Invariance of evoked-potential echo-responses to target strength and distance in an echolocating false killer whale

Brain auditory evoked potentials (AEPs) were recorded in a false killer whale Pseudorca crassidens trained to accept suction-cup EEG electrodes and to detect targets by echolocation. AEP collection was triggered by echolocation pulses transmitted by the animal. The target strength varied from -22 to -40 dB; the distance varied from 1.5 to 6 m. All the records contained two AEP sets: the first one of a constant latency (transmission-related AEP) and a second one with a delay proportional to the distance (echo-related AEP). The amplitude of echo-related AEPs was almost independent of both target strength and distance, though combined variation of these two parameters resulted in echo intensity variation within a range of 42 dB. The amplitude of transmission-related AEPs was independent of distance but dependent on target strength: the less the target strength, the higher the amplitude. Recording of transmitted pulses has not shown their intensity dependence on target strength. It is supposed that the constancy of echo-related AEP results from variation of hearing sensitivity depending on the target strength and release of echo-related responses from masking by transmitted pulses depending on the distance.     

Forward masking as a mechanism of automatic gain control in odontocete biosonar: a psychophysical study

In a false killer whale Pseudorca crassidens, echo perception thresholds were measured using a go/no-go psychophysical paradigm and one-up-one-down staircase procedure. Computer controlled echoes were electronically synthesized pulses that were played back through a transducer and triggered by whale emitted biosonar pulses. The echo amplitudes were proportional to biosonar pulse amplitudes; echo levels were specified in terms of the attenuation of the echo sound pressure level near the animal's head relative to the source level of the biosonar pulses. With increasing echo delay, the thresholds (echo attenuation factor) decreased from -49.3 dB at 2 ms to -79.5 dB at 16 ms, with a regression slope of -9.5 dB per delay doubling (-31.5 dB per delay decade). At the longer delays, the threshold remained nearly constant around -80.4 dB. Levels of emitted pulses slightly increased with delay prolongation (threshold decrease), with a regression slope of 3.2 dB per delay doubling (10.7 dB per delay decade). The echo threshold dependence on delay is interpreted as a release from forward masking by the preceding emitted pulse. This release may compensate for the echo level decrease with distance, thus keeping the echo sensation level for the animal near constant within a certain distance range.     

The interaction of outgoing echolocation pulses and echoes in the false killer whale's auditory system: evoked-potential study

Brain auditory evoked potentials (AEP) associated with echolocation were recorded in a false killer whale Pseudorca crassidens trained to accept suction-cup EEG electrodes and to detect targets by echolocation. AEP collection was triggered by echolocation pulses transmitted by the animal. The target was a hollow aluminum cylinder of strength of -22 dB at a distance from 1 to 8 m. Each AEP record was obtained by averaging more than 1000 individual records. All the records contained two AEP sets: the first one of a constant latency and a second one with a delay proportional to the distance. The timing of these two AEP sets was interpreted as responses to the transmitted echolocation pulse and echo, respectively. The echo-related AEP, although slightly smaller, was comparable to the outgoing click-related AEP in amplitude, even though at a target distance as far as 8 m the echo intensity was as low as -64 dB relative to the transmitted pulse in front of the head. The amplitude of the echo-related AEP was almost independent of distance, even though variation of target distance from 1 to 8 m influenced the echo intensity by as much as 36 dB.     

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.     

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.     

Vertical-plane sound localization probed with ripple-spectrum noise

Ripple-spectrum stimuli were used to investigate the scale of spectral detail used by listeners in interpreting spectral cues for vertical-plane localization. In three experiments, free-field localization judgments were obtained for 250-ms, 0.6-16-kHz noise bursts with log-ripple spectra that varied in ripple density, peak-to-trough depth, and phase. When ripple density was varied and depth was held constant at 40 dB, listeners' localization error rates increased most (relative to rates for flat-spectrum targets) for densities of 0.5-2 ripples/oct. When depth was varied and density was held constant at 1 ripple/oct, localization accuracy was degraded only for ripple depths > or = 20 dB. When phase was varied and density was held constant at 1 ripple/oct and depth at 40 dB, three of five listeners made errors at consistent locations unrelated to the ripple phase, whereas two listeners made errors at locations systematically modulated by ripple phase. Although the reported upper limit for ripple discrimination is 10 ripples/oct [Supin et al., J. Acoust. Soc. Am. 106, 2800-2804 (1999)], present results indicate that details finer than 2 ripples/oct or coarser than 0.5 ripples/oct do not strongly influence processing of spectral cues for sound localization. The low spectral-frequency limit suggests that broad-scale spectral variation is discounted, even though components at this scale are among those contributing the most to the shapes of directional transfer functions.     

Effects of peripheral nonlinearity on psychometric functions for forward-masked tones

Psychometric functions (PFs) for forward-masked tones were obtained for conditions in which signal level was varied to estimate threshold at several masker levels (variable-signal condition), and in which masker level was varied to estimate threshold at several signal levels (variable-masker condition). The changes in PF slope across combinations of masker frequency, masker level, and signal delay were explored in three experiments. In experiment 1, a 2-kHz, 10-ms tone was masked by a 50, 70 or 90 dB SPL, 20-ms on-frequency forward masker, with signal delays of 2, 20, or 40 ms, in a variable-signal condition. PF slopes decreased in conditions where signal threshold was high. In experiments 2 and 3, the signal was a 4-kHz, 10-ms tone, and the masker was either a 4- or 2.4-kHz, 200-ms tone. In experiment 2, on-frequency maskers were presented at 30 to 90 dB SPL in 10-dB steps and off-frequency maskers were presented at 60 to 90 dB SPL in 10-dB steps, with signal delays of 0, 10, or 30 ms, in a variable-signal condition. PF slopes decreased as signal level increased, and this trend was similar for on- and off-frequency maskers. In experiment 3, variable-masker conditions with on- and off-frequency maskers and 0-ms signal delay were presented. In general, the results were consistent with the hypothesis that peripheral nonlinearity is reflected in the PF slopes. The data also indicate that masker level plays a role independent of signal level, an effect that could be accounted for by assuming greater internal noise at higher stimulus levels.     

Hearing sensitivity during target presence and absence while a whale echolocates

Hearing sensitivity was measured in a false killer whale during echolocation. Sensitivity was measured using probe stimuli as sinusoidally amplitude modulated signals with a 22.5-kHz carrier frequency and recording auditory evoked potentials as envelope-following responses. The probes were presented and responses were recorded during short 2-s periods when the animal echolocated to detect the presence or absence of a target in a go/no-go paradigm. In the target-absent trials, a hearing threshold of 90.4 dB re 1 muPa was found; in the target-present trials, the threshold was 109.8 dB. Thus, a 19.4-dB difference was found between thresholds in the target-present and target-absent trials. To check the possibility that this difference was the result of different masking degree of the probe by the emitted sonar clicks, click statistics were investigated in similar trials. No indication was found that the energy of the emitted clicks was higher in the target-present than in target-absent trials; on the contrary, mean click level, mean number of clicks per train, and overall train energy was slightly higher in the target-absent trials. Thus the data indicate that the hearing sensitivity of the whale varied depending on target presence or absence.     

Source-to-sensation level ratio of transmitted biosonar pulses in an echolocating false killer whale

Transmitted biosonar pulses, and the brain auditory evoked potentials (AEPs) associated with those pulses, were synchronously recorded in a false killer whale Pseudorca crassidens trained to accept suction-cup EEG electrodes and to detect targets by echolocation. AEP amplitude was investigated as a function of the transmitted biosonar pulse source level. For that, a few thousand of the individual AEP records were sorted according to the spontaneously varied amplitude of synchronously recorded biosonar pulses. In each of the sorting bins (in 5-dB steps) AEP records were averaged to extract AEP from noise; AEP amplitude was plotted as a function of the biosonar pulse source level. For comparison, AEPs were recorded to external (in free field) sound pulses of a waveform and spectrum similar to those of the biosonar pulses; amplitude of these AEPs was plotted as a function of sound pressure level. A comparison of these two functions has shown that, depending on the presence or absence of a target, the sensitivity of the whale's hearing to its own transmitted biosonar pulses was 30 to 45 dB lower than might be expected in a free acoustic field.     

Detection of intensity decrements by the chinchilla.

Three monaural chinchillas were trained to detect intensity decrements in broadband noise (20 kHz) using a shock-avoidance conditioning procedure. The intensity decrements were presented at one of nine different durations between 2 and 35 ms at noise levels of 25, 45, and 65 dB SPL. At each intensity-duration combination, the level of the decrement was varied to obtain a decrement threshold. The minimal detectable decrement decreased from approximately 20 dB at the shortest duration to an asymptote of roughly 4 dB at approximately 30 ms. The data were modeled by a low-pass filter with an 11-ms time constant. The decrement detection function of the chinchilla is similar to that of humans. However, long-duration decrement thresholds are larger in the chinchilla, as would be predicted from the large intensity difference limen of the chinchilla. In general, there was little change in the decrement function across background intensities except that 2-ms decrements were not detected at the 25-dB SPL background intensity.     

Behavioral and auditory evoked potential audiograms of a false killer whale (Pseudorca crassidens)

Behavioral and auditory evoked potential (AEP) audiograms of a false killer whale were measured using the same subject and experimental conditions. The objective was to compare and assess the correspondence of auditory thresholds collected by behavioral and electrophysiological techniques. Behavioral audiograms used 3-s pure-tone stimuli from 4 to 45 kHz, and were conducted with a go/no-go modified staircase procedure. AEP audiograms used 20-ms sinusoidally amplitude-modulated tone bursts from 4 to 45 kHz, and the electrophysiological responses were received through gold disc electrodes in rubber suction cups. The behavioral data were reliable and repeatable, with the region of best sensitivity between 16 and 24 kHz and peak sensitivity at 20 kHz. The AEP audiograms produced thresholds that were also consistent over time, with range of best sensitivity from 16 to 22.5 kHz and peak sensitivity at 22.5 kHz. Behavioral thresholds were always lower than AEP thresholds. However, AEP audiograms were completed in a shorter amount of time with minimum participation from the animal. These data indicated that behavioral and AEP techniques can be used successfully and interchangeably to measure cetacean hearing sensitivity.     

Temporal resolution in sensorineural hearing-impaired listeners

Temporal masking curves were obtained from 12 normal-hearing and 16 hearing-impaired listeners using 200-ms, 1000-Hz pure-tone maskers and 20-ms, 1000-Hz fixed-level probe tones. For the delay times used here (greater than 40 ms), temporal masking curves obtained from both groups can be well described by an exponential function with a single level-independent time constant for each listener. Normal-hearing listeners demonstrated time constants that ranged between 37 and 67 ms, with a mean of 50 ms. Most hearing-impaired listeners, with significant hearing loss at the probe frequency, demonstrated longer time constants (range 58-114 ms) than those obtained from normal-hearing listeners. Time constants were found to grow exponentially with hearing loss according to the function tau = 52e0.011(HL), when the slope of the growth of masking is unity. The longest individual time constant was larger than normal by a factor of 2.3 for a hearing loss of 52 dB. The steep slopes of the growth of masking functions typically observed at long delay times in hearing-impaired listeners' data appear to be a direct result of longer time constants. When iterative fitting procedures included a slope parameter, the slopes of the growth of masking from normal-hearing listeners varied around unity, while those from hearing-impaired listeners tended to be less (flatter) than normal. Predictions from the results of these fixed-probe-level experiments are consistent with the results of previous fixed-masker-level experiments, and they indicate that deficiencies in the ability to detect sequential stimuli should be considerable in hearing-impaired listeners, partially because of extended time constants, but mostly because forward masking involves a recovery process that depends upon the sensory response evoked by the masking stimulus. Large sensitivity losses reduce the sensory response to high SPL maskers so that the recovery process is slower, much like the recovery process for low-level stimuli in normal-hearing listeners.     

Echo-intensity compensation in echolocating bats (Pipistrellus abramus) during flight measured by a telemetry microphone

An onboard microphone (Telemike) was developed to examine changes in the basic characteristics of echolocation sounds of small frequency-modulated echolocating bats, Pipistrellus abramus. Using a dual high-speed video camera system, spatiotemporal observations of echolocation characteristics were conducted on bats during a landing flight task in the laboratory. The Telemike allowed us to observe emitted pulses and returning echoes to which the flying bats listened during flight, and the acoustic parameters could be precisely measured without traditional problems such as the directional properties of the recording microphone and the emitted pulse, or traveling loss of the sound in the air. Pulse intensity in bats intending to land exhibited a marked decrease by 30 dB within 2 m of the target wall, and the reduction rate was approximately 6.5 dB per halving of distance. The intensity of echoes returning from the target wall indicated a nearly constant intensity (-42.6 +/- 5.5 dB weaker than the pulse emitted in search phase) within a target distance of 2 m. These findings provide direct evidence that bats adjust pulse intensity to compensate for changes in echo intensity to maintain a constant intensity of the echo returned from the approaching target at an optimal range.     

Auditory brainstem response in a harbor porpoise show lack of automatic gain control for simulated echoes

The auditory brainstem response (ABR) response to simulated echolocation clicks was studied in a harbor porpoise, Phocoena phocoena, to determine the relationship between the animal's perceived echo strength and the simulated target distance. In one experiment the click level at the listening post was kept constant while delay was changed, in another, the level was varied to approximate spreading losses. Results of both experiments indicate that there is no automatic gain control in the hearing system of this harbor porpoise.     

Auditory brainstem responses to chirps delivered by different insert earphones

The frequency response and sensitivity of the ER-3A and ER-2 insert earphones are measured in the occluded-ear simulator using three ear canal extensions. Compared to the other two extensions, the DB 0370 (Bru?el & Kj?r), which is recommended by the international standards, introduces a significant resonance peak around 4500 Hz. The ER-3A has an amplitude response like a band-pass filter (1400 Hz, 6 dB/octave -4000 Hz, -36 dB/octave), and a group delay with "ripples" of up to ±0.5 ms, while the ER-2 has an amplitude response, and a group delay which are flat and smooth up to above 10000 Hz. Both earphones are used to record auditory brainstem responses, ABRs, from 22 normal-hearing ears in response to two chirps and a click at levels from 20 to 80 dB nHL. While the click-ABRs are slightly larger for ER-2 than for ER-3A, the chirp-ABRs are much larger for ER-2 than for ER-3A at levels below 60 dB nHL. With a simulated amplitude response of the ER-3A and the smooth group delay of the ER-2 it is shown that the increased chirp-ABR amplitude with the ER-2 is caused by its broader amplitude response and not by its smoother group delay.     

The effect of broadband noise on the human brainstem auditory evoked response. I. Rate and intensity effects

A series of experiments investigated the effects of continuous broadband noise (ipsilateral) on wave V of the click-evoked brainstem auditory evoked response (BAER). In general, a broadband noise masker increases the latency and decreases the amplitude of wave V. Varying both click and noise intensity, it was found that noise levels above about 40 dB SPL increase the latency and decrease the amplitude of wave V, regardless of click intensity. The effects of noise on wave V amplitude appear constant across click intensity, whereas the effects of a constant noise level on wave V latency decrease at higher click intensities. Both masking and adaptation increase wave V latency, but their combined effects are occlusive: rate-induced wave V latency shift decreases in the presence of continuous broadband noise. The clinical and theoretical implications of these findings are discussed.     

The time required to focus on a cued signal frequency

How quickly can a listener focus on a single tonal cue that indicates the frequency of an upcoming signal? Initial measurements were made with frequency uncertainty (signal frequency varies randomly from trial to trial) and with certainty (same frequency on all trials). Measured by a yes-no procedure, thresholds for 40- and 20-ms signals presented in continuous broadband noise at 50 dB SPL were higher in uncertainty than in certainty; the difference decreased monotonically from 5 dB at frequencies below 500 Hz to under 3 dB above about 2500 Hz. This decrease in the detrimental effect from uncertainty, which comes about with increasing signal frequency, may result from preferential attention to higher frequencies. In a second experiment, frequency again varied randomly, but each trial now began with a cue at the signal frequency. The critical variable was the delay from cue onset to signal onset. A delay of 352 ms eliminated the detrimental effect of frequency uncertainty at all frequencies. At the shortest delays of 52 and 82 ms the detrimental effect was reduced primarily at lower frequencies. Our analysis suggests that shifting focus to a cued frequency region, under optimal stimulus conditions, requires less than 52 ms.     

Simultaneously measured behavioral and electrophysiological hearing thresholds in a bottlenose dolphin (Tursiops truncatus)

Dolphin auditory thresholds obtained via evoked potential audiometry may deviate from behavioral estimates by 20 dB or more. Differences in the sound source, stimulus presentation method, wave form, and duration may partially explain these discrepancies. To determine the agreement between behavioral and auditory evoked potential (AEP) threshold estimates when these parameters are held constant, behavioral and AEP hearing tests were simultaneously conducted in a bottlenose dolphin. Measurements were made in-air, using sinusoidal amplitude-modulated tones continuously projected via a transducer coupled to the pan region of the dolphin's lower jaw. Tone trials were presented using the method of constant stimuli. Behavioral thresholds were estimated using a 50% correct detection. AEP thresholds were based on the envelope following response and 50% correct detection. Differences between AEP and behavioral thresholds were within +/-5 dB, except at 10 kHz (12 dB), 20 kHz (8 dB), 30 kHz (7 dB), and 150 kHz (24 dB). In general, behavioral thresholds were slightly lower, though this trend was not significant. The results demonstrate that when the test environment, sound source, stimulus wave form, duration, presentation method, and analysis are consistent, the magnitude of the differences between AEP and behavioral thresholds is substantially reduced.     

Click-evoked potentials in a large marine mammal, the adult male northern elephant seal (Mirounga angustirostris)

Auditory evoked potential (AEP) hearing studies in marine mammals should consider an expected size-dependent reduction in AEP amplitude. This study is the first to measure the click-evoked response in a large marine mammal, the adult male elephant seal (Mirounga angustirostris). Click stimuli were presented at peak-peak equivalent sound pressure levels of 117-118 dB re: 20 microPa. Three positive peaks (P1-P3) and two negative peaks (N4 and N5) were observed in the AEP. Response latencies were longer than previously observed in a 1.8 yr old seal and the maximum peak-peak amplitude was comparatively reduced by more than 60%. The inverse relationship between size and AEP amplitude will likely require increased averaging with larger subjects and possibly modifications to electrode placement and design in order to increase the quality of recorded evoked responses.     

Group delay measurement from spiral ganglion cells in the basal turn of the guinea pig cochlea

Measurements of group delay were made extracellularly from spiral ganglion cells in the 3.7 to 5.0-mm region of the guinea pig cochlea, using sinusoidally amplitude modulated tones with constant modulating frequency (100 Hz) and depth of modulation (0.19). Threshold cochlear tuning was accompanied by frequency-dependent group delays. The group delay on the low-frequency tail was independent of carrier frequency; the interunit variation was 0.28-1.28 ms. The difference in group delay between CF and the low-frequency tail decreased as the CF threshold increased (-0.09 +/- 0.02 ms per 10 dB, beginning at 0.62 +/- 0.07 ms at 0 dB SPL). The group delay decreased above CF; at the units' maximum frequency it was less than the low-frequency tail value, and was sometimes negative. Following arterial injections of furosemide the CF threshold increased and the group delay peak decreased; the low-frequency tail was unaffected. The group delay decreased with increasing intensity; the reduction near and above CF was not only larger than that on the low-frequency tail, but also the change at 5-10 dB above threshold was far greater than expected from the Q10dB of the suprathreshold iso-rate tuning curves. A minimum-phase analysis suggested that the group delay response above CF, together with its nonlinear behavior, can be accounted for by a high-frequency, level-independent, amplitude plateau, in combination with the single unit, amplitude nonlinearity which is known to exist above CF.