Detection of interaural differences of intensity in trains of high-frequency clicks as a function of interclick interval and number

Listeners were asked to detect interaural differences of intensity in trains of 4000-Hz clicks as the interclick interval (ICI) was varied from 10 to 1 ms and the number of clicks in a train (n) was varied from 1 to 32. As has previously been shown for differences of time [Hafter and Dye, J. Acoust. Soc. Am. 73, 644-651 (1983)], plots of log interaural threshold versus log n produced slopes that decrease with ICI. These results are explained in terms of a saturation model which argues that as the click rate increases, the evoked neural activity changes from what is essentially a tonic response toward one that is more phasic.     

The ontogeny of echolocation in a Yangtze finless porpoise (Neophocaena phocaenoides asiaeorientalis)

Acoustic and concurrent behavioral data from one neonatal male Yangtze finless porpoise (Neophocaena phocaenoides asiaeorientalis) in captivity were presented. The calf click train was first recorded at 22 days postnatal, and the frequency of hydrophone-exploration behavior with head scanning motions in conjunction with emissions of click trains by the calf increased gradually with age. The echolocation clicks in the first recorded click train were indistinguishable from those of adults. Calf echolocation trains were found to decrease in maximum click-repetition rate, duration, and number of clicks per train with age while the minimum click-repetition rate remained more consistent.     

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.     

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."     

On the performance of automated porpoise-click-detectors in experiments with captive harbor porpoises (Phocoena phocoena)

Recently, automated porpoise-click-detectors (T-PODs, Chelonia-Marine-Research) have been used intensively in monitoring harbor porpoises (Phocoena phocoena) in the wild. However, the automated click-detection-mechanism of the T-POD leads to questions on the characteristics of the detection process. We undertook experiments with six captive harbor porpoises (four subadult males in one pool, two adult males in another) at the Dolfinarium Harderwijk (Netherlands). One T-POD was placed for over a week in each pool, while the behavior of the porpoises was logged by visual observation. Data were analyzed using the T-POD software. A total of 725 431 clicks in 30 090 trains were recorded with 32% of the trains classified as CET HI, 27% as CET LO, and 41% as DOUBTFUL. All three train classes differed significantly in all parameters, except for click duration. We conclude that T-PODs perform generally well in detecting click trains of harbor porpoises but that in any future study trains classified as being of lower probability should be investigated very carefully to avoid the risk of losing valuable information.     

Free-field release from masking

Free-field release from masking was studied as a function of the spatial separation of a signal and masker in a two-interval, forced-choice (2IFC) adaptive paradigm. The signal was a 250-ms train of clicks (100/s) generated by filtering 50-microseconds pulses with a TDH-49 speaker (0.9 to 9.0 kHz). The masker was continuous broadband (0.7 to 11 kHz) white noise presented at a level of 44 dBA measured at the position of the subject's head. In experiment I, masked and absolute thresholds were measured for 36 signal source locations (10 degree increments) along the horizontal plane as a function of seven masking source locations (30 degree increments). In experiment II, both absolute and masked thresholds were measured for seven signal locations along three vertical planes located at azimuthal rotations of 0 degrees (median vertical plane), 45 degrees, and 90 degrees. In experiment III, monaural absolute and masked thresholds were measured for various signal-masker configurations. Masking-level differences (MLDs) were computed relative to the condition where the signal and mask were in front of the subjects after using absolute thresholds to account for differences in the signal's sound-pressure level (SPL) due to direction. Maximum MLDs were 15 dB along the horizontal plane, 8 dB along the vertical, and 9 dB under monaural conditions.     

3D tracking of foraging belugas from their clicks: Experiment from a coastal hydrophone array

A simple passive acoustic monitoring (PAM) setup was used to localize and track beluga whales underwater in three dimensions (3D) in a fjord. In June 2009, beluga clicks were recorded from a cabled hydrophone array in a regularly frequented habitat in Eastern Canada. Beluga click energy was concentrated in the 30-50 kHz frequency band. The click trains detected on several hydrophones were localized from their time difference of arrivals. Cluster analysis linked localizations into tracks based on criteria of spatial and temporal proximity. At close ranges from the array, the localized click-train series allowed three-dimensional tracking of a beluga during its dive. Clicks within a train spanned a large range of durations, inter-click intervals, source levels and bandwidths. Buzzes sometimes terminated the trains. Repeated click packets were frequent. All click characteristics are consistent with oblique observations from the beam axis, and ordered variation of the source pattern during a train, likely resulting from a scan of angles from the beam axis, was observed before click trains indicated focusing of the echolocation clicks in one direction. The click-train series is interpreted as echolocation chasing for preys during a foraging dive. Results show that a simple PAM system can be configured to passively and effectively 3D track wild belugas and small odontocetes in their regularly frequented habitat.     

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.     

Binaural detection at high frequencies with time-delayed waveforms

Recent research has demonstrated that the binaural system can utilize ongoing interaural time differences for lateralization at high frequencies as well as at low frequencies. The requirement is that the signal be complex so that the time difference appears as a delay in the envelope of the waveform at one ear. Reported here are several masking experiments that examine detection performance with time-delayed signals or maskers. In the first experiment, the signal was a 50-Hz band of noise centered at 4000 Hz that was time delayed by different amounts on different blocks of trials; the masker was similar band of noise, presented diotically. Large masking-level differences (MLDs) were obtained for some values of time delay, but the MLDs did not increase monotonically within time delay as they should were envelope time delay the basis for detection performance. Subsequent experiments in which the masker was time delayed and the signal was a diotic, high-frequency tone, revealed that detectability follows the autocorrelation function, and that MLDs as large as 24 dB can be obtained at 4000 Hz at time delays corresponding to negative values in the autocorrelation function. Examination of the signal-plus masker waveforms in these conditions reveals that ongoing interaural differences in level and cycle-by-cycle time exist in those conditions that yield MLDs. Since the time differences are small by usual standards, the basis for detection performance in these conditions appears to be the ongoing interaural level differences. In a final experiment, lateralization performance was measured for a time-delayed, complex waveform in the presence of maskers of various intensities. The results show that subjects are able to extract information about the time delay in the envelope even when the signal is added to a masker of equal intensity or greater. Thus, at the small signal-to-noise ratios used in our detection experiments, extraction of envelope time information was impossible, but also unnecessary, for detection was accomplished on the basis of another cue--most likely the ongoing interaural level differences.     

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.     

Sound localization in noise in hearing-impaired listeners.

The present study assesses the ability of four listeners with high-frequency, bilateral symmetrical sensorineural hearing loss to localize and detect a broadband click train in the frontal-horizontal plane, in quiet and in the presence of a white noise. The speaker array and stimuli are identical to those described by Lorenzi et al. (in press). The results show that: (1) localization performance is only slightly poorer in hearing-impaired listeners than in normal-hearing listeners when noise is at 0 deg azimuth, (2) localization performance begins to decrease at higher signal-to-noise ratios for hearing-impaired listeners than for normal-hearing listeners when noise is at +/- 90 deg azimuth, and (3) the performance of hearing-impaired listeners is less consistent when noise is at +/- 90 deg azimuth than at 0 deg azimuth. The effects of a high-frequency hearing loss were also studied by measuring the ability of normal-hearing listeners to localize the low-pass filtered version of the clicks. The data reproduce the effects of noise on three out of the four hearing-impaired listeners when noise is at 0 deg azimuth. They reproduce the effects of noise on only two out of the four hearing-impaired listeners when noise is at +/- 90 deg azimuth. The additional effects of a low-frequency hearing loss were investigated by attenuating the low-pass filtered clicks and the noise by 20 dB. The results show that attenuation does not strongly affect localization accuracy for normal-hearing listeners. Measurements of the clicks' detectability indicate that the hearing-impaired listeners who show the poorest localization accuracy also show the poorest ability to detect the clicks. The inaudibility of high frequencies, "distortions," and reduced detectability of the signal are assumed to have caused the poorer-than-normal localization accuracy for hearing-impaired listeners.     

Discrimination of amplitude-modulated synthetic echo trains by an echolocating bottlenose dolphin

Bottlenose dolphins (Tursiops truncatus) have an acute ability to use target echoes to judge attributes such as size, shape, and material composition. Most target recognition studies have focused on features associated with individual echoes as opposed to information conveyed across echo sequences (feature envelope of the multi-echo train). One feature of aspect-dependent targets is an amplitude modulation (AM) across the return echoes in the echo train created by relative movement of the target and dolphin. The current study examined whether dolphins could discriminate targets with different AM envelopes. "Electronic echoes" triggered by a dolphin's outgoing echolocation clicks were manipulated to create sinusoidal envelopes with varying AM rate and depth. Echo trains were equated for energy, requiring the dolphin to extract and retain information from multiple echoes in order to detect and report the presence of AM. The dolphin discriminated amplitude-modulated echo trains from those that were not modulated. AM depth thresholds were approximately 0.8 dB, similar to other published amplitude limens. Decreasing the rate of modulation from approximately 16 to 2 cycles per second did not affect the dolphin's AM depth sensitivity. The results support multiple-echo processing in bottlenose dolphin echolocation. This capability provides additional theoretical justification for exploring synthetic aperture sonar concepts in models of animal echolocation that potentially support theories postulating formation of images as an ultimate means for target identification.     

The precedence effect: no evidence for an "active" release process found

Hafter et al. [Auditory Function: The Neurobiological Bases for Hearing (Wiley, New York, 1988)] have reported that "post-onset adaptation," as measured with trains of high-frequency clicks in a lateralization paradigm, can instantly be released by presentation of an additional, short trigger signal, which is spectrally different from the click trains. As post-onset saturation may be one of the psychoacoustic components of the precedence effect, it has been investigated whether suitable trigger signals might also induce a release from echo inhibition. However, no evidence for such an "active" release process could be observed in a number of exploratory precedence-effect settings.     

Effects of relative starting phase and frequency separation of two-tone stimuli on the brain-stem auditory-evoked response

Brain-stem auditory-evoked responses (BAERs) were obtained in six normal-hearing adults using single-tone and two-tone stimuli arithmetically centered around 4000 Hz. Two-tone stimuli varied in frequency separation from 200 to 3200 Hz, and started in-phase (homophasic) or 180 deg out-of-phase (antiphasic) with each other. Responses to each of the single-tone components of the two-tone stimuli were elicited and then summed for comparison with responses to the two-tone stimuli. Results indicated no significant difference in wave V latency between homophasic or antiphasic two-tone conditions, and summed single-tone conditions. Under the homophasic condition, the mean latency for the widest frequency separation of the tones was significantly longer than those for narrower separations. A significant difference in wave V amplitude between two-tone phase conditions was found for frequency separations of 200, 400, and 3200 Hz only. Summed single-tone BAERs demonstrated a significantly larger wave V amplitude than responses from either two-tone phase condition at all frequency separations.     

Automatic detection of bioacoustics impulses based on kurtosis under weak signal to noise ratio

Passive acoustic monitoring (PAM) of marine mammal vocalizations has been efficiently used in a wide set of applications ranging from marine wildlife surveys to risk mitigation of military sonar emissions. The primary use of PAM is for detecting bioemissions, a good proportion of which are impulse sounds or clicks. A click detection algorithm based on kurtosis estimation is proposed as a general automatic click detector. The detector works under the assumption that click trains are embedded in stochastic but Gaussian noise. Under this assumption, kurtosis is used as a statistical test for detection. The algorithm explores acoustic sequences with the optimal frequency bandwidth for focusing on impulse sounds. The detector is successfully applied to field observations, and operates under weak signal to noise ratios and in presence of stochastic background noise. The algorithm adapts to varying click center frequency. Kurtosis appears as a promising approach to detect click trains, alone or in combination with other clicks detector, and to isolate individual clicks.     

Periodic pulse train conformation based on the temporal Radon-Wigner transform

By using the Radon-Wigner transform (RWT), we analyze the temporal selfimaging or Talbot effect for producing well-conformed pulse trains with variable repetition rates and duty-cycles. The relationships linking the selfimaging conditions with the fractional orders of the RWT are first obtained for unchirped pulse trains. Then, we extend the analysis to chirped pulse sequences by deriving the conditions to be fulfilled by an equivalent unchirped pulse train producing the same selfimage irradiances. This result becomes relevant for observing well-defined high order fractional selfimaging, which are of interest due to their repetition rate multiplication. Besides, the effect of the finite extension of the pulse train on the selfimage quality is analyzed and a condition is found for relating the required minimum pulse number with the chirp parameter of the pulses.     

Dual mechanisms in the perceptual processing of click train temporal regularity

Two experiments measured human sensitivity to temporal jitter in 25-click trains with inter-click intervals (ICIs) between 5 and 100 ms. In a naturalistic experiment using wideband clicks, jitter thresholds were a nonmonotonic function of ICI, peaking for ICIs near 40-60 ms. In a subsequent experiment, clicks were high-passed and presented against a low-frequency noise masker. Jitter threshold vs ICI functions lost the positive slope over short ICIs but retained the negative slope at long ICIs. The same behavior was seen in click rate discrimination tasks. Different processes mediate regularity analysis for click trains with ICIs above and below 40-60 ms.     

Acoustically dependent latency shifts of BSER (wave V) in man

The latencies of wave V in Brain Stem Evoked Responses (BSER) elicited by a set of acoustic transients were measured. The stimuli were produced by delivering pulses to two filters, arranged in series. The filters were set so that the maximum acoustic energy in the transients, i.e., filtered clicks, occurred at 0.5, 1, 2, 4, or 8 kHz. The filtered clicks were presented via earphones at a rate of 30/s at 20, 40, or 60 dB HL to ten subjects with normal hearing. The latencies of wave V varied systematically with center frequency of the filtered clicks when they were each at the same HL. Stimuli presented at 40 dB HL produced the greatest opportunity for relating stimulus frequency to latency. The latencies for a smaller set of responses to stimuli presented at 10/s were the same as those for the principal data taken at 30/s. The changes in latency of wave V due to frequency are similar to those observed by other investigators in whole-nerve responses recorded in man.     

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.