measured for maskers of the same center frequency and bandwidths. For bandwidths of noise up to about 400 Hz, listeners were similarly sensitive to interaural decorrelation in both types of task. However, for bandwidths greater than 400 Hz or so, while sensitivity in the discrimination task was unaffected, sensitivity was reduced in the NoS pi conditions. Additional data suggested that listeners were able to maintain their sensitivity independent of bandwidth in the discrimination task by focusing on binaural information within select spectral regions of the stimuli.     

Effects of temporal separation and masker level on binaural analysis in forward masking.

In the present study detection under diotic (NoSo) and dichotic (NoS pi) listening conditions in a forward masking paradigm was investigated. Both the level of a noise masker and the temporal separation between the masker and a 250-Hz tone burst served as independent variables. Results showed that most of the variance in the data could be accounted for by the amount of masking in the NoSo condition, independent of the value of the temporal parameter, which itself accounted for only 1.4% of the variance that remained. Once the data were corrected for NoSo masking effectiveness, the MLD was found to decrease by only 1.4 dB as temporal separation increased from 5-100 ms, which is consistent with a very long time constant for the binaural system. Consistent with this finding, it was shown that slope changes of the growth of masking functions, for simultaneous as compared to forward masking, were similar for both the NoSo and NoS pi conditions.     

Interaural octave phase-shift detection and aural harmonic distortion

A continuous 400-Hz tone (60-75 dB SPL) and 250-msec bursts of an 800-Hz tone (10 dB SL) were delivered dichotically. Four out of nine listeners were able to detect a 180 degree interaural phase shift. When a subaudible continuous 800-Hz tone was added to the ear with the 400-Hz tone, interaural phase-shift detection depended on the phase relation of the added tone to the 400-Hz tone. These results are shown to be consistent with the hypothesis of aural harmonic distortion.     

Detection of tones in reproducible narrow-band noise

Hit and false-alarm rates were measured for detection of a 500-Hz tone target in each of ten reproducible samples of 1/3-oct bandwidth noise centered at 500 Hz for both NoS pi and NoSo conditions. The effects on hit rates of the starting phase of the target relative to individual noise samples were investigated with two target phase angles for three subjects. The major results are: (1) performance varies significantly over masker waveforms; (2) for NoS pi conditions, the effect of target-to-marker phase angle on hit rates is not significant for these narrow-band maskers; (3) for NoSo conditions, the target-to-masker phase angle has a large effect; (4) no significant correlation between NoSo performance and NoS pi performance is seen across masker waveforms. These results are generally consistent wuth previously reported results for wideband maskers [R.H. Gilkey, D.E. Robinson, and T.E. Hanna, "Effects of masker waveform and signal-to-masker phase relation on diotic and dichotic masking by reproducible noise," J. Acoust. Soc. Am. 78, 1207-1219 (1985)] with an important exception. Specifically, in the wideband experiment, significant correlation between NoSo and NoS pi performance across noise samples was found. In addition, in the wideband experiment, a small yet statistically significant effect of target-to-masker phase was observed in the NoS pi condition.     

Speech perception from monaural and binaural information

Two experiments explored the concept of the binaural spectrogram [Culling and Colburn, J. Acoust. Soc. Am. 107, 517-527 (2000)] and its relationship to monaurally derived information. In each experiment, speech was added to noise at an adverse signal-to-noise ratio in the NoS pi binaural configuration. The resulting monaural and binaural cues were analyzed within an array of spectro-temporal bins and then these cues were resynthesized by modulating the intensity and/or interaural correlation of freshly generated noise. Experiment 1 measured the intelligibility of the resynthesized stimuli and compared them with the original NoSo and NoS pi stimuli at a fixed signal-to-noise ratio. While NoS pi stimuli were approximately equal to 50% intelligible, each cue in isolation produced similar (very low) intelligibility to the NoSo condition. The resynthesized combination produced approximately equal to 25% intelligibility. Modulation of interaural correlation below 1.2 kHz and of amplitude above 1.2 kHz was not as effective as their combination across all frequencies. Experiment 2 measured three-point psychometric functions in which the signal-to-noise ratio of the original NoS pi stimulus was increased in 3-dB steps from the level used in experiment 1. Modulation of interaural correlation alone proved to have a flat psychometric function. The functions for NoS pi and for combined monaural and binaural cues appeared similar in slope, but shifted horizontally. The results indicate that for sentence materials, neither fluctuations in interaural correlation nor in monaural intensity are sufficient to support speech recognition at signal-to-noise ratios where 50% intelligibility is achieved in the NoS pi configuration; listeners appear to synergistically combine monaural and binaural information in this task, to some extent within the same frequency region.     

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.     

The effect of random intensity fluctuation on monaural and binaural detection

Two experiments were performed to determine the effects of random intensity fluctuation on NoSo and NoS pi performance. Noise was used as both signal and masker, and stimuli were bands of noise from either 0-2.0 or 2.0-4.0kHz. Signal and masker were either coherent (from the same source) or noncoherent (from independent sources). In the first experiment, noise fluctuation was achieved by modulating a wide band of noise. In the second experiment, fluctuation was achieved by narrowing the noise bandwidth. Results from both experiments indicated that NoSo performance was adversely affected by fluctuation and by noncoherent relation between signal and masker. NoS pi detection was not adversely affected by fluctuation at low frequency, and was affected less adversely than was NoSo detection at high frequency. This difference between NoSo and NoS pi performance is an important consideration when making inferences about monaural and binaural processing when the stimuli are fluctuating rather than temporally steady.     

Frequency dependence of binaural performance in listeners with impaired binaural hearing.

Binaural performance was measured as a function of stimulus frequency for four impaired listeners, each with bilaterally symmetric audiograms. The subjects had various degrees and configurations of audiometric losses: two had high-frequency, sensorineural losses; one had a flat sensorineural loss; and one had multiple sclerosis with normal audiometric thresholds. Just noticeable differences (jnd's) in interaural time, interaural intensity, and interaural correlation as well as detection thresholds for NoSo and NoS pi conditions were obtained for narrow-band noise stimuli at octave frequencies from 250-4000 Hz. Performance of the impaired listeners was generally poorer than that of normal-hearing listeners, although it was comparable to normal in a few instances. The patterns of binaural performance showed no apparent relation to the audiometric patterns; even the two subjects with similar degree and configuration of hearing loss have very different binaural performance, both in the level and frequency dependence of their performance. The frequency dependence of performance on individual tests is irregular enough that one cannot confidently interpolate between octaves. In addition, it appears that no subset of the measurements is adequate to characterize the performance in the rest of the measurements with the exception that, within limits, interaural correlation discrimination and NoS pi detection performance are related.     

Detection of interaural phase shift between a subaudible and an audible tone

Can a shift in interaural phase between a subthreshold signal and an audible contralateral probe tone affect perception of the probe? To obtain an answer, an 800-Hz tone was presented to both ears. The tone was presented continuously to one ear (-25 to + 10 dB SL) and in a sequence of four bursts per trial to the other ear (+ 10 dB SL). Interaural phase was reversed for either the second or the fourth burst in a 2 AFC task. Interaural phase-shift detection threshold (65% correct) varied with the intensity of the continuous signal; across subjects, this threshold varied from -21 to + 1 dB SL. When a 300-or 500-Hz masking tone was added to the ear with the continuous signal, phase-shift detection accuracy depended primarily upon the sensation level of the signal rather than its sound pressure level. These findings demonstrate temporal encoding at signal levels well below hearing threshold.     

Frequency resolution for diotic and dichotic listening conditions compared using the bandlimiting measure and a modified bandlimiting measure

Two experiments were performed that examined the relation between frequency selectivity for diotic and dichotic stimuli. Subjects were eight normal-hearing listeners. In each experiment, a 500-Hz pure tone of 400-ms duration was presented in continuous noise. In the diotic listening conditions, a signal and noise were presented binaurally with no interaural differences (So and No, respectively). In the dichotic listening conditions, the signal or noise at one ear was 180 degrees out-of-phase relative to the respective stimulus at the other ear (S pi and N pi, respectively). The first experiment examined frequency selectivity using the bandlimiting measure. Here, signal thresholds were determined as a function of masker bandwidth (50, 100, 250, 500, and 1000 Hz) for SoNo, S pi No, and SoN pi listening conditions. The second experiment used a modified bandlimiting measure. Here, signal thresholds (So and S pi) were determined with a relatively narrow No band of masker energy (50 Hz wide) centered about the signal. Then, a second No narrow-band masker (30 Hz wide) was added at another frequency region, and signal thresholds were reestablished. The results of the two experiments indicated that listeners process a wider band of frequencies when resolving dichotic stimuli than when resolving diotic or monotic stimuli. The results also indicated that the bandlimiting measure may underestimate the spectral band processed upon dichotic stimulation. Results are interpreted in terms of an across-ear and across-frequency processing of waveform amplitude envelope.     

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.     

Prior stimulation and the masking-level difference

Signal detection in diotic (NoSo) and dichotic (NoS pi) conditions was measured as a function of the stimulus parameters of the noise that preceded the signal-plus-masker. When the signal and masker were both pulsed, dichotic signal detection was worse than when the masker was continuous or when the onset of the masker preceded the signal-plus-masker by at least 500 ms. The dichotic detection thresholds decreased as the duration of the pulsed signal plus pulsed masker was increased. The level, spectrum, interaural configuration, duration, and temporal proximity of the prior noise (forward fringe) relative to the masker and/or signal and masker were all investigated. Almost any difference between the parameters of the fringe and the masker resulted in poorer signal detection in the dichotic conditions. These same stimulus conditions produced small (less than 2.2 dB) changes in the diotic detection thresholds. The various models of the Masking-Level Difference (MLD) may be modified to qualitatively describe some of these results.     

Spectral integration of broadband signals in diotic and dichotic masking experiments.

The method of G?ssler [Acustica 4, 408-414 (1954)] was used to measure the audibility of multicomponent signals as a function of their bandwidth against a broadband, white-noise masker. Test signals were composed of 1 to 41 sinusoids with a spectral spacing of 10 Hz and were always spectrally centered around 400 Hz. Masker duration was 400 ms and the 300-ms signals were centered within the noise intervals. A three-interval forced-choice procedure with adaptive level adjustment was applied. NoSo, NoSm, NoS pi, and N pi So masked thresholds were obtained for four subjects. A comparison of the diotic and the three dichotic conditions yields no significant difference in the bandwidth dependence and suggests equal integration bandwidths for all conditions. However, the original results of G?ssler could not be replicated: Neither were the overall levels of signals with a bandwidth below the critical bandwidth constant nor were the results for broadband signals in accordance with a single-band model of detection. The narrow-band data are much better described by calculating the overall signal level at the output of a rounded exponential filter [Patterson et al., J. Acoust. Soc. Am. 72, 1788-1803 (1982)] with an equivalent rectangular bandwidth of 65 Hz. For broader signal bandwidths, the signal level at threshold increases as predicted by a multiband model.     

Detectability of tonal signals with changing interaural phase differences in noise

Detectability of binaurally presented 400- and 800-Hz tonal signals was investigated in an adaptive, two-interval forced-choice experiment. A continuous 3150-Hz low-pass noise masker was presented either diotically (No), interaurally uncorrelated (NU), or interaurally phase-reversed (N pi), at an overall level of 70 dB SPL. Signal duration was either 100 or 1000 ms. The interaural phase difference (IAPD) of the signal was either fixed (0 degree-180 degrees) or time-varying (slightly different frequencies were presented to the two ears). The range of interaural phase variations was selected to yield the same varying interaural temporal differences that would be produced if real auditory targets moved through various arcs in the horizontal plane. In no case was a signal with varying IAPD any more (or less) detectable than would be expected from averaging subjects' performance in the corresponding fixed-IAPD conditions through which the variation occurred. However, in detecting these signals, subjects placed relatively more weight on the temporal central portion than on either the onset or offset. It is proposed that this weighting effect is based on two factors: (1) the signal's 20-ms rise-decay time (i.e., the onset and offset receive less binaural weight because of monaural attenuation); and (2) the very low-pass filtering effected by the binaural system, which results in some minimum time required for it to become "fully engaged." Another finding was that signal detectability became gradually worse as the antiphasic moment in a varying-IAPD signal was moved from the temporal midpoint toward the onset. No evidence was found that a signal's onset and offset were weighted differently in a binaural signal detection task.     

Temporal structure model of binaural masking level difference.

It is shown that a simple cross-correlation model is not adequate to explain both binaural masking level difference (MLD) and spatial selective attention. The reason is that for a low-intensity signal in NoS(pi) condition the maximal activity in the binaural analyzer as a function of interaural delay in single spectral channel is independent of signal intensity. On the other hand, if detection ability is associated with the isolation of tonically firing units, MLD is simply explained as the increase in firing synchronization as a function of the signal's interaural phase difference (IPD). Quantitatively results are presented based on numerical solutions of the model.     

Binaural sluggishness in the perception of tone sequences and speech in noise

The binaural system is well-known for its sluggish response to changes in the interaural parameters to which it is sensitive. Theories of binaural unmasking have suggested that detection of signals in noise is mediated by detection of differences in interaural correlation. If these theories are correct, improvements in the intelligibility of speech in favorable binaural conditions is most likely mediated by spectro-temporal variations in interaural correlation of the stimulus which mirror the spectro-temporal amplitude modulations of the speech. However, binaural sluggishness should limit the temporal resolution of the representation of speech recovered by this means. The present study tested this prediction in two ways. First, listeners' masked discrimination thresholds for ascending vs descending pure-tone arpeggios were measured as a function of rate of frequency change in the NoSo and NoSpi binaural configurations. Three-tone arpeggios were presented repeatedly and continuously for 1.6 s, masked by a 1.6-s burst of noise. In a two-interval task, listeners determined the interval in which the arpeggios were ascending. The results showed a binaural advantage of 12-14 dB for NoSpi at 3.3 arpeggios per s (arp/s), which reduced to 3-5 dB at 10.4 arp/s. This outcome confirmed that the discrimination of spectro-temporal patterns in noise is susceptible to the effects of binaural sluggishness. Second, listeners' masked speech-reception thresholds were measured in speech-shaped noise using speech which was 1, 1.5, and 2 times the original articulation rate. The articulation rate was increased using a phase-vocoder technique which increased all the modulation frequencies in the speech without altering its pitch. Speech-reception thresholds were, on average, 5.2 dB lower for the NoSpi than for the NoSo configuration, at the original articulation rate. This binaural masking release was reduced to 2.8 dB when the articulation rate was doubled, but the most notable effect was a 6-8 dB increase in thresholds with articulation rate for both configurations. These results suggest that higher modulation frequencies in masked signals cannot be temporally resolved by the binaural system, but that the useful modulation frequencies in speech are sufficiently low (<5 Hz) that they are invulnerable to the effects of binaural sluggishness, even at elevated articulation rates.