The effects of intensity on the detection of interaural differences of time in high-frequency trains of clicks

Threshold values of interaural differences of time (delta IDTs ) were measured for trains of dichotic clicks whose levels were 20, 40, or 60 dB SPL. All clicks were bandpass filtered at 4 kHz, and the number of clicks in the train (n) was 1, 2, 4, 8, 16, or 32. The interclick interval (ICI) was 5, 2, or 1 ms. Performance was compared to that of an ideal integrator of information, which produces slopes of - 0.5 when log delta IDT versus log n is plotted. The results showed that increases in level had no effect on the slopes of the log-log functions regardless of the ICI but did decrease the intercepts. Shortening the ICI caused the slopes to go from nearly - 0.5 towards 0.0. The improvement with level could be explained by either a decrease in the temporal variability of neural discharges, or by an increase in the number of samples of IDT at higher intensities brought on by increased firing rates or the activation of more auditory units. A review of the physiological literature found the most parsimonious explanation to be that the decline in threshold IDT was mediated by an increase in the number of active units, each possessing the same degree of adaptation.     

Temporal weighting of binaural cues revealed by detection of dynamic interaural differences in high-rate Gabor click trains

Listeners detected interaural differences of time (ITDs) or level (ILDs) carried by single 4000-Hz Gabor clicks (Gaussian-windowed tone bursts) and trains of 16 such clicks repeating at an interclick interval (ICI) of 2, 5, or 10 ms. In separate conditions, target interaural differences favored the right ear by a constant amount for all clicks (condition RR), attained their peak value at onset and diminished linearly to 0 at offset (condition R0), or grew linearly from 0 at onset to a peak value at offset (condition 0R). Threshold ITDs and ILDs were determined adaptively in separate experiments for each of these conditions and for single clicks. ITD thresholds were found to be lower for 16-click trains than for single clicks at 10-ms ICI, regardless of stimulus condition. At 2-ms ICI, thresholds in RR and R0 conditions were similar to single click thresholds at 2-ms ICI; thresholds in the 0R condition were significantly worse than for single clicks at 2-ms ICI, consistent with strong rate-dependent onset dominance in listeners' temporal weighting of ITD. ILD thresholds, in contrast, were predominantly unaffected by ICI, suggesting little or no onset dominance for ILD of high-rate stimuli.     

Temporal weighting functions for interaural time and level differences. II. The effect of binaurally synchronous temporal jitter

Recent work has demonstrated that sensitivity to interaural time differences (ITD) carried by high-rate cochlear implant pulse trains or analogous acoustic signals can be enhanced by imposing random temporal variation on the stimulus rate [see Goupell et al. (2009). J. Acoust. Soc. Am. 126, 2511-2521]. The present study characterized the effect of such "temporal jitter" on normal-hearing listeners' weighting of ITD and interaural level differences (ILD) applied to brief trains of Gabor clicks (4 kHz center frequency) presented at nominal interclick intervals (ICI) of 1.25 and 2.5 ms. Lateral discrimination judgments were evaluated on the basis of the ITD or ILD carried by individual clicks in each train. Random perturbation of the ICI significantly reduced listeners' weighting of onset cues for both ITD and ILD discrimination compared to corresponding isochronous conditions, consistent with enhanced sensitivity to post-onset binaural cues in jittered stimuli, although the reduction of onset weighting was not statistically significant at 1.25 ms ICI. An additional analysis suggested greater weighting of ITD or ILD presented following lengthened versus shortened ICI, although weights for such "gaps" and "squeezes" were comparable to other post-onset weights. Results are discussed in terms of binaural information available in jittered versus isochronous stimuli.     

The combination of interaural time and intensity in the lateralization of high-frequency complex signals

In an effort to examine the rules by which information arising from interaural differences of time (IDT) and interaural differences of intensity (IDI) is combined, d"s were measured for trains of high-frequency clicks (4000 Hz, bandpass) possessing various combinations of IDT and IDI. The number of clicks was either 1 or 8, with the interclick interval either 2 or 10 ms. A 2-IFC task was employed in which the paired values of IDT and IDI favored one side during one interval and the other side during the other interval. Data obtained with the combined cues are compared to those obtained with IDTs or IDIs alone in order to determine the degree to which processing of the two cues is done independently. Results suggest that lateralization with such stimuli is based on the sum of the temporal and intensive differences and not on independent evaluations of their separate presences.     

Source levels of clicks from free-ranging white-beaked dolphins (Lagenorhynchus albirostris Gray 1846) recorded in Icelandic waters

This study reports the source levels of clicks recorded from free-ranging white-beaked dolphins (Lagenorhynchus albirostris Gray 1846). A four-hydrophone array was used to obtain sound recordings. The hydrophone signals were digitized on-line and stored in a portable computer. An underwater video camera was used to visualize dolphins to help identify on-axis recordings. The range to a dolphin was calculated from differences in arrival times of clicks at the four hydrophones, allowing for calculations of source levels. Source levels in a single click train varied from 194 to 211 dB peak-to-peak (p-p) re: 1 microPa. The source levels varied linearly with the log of range. The maximum source levels recorded were 219 dB (p-p) re: 1 microPa.     

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.     

Masking-level differences for trains of clicks

Masking-level differences (MLDs) were measured for trains of 2000-Hz bandpass clicks as a function of the interclick interval (ICI) and the number of clicks in the train. The magnitude of the MLD grew as the number of clicks in the train was increased from 1 to 32. While the MLDs tended to be larger at longer ICIs, the effect was mediated by changes in detectability in the homophasic conditions. For click trains consisting of 4-32 clicks, the improvement in detectability in the antiphasic conditions with increases in the number of clicks appears to be the result of integration of acoustic power, as is the case for the homophasic conditions. The absence of MLDs for short trains of high-frequency transients remains quite puzzling, since large MLDs are found with single, low-frequency transients.     

Click lateralization is related to the beta component of the dichotic brainstem auditory evoked potentials of human subjects

The changes in perception and in the binaural difference waveform (BD) for dichotic clicks with interaural time and level differences (ITDs and ILDs) are compared. Only beta, the first major peak of the BD, correlated with the perceptual measurements. Whenever beta is clearly present, the clicks are perceived as a unitary fused image. Whenever the clicks are perceived as not fused, beta is undetectable by our methods. The amplitude of beta remains nearly constant as the ITD is increased to about 1 ms, while the click's position is perceived as moving from midline toward the leading ear. Over about the next 0.2 ms, beta becomes undetectable, as the image stops moving and loses its fused quality. As the ILD is increased, beta amplitude decreases gradually, while the image remains unitary and moves toward the unattenuated earphone. Thus beta becomes undetectable for ILDs of 30 to 35 dB, and the dichotic clicks become indistinguishable from monotic clicks for ILDs of 44 to 53 dB. The ITD and ILD matching curve for beta latency is similar to the ITD and ILD psychophysical matching curve for lateralization. These results suggest that beta is a physiological correlate of the categorical percept, binaural fusion, and is generated by a brainstem structure essential for the preception of click lateralization.     

Restarting the adapted binaural system

Previous experiments using trains of high-frequency filtered clicks have shown that for lateralization based on interaural difference of time or level, there is a decline in the usefulness of interaural information after the signal's onset when the clicks are presented at a high rate. This process has been referred to as "binaural adaptation." Of interest here are the conditions that produce a recovery from adaptation and allow for a resampling of the interaural information. A train of clicks with short interclick intervals is used to produce adaptation. Then, during its course, a treatment such as the insertion of a temporal gap or the addition of another "triggering" sound is tested for its ability to restart the binaural process. All of the brief triggers tested are shown to be capable of promoting recovery from adaptation. This suggests that, while the binaural system deals with the demands of high-frequency stimulation with rapid adaptation, it quickly cancels the adaptation in response to stimulus change.     

Discrimination of interaural differences of time in the envelopes of high-frequency signals: integration times

Listeners detected interaural differences of time in trains of high-frequency clicks. The manipulated variables were the number of clicks in the train and the period between clicks. Thresholds were compared to an optimal integrator, where the binaural information accrued from each click in the stimulus train is equivalent. In agreement with data reported in the past, integration is optimal only when the period between clicks exceeds approximately 10 ms and when the duration of the entire stimulus train is less than about 250 ms. The first constraint represents a limitation due to a form of "binaural adaptation" and the second is due to a limited "integration period."     

Buildup and breakdown of echo suppression for stimuli presented over headphones-the effects of interaural time and level differences

The current study investigates buildup and breakdown of echo suppression for stimuli presented over headphones. The stimuli consisted of pairs of 120-micros clicks. The leading click (lead) and the lagging click (lag) in each pair were lateralized on opposite sides of the midline by means of interaural level differences (ILDs) of +/-10 dB or interaural time differences (ITDs) of +/-300 micros. Echo threshold was measured with an adaptive one-interval, two-alternative, forced-choice procedure with a subjective decision criterion, in which listeners had to report whether they heard a single, fused auditory event on one side of the midline, or two separate events on both sides. In the control conditions, referred to as the "single" conditions, echo threshold was measured for a single click pair, the test pair, presented in isolation. In addition to the control conditions, two kinds of test conditions were investigated, in which the test pair was preceded by 12 identical conditioning pairs: in the "same" conditions, the interaural configuration (ILDs or ITDs) of the conditioning pairs was identical to that of the test pair; in the "switch" conditions, the interaural configuration of lead and lag was reversed between the conditioning pairs and the test pair, in order to produce a switch in the lateralizations of the stimuli between the conditioning train and the test pair. No matter whether the lateralization of the clicks was produced by ILDs or by ITDs, most listeners experienced a buildup of echo suppression in the "same" conditions, manifested by a prolongation of echo threshold relative to the respective "single" conditions. However, the breakdown of echo suppression was much stronger in the ILD-switch than in the ITD-switch conditions. In five out of six listeners, the ITD switch had hardly any effect on echo threshold, although the ITDs (+/-300 micros) produced roughly the same degree of lateral displacement as the ILDs (+/-10 dB). These results suggest that the dynamic processes in echo suppression operate differentially in pathways responsible for the processing of interaural time and level differences.     

Perception of the sound source position

This paper presents several experiments on sound source localization. They are based on monaural click presented at different interclick intervals (ICI), from 10 to 100 ms. Trains of clicks were presented to 10 healthy subjects. At short interclick intervals the clicks were perceived as a blur of clicks having a buzzy quality. Moreover, it was proven that the accurateness in the response improves with the increase of the length of ICI. The present results imply the usefulness of the interclick interval in estimating the perceptual accuracy. An important benefit of this task is that this enables a careful examination of the sound source perception threshold. This allows detecting, localizing and dividing with a high accuracy the sounds in the environment. The text was submitted by the authors in English.     

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.     

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.     

Precedence-effect thresholds for a population of untrained listeners as a function of stimulus intensity and interclick interval

Data are reported from 127 untrained individuals under lag- and single-click conditions in a precedence-effect task. In experiment I, each subject completed ten runs in a two-interval forced-choice design under a lag-click condition and three runs under a single-click condition. The cue to be discriminated was an interaural time difference (ITD). Stimuli were 125-micros rectangular pulses and the interclick interval (ICI) was 2 ms. Subjects were randomly assigned to three groups of approximately 30. Each group was tested at one stimulus intensity (43, 58, or 73 dB). Mean threshold within each group was greater than 500 micros for lag-click ITD conditions, although substantial intersubject variability and a clear effect of stimulus intensity on lag-click ITD thresholds were observed, with lower thresholds for higher intensities. In experiment II, the ICI was varied from 0.3 to 10 ms, and thresholds were obtained from groups of approximately 20 untrained subjects. Data were also collected from three highly experienced observers as a function of ICI. The best naive subject produced mean thresholds near, but not as low as those obtained from experienced subjects. Analysis of adaptive-track patterns revealed abrupt irregularities in threshold tracking, consistent with either losing the cue or listening to the wrong cue in an ambiguous stimulus.     

The effect of an additional reflection in a precedence effect experiment

Studies on the precedence effect typically utilize a two-source paradigm, which is not realistic relative to real world situations where multiple reflections exist. A step closer to multiple-reflection situations was studied using a three-source paradigm. Discrimination of interaural time differences (ITDs) was measured for one-, two-, and three-source stimuli, using clicks presented over headphones. The ITD was varied in either the first, second, or the third source. The inter-source intervals ranged from 0-130 ms. A perceptual weighting model was extended to incorporate the three-source stimuli and used to interpret the data. The effect of adding a third source could mostly, but not entirely, be understood by the interaction of effects observed in the precedence effect with two sources. Specifically, for delays between 1 and 8 ms, the ITD information of prior sources was typically weighted more heavily than subsequent sources. For delays greater than 8 ms, subsequent sources were typically weighted slightly more heavily than prior sources. However, there were specific conditions that showed a more complex interaction between the sources. These findings suggest that the two-source paradigm provides a strong basis for understanding how the auditory system processes reflections in spatial hearing tasks.     

The influence of later-arriving sounds on the ability of listeners to judge the lateral position of a source

This study examined the deleterious effects of a later-arriving sound on the processing of interaural differences of time (IDTs) from a preceding sound. A correlational analysis assessed the relative weight given to IDTs of source and echo clicks for echo delays of 1-64 ms when the echo click was attenuated relative to the source click (0-36 dB). Also measured were proportion correct and the proportion of responses predicted from the weights. The IDTs of source and echo clicks were selected independently from Gaussian distributions (mu=0 s, sigma = 100/s). Listeners were instructed to indicate the laterality of the source click. Equal weight was given to the source and echo clicks for echo delays of 64 ms with no echo attenuation. For echo delays of 16-64 ms, attenuating the echo had no substantial effect on source weight or proportion correct until the echo was attenuated by 18-30 dB. At echo delays < or =4 ms, source weights and proportions correct remained high regardless of echo attenuation. The proportions of responses predicted from the weights were lower at echo delays > or =16 ms. Results were discussed in terms of backward recognition masking and binaural sluggishness and compared to measurements of echo disturbance.     

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.     

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.     

Evoked-potential recovery during double click stimulation in a whale: a possibility of biosonar automatic gain control

False killer whale Pseudorca crassidens auditory brainstem responses (ABR) were recorded using a double-click stimulation paradigm specifically measuring the recovery of the second response (to the test click) as a function of the inter-click interval (ICI) at various levels of the conditioning and test click. At all click intensities, the slopes of recovery functions were almost constant: 0.6-0.8 microV per ICI decade. Therefore, even when the conditioning-to-test-click level ratio was kept constant, the duration of recovery was intensity-dependent: The higher intensity the longer the recovery. The conditioning-to-test-click level ratio strongly influenced the recovery time: The higher the ratio, the longer the recovery. The dependence was almost linear using a logarithmic ICI scale with a rate of 25-30 dB per ICI decade. These data were used for modeling the interaction between the emitted click and the echo during echolocation, assuming that the two clicks simulated the transmitted and echo clicks. This simulation showed that partial masking of the echo by the preceding emitted click may explain the independence of echo-response amplitude of target distance. However, the distance range where this mechanism is effective depends on the emitted click level: The higher the level, the greater the range. @ 2007 Acoustical Society of America.