Stephanie Vlachos

Predicting temporary threshold shifts in a bottlenose dolphin (Tursiops truncatus): The effects of noise level and duration

Noise levels in the ocean are increasing and are expected to affect marine mammals. To examine the auditory effects of noise on odontocetes, a bottlenose dolphin (Tursiops truncatus) was exposed to octave-band noise (4-8 kHz) of varying durations (<2-30 min) and sound pressures (130-178 dB re 1 microPa). Temporary threshold shift (TTS) occurrence was quantified in an effort to (i) determine the sound exposure levels (SELs) (dB re 1 microPa(2) s) that induce TTS and (ii) develop a model to predict TTS onset. Hearing thresholds were measured using auditory evoked potentials. If SEL was kept constant, significant shifts were induced by longer duration exposures but not for shorter exposures. Higher SELs were required to induce shifts in shorter duration exposures. The results did not support an equal-energy model to predict TTS onset. Rather, a logarithmic algorithm, which increased in sound energy as exposure duration decreased, was a better predictor of TTS. Recovery to baseline hearing thresholds was also logarithmic (approximately -1.8 dB/doubling of time) but indicated variability including faster recovery rates after greater shifts and longer recoveries necessary after longer duration exposures. The data reflected the complexity of TTS in mammals that should be taken into account when predicting odontocete TTS.

Atlantic bottlenose dolphin (Tursiops truncatus) hearing threshold for brief broadband signals.

The hearing sensitivity of an Atlantic bottlenose dolphin (Tursiops truncatus) to both pure tones and broadband signals simulating echoes from a 7.62-cm water-filled sphere was measured. Pure tones with frequencies between 40 and 140 kHz in increments of 20 kHz were measured along with broadband thresholds using a stimulus with a center frequency of 97.3 kHz and 88.2 kHz. The pure-tone thresholds were compared with the broadband thresholds by converting the pure-tone threshold intensity to energy flux density. The results indicated that dolphins can detect broadband signals slightly better than a pure-tone signal. The broadband results suggest that an echolocating bottlenose dolphin should be able to detect a 7.62-cm diameter water-filled sphere out to a range of 178 m in a quiet environment.

A re-evaluation of auditory filter shape in delphinid odontocetes: Evidence of constant-bandwidth filters

The auditory filter shape of delphinid odontocetes was previously considered to be typically mammalian constant-quality in which filter bandwidths increase proportionally with frequency. Recent studies with porpoises demonstrate constant-bandwidth portions of the auditory filter. The critical ratios for a bottlenose dolphin were measured between 40 and 120  kHz by behaviorally determining the subjects ability to detect pure tones in the presence of white noise. Critical ratios as a function of frequency were constant, indicating the auditory filter acts as a constant-bandwidth system in this frequency range. Re-analysis of past studies supports these findings, and suggests the delphinid auditory system is best characterized as a constant-Q system below 40  kHz and a constant-bandwidth-like system between 40  kHz and 120  kHz before returning to a constant Q pattern at the highest frequencies.

Single-lobed frequency-dependent beam shape in an echolocating false killer whale (Pseudorca crassidens).

Recent studies indicate some odontocetes may produce echolocation beams with a dual-lobed vertical structure. The shape of the odontocete echolocation beam was further investigated in a false killer whale performing an echolocation discrimination task. Clicks were recorded with an array of 16 hydrophones and frequency-dependent amplitude plots were constructed to assess beam shape. The majority of the echolocation clicks were single-lobed in structure with most energy located between 20 and 80 kHz. These data indicate the false killer whale does not produce a dual-lobed structure, as has been shown in bottlenose dolphins, which may be a function of lowered frequencies in the emitted signal due to hearing loss.

High-frequency auditory filter shape for the Atlantic bottlenose dolphin

High-frequency auditory filter shapes of an Atlantic bottlenose dolphin (Tursiops truncatus) were measured using a notched noise masking source centered on pure tone signals at frequencies of 40, 60, 80 and 100 kHz. A dolphin was trained to swim into a hoop station facing the noise/signal transducer located at a distance of 2 m. The dolphins masked threshold was determined using an up-down staircase method as the width of the notched noise was randomly varied from 0, 0.2, 04, 0.6, and 0.8 times the test tone frequency. The masked threshold decreased as the width of the notched increased and less noise fell within the auditory filter associated with the test tone. The auditory filter shapes were approximated by fitting a roex (p,r(r)) function to the masked threshold results. A constant-Q value of 8.4 modeled the results within the frequency range of 40 to 100 kHz relatively well. However, between 60 and 100 kHz, the 3 dB bandwidth was relatively similar between 9.5 and 10 kHz, indicating a constant-bandwidth system in this frequency range The mean equivalent rectangular bandwidth calculated from the filter shape was approximately 16.0%, 17.0%, 13.6% and 11.3% of the tone frequencies of 40, 60, 80, and 100 kHz.

The perception of complex tones by a false killer whale (Pseudorca crassidens)

Complex tonal whistles are frequently produced by some odontocete species. However, no experimental evidence exists regarding the detection of complex tones or the discrimination of harmonic frequencies by a marine mammal. The objectives of this investigation were to examine the ability of a false killer whale to discriminate pure tones from complex tones and to determine the minimum intensity level of a harmonic tone required for the whale to make the discrimination. The study was conducted with a go/no-go modified staircase procedure. The different stimuli were complex tones with a fundamental frequency of 5 kHz with one to five harmonic frequencies. The results from this complex tone discrimination task demonstrated: (1) that the false killer whale was able to discriminate a 5 kHz pure tone from a complex tone with up to five harmonics, and (2) that discrimination thresholds or minimum intensity levels exist for each harmonic combination measured. These results indicate that both frequency level and harmonic content may have contributed to the false killer whales discrimination of complex tones.

High‐frequency auditory filter shapes in an Atlantic bottlenose dolphin

High‐frequency auditory filter shapes were calculated for a 20‐year‐old female Atlantic bottlenose dolphin (Tursiops truncatus). Thresholds were determined for tones of 40, 60, 80, and 100 kHz masked by notched noise. Auditory filter shapes were determined by fitting the integral of the roex(p,r) filter shape to the functions relating masked tonal threshold to notch width. Filter shapes were found to be approximately symmetric at the moderate noise level used. Equivalent rectangular bandwidths of the auditory filters ranged from 16% of center frequency at 40 kHz to 11% of center frequency at 100 kHz. There was very little change in the bandwidths of the filters between 60 and 100 kHz, indicating that relative tuning sharpness increases as a function of frequency in this range. Efficiency of processing after the periphery was found to be maximal at 40 and 60 kHz (better than 12‐dB SNR) and to decrease gradually above 60 kHz. The efficiency estimates allowed for the reanalysis of critical ratio data collect...

Spatial orientation of different frequencies within the echolocation beam of a Tursiops truncatus and Pseudorca crassidens

A two-dimensional array of 16 hydrophones was created to map the spatial distribution of different frequencies within the echolocation beam of a Tursiops truncatus and a Pseudorca crassidens. It was previously shown that both the Tursiops and Pseudorca only paid attention to frequencies between 29 and 42 kHz while echolocating. Both individuals tightly focused the 30 kHz frequency and the spatial location of the focus was consistently pointed toward the target. At 50 kHz the beam was less focused and less precisely pointed at the target. At 100 kHz the focus was often completely lost and was not pointed at the target. This indicates that these individuals actively focused the beam toward the target only in the frequency range they paid attention to. Frequencies outside this range were left unfocused and undirected. This focusing was probably achieved through sensorimotor control of the melon morphology and nasal air sacs. This indicates that both morphologically different species can control the spatial distribution of different frequency ranges within the echolocation beam to create consistent ensonation of desired targets.

Temporary threshold shifts in the bottlenose dolphin (Tursiopstruncatus), varying noise duration and intensity

There is much concern regarding increasing noise levels in the ocean and how it may affect marine mammals. However, there is a little information regarding how sound affects marine mammals and no published data examining the relationship between broadband noise intensity and exposure duration. This study explored the effects of octave‐band noise on the hearing of a bottlenose dolphin by inducing temporary hearing threshold shifts (TTS). Sound pressure level (SPL) and exposure duration were varied to measure the effects of noise duration and intensity. Hearing thresholds were measured using auditory evoked potentials before and after sound exposure to track and map TTS and recovery. Shifts were frequency dependent and recovery time depended on shift and frequency, but full recovery was relatively rapid, usually within 20 min and always within 40 min. As exposure time was halved, TTS generally occurred with an increase in noise SPL. However, with shorter, louder noise, threshold shifts were not linear but r...

Auditory frequency selectivity and masked hearing capabilities in an Atlantic bottlenose dolphin

Frequency selectivity capabilities of an Atlantic bottlenose dolphin were examined by calculating critical ratios from masked hearing data. Absolute sensitivity to pure tones from 40–140 kHz was measured as a base line, and masked sensitivity was determined for the same signals masked by three levels of white noise (52, 42, and 32 dB re:1 μ Pa2/Hz). Absolute and masked sensitivity were essentially constant between 40 and 120 kHz at each of the masking conditions. Sensitivity decreased approximately 100 dB per octave between 120 and 140 kHz. Critical ratios averaged across noise levels were constant between 40 and 120 kHz, averaging 26 dB. The critical ratio at 140 kHz was 42 dB. This flat trend in critical ratios between 40–120 kHz does not agree with the constant‐Q filter‐bank models used to account for earlier critical ratio and bandwidth measurements for the species [W. W. L. Au and P. W. B. Moore, J. Acoust. Soc. Am. 88, 1635–1638 (1990)], but comparison to other cetacean masked hearing work [Johnson ...