Sequential streaming due to manipulation of interaural time differences

The effect of apparent spatial location on sequential streaming was investigated by manipulating interaural time differences (ITDs). The degree of obligatory stream segregation was inferred indirectly from the threshold for detecting a rhythmic irregularity in an otherwise isochronous sequence of interleaved "A" and "B" tones. Stimuli were bandpass-filtered harmonic complexes with a 100-Hz fundamental. The A and B tones had equal but opposite ITDs of 0, 0.25, 0.5, 1, or 2 ms and had the same or different passbands. The passband ranges were 1250-2500 Hz and 1768-3536 Hz in experiment 1, and 353-707 Hz and 500-1000 Hz in experiment 2. In both experiments, increases in ITD led to increases in threshold, mainly when the passbands of A and B were the same. The effects were largest for ITDs above 0.5 ms, for which rhythmic irregularities in the timing of the A or B tones alone may have disrupted performance. It is concluded that the differences in apparent spatial location produced by ITD have only weak effects on obligatory streaming.     

Interaural correlation sensitivity

Sensitivity to differences in interaural correlation was measured for 1.3-ERB-wide bands of noise using a 2IFC task at six frequencies: 250, 500, 750, 1000, 1250, and 1500 Hz. The sensitivity index, d', was measured for discriminations between a number of fixed pairs of correlation values. Cumulative d' functions were derived for each frequency and condition. The d' for discriminating any two values of correlation may be recovered from the cumulative d' function by the difference between cumulative d's for these values. Two conditions were employed: the noisebands were either presented in isolation (narrow-band condition) or in the context of broad, contiguous flanking bands of correlated noise (fringed condition). The cumulative d' functions showed greater sensitivity to differences in correlation close to 1 than close to 0 at low frequencies, but this difference was less pronounced in the fringed condition. Also, a more linear relationship was observed when cumulative d' was plotted as a function of the equivalent signal-to-noise ratio (SNR) in dB for each correlation value, rather than directly against correlation. The equivalent SNR was the SNR at which the interaural correlation in an NoS(pi) stimulus would equal the interaural correlation of the noise used in the experiment. The maximum cumulative d' declined above 750 Hz. This decline was steeper for the fringed than for the narrow-band condition. For the narrow-band condition, the total cumulative d' was variable across listeners. All cumulative d' functions were closely fitted using a simple two-parameter function. The complete data sets, averaged across listeners, from the fringed and narrow-band conditions were fitted using functions to describe the changes in these parameters over frequency, in order to produce an interpolated family of curves that describe sensitivity at frequencies between those tested. These curves predict the spectra recovered by the binaural system when complex sounds, such as speech, are masked by noise.     

Localization of sound in rooms. V. Binaural coherence and human sensitivity to interaural time differences in noise

Binaural recordings of noise in rooms were used to determine the relationship between binaural coherence and the effectiveness of the interaural time difference (ITD) as a cue for human sound localization. Experiments showed a strong, monotonic relationship between the coherence and a listener's ability to discriminate values of ITD. The relationship was found to be independent of other, widely varying acoustical properties of the rooms. However, the relationship varied dramatically with noise band center frequency. The ability to discriminate small ITD changes was greatest for a mid-frequency band. To achieve sensitivity comparable to mid-band, the binaural coherence had to be much larger at high frequency, where waveform ITD cues are imperceptible, and also at low frequency, where the binaural coherence in a room is necessarily large. Rivalry experiments with opposing interaural level differences (ILDs) found that the trading ratio between ITD and ILD increasingly favored the ILD as coherence decreased, suggesting that the perceptual weight of the ITD is decreased by increased reflections in rooms.     

Interaural time sensitivity of high-frequency neurons in the inferior colliculus

Recent psychoacoustic experiments have shown that interaural time differences provide adequate cues for lateralizing high-frequency sounds, provided the stimuli are complex and not pure tones. We present here physiological evidence in support of these findings. Neurons of high best frequency in the cat inferior colliculus respond to interaural phase differences of amplitude modulated waveforms, and this response depends upon preservation of phase information of the modulating signal. Interaural phase differences were introduced in two ways: by interaural delays of the entire waveform and by binaural beats in which there was an interaural frequency difference in the modulating waveform. Results obtained with these two methods are similar. Our results show that high-frequency cells can respond to interaural time differences of amplitude modulated signals and that they do so by a sensitivity to interaural phase differences of the modulating waveform.     

Sensitivity to brief changes of interaural time and interaural intensity

The purpose of this study was to measure listeners' abilities to detect brief changes in interaural temporal disparities (ITDs) or interaural intensitive disparities (IIDs) conveyed by bursts of noise (probes) temporally and symmetrically flanked by segments of diotic or uncorrelated noise. Thresholds were measured using a four-interval, two-alternative, forced-choice adaptive task and the total duration of the bursts of noise was either 20, 40, or 100 ms. Probes were temporally centered within each burst and the durations of the probes ranged from 2 to 100 ms, depending upon the duration of the (longer) total burst of noise within which they were embedded. The results indicate that, for a given total duration of noise, there is a rapid decrease in threshold ITD or threshold IID as the duration of the probe is increased so that it occupies a larger portion of the total burst of noise. Mathematical analyses revealed that both threshold ITDs and threshold IIDs could be well accounted for by assuming that the listener processes both types of binaural cues via a single, symmetric, double-exponential temporal window. Interestingly, the shapes of the temporal windows that fit the data obtained from the human listeners resemble the shapes of the temporal windows derived by Wagner [H. Wagner, J. Comp. Physiol. A 169, 281-289 (1991)], who studied the barn owl. The time constants and relative weightings yielded temporal window functions that heavily emphasize information occurring within the very temporal center of the window. This temporal window function was found to be generalizable in the sense that it also accounts for classic data reported by Grantham and Wightman [D.W. Gratham and F.L. Wightman, J. Acoust. Soc. Am. 63, 511-523 (1978)] concerning sensitivity to dynamically changing interaural disparities.     

Interaural fluctuations and the detection of interaural incoherence: bandwidth effects

One-hundred left-right noise-pairs were generated, all with a fixed value of long-term interaural coherence, 0.9922. The noises had a center frequency of 500 Hz, a bandwidth of 14 Hz, and a duration of 500 ms. Listeners were required to discriminate between these slightly incoherent noises and diotic noises, with a coherence of 1.0. It was found that the value of interaural coherence itself was an inadequate predictor of discrimination. Instead, incoherence was much more readily detected for those noise-pairs with the largest fluctuations in interaural phase or level differences (as measured by the standard deviations). One-hundred noise-pairs with the same value of coherence, 0.9922, and geometric mean frequency of 500 Hz were also generated for bandwidths of 108 and 2394 Hz. It was found that for increasing bandwidth, fluctuations in interaural differences varied less between different noise-pairs and that detection performance varied less as well. The results suggest that incoherence detection is based on the size of interaural fluctuations and that the value of coherence itself predicts performance only in the wideband limit.     

Comparing monaural and interaural temporal windows: effects of a temporal fringe on sensitivity to intensity differences

In an effort to provide a unifying framework for understanding monaural and binaural processing of intensity differences, an experiment was performed to assess whether temporal weighting functions estimated in two-interval monaural intensity-discrimination tasks could account for data in single-interval interaural intensity-discrimination tasks. In both tasks, stimuli consisted of a 50-ms burst of noise with a 5-ms probe segment at temporal positions ranging between the onset and offset of the overall stimulus. During the probe segment, one monaural interval or binaural channel of each trial contained an intensity increment and the other contained a decrement. Listeners were instructed to choose the interval/channel containing the increment. The pattern of monaural thresholds was roughly symmetrical (an inverted U) across temporal position of the probe but interaural thresholds were substantially higher for a brief time interval following stimulus onset. A two-sided exponential temporal window fit to the monaural data accounted for the interaural data well when combined with a post-onset-weighting function that described greatest weighting of binaural information at stimulus onset. A second experiment showed that the specific procedure used in measuring fringed interaural-intensity-difference-discrimination thresholds affects thresholds as a function of fringe duration and influences the form of the best-fitting post-onset-weighting function.     

The influence of different segments of the ongoing envelope on sensitivity to interaural time delays

The auditory system is sensitive to interaural timing disparities in the fine structure and the envelope of sounds, each contributing important cues for lateralization. In this study, psychophysical measurements were conducted with customized envelope waveforms in order to investigate the isolated effect of different segments of a periodic, ongoing envelope on lateralization. One envelope cycle was composed of the four segments attack flank, hold duration, decay flank, and pause duration, which were independently varied to customize the envelope waveform. The envelope waveforms were applied to a 4-kHz sinusoidal carrier, and just noticeable envelope interaural time differences were measured in six normal hearing subjects. The results indicate that attack durations and pause durations prior to the attack are the most important stimulus characteristics for processing envelope timing disparities. The results were compared to predictions of three binaural lateralization models based on the normalized cross correlation coefficient. Two of the models included an additional stage to mimic neural adaptation prior to binaural interaction, involving either a single short time constant (5 ms) or a combination of five time constants up to 500 ms. It was shown that the model with the single short time constant accounted best for the data.     

The dominant role of low-frequency interaural time differences in sound localization.

Two experiments are described in which listeners judge the apparent directions of virtual sound sources-headphone-presented sounds that are processed in order to simulate free-field sounds. Previous results suggest that when the cues to sound direction are preserved by the simulation, the apparent directions of virtual sources are nearly the same as the apparent directions of real free-field sources. In the experiments reported here, the interaural phase relations in the processing algorithms are manipulated in order to produce stimuli in which the interaural time difference cues signal one direction and interaural intensity and pinna cues signal another direction. The apparent directions of these conflicting cue stimuli almost always follow the interaural time cue, as long as the wideband stimuli include low frequencies. With low frequencies removed from the stimuli, the dominance of interaural time difference disappears, and apparent direction is determined primarily by interaural intensity difference and pinna cues.     

Interaural fluctuations and the detection of interaural incoherence. II. Brief duration noises

Listeners detected a small amount of interaural incoherence in reproducible noises with narrow bandwidths and a center frequency of 500 Hz. The durations of the noise stimuli were 100, 50, or 25 ms, and every one of the noises had the same value of interaural coherence, namely 0.992. When the nominal noise bandwidth was 14 Hz, the ability to detect incoherence was found to depend strongly on the size of the fluctuations in interaural phase and level for durations of 100 and 50 ms. For the duration of 25 ms, performance did not appear to depend entirely on fluctuations. Instead, listeners sometimes recognized incoherence on the basis of laterality. However, when the nominal bandwidth was doubled, leading to a greater number of fluctuations, detection performance at 25 ms resembled that at 50 ms for the smaller bandwidth. It is concluded that the detection of a small amount of interaural incoherence is mediated by fluctuations in phase and level for brief stimulus durations, so long as such fluctuations exist physically. This conclusion presents a promising alternative to models of binaural detection that are based on the short-term cross-correlation in the stimulus.     

The effect of head-induced interaural time and level differences on speech intelligibility in noise

A study was made of the effect of interaural time delay (ITD) and acoustic headshadow on binaural speech intelligibility in noise. A free-field condition was simulated by presenting recordings, made with a KEMAR manikin in an anechoic room, through earphones. Recordings were made of speech, reproduced in front of the manikin, and of noise, emanating from seven angles in the azimuthal plane, ranging from 0 degree (frontal) to 180 degrees in steps of 30 degrees. From this noise, two signals were derived, one containing only ITD, the other containing only interaural level differences (ILD) due to headshadow. Using this material, speech reception thresholds (SRT) for sentences in noise were determined for a group of normal-hearing subjects. Results show that (1) for noise azimuths between 30 degrees and 150 degrees, the gain due to ITD lies between 3.9 and 5.1 dB, while the gain due to ILD ranges from 3.5 to 7.8 dB, and (2) ILD decreases the effectiveness of binaural unmasking due to ITD (on the average, the threshold shift drops from 4.6 to 2.6 dB). In a second experiment, also conducted with normal-hearing subjects, similar stimuli were used, but now presented monaurally or with an overall 20-dB attenuation in one channel, in order to simulate hearing loss. In addition, SRTs were determined for noise with fixed ITDs, for comparison with the results obtained with head-induced (frequency dependent) ITDs.(ABSTRACT TRUNCATED AT 250 WORDS)     

Bimodal listeners are not sensitive to interaural time differences in unmodulated low-frequency stimuli (L)

Sensitivity to interaural time differences (ITDs) with unmodulated low-frequency stimuli was assessed in bimodal listeners who had previously shown to be good performers in ITD experiments. Two types of stimuli were used: (1) an acoustic sinusoid combined with an electric transposed signal and (2) an acoustic sinusoid combined with an electric clicktrain. No or very low sensitivity to ITD was found for these stimuli, even though subjects were highly trained on the task and were intensively tested in multiple test sessions. In previous studies with users of a cochlear implant (CI) and a contralateral hearing aid (HA) (bimodal listeners), sensitivity was shown to ITD with modulated stimuli with frequency content between 600 and 3600 Hz. The outcomes of the current study imply that in speech processing design for users of a CI in combination with a HA on the contralateral side, the emphasis should be more on providing salient envelope ITD cues than on preserving fine-timing ITD cues present in acoustic signals.     

Dependence of binaural loudness summation on interaural level differences, spectral distribution, and temporal distribution

Loudness of interaurally correlated narrow- and broadband noises was investigated using a loudness estimation paradigm (with two anchors) presented via headphones. Throughout the experiments (most performed by 12 subjects), the results from both anchors agreed very well. In the first experiment, third-octave-band noises centered around 250, 710, or 2000 Hz, as well as broadband red (-10 dB/oct), pink (-3 dB/oct), and blue (+10 dB/oct) noises, with interaural level differences of delta L = 0, 4, 10, 20, and infinity dB, were presented as test signals while the same sound presented monaurally or diotically served as anchor. The binaurally summed loudness at delta L = 0 dB was found to be larger than the loudness of a monaural signal of the same SPL by a factor of about 1.5 and decreased with increasing delta L. No dependence of this behavior on frequency, level, or spectral shape was found. In a second experiment, abutting frequency bands of varying width were alternately presented to the subject's left and right ears with the overall spectrum encompassing the whole audio range. The binaural loudness was larger for fewer but broader frequency bands. In a third experiment, uniform exciting noise was switched between the two ears at various speeds. Increasing the switching frequency gave rise to an increase in loudness of about 20%. All results are discussed from the viewpoint of the use of the standardized loudness meter. At this point, there is no evidence that any significant systematic errors due to single-channel evaluation (in contrast to the human two-channel processing) are made by measuring loudness using these meters.     

Interaural time and amplitude discrimination in noise

Just-noticeable differences (jnds) of both interaural time delay (ITD) and interaural intensity difference (IID) were measured for binaural tones in the presence of broadband maskers. The tones were presented at 50 dB SPL, the target frequency was 500 Hz, and the masker frequency was 100-1000 Hz, with various combinations of ITD and IID. The time and amplitude jnds exhibit similar dependencies on target-to-masker ratio and masker type. At a given target-to-masker ratio, discrimination was generally best in the presence of diotic maskers and worst in the presence of the interaurally out-of-phase maskers. Results for the other masker types examined tended to fall in between these two extremes. Many of these data trends are consistent with predictions of the lateralization model and the position-variable model based on auditory-nerve activity.