Threshold differences for interaural time delays carried by double vowels.

Klumpp and Eady (1956, J Acoust Soc Am 28, p.859-860) reported preliminary data on human sensitivity to interaural time differences (ITD) with various stimuli. At 10 μs ITD the best discrimination of 79% correct was reported for band-pass filtered (150-1700 Hz) noise. Despite the preliminary nature, and presentation methods different from todays, the above is still the best available reference for optimal ITD discrimination. The goal of the current study is to systematically determine the stimulus and the experimental paradigm that results in the smallest threshold ITD and to provide an accurate reference value. We varied seven stimulus and procedure parameters: stimulus waveform, stimulation level, stimulus duration, adaptive versus constant stimulus procedure, alternative-forced-choice (AFC) procedure, inter-stimulus pause duration, and complete waveform versus ongoing ITD. The condition yielding the lowest threshold ITD was Gaussian noise band-pass filtered from 20 to 1400 Hz, presented at 70 dB SPL, with a short inter-stimulus pause of 50 ms, and an interval duration of 0.5 s. Averaged across 8 trained subjects, the threshold ITD for this condition at the 79% correct level was 7 μs. The influence of each parameter will be discussed together with the obstacles of accurately determining this value.

Room reflections alter the envelope of sounds differently at both ears, reducing binaural coherence. Experiment 1 measured interaural time difference (ITD) discrimination thresholds for broadband and transposed speech. A sentence was convolved with binaural room impulse responses for different source-receiver distances, and the envelopes extracted and multiplied with 4-kHz tones. Preliminary results indicate that envelope ITD thresholds increased from <100 μs without reverberation to >700 μs at four times the reverberation radius. ITD thresholds for unprocessed speech were more robust to the addition of reverberation, only rising above 350 μs at the same distance. To investigate whether reverberation is detrimental owing to reduction in binaural coherence, stimulus envelopes were created by temporally jittering raised cosine pulses around a 10-ms separation. Bilaterally independent jittering allowed variation in coherence while minimizing change in other envelope parameters. Preliminary results show ITD thresholds <100 μs for coherent envelopes, increasing to >700 μs for coherences of 0.6, a value consistent with the room simulations above. Envelope coherence strongly affects ITD discrimination, suggesting that ITDs extracted from high-frequency channels may not provide useful information in many realistic situations. This has implications for bilateral cochlear implant users, as current devices provide ITDs only in envelopes.

Background: The temporal envelope (ENV) plays a vital role in conveying inter-aural time difference (ITD) in many clinical populations. However, the presence of background noise and electronic features, such as compression, reduces the modulation depth of ENV to a different degree in both ears. The effect of ENV modulation depth differences between the ears on ITD thresholds is unknown; therefore, this was the aim of the current study's investigation. Methods: Six normally hearing young adults (age range 20-30 years) participated in the current study. Six vowel-consonant-vowel (VCV) (/aka/, /aga/, /apa/, /aba/, /ata/, /ada/) tokens were used as the probe stimuli. ENV depth of VCV tokens was smeared by 0%, 29%, and 50%, which results in 100%, 71%, and 50% of the original modulation depth. ITD thresholds were estimated as a function of the difference in temporal ENV depth between the ears, wherein in one ear the modulation depth was retained at 100% and in the other ear, the modulation depth was changed to 100%, 71%, and 50%. Results: Repeated measures of ANOVA revealed a significant main effect of interaural modulation depth differences on the ITD threshold (F(2,10)= 9.04, p= 0.006). ITD thresholds increased with an increase in the inter-aural modulation depth difference. Conclusion: Inter-aural ENV depth is critical for ITD perception.

Effects of simulated reverberation on the use of binaural cues and fundamental-frequency differences for separating concurrent vowels

When acoustic stimuli are presented over headphones listeners usually report that a sound image appears within their head. Interaural differences in time and intensity can move the position of this image within the head. This effect is usually referred to as lateralization. We will review some of the basic results on the lateralization of simple stimuli. The relationship between lateral position and the interaural differences of time and intensity will be summarized. These results are used to predict those values of interaural time and intensity that yield an equivalent lateral position. The results from experiments which measure the just discriminable differences in interaural time and intensity will also be reviewed. The relationship between these lateral discrimination results and those from the lateral position studies will be described. The various models of lateralization all assume some type of cross-correlation operation. The cross-correlation approach to modeling these data has some strengths and some weaknesses. Results concerning the interaural differences of onset time and ongoing time will be compared. An attempt will be made to provide a general framework for the basic data concerning the lateral position and lateral discrimination of simple stimuli. The extent to which results from these lateralization experiments can account for our ability to localize sound sources in the free field will be discussed.

The interaural time difference threshold for speech has been reported to be approximately 35 μsec (Cherry & Sayers, 1956), a value substantially larger than the 6 ~sec reported for broadband noise signals by other experimenters (e.g., Tobias & Zerlin, 1959). In the two studies just mentioned, however, different subjects and psychoacoustical methods were employed; thus, it is unclear whether larger interaural time differences are needed to lateralize speech signals. The purpose of this experiment, therefore, was to compare lateralization performance for speech and nonspeech stimuli. Interaural time difference thresholds were obtained for speech, speech spectrum noise, speech multiplied noise, and 200-, 500-, and 1,000-Hz sinusoids for the same subjects using a 2 IFC experimental paradigm. Under these conditions, speech and speech-multiplied noise yielded essentially the same interaural time difference thresholds.

Interaural time differences (ITD) and interaural level differences (ILD) both signal horizontal sound source location. To achieve a unified percept of our acoustic environment, these two cues require integration. In the present study, we tested this integration of ITD and ILD with electroencephalography (EEG) by measuring the mismatch negativity (MMN). The MMN can arise in response to spatial changes and is at least partly generated in auditory cortex. In our study, we aimed at testing for an MMN in response to stimuli with counter-balanced ITD/ILD cues. To this end, we employed a roving oddball paradigm with alternating sound sequences in two types of blocks: (a) lateralized stimuli with congruently combined ITD/ILD cues and (b) midline stimuli created by counter-balanced, incongruently combined ITD/ILD cues. We observed a significant MMN peaking at about 112–128 ms after change onset for the congruent ITD/ILD cues, for both lower (0.5 kHz) and higher carrier frequency (4 kHz). More importantly, we also observed significant MMN peaking at about 129 ms for incongruently combined ITD/ILD cues, but this effect was only detectable in the lower frequency range (0.5 kHz). There were no significant differences of the MMN responses for the two types of cue combinations (congruent/incongruent). These results suggest that—at least in the lower frequency ranges (0.5 kHz)—ITD and ILD are processed independently at the level of the MMN in auditory cortex.

For high-frequency complex stimuli, detection thresholds for envelope-based interaural time differences (ITDs) decrease with overall level. Substantial heterogeneity is, however, evident among the findings concerning the rate at which thresholds decline with level. This study investigated factors affecting the influence of overall level on threshold ITDs. Thresholds were measured as a function of overall level for 4-kHz-centered "targets" in three experiments focusing, respectively, on stimulus-type (sinusoidally amplitude-modulated or "transposed" tones), modulation frequency, and details concerning low-pass noise used to mask low-frequency distortion products. Results indicated that (1) log-ITD thresholds decreased linearly with overall level; (2) slopes relating log-ITD thresholds to level did not depend significantly on stimulus type; (3) lower modulation frequencies produced greater dependencies of thresholds on overall level than did higher modulation frequencies; (4) the effect of overall level on threshold-ITDs was independent of the interaural configuration and levels of the low-pass noise maskers tested; (5) synchronously gating the low-pass noise and target produced a greater dependency of thresholds on the overall level of the target than did continuous or temporally "fringed" presentation of the noise. A fourth experiment showed that threshold interaural level differences were somewhat less affected by changes in overall level than were threshold ITDs.

In environments with many concurrent sound sources, it is often possible to hear out individual sounds and identify their sources. To accomplish this perceptual organization of the sound sources, listeners analyze the mixture of sounds to estimate the constituent components that correspond to one acoustic sound source. Signal components that are likely to come from the same source are grouped into an auditory object, that is segregated from the acoustic complex. The grouping of signal components into sources is based on identifying shared features of the components, like common spectral cues (the same fundamental frequency or frequency modulation), temporal cues (the same temporal envelope), or spatial cues (the same interaural differences). When signal components’ onsets fall within such a time window that they are perceived as being synchronous, the components are combined to form a new auditory object, that is monitored across time on cues that sustain their grouping. Given that spatial cues are inherent to every sound and already present at the onset, and that they enhance detection of a sound by a spatial release from masking, it is unclear whether and how much temporal perception depends on the presence of spatial cues. The research presented here addressed the contribution of the binaural hearing system, which processes a sound’s spatial cues, to the processing of its temporal cues, and the contribution of the sounds’ temporal cues to the processing of their spatial cues. In order to assess the contribution of spatial cues to temporal perception, experiments were conducted in which the temporal position of a target sound’s onset was controlled by the subjects to find its desired position relative to a regular series of reference sound onsets. Manipulation of the target’s interaural differences, its sensation level, and the target and reference sounds’ durations allowed comparison of their effects on temporal perception of the target’s onset. The main finding was that temporal perception precision degraded below a critical level, depending on the absolute target and masker levels and the difference in spatial configuration between target and masker that facilitated perception at much lower signal-to-noise ratios. The implication of a sound’s signal-to-noise ratio getting below this critical level is that, when the temporal window for grouping signal components’ onsets is enlarged due to the degraded precision, there is an increased probability for signal components to be grouped incorrectly into an auditory object, or to be prevented from grouping into a new auditory object and remain part of the acoustic complex until other evidence is gathered for the existence of their source. This may be a contributing factor to the decrease of speech intelligibility in noisy environments, where precise temporal perception of transients is required for the processing of speech signals. In order to assess the contribution of temporal cues to binaural processing, thresholds were measured for discrimination between the spatial configurations of two sounds that had identical spectral range and onset and offset times, but differed in their temporal envelope structure and had equally-sized directional information to opposite sides of the medial plane. The measured minimum interaural differences in time or level at which the sounds’ spatial configurations could successfully be discriminated represented the ability to process binaural cues, and allowed comparison between the effects of the various manipulations of bandwidths, center frequencies, and temporal envelopes to the sounds. The main findings were that discrimination based on interaural level differences was good for all tested conditions (although absolute thresholds depended on the specific conditions), and that discrimination based on interaural time differences was only possible when the temporal envelopes of the sounds were sufficiently different. The binaural system must, therefore, have been capable of processing the interaural differences within short periods of time when one of the sounds was dominant in the monaural temporal envelopes of the combined signals at each ear. Analysis of the monaural temporal envelopes influences the ability to use the binaural information to separate individual signal components, and allow tracking across time of the signals components with identical spatial cues to form a perceptual unity, the auditory object. In conditions where discrimination was not possible, the temporal envelopes of the individual sounds were too similar and such periods did not occur systematically in the temporal envelopes of the combined sounds. This finding suggests that a monaural analysis of a composite signal’s temporal envelope is a prerequisite for the binaural processing of the constituent signal components’ interaural time differences. The implication is that, if the signals’ temporal envelopes are too similar, their signal components may not be correctly grouped into separate and individually lateralized auditory objects, but remain fused as a single object with an averaged spatial image.

The ability to detect amplitude modulation (AM) of a tonal probe can be disrupted by the presence of modulated masking tones. Two experiments examined whether a disparity in the interaural parameters of the probe and masker can reduce the amount of interference. In the first experiment, the effects of interaural time and intensity differences were studied in separate sets of conditions. With low-frequency carriers, the detection of 10-Hz probe modulation in the presence of 10-Hz masker modulation was not significantly affected by interaural time differences. With higher-frequency carriers, dichotic stimuli were generated through combinations of diotic, dichotic, or monotic probe and masker presentations in which the probe and masker did not share a common interaural intensity difference. In these conditions, the amount of interference was affected by the interaural configuration. However, monotic level differences between the probe and masker may have contributed to the effect of interaural configuration. In the second experiment, the probe and masker were presented through separate speakers in an enclosed listening environment. Spatial separation between the sources for the probe and masker led to a small reduction in the amount of interference. When the masker modulation rate was varied with the probe AM rate fixed at 10 Hz, the extent of tuning in the modulation domain in the sound-field conditions was similar to that obtained with diotic stimulus presentation over headphones.

1. Interaural level differences (ILDs), created by the head and pinna, have long been known to be the dominant acoustic cue for azimuthal localization of high-frequency tones. However, psychophysical experiments have demonstrated that human subjects can also lateralize complex high-frequency sounds on the basis of interaural time differences (ITDs) of the signal envelope. The lateral superior olive (LSO) is one of two pairs of binaural nuclei where the primary extraction of binaural cues for sound source location occurs. "IE" cells in LSO are inhibited by stimuli to the contralateral and excited by stimuli to the ipsilateral ear, and their response rate is therefore dependent on ILD. Anatomic specializations in the afferent pathways to the LSO suggest that this circuit also has a function in the detection of timing cues. We hypothesized that, besides ILD sensitivity, the IE property also conveys a sensitivity to ITDs of amplitude-modulated (AM) tones and could provide the physiological substrate for the psychophysical effect mentioned above. 2. In extracellular recordings from binaural LSO cells in barbiturate-anesthetized cats, response rate was a periodic function of ITDs of AM stimuli, i.e., all cells displayed ITD sensitivity. Binaural responses were smaller than responses to stimulation of the ipsilateral ear alone and were minimal when the envelopes in both ears were in-phase or nearly so. There was good correspondence between responses to ITDs and to dynamic interaural phase differences (IPDs), created by a difference in the envelope frequency to the two ears. Qualitatively, the responses were consistent with the outcome of an IE operation on temporally structured inputs. 3. To compare the relative importance of ILD and ITD, responses to combinations of the two cues were obtained. Despite robust ITD sensitivity in all binaural LSO cells encountered, the changes in response rate that would occur in response to naturally occurring ITDs were small in comparison with the changes expected for naturally occurring ILDs. The main limitation on ITD sensitivity was a steep decline in average discharge rate as the modulation frequency exceeded several hundred Hertz. 4. ITD sensitivity was also present to broadband stimuli, again with minimal rates occurring near 0 ITD. The sensitivity depended in a predictable fashion on the passband of filtered noise and was absent to binaurally uncorrelated noise bands. In response to clicks, ILDs interacted with ITD in a complicated fashion involving amplitude and latency effects. 5. Three low-characteristic frequency (CF) LSO cells were encountered that were IE and showed ITD sensitivity to the fine structure of low-frequency stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

The localization of low-frequency sounds (less than 3 kHz) along the azimuth involves comparing the ongoing difference in the time of arrival of a sound at the two ears. Information about interaural time differences (ITDs) is derived from an initial comparison performed in the superior olivary complex. However, little is known about which aspects of this information are transformed as it ascends the brainstem. To address this issue, we compared the ITD sensitivity of neurons in the inferior colliculus (IC) and auditory thalamus, successive stations in the auditory pathway. We found ITD sensitivity in the IC and thalamus to be similar in several respects. At both levels, the large majority of neurons responded maximally to ITDs within the range that a rabbit would normally encounter (+/- 300 microseconds) and preferred ipsilateral delays, delays that would be created by sounds in the contralateral sound field. The range of frequencies over which ITD sensitivity was expressed was also similar in the midbrain and thalamus. Several differences were also apparent. In comparison to IC neurons, neurons in the thalamus responded over more restricted ranges of ITD, responded at lower rates, and, in response to monaural stimulation, showed an increased influence of inhibition. Finally, a greater proportion of thalamic units had characteristic delays corresponding to intermediate discharge rates. The preservation of a bias for ipsilateral delays from IC to thalamus suggests that a representation of contralateral azimuths is present at both levels. Similarities between the two levels suggest that information about ITDs is faithfully transmitted from midbrain to thalamus. Differences in ITD sensitivity, such as the sharper tuning for ITDs, suggest that the thalamus is not a simple relay. Enhanced sensitivity to ITDs should translate to better-defined azimuthal receptive fields, and therefore may be a step toward achieving an optimal representation of azimuth within the auditory pathway.

Auditory evoked magnetic fields in relation to interaural time delay and interaural correlation

Interaural time difference (ITDs) are the most important cue to the location of sounds containing low-frequency energy (Wightman and Kistler 1992). ITDs are encoded centrally in the medial (MSO) and lateral (LSO) superior olives which transmit the code to the inferior colliulus (IC) (Batra et al. 1997; Goldberg and Brown 1969). Each ITD-sensitive neuron is characterized by its best ITD (BD), the one producing maximal discharge rate. It is a longstanding view that these neurons are conceptually arranged in an array with best frequency (BF) on one axis and BD on the other to form a labeled-line code. According to this model, the stimulus ITD corresponds to the BD (i.e. the label) of the most active neuron in the array (Jeffress 1948). The labeled-line model is challenged by physiological data from guinea pig (confirmed in cat and gerbil) showing that the distribution of BD is highly dependent on BF, and in general does not correspond to the range of naturally-occurring ITDs (Brand et al. 2002; Hancock and Delgutte 2004; McAlpine et al. 2001). Instead, best interaural phase (BP = BD × BF) is more nearly independent of BF, such that the steepest slopes of neural rate-ITD curves tend to occur near the midline. Because the slopes, not the peaks, align near the midline (where perceptual ITD acuity is finest), it has been suggested that ITD is encoded by the discharge rate itself rather than by the locus of maximal activity (McAlpine et al. 2001). Thus, ITD may be represented by a population rate code, in which the activity of many neurons pool to form monolithic ITD channels on each side of the brain, and the stimulus ITD may be inferred by comparing the relative activity of the two channels (van Bergeijk 1962; von Bekesy 1960). Though the physiological data suggest the existence of a rate code, analysis of its viability has barely begun (Marquardt and McAlpine 2001). Here, we demonstrate that a population rate code model can account for the dependence of perceived laterality on stimulus bandwidth.

Interaural time difference (ITD) detection plays a critical role in sound localization. Some neurophysiology experiments found that the ITD discrimination threshold for single neuron in medial superior olive (MSO) is about 20 μs. In order to reveal the ITD coding mechanism essentially, an ITD detection model has been proposed in this paper based on the anatomical structure of the mammalian auditory system. Most previous research only discuss the relationship between the ITDs and the spike rate of the MSO neuron. In these results, the spike rate difference is under 1 spike/s when the ITD changes in 20 μs. It means the neuron needs more than 1 s to distinguish the ITD by spike rate coding. This explanation contradicts with the rapid and accurate sound localization response in animal psychoacoustics experiments. In our study, the neural coding in ITD detection model has been investigated via the approach of circle maps and symbolic dynamics. With these two methods, the ITDs under 20 μs can be detected in 1 s. This neural coding hypothesis agrees with the ITD detection response time in the animal behavior experiments.