Vocal reporting of echolocation targets: dolphins often report before click trains end

Bottlenose dolphins (Tursiops truncatus) wore opaque suction cups over their eyes while stationing behind an acoustically opaque door. This put the dolphins in a known position and orientation. When the door opened, the dolphin clicked to detect targets. Trainers specified that Dolphin S emit a whistle if the target was a 7.5 cm water filled sphere, or a pulse burst if the target was a rock. S remained quiet if there was no target. Dolphin B whistled for the sphere. She remained quiet for rock and for no target. Thus, S had to choose between three different responses, whistle, pulse burst, or remain quiet. B had to choose between two different responses, whistle or remain quiet. S gave correct vocal responses averaging 114 ms after her last echolocation click (range 182 ms before and 219 ms after the last click). Average response for B was 21 ms before her last echolocation click (range 250 ms before and 95 ms after the last click in the train). More often than not, B began her whistle response before her echolocation train ended. The findings suggest separate neural pathways for generation of response vocalizations as opposed to echolocation clicks.     

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.     

Auditory nerve fiber responses to electric stimulation: modulated and unmodulated pulse trains

Many modern cochlear implants use sound processing strategies that stimulate the cochlea with modulated pulse trains. Rubinstein et al. [Hear. Res. 127, 108 (1999)] suggested that representation of the modulator in auditory nerve responses might be improved by the addition of a sustained, high-rate, desynchronizing pulse train (DPT). In addition, activity in response to the DPT may mimic the spontaneous activity (SA) in a healthy ear. The goals of this study were to compare responses of auditory nerve fibers in acutely deafened, anesthetized cats elicited by high-rate electric pulse trains delivered through an intracochlear electrode with SA, and to measure responses of these fibers to amplitude-modulated pulse trains superimposed upon a DPT. Responses to pulse trains showed variability from presentation to presentation, but differed from SA in the shape of the envelope of the interval histogram (IH) for pulse rates above 4.8 kpps (kilo pulses per second). These IHs had a prominent mode near 5 ms that was followed by a long tail. Responses to modulated biphasic pulse trains resembled responses to tones in intact ears for small (<10%) modulation depths, suggesting that acousticlike responses to sinusoidal stimuli might be obtained with a DPT. However, realistic responses were only observed over a narrow range of levels and modulation depths. Improved coding of complex stimulus waveforms may be achieved by signal processing strategies for cochlear implants that properly incorporate a DPT.     

Echolocation range of captive and free-ranging baiji (Lipotes vexillifer), finless porpoise (Neophocaena phocaenoides), and bottlenose dolphin (Tursiops truncatus).

The interclick intervals of captive dolphins are known to be longer than the two-way transit time between the dolphin and a target. In the present study, the interclick intervals of free-ranging baiji, finless porpoises, and bottlenose dolphins in the wild and in captivity were compared. The click intervals in open waters ranged up to 100-200 ms, whereas the click intervals in captivity were in the order of 4-28 ms. Echolocation of free-ranging dolphins appears to adapt to various distance in navigation or ranging, sometimes up to 140 m. Additionally, the difference of waveform characteristics of clicks between species was recognized in the frequency of maximum energy and the click duration.     

A datalogger to identify vocalizing dolphins

A datalogger was developed to identify vocalizing dolphins within socially interacting captive groups. Every 50 ms the logger stores data on the level and frequency of detected sound. Dataloggers are temporarily attached to dolphins by suction cups for data collection sessions lasting up to 45 min. Later, computer analysis of data from the dataloggers reveals which dolphin produced each vocalization recorded during the session. Results from use of dataloggers with two captive bottlenose dolphins (Tursiops truncatus) at the New England Aquarium in Boston, MA are presented. The possible use of dataloggers with wild dolphins is discussed.     

Who is whistling? Localizing and identifying phonating dolphins in captivity

Acoustic communication through whistles is well developed in dolphins. However, little is known on how dolphins are using whistles because localizing the sound source is not an easy task. In the present study, the hyperbola method was used to localize the sound source using a two-hydrophone array. A combined visual and acoustic method was used to determine the identity of the whistling dolphin. In an aquarium in Mexico City where two adult bottlenose dolphins were housed we recorded 946 whistles during 22 days. We found that a dolphin was located along the calculated hyperbola for 72.9% of the whistles, but only for 60.3% of the whistles could we determine the identity of the whistling dolphin. However, sometimes it was possible to use other cues to identify the whistling dolphin. It could be the animal that performed a behavior named “observation” at the time whistling occurred or, when a whistle was only recorded on one channel, the whistling dolphin could be the animal located closest to the hydrophone that captured the whistle. Using these cues, 15.4% of the whistles were further ascribed to either dolphin to obtain an overall identification efficiency of 75.7%. Our results show that a very simple and inexpensive acoustic setup can lead to a reasonable number of identifications of the captive whistling dolphin: this is the first study to report such a high rate of whistles identified to the free swimming, captive dolphin that produced them. Therefore, we have a data set with which we can investigate how dolphins are using whistles. This method can be applied in other aquaria where a small number of dolphins is housed; though, the actual efficiency of this method will depend on how often dolphins spend time next to each other and on the reverberation conditions of the pool.     

Test of a model of auditory object formation using intensity and interaural time difference discrimination.

A model of auditory object formation and an experimental evaluation of the model are described. Specifically, predictions for intensity discrimination and interaural time difference discrimination for the central component of a three-component harmonic complex are evaluated empirically. The onset time of the central, target component is varied relative to the onset times of the remaining, interferer components in order to vary the degree of fusion (versus perceptual segregation) of the target and the interferers. The model, which is based on the idea of attenuation of the components in the nonattended auditory image (in the case of segregated images), predicts lower sensitivity to information at the target component for the fused versus segregated target, and equal sensitivities for completely segregated targets and targets presented in isolation. Results are presented for four subjects with component frequencies of 400, 600, and 800 Hz and with onset time differences of 0 or 250 ms. The target duration was always 100 ms and offset times were the same for all components. The subjective results were as expected, with synchronous onsets yielding one sound object, and asynchrony of the central component yielding two sound objects. Also, the empirical results on interference in the synchronous case were in qualitative agreement with the above predictions. However, significantly more interference was found than was predicted for both synchronous and asynchronous conditions. In fact, the amount of interference found contradicts the simple attenuation model of object formation.     

时频特征的海豚发声端点检测方法研究*

为在复杂的海洋环境噪声场中检测出目标海豚的某类声信号,设计了一种基于海豚声学特征的端点检测方法。根据海洋环境中噪声能量大且分布频率范围广,而海豚声音的时频特征具有持续时间短,、频率高且集中,、发声行为持续时间长的特点,将采集到的海洋声音进行分帧,计算单帧信号短时能量、谱质心和谱质心二阶偏移率,当海豚发出声信号时,谱质心和能量相应发生突变,截取发生突变的信号实现端点检测。通过与门限法等其他常见端点检测方法进行对比,结果表明,此方法在低信噪比的海洋环境下对单一海豚的某一类声信号检测准确度更高,同时具有较强的抗干扰能力。     

浅水海豚三维空间定位与发声源级测量

详细阐述了单水听器和水听器阵列进行海豚定距定位的几何模型,以及利用不同接收通道间的时延差计算空间坐标的方法,结合实验数据计算了海豚在水中的三维空间坐标和回声定位信号的声源级,并开展了海豚运动轨迹追踪分析。结果表明,相对于单水听器,水听器阵列对测量环境和海豚行为的限制较少,但对采集设备的同步性和硬件连接有较高要求。误差分析表明水听器阵列在大于3 m的距离,定位误差可降低到5%以下。研究结果为精确计算海豚发声源级及海豚的声呐性能及行为研究提供了技术支撑。      

Temporal resolution and temporal integration of short pulses at the auditory periphery of echolocating animals

To explain the temporal integration and temporal resolution abilities revealed in echolocating animals by behavioral and electrophysiological experiments, the peripheral coding of sounds in the high-frequency auditory system of these animals is modeled. The stimuli are paired pulses similar to the echolocating signals of the animals. Their duration is comparable with or smaller than the time constants of the following processes: formation of the firing rate of the basilar membrane, formation of the receptor potentials of internal hair cells, and recovery of the excitability of spiral ganglion neurons. The models of auditory nerve fibers differ in spontaneous firing rate, response thresholds, and abilities to reproduce small variations of the stimulus level. The formation of the response to the second pulse of a pair of pulses in the multitude of synchronously excited high-frequency auditory nerve fibers may occur in only two ways. The first way defined as the stochastic mechanism implies the formation of the response to the second pulse as a result of the responses of the fibers that did not respond to the first pulse. This mechanism is based on the stochastic nature of the responses of auditory nerve fibers associated with the spontaneous firing rate. The second way, defined as the repeatition mechanism, implies the appearance of repeated responses in fibers that already responded to the first pulse but suffered a decrease in their response threshold after the first spike generation. This mechanism is based on the deterministic nature of the responses of fibers associated with refractoriness. The temporal resolution of pairs of short pulses, which, according to the data of behavioral experiments, is about 0.1–0.2 ms, is explained by the formation of the response to the second pulse through the stochastic mechanism. A complete recovery of the response to the second pulse, which, according to the data of electrophysiological studies of short-latency evoked brainstem potentials in dolphins, occurs within 5 ms, is explained by the formation of the response to the second pulse through the repetition mechanism. The time constant of temporal integration, which, according to the behavioral experiments at threshold levels of pulses, is about 0.2–0.3 ms, is explained by the integrating properties of internal hair cells, etc. It is shown that, at the high-frequency auditory periphery, the temporal integration imposes no limitations on the temporal resolution, because both integration and resolution are different characteristics of the same multiple response of synchronously excited fibers.     

Responses of neurons to click-pairs as simulated echoes: auditory nerve to auditory cortex

When two identical sounds are presented from different locations with a short interval between them, the perception is of a single sound source at the location of the leading sound. This "precedence effect" is an important behavioral phenomenon whose neural basis is being increasingly studied. For this report, neural responses were recorded to paired clicks with varying interstimulus intervals, from several structures of the ascending auditory system in unanesthetized animals. The structures tested were the auditory nerve, anteroventral cochlear nucleus, superior olivary complex, inferior colliculus, and primary auditory cortex. The main finding is a progressive increase in the duration of the suppressive effect of the leading sound (the conditioner) on the response to the lagging sound (the probe). The first major increase occurred between the lower brainstem and inferior colliculus, and the second between the inferior colliculus and auditory cortex. In neurons from the auditory nerve, cochlear nucleus, and superior olivary complex, 50% recovery of the response to the probe occurred, on average, for conditioner and probe intervals of approximately 2 ms. In the inferior colliculus, 50% recovery occurred at an average separation of approximately 7 ms, and in the auditory cortex at approximately 20 ms. Despite these increases in average recovery times, some neurons in every structure showed large responses to the probe within the time window for precedence (approximately 1-4 ms for clicks). This indicates that during the period of the precedence effect, some information about echoes is retained. At the other extreme, for some cortical neurons the conditioner suppressed the probe response for intervals of up to 300 ms. This is in accord with behavioral results that show dominance of the leading sound for an extended period beyond that of the precedence effect. Other transformations as information ascended included an increased variety in the shapes of the recovery functions in structures subsequent to the nerve, and neurons "tuned" to particular conditioner-probe intervals in the auditory cortex. These latter are reminiscent of neurons tuned to echo delay in bats, and may contribute to the perception of the size of the acoustic space.     

Assessment of dolphin (Tursiops truncatus) auditory sensitivity and hearing loss using jawphones

Devices known as jawphones have previously been used to measure interaural time and intensity discrimination in dolphins. This study introduces their use for measuring hearing sensitivity in dolphins. Auditory thresholds were measured behaviorally against natural background noise for two bottlenose dolphins (Tursiops truncatus); a 14-year-old female and a 33-year-old male. Stimuli were delivered to each ear independently by placing jawphones directly over the pan bone of the dolphin's lower jaw, the assumed site of best reception. The shape of the female dolphin's auditory functions, including comparison measurements made in the free field, favorably matches that of the accepted standard audiogram for the species. Thresholds previously measured for the male dolphin at 26 years of age indicated a sensitivity difference between the ears of 2-3 dB between 4-10 kHz, which was considered unremarkable at the time. Thresholds for the male dolphin reported in this study suggest a high-frequency loss compared to the standard audiogram. Both of the male's ears have lost sensitivity to frequencies above 55 kHz and the right ear is 16-33 dB less sensitive than the left ear over the 10-40 kHz range, suggesting that males of the species may lose sensitivity as a function of age. The results of this study support the use of jawphones for the measurement of dolphin auditory sensitivity.     

The temporal growth and decay of the auditory motion aftereffect

The present work investigated the temporal tuning of the auditory motion aftereffect (aMAE) by measuring the time course of adaptation and recovery to auditory motion exposure. On every trial, listeners were first exposed to a broadband, horizontally moving sound source for either 1 or 5 seconds, then presented moving test stimuli after delays of 0, 2/3, or 1 2/3 seconds. All stimuli were synthesized from head related transfer functions recorded for each participant. One second of motion exposure (i.e., a single pass of the moving source) produced clearly measurable aMAEs which generally decayed monotonically after adaptation ended, while five seconds exposure produced stronger aftereffects that remained largely unattenuated across test delays. These differences may imply two components to the aMAE: a short time-constant motion illusion and a longer time-constant response bias. Finally, aftereffects were produced only by adaptor movement toward but not away from listener midline. This aftereffect asymmetry may also be a consequence of brief adaptation times and reflect initial neural response to auditory motion in primate auditory cortex.     

Detection and discrimination of simulated motion of auditory targets in the horizontal plane

Three experiments investigated subjects' ability to detect and discriminate the simulated horizontal motion of auditory targets in an anechoic environment. "Moving" stimuli were produced by dynamic application of stereophonic balancing algorithms to a two-loudspeaker system with a 30 degree separation. All stimuli were 500-Hz tones. In experiment 1, subjects had to discriminate a left-to-right moving stimulus from a stationary stimulus pulsed for the same duration (300 or 600 ms). For both durations, minimum audible "movement" angles ("MAMA's") were on the order of 5 degrees for stimuli presented at 0 degrees azimuth (straight ahead), and increased to greater than 30 degrees for stimuli presented at +/- 90 degrees azimuth. Experiment 2 further investigated MAMA's at 0 degrees azimuth, employing two different procedures to track threshold: holding stimulus duration constant (at 100-600 ms) while varying velocity; or holding the velocity constant (at 22 degrees-360 degrees/s) while varying duration. Results from the two procedures agreed with each other and with the MAMA's determined by Perrott and Musicant for actually moving sound sources [J. Acoust. Soc. Am. 62, 1463-1466 (1977b)]: As stimulus duration decreased below 100-150 ms, the MAMA's increased sharply from 5 degrees-20 degrees or more, indicating that there is some minimum integration time required for subjects to perform optimally in an auditory spatial resolution task. Experiment 3 determined differential "velocity" thresholds employing simulated reference velocities of 0 degrees-150 degrees/s and stimulus durations of 150-600 ms. As with experiments 1 and 2, the data are more easily summarized by considering angular distance than velocity: For a given "extent of movement" of a reference target, about 4 degrees-10 degrees additional extent is required for threshold discrimination between two "moving" targets, more or less independently of stimulus duration or reference velocity. These data suggest that for the range of simulated velocities employed in these experiments, subjects respond to spatial changes--not velocity per se--when presented with a "motion" detection or discrimination task.     

Auditory brainstem response recovery in the dolphin as revealed by double sound pulses of different frequencies.

Recovery of auditory brainstem responses (ABR) in a bottlenose dolphin was studied in conditions of double-pip stimulation when two stimuli in a pair differed in frequency and intensity. When the conditioning and test stimuli were of equal frequencies, the test response was markedly suppressed at short interstimulus intervals; complete recovery appeared at intervals from about 2 ms (when two stimuli were of equal intensity) to 10-20 ms (when the conditioning stimulus exceeded the test by up to 40 dB). When the two stimuli were of different frequencies, the suppression diminished and was almost absent at a half-octave difference even if the conditioning stimulus exceeded the test one by 40 dB. Frequency-dependence curves (ABR amplitude dependence on frequency difference between the two stimuli) had equivalent rectangular bandwidth from +/-0.2 oct at test stimuli of 20 dB above threshold to +/-0.5 oct at test stimuli of 50 dB above threshold.     

Urgency is a non-monotonic function of pulse rate

Magnitude estimation was used to assess the experience of urgency in pulse-train stimuli (pulsed white noise) ranging from 3.13 to 200 Hz. At low pulse rates, pulses were easily resolved. At high pulse rates, pulses fused together leading to a tonal sensation with a clear pitch level. Urgency ratings followed a nonmonotonic (polynomial) function with local maxima at 17.68 and 200 Hz. The same stimuli were also used in response time and pitch scaling experiments. Response times were negatively correlated with urgency ratings. Pitch scaling results indicated that urgency of pulse trains is mediated by the perceptual constructs of speed and pitch.     

Neural activity associated with distinguishing concurrent auditory objects

The neural processes underlying concurrent sound segregation were examined by using event-related brain potentials. Participants were presented with complex sounds comprised of multiple harmonics, one of which could be mistuned so that it was no longer an integer multiple of the fundamental. In separate blocks of trials, short-, middle-, and long-duration sounds were presented and participants indicated whether they heard one sound (i.e., buzz) or two sounds (i.e., buzz plus another sound with a pure-tone quality). The auditory stimuli were also presented while participants watched a silent movie in order to evaluate the extent to which the mistuned harmonic could be automatically detected. The perception of the mistuned harmonic as a separate sound was associated with a biphasic negative-positive potential that peaked at about 150 and 350 ms after sound onset, respectively. Long duration sounds also elicited a sustained potential that was greater in amplitude when the mistuned harmonic was perceptually segregated from the complex sound. The early negative wave, referred to as the object-related negativity (ORN), was present during both active and passive listening, whereas the positive wave and the mistuning-related changes in sustained potentials were present only when participants attended to the stimuli. These results are consistent with a two-stage model of auditory scene analysis in which the acoustic wave is automatically decomposed into perceptual groups that can be identified by higher executive functions. The ORN and the positive waves were little affected by sound duration, indicating that concurrent sound segregation depends on transient neural responses elicited by the discrepancy between the mistuned harmonic and the harmonic frequency expected based on the fundamental frequency of the incoming stimulus.     

Insights into dolphin sonar discrimination capabilities from human listening experiments

A variety of dolphin sonar discrimination experiments have been conducted, yet little is known about the cues utilized by dolphins in making fine target discriminations. In order to gain insights on cues available to echolocating dolphins, sonar discrimination experiments were conducted with human subjects using the same targets employed in dolphin experiments. When digital recordings of echoes from targets ensonified with a dolphinlike signal were played back at a slower rate to human subjects, they could also make fine target discriminations under controlled laboratory conditions about as well as dolphins under less controlled conditions. Subjects reported that time-separation-pitch and duration cues were important. They also reported that low-amplitude echo components 32 dB below the maximum echo component were usable. The signal-to-noise ratio had to be greater than 10 dB above the detection threshold for simple discrimination and 30 dB for difficult discrimination. Except for two cases in which spectral cues in the form of "click pitch" were important, subjects indicated that time-domain rather than frequency-domain processing seemed to be more relevant in analyzing the echoes.