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

High Frequency hearing in echolocating dolphins

The Reflections series takes a look back on historical articles from The Journal of the Acoustical Society of America that have had a significant impact on the science and practice of acoustics.

Mechanisms of auditory masking in marine mammals

Anthropogenic noise is an increasing threat to marine mammals that rely on sound for communication, navigation, detecting prey and predators, and finding mates. Auditory masking is one consequence of anthropogenic noise, the study of which is approached from multiple disciplines including field investigations of animal behavior, noise characterization from in-situ recordings, computational modeling of communication space, and hearing experiments conducted in the laboratory. This paper focuses on laboratory hearing experiments applying psychophysical methods, with an emphasis on the mechanisms that govern auditory masking. Topics include tone detection in simple, complex, and natural noise; mechanisms for comodulation masking release and other forms of release from masking; the role of temporal resolution in auditory masking; and energetic vs informational masking.

Auditory masking in killer whales (Orcinus orca): Critical ratios for tonal signals in Gaussian noise

Masked detection thresholds were measured for two killer whales (Orcinus orca) using a psychoacoustic, adaptive-staircase procedure. Noise bands were 1-octave wide continuous Gaussian noise. Tonal signals extended between 500 Hz and 80 kHz. Resulting critical ratios increased with the signal frequency from 15 dB at 500 Hz up to 32 dB at 80 kHz. Critical ratios for killer whales were similar to those of other odontocetes despite considerable differences in size, hearing morphology, and hearing sensitivity between species.

The rate of cochlear compression in a dolphin: a forward-masking evoked-potential study

The "active" cochlear mechanism of hearing manifests in the cochlear compression. Investigations of compression in odontocetes help to determine the frequency limit of the active mechanism. The compression may be evaluated by comparison of low- and on-frequency masking. In a bottlenose dolphin, forward masking of auditory evoked potentials to tonal pips was investigated. Measurements were performed for test frequencies of 45 and 90 kHz. The low-frequency maskers were - 0.25 to - 0.75 oct relative the test. Masking efficiency was varied by masker-to-test delay variation from 2 to 20 ms, and masker levels at threshold (MLTs) were evaluated at each of the delays. It was assumed that low-frequency maskers were not subjected or little subjected to compression whereas on-frequency maskers were subjected equally to the test. Therefore, the compression rate was assessed as the slope of low-frequency MLT dependence on on-frequency MLT. For the 90-kHz test, the slopes were 0.63 and 0.18 dB/dB for masker of - 0.25 and - 0.5 oct, respectively. For the 45 kHz test, the slopes were 0.69 and 0.39 dB/dB for maskers of - 0.25 and - 0.5 oct. So, compression did not decay at the upper boundary of the hearing frequency range in the dolphin.

How humans discriminate acoustically among bottlenose dolphin signature whistles with and without masking by boat noise

Anthropogenic noise in the world's oceans is known to impede many species' ability to perceive acoustic signals, but little research has addressed how this noise affects the perception of bioacoustic signals used for communication in marine mammals. Bottlenose dolphins (Tursiops truncatus) use signature whistles containing identification information. Past studies have used human participants to gain insight into dolphin perception, but most previous research investigated echolocation. In Experiment 1, human participants were tested on their ability to discriminate among signature whistles from three dolphins. Participants' performance was nearly errorless. In Experiment 2, participants identified signature whistles masked by five different samples of boat noise utilizing different signal-to-noise ratios. Lower signal-to-noise ratio and proximity in frequency between the whistle and noise both significantly decreased performance. Like dolphins, human participants primarily identified whistles using frequency contour. Participants reported greater use of amplitude in noise-present vs noise-absent trials, but otherwise did not vary cue usage. These findings can be used to generate hypotheses about dolphins' performance and auditory cue use for future research. This study may provide insight into how specific characteristics of boat noise affect dolphin whistle perception and may have implications for conservation and regulations.

Level-dependent masking of the auditory evoked responses in a dolphin: manifestation of the compressive nonlinearity

At suprathreshold sound levels, interactions between masking noise and sound signals are liable to compressive nonlinearity in the auditory system. The compressive nonlinearity is a property of the "active" cochlear mechanism. It is not known whether this mechanism is capable to function at frequencies close to or above 100 kHz that are available to odontocetes (toothed whales, dolphins, and porpoises). This question may be answered by the use of the frequency-specific masking. Auditory evoked potentials to sound stimuli in a bottlenose dolphin, Tursiops truncatus, were recorded in the presence of simultaneous maskers. Stimulus frequencies were 45, 64, or 90 kHz. Maskers were on-frequency bandlimited noise or low-frequency noise of frequencies 0.25-1 oct below the stimulus frequency. The stimuli provoked responses as a series of brain-potential waves following the pip-train rate. For the on-frequency masker, the masker level at threshold dependence on the signal level was 1.1 dB/dB. For maskers of 1 oct below the stimulus, the dependence was 0.53-0.57 dB/dB. The data considered evidence for the compressive nonlinearity of responses to stimuli, and therefore, are indicative of the functioning of the active mechanism at frequencies up to 90 kHz.

Modulation rate transfer functions from four species of stranded odontocete (Stenella longirostris, Feresa attenuata, Globicephala melas, and Mesoplodon densirostris)

Odontocete marine mammals explore the environment by rapidly producing echolocation signals and receiving the corresponding echoes, which likewise return at very rapid rates. Thus, it is important that the auditory system has a high temporal resolution to effectively process and extract relevant information from click echoes. This study used auditory evoked potential methods to investigate auditory temporal resolution of individuals from four different odontocete species, including a spinner dolphin (Stenella longirostris), pygmy killer whale (Feresa attenuata), long-finned pilot whale (Globicephala melas), and Blainville's beaked whale (Mesoplodon densirostris). Each individual had previously stranded and was undergoing rehabilitation. Auditory Brainstem Responses (ABRs) were elicited via acoustic stimuli consisting of a train of broadband tone pulses presented at rates between 300 and 2000 Hz. Similar to other studied species, modulation rate transfer functions (MRTFs) of the studied individuals followed the shape of a low-pass filter, with the ability to process acoustic stimuli at presentation rates up to and exceeding 1250 Hz. Auditory integration times estimated from the bandwidths of the MRTFs ranged between 250 and 333 µs. The results support the hypothesis that high temporal resolution is conserved throughout the diverse range of odontocete species.

Composite critical ratio functions for odontocete cetaceans

Critical ratios (CRs) are useful for estimating detection thresholds of tonal signals when the spectral density of noise is known. In cetaceans, CRs have only been measured for a few animals representing four odontocete species. These data are sparse, particularly for lower frequencies where anthropogenic noise is concentrated. There is currently no systematic method for implementing CR predictions (e.g., a composite frequency-dependent CR function). The current study measures CRs for two bottlenose dolphins (Tursiops truncatus) and estimates composite CR functions. The composite models can aid in predicting and extrapolating auditory masking for a broad range of frequencies.

Killer whale (Orcinus orca) behavioral audiograms

Killer whales (Orcinus orca) are one of the most cosmopolitan marine mammal species with potential widespread exposure to anthropogenic noise impacts. Previous audiometric data on this species were from two adult females [Szymanski, Bain, Kiehl, Pennington, Wong, and Henry (1999). J. Acoust. Soc. Am. 108, 1322-1326] and one sub-adult male [Hall and Johnson (1972). J. Acoust. Soc. Am. 51, 515-517] with apparent high-frequency hearing loss. All three killer whales had best sensitivity between 15 and 20 kHz, with thresholds lower than any odontocete tested to date, suggesting this species might be particularly sensitive to acoustic disturbance. The current study reports the behavioral audiograms of eight killer whales at two different facilities. Hearing sensitivity was measured from 100 Hz to 160 kHz in killer whales ranging in age from 12 to 52 year. Previously measured low thresholds at 20 kHz were not replicated in any individual. Hearing in the killer whales was generally similar to other delphinids, with lowest threshold (49 dB re 1 μPa) at approximately 34 kHz, good hearing (i.e., within 20 dB of best sensitivity) from 5 to 81 kHz, and low- and high-frequency hearing cutoffs (>100 dB re μPa) of 600 Hz and 114 kHz, respectively.

Auditory evoked potentials in the auditory system of a beluga whale Delphinapterus leucas to prolonged sound stimuli

The effects of prolonged (up to 1500 s) sound stimuli (tone pip trains) on evoked potentials (the rate following response, RFR) were investigated in a beluga whale. The stimuli (rhythmic tone pips) were of frequencies of 45, 64, and 90 kHz at levels from 20 to 60 dB above threshold. Two experimental protocols were used: short- and long-duration. For the short-duration protocol, the stimuli were 500-ms-long pip trains that repeated at a rate of 0.4 trains/s. For the long-duration protocol, the stimuli were continuous pip successions lasting up to 1500 s. The RFR amplitude gradually decreased by three to seven times from 10 ms to 1500 s of stimulation. Decrease of response amplitude during stimulation was approximately proportional to initial (at the start of stimulation) response amplitude. Therefore, even for low stimulus level (down to 20 dB above the baseline threshold) the response was never suppressed completely. The RFR amplitude decay that occurred during stimulation could be satisfactorily approximated by a combination of two exponents with time constants of 30-80 ms and 3.1-17.6 s. The role of adaptation in the described effects and the impact of noise on the acoustic orientation of odontocetes are discussed.

Communication masking in marine mammals: A review and research strategy

Underwater noise, whether of natural or anthropogenic origin, has the ability to interfere with the way in which marine mammals receive acoustic signals (i.e., for communication, social interaction, foraging, navigation, etc.). This phenomenon, termed auditory masking, has been well studied in humans and terrestrial vertebrates (in particular birds), but less so in marine mammals. Anthropogenic underwater noise seems to be increasing in parts of the world's oceans and concerns about associated bioacoustic effects, including masking, are growing. In this article, we review our understanding of masking in marine mammals, summarise data on marine mammal hearing as they relate to masking (including audiograms, critical ratios, critical bandwidths, and auditory integration times), discuss masking release processes of receivers (including comodulation masking release and spatial release from masking) and anti-masking strategies of signalers (e.g. Lombard effect), and set a research framework for improved assessment of potential masking in marine mammals.

Frequency Tuning of Hearing in the Beluga Whale

Data on frequency tuning in odontocetes are contradictory: different authors have reported filter qualities from 2 to almost 50. In this study, frequency tuning was measured in a beluga whale (Delphinapterus leucas) using a rippled-noise test stimulus in conjunction with the auditory evoked potential (AEP) technique. The response to ripple reversions was considered to indicate resolvability of the ripple pattern. The limit of ripple-pattern resolution ranged from 20 to 32 ripples per octave (rpo). A model of interaction of the ripple spectrum with frequency-tuned filters suggests that this resolution limit requires a filter quality of 29-46.

Spectrum pattern resolution after noise exposure in a beluga whale, Delphinapterus leucas: Evoked potential study

Temporary threshold shift (TTS) and the discrimination of spectrum patterns after fatiguing noise exposure (170 dB re 1 μPa, 10 min duration) was investigated in a beluga whale, Delphinapterus leucas, using the evoked potential technique. Thresholds were measured using rhythmic (1000/s) pip trains of varying levels and recording the rhythmic evoked responses. Discrimination of spectrum patterns was investigated using rippled-spectrum test stimuli of various levels and ripple densities, recording the rhythmic evoked responses to ripple phase reversals. Before noise exposure, the greatest responses to rippled-spectrum probes were evoked by stimuli with a low ripple density with a decrease in the response magnitude occurring with an increasing ripple density. After noise exposure, both a TTS and a reduction of the responses to rippled-spectrum probes appeared and recovered in parallel. The reduction of the responses to rippled-spectrum probes was maximal for high-magnitude responses at low ripple densities and was negligible for low-magnitude responses at high ripple densities. It is hypothesized that the impacts of fatiguing sounds are not limited by increased thresholds and decreased sensitivity results in reduced ability to discriminate fine spectral content with the greatest impact on the discrimination of spectrum content that may carry the most obvious information about stimulus properties.

Frequency tuning of hearing in the beluga whale: discrimination of rippled spectra

Frequency tuning was measured in the beluga whale (Delphinapterus leucas) using rippled-noise test stimuli in conjunction with an auditory evoked potential (AEP) technique. The test stimulus was a 2-octave-wide rippled noise with frequency-proportional ripple spacing. The rippled-noise signal contained either a single reversal or rhythmic (1-kHz rate) reversals of the ripple phase. Single or rhythmic phase reversals evoked, respectively, a single auditory brainstem response (ABR) or a rhythmic AEP sequence-the envelope following response (EFR). The response was considered as an indication of resolvability of the ripple pattern. The rhythmic phase-reversal test with EFR recording revealed higher resolution than the single phase-reversal test with single ABR recording. The limit of ripple-pattern resolution with the single phase-reversal test ranged from 17 ripples per octave (rpo) at 32 kHz to 24 rpo at 45 to 64 kHz; for the rhythmic phase-reversal test, the limit ranged from 20 to 32 rpo. An interaction model of a ripple spectrum with frequency-tuned filters suggests that the ripple-pattern resolution limit of 20 to 32 rpo requires a filter quality Q of 29 to 46. Possible causes of disagreement of these estimates with several previously published data are discussed.

Time and frequency metrics related to auditory masking of a 10 kHz tone in bottlenose dolphins (Tursiops truncatus)

Metrics related to the frequency spectrum of noise (e.g., critical ratios) are often used to describe and predict auditory masking. In this study, detection thresholds for a 10 kHz tone were measured in the presence of anthropogenic, natural, and synthesized noise. Time-domain and frequency-domain metrics were calculated for the different noise types, and regression models were used to determine the relationship between noise metrics and masked tonal thresholds. Statistical models suggested that detection thresholds, masked by a variety of noise types at a variety of noise levels, can be explained with metrics related to the spectral density of noise and the degree to which amplitude modulation is correlated across frequency regions of the noise. The results demonstrate the need to include time-domain metrics when describing and predicting auditory masking.

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 dolphin's 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.

Hearing in cetaceans: from natural history to experimental biology

Sound is a primary sensory cue for most marine mammals, and this is especially true for cetaceans. To passively and actively acquire information about their environment, cetaceans have some of the most derived ears of all mammals, capable of sophisticated, sensitive hearing and auditory processing. These capabilities have developed for survival in an underwater world where sound travels five times faster than in air, and where light is quickly attenuated and often limited at depth, at night, and in murky waters. Cetacean auditory evolution has capitalized on the ubiquity of sound cues and the efficiency of underwater acoustic communication. The sense of hearing is central to cetacean sensory ecology, enabling vital behaviours such as locating prey, detecting predators, identifying conspecifics, and navigating. Increasing levels of anthropogenic ocean noise appears to influence many of these activities. Here, we describe the historical progress of investigations on cetacean hearing, with a particular focus on odontocetes and recent advancements. While this broad topic has been studied for several centuries, new technologies in the past two decades have been leveraged to improve our understanding of a wide range of taxa, including some of the most elusive species. This chapter addresses topics including how sounds are received, what sounds are detected, hearing mechanisms for complex acoustic scenes, recent anatomical and physiological studies, the potential impacts of noise, and mysticete hearing. We conclude by identifying emerging research topics and areas which require greater focus.