Extension of a binaural cross-correlation model by contralateral inhibition. II. The law of the first wave front

This paper explains the "law of the first wave front" and related binaural phenomena on the basis of the model presented in the previous paper [Lindemann, J. Acoust. Soc. Am. 80, 1608-1622 (1986)] in which a contralateral inhibition mechanism was added to the well-known model of binaural cross correlation. In order to verify the predictions of the extended model, psychoacoustic experiments were performed with pairs of narrow-band impulses which were presented through headphones. The test signals consisted of a diotic primary sound and an "echo" with an interaural arrival-time difference. Lateralization was measured as a function of the time delay between primary sound and echo. For delays below the echo threshold, summing localization and the law of the first wave front were simulated; for delays above the echo threshold, the model predicts an influence of the primary sound on the lateralization of the echo.     

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.     

A central spectrum theory of binaural processing. Evidence from dichotic pitch

A theory is presented that describes the binaural processing of interaural time or phase differences. It is an elaboration of the central spectrum concept for the explanation of dichotic pitch phenomena [F. A. Bilsen, "Pitch of noise signals: Evidence for a 'central spectrum'," J. Acoust. Soc. Am. 61, 150-161 (1977)]. The generation is postulated for central activity patterns (CAP) due to binaural interaction. From these CAPs the central processor selects specific spectral information that constitutes the information for lateralization, dichotic pitch, binaural masking, etc. Here, a strategy is assumed to be based on central spectra (CS) rather than on interaural cross correlation. For the calculation of the central activity patterns a number of assumptions have been introduced. The peripheral filters are supposed to be infinitesimally narrow. The analog filter outputs from corresponding filters at both ears are thought to interact by means of a linear delay-and-add mechanism. The squared output (power) of such a binaural (addition) network constitutes the CAP. The theory has been tested with lateralization and BMLD measurements using dichotic stimulus configurations characteristic of the perception of dichotic pitch. The predictions of the model concerning the pitch and the lateralization of the pitch images as well as the BMLD patterns for this kind of stimuli are confirmed.     

Lateralization of acoustic signals by dichotically listening budgerigars (Melopsittacus undulatus)

Sound localization allows humans and animals to determine the direction of objects to seek or avoid and indicates the appropriate position to direct visual attention. Interaural time differences (ITDs) and interaural level differences (ILDs) are two primary cues that humans use to localize or lateralize sound sources. There is limited information about behavioral cue sensitivity in animals, especially animals with poor sound localization acuity and small heads, like budgerigars. ITD and ILD thresholds were measured behaviorally in dichotically listening budgerigars equipped with headphones in an identification task. Budgerigars were less sensitive than humans and cats, and more similar to rabbits, barn owls, and monkeys, in their abilities to lateralize dichotic signals. Threshold ITDs were relatively constant for pure tones below 4 kHz, and were immeasurable at higher frequencies. Threshold ILDs were relatively constant over a wide range of frequencies, similar to humans. Thresholds in both experiments were best for broadband noise stimuli. These lateralization results are generally consistent with the free field localization abilities of these birds, and add support to the idea that budgerigars may be able to enhance their cues to directional hearing (e.g., via connected interaural pathways) beyond what would be expected based on head size.     

Binaural summation and lateralization of transients: a combined analysis

Subjects judged the loudness and the lateral position of dichotic transient signals, which were presented at equal and unequal levels, synchronously and asynchronously, to the two ears. Binaural loudness summation of clicks does not obey a law of linear addition: It is partial at low level and superadditive at high level. Supersummation is greater for interaurally delayed clicks than for coincidental ones. The relation between click loudness and sound pressure (over moderate SLs) can be described as a power function with a greater exponent for the binaural function. Lateral positions spread over a greater range for interaural level differences than for interaural time differences. The time-intensity trading ratio was greater than is typically reported for tones. When sound lateralization was induced by interaural time difference, but not by intensity difference, a virtually perfect negative correlation between loudness and extent of off-center displacement existed.     

Lateralization of sine tones-interaural time vs phase (L)

Listeners estimated the lateral positions of 50 sine tones with interaural phase differences ranging from -150 degrees to +150 degrees and with different frequencies, all in the range where signal fine structure supports lateralization. The estimates indicated that listeners lateralize sine tones on the basis of interaural time differences and not interaural phase differences.     

Discrimination of interaural differences of level as a function of frequency

Discrimination of interaural differences of level (IDLs) was measured for pure tones as a function of frequency and as a function of the interaural difference of phase or level of a standard. Varying the interaural difference of the standard was assumed to change the lateral position of its intracranial image. Threshold IDLs were approximately constant over a frequency range from 200-5000 Hz, except in a region near 1000 Hz where they were slightly elevated. Thresholds increased as the value of the standard interaural differences of phase or level increased, implying that interaural resolution declines as the lateral image moves away from midline. The results are generally consistent with the predictions of current models of lateralization, but additions to these models are required in order for them to account for the slight frequency dependence of threshold IDLs.     

Auditory lateralization in monkeys: an examination of two cues serving directional hearing.

In assigning binaural ongoing time differences (phase) as the cue for localization of low frequencies, and binaural intensity differences as the cue for localization of high frequencies, the duplex theory has successfully accounted for human directional hearing of tones. Sensitivity of monkeys to these cues was examined in two experiments. The dependencies on frequency of interaural intensity difference thresholds (lateralization experiment I) and time difference thresholds (lateralization experiment II) were determined behaviorally on three monkeys (M. nemestrina). The range of frequencies was from 125 Hz to 8 kHz in experiment I and from 250 Hz to 2 kHz in experiment II. The results indicate that the duplex theory is applicable to monkeys. However, monkeys are less sensitive than man to both binaural cues. The shortest time disparity monkeys discriminate is 42 microseconds at 1.5 kHz and the smallest intensity difference is 3.5 dB at 500 Hz. Good agreement between the present findings and localization measurements [C. H. Brown et al., J. Acoust. Soc. Am. 63, 1484-1492 (1978)] suggests: (a) that monkeys utilize time disparity cues through higher frequencies than man; and (b) that inaccurate localization by monkeys at high frequencies reflects decreasing sensitivity to interaural intensity cues.     

Interaural intensity discrimination: insensitivity at 1000 Hz

Recent data from three laboratories have replicated Mills' [J. Acoust. Soc. Am. 32, 132-134 (1960)] finding that interaural intensity discrimination is relatively poorer for tones of 1000 Hz than for tones of either higher or lower frequencies. To get a finer look at this frequency effect, interaural intensity difference thresholds were obtained from four subjects for tones of several frequencies around 1000 Hz. An adaptive two-interval forced-choice procedure was employed, in which the overall intensity of the signals was varied randomly in order to prevent subjects from listening to monaural loudness changes. Despite large intersubject differences in overall sensitivity to interaural intensity differences, all four subjects showed a local peak in their threshold functions at or near 1000 Hz. This curious "1000-Hz effect" might be explained by imagining that an interaural intensity comparator operates more efficiently as frequency increases, but that a peripheral interaural intensity difference to interaural-time difference conversion contributes to laterality judgments for low-frequency tones, thus acting to lower thresholds again for frequencies below 1000 Hz.     

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.     

Stereausis: binaural processing without neural delays

A neural network model is proposed for the binaural processing of interaural-time and level cues. The two-dimensional network measures interaural differences by detecting the spatial disparities between the instantaneous outputs of the two ears. The network requires no neural delay lines to generate such attributes of binaural hearing as the lateralization of all frequencies, and the detection and enhancement of noisy signals. It achieves this by comparing systematically, at various horizontal shifts, the spatiotemporal responses of the tonotopically ordered array of auditory-nerve fibers. An alternative view of the network operation is that it computes approximately the cross correlation between the responses of the two cochleas by combining an ipsilateral input at a given characteristic frequency (CF) with contralateral inputs from locally off-CF locations. Thus the network utilizes the delays already present in the traveling waves of the basilar membrane to extract the correlation function. Simulations of the network operation with various signals are presented as are comparisons to computational schemes suggested for stereopsis in vision. Physiological arguments in support of this scheme are also discussed.     

Lateralization of tonal signals which have neither onsets nor offsets.

In order to ascertain the special importance of binaural cues conveyed in the transient portions of dichotic signals, thresholds for interaural differences of time (delta t) and intensity (delta I) were studied using stimuli whose onsets and offsets were masked. Intense noise was used to mask all portions of each experimental trial except for the two intervals of a two-interval, forced-choice detection task. During the intervals, the noise was turned off with decay-rise times of 10 ms. What remained were tones whose interaural phase or intensity was different for intervals one and two. Performance was compared to control conditions which used unmasked gated sinusoids. For longer durations, detection without onsets and offsets was about as good as that with no masker. For the shorter signals, detection without transients was poorer than with standard lateralization, but this is attributed to forward and backward masking which reduced the effective durations of those stimuli. The ability to detect interaural differences of time with the onsets and offsets masked was extended to conditions in which the decay times of the noise were 100 ms. Performance here was slightly worse, but not by so much as to change the basic result. This is interpreted as showing that performance with the faster decay-rise times was not a product of momentary undershoots in neural following, but depended, rather, upon a true encoding of the interaural information in the stimulus fine-structure.     

Lateralization of complex binaural stimuli: a weighted-image model

This article describes a new model that predicts the subjective lateral position of bandpass stimuli. It is assumed, as in other models, that stimuli are bandpass filtered and rectified, and that the rectified outputs of filters with matching center frequencies undergo interaural cross correlation. The model specifies and utilizes the shape and location of assumed patterns of neural activity that describe the cross-correlation function. Individual modes of this function receive greater weighting if they are straighter (describing consistent interaural delay over frequency) and/or more central (describing interaural delays of smaller magnitude). This weighting of straightness and centrality is used by the model to predict the perceived laterality of several types of low-frequency bandpass stimuli with interaural time delays and/or phase shifts, including bandpass noise, amplitude-modulated stimuli with time-delayed envelopes, and bandpass-filtered clicks. This model is compared to other theories that describe lateralization in terms of the relative contributions of information in the envelopes and fine structures of binaural stimuli.     

A model for sound lateralization.

Recent studies of multiple sclerosis (MS) and stroke patients suggested a correlation between two patterns of abnormal performance in lateralization tasks and two sites of pontine lesions. Most patients who had lesions below or at the superior olivary complex (SOC) perceived all interaural differences in binaural stimuli as small, while most patients who had lesions above the SOC perceived all interaural differences as large. The two abnormal performance patterns occurred for interaural time differences (ITD) and/or for interaural level differences (ILD). The present model proposes a multi-level hierarchical brainstem structure that estimates ITD and ILD. The first level seeks dissimilarity between the left and right inputs and a second level looks for similarity between the two sides' inputs. Each level is modeled as an ensemble of neural arrays in which each unit performs a logic or arithmetic function. The inputs are simulations of auditory nerve responses to broadband stimuli. Simulations yield good correspondence to the effect of both locations of pontine lesions on binaural performance.     

Stereophonic channel decorrelation using a binaural masking model

Traditional methods often only use monaural masking models to decorrelate input signals for stereo acoustic echo cancellation. Whereas, it seems more reasonable to use binaural masking models for the following two reasons. First, stereo signals are heard by two ears rather than just one. Second, psychoacoustic researchers have already shown that there are obvious masking level differences between binaural masking models and monaural masking models. By studying binaural masking level difference models, we first introduce a simplified binaural masking model for stereo acoustic echo cancellation. Considering that the interaural time difference is dominant at low frequencies (?$?$1.5  kHz) and the interaural level difference is a major cue at higher frequencies, we propose to use different signal decorrelation schemes at these two frequency bands. In the low-frequency band, a pitch-driven sinusoidal injection scheme is proposed to maintain the interaural time difference, where the amount of injection is determined by the proposed binaural masking model. In the high-frequency band, a modified sinusoidal phase modulation scheme is applied to make a trade-off between preserving the interaural level difference and decorrelating the stereophonic input signals. Assessment results show that the proposed method can effectively improve the non-unique problem and retain good speech quality.     

Sound segregation based on temporal envelope structure and binaural cues

The ability to segregate two spectrally and temporally overlapping signals based on differences in temporal envelope structure and binaural cues was investigated. Signals were a harmonic tone complex (HTC) with 20 Hz fundamental frequency and a bandpass noise (BPN). Both signals had interaural differences of the same absolute value, but with opposite signs to establish lateralization to different sides of the medial plane, such that their combination yielded two different spatial configurations. As an indication for segregation ability, threshold interaural time and level differences were measured for discrimination between these spatial configurations. Discrimination based on interaural level differences was good, although absolute thresholds depended on signal bandwidth and center frequency. Discrimination based on interaural time differences required the signals' temporal envelope structures to be sufficiently different. Long-term interaural cross-correlation patterns or long-term averaged patterns after equalization-cancellation of the combined signals did not provide information for the discrimination. The binaural system must, therefore, have been capable of processing changes in interaural time differences within the period of the harmonic tone complex, suggesting that monaural information from the temporal envelopes influences the use of binaural information in the perceptual organization of signal components.     

前方空间环绕声的四扬声器虚拟重放

考虑头部转动带来的动态因素对听觉垂直定位的贡献,提出了前方空间环绕声的四扬声器虚拟重放方法。4个扬声器分别布置在水平面左前、右前以及高仰角的左前上、右前上方向,并采用听觉传输信号处理的方法将多通路空间环绕声信号转换为4个扬声器的重放信号。以9.1通路空间环绕声虚拟重放为例,采用头相关传输函数对双耳声压及其包含的定位因素进行分析表明,该方法可以产生正确的双耳时间差及其随头部转动的变化,从而产生合适的侧向定位双耳因素和垂直定位的动态因素。而心理声学实验结果表明,该方法可以重放稳定的前方空间的水平和垂直虚拟源。因此,四扬声器布置结合听觉传输处理足以重放前方空间环绕声的垂直定位信息,实现多通路空间环绕声的向下混合与简化。     

The lateralization of simple dichotic pitches

A "simple" dichotic pitch arises when a single narrow band possesses a different interaural configuration from a surrounding broadband noise whose interaural configuration is uniform and correlated. Such pitches were created by interaurally decorrelating a narrow band (experiment 1) or by giving a narrow band a different interaural time difference from the noise (experiment 2). Using an adaptive forced-choice procedure, listeners adjusted the interaural intensity difference of "pointers" to match their lateralization to that of the dichotic pitches. The primary determinants of lateralization were the interaural configuration of the broadband noise (experiment 1), the center frequency of the narrow band (experiment 1), and its interaural configuration (experiment 2). The ability of two computational models to predict these results was evaluated. A version of the central-spectrum model [J. Raatgever and F. A. Bilsen, J. Acoust. Soc. Am. 80, 429-441 (1986)] incorporating realistic frequency selectivity accounted for the main results of experiment 1 but not experiment 2. A new "reconstruction-comparison" model accounted for the main results of both experiments. To accommodate the variables shown to influence lateralization, this model segregates evidence of the dichotic pitch from the noise, reconstructs the cross-correlogram of the noise, and compares it with the cross-correlogram of the original stimulus.     

Interaural correlation discrimination: II. Relation to binaural unmasking

Many theoretical models of binaural interaction assume that sensitivity to interaural correlation underlies binaural unmasking. This paper explores the extent to which sensitivity to changes in interaural correlation implied by results from binaural detection experiments are consistent with sensitivity to changes in interaural correlation implied by results from binaural detection experiments are consistent with sensitivity to changes in interaural correlation measured directly in correlation discrimination experiments. The vehicle for this exploration is a simplified model of the underlying processes assumed by many models of binaural unmasking for the detection of narrow-band signals in the presence of broadband noise. Consideration is given to psychometric function slopes, detection thresholds, bandwidth effects, duration effects, level effects, and interaural-parameter effects. Although many of the results obtained from our analysis are consistent with the notion that the cue in binaural detection tasks is a change in interaural correlation, some significant inconsistencies are noted.