Vladimir V. Popov

Forward masking was investigated by the auditory evoked potentials (AEP) method in a bottlenose dolphin Tursiops truncatus using stimulation by two successive acoustic pulses (the masker and test) projected from spatially separated sources. The positions of the two sound sources either coincided with or were symmetrical relative to the head axis at azimuths from 0 to ± 90°. AEPs were recorded either from the vertex or from the lateral head surface next to the auditory meatus. In the last case, the test source was ipsilateral to the recording side, whereas the masker source was either ipsi- or contralateral. For lateral recording, AEP release from masking (recovery) was slower for the ipsi- than for the contralateral masker source position. For vertex recording, AEP recovery was equal both for the coinciding positions of the masker and test sources and for their symmetrical positions relative to the head axis. The data indicate that at higher levels of the auditory system of the dolphin, binaural convergence makes the forward masking nearly equal for ipsi- and contralateral positions of the masker and test.

Forward masking was investigated by the auditory evoked potentials (AEP) method in a bottlenose dolphin Tursiops truncatus using stimulation by two successive acoustic pulses (the masker and test) projected from spatially separated sources. The positions of the two sound sources either coincided with or were symmetrical relative to the head axis at azimuths from 0 to ±90°. AEPs were recorded either from the vertex or from the lateral head surface next to the auditory meatus. In the last case, the test source was ipsilateral to the recording side, whereas the masker source was either ipsi- or contralateral. For lateral recording, AEP release from masking (recovery) was slower for the ipsi- than for the contralateral masker source position. For vertex recording, AEP recovery was equal both for the coinciding positions of the masker and test sources and for their symmetrical positions relative to the head axis. The data indicate that at higher levels of the auditory system of the dolphin, binaural convergence makes the forward masking nearly equal for ipsi- and contralateral positions of the masker and test.

Forward masking was investigated by the auditory evoked potentials (AEP) method in a bottlenose dolphin Tursiops truncatus using stimulation by two successive acoustic pulses (the masker and test) projected from spatially separated sources. The positions of the two sound sources either coincided with or were symmetrical relative to the head axis at azimuths from 0 to ±90°. AEPs were recorded either from the vertex or from the lateral head surface next to the auditory meatus. In the last case, the test source was ipsilateral to the recording side, whereas the masker source was either ipsi- or contralateral. For lateral recording, AEP release from masking (recovery) was slower for the ipsi-than for the contralateral masker source position. For vertex recording, AEP recovery was equal both for the coinciding positions of the masker and test sources and for their symmetrical positions relative to the head axis. The data indicate that at higher levels of the auditory system of the-dolphin, binaural convergence makes the forward masking nearly equal for ipsi- and contralateral positions of the masker and test.

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.

Auditory adaptation manifests as temporary change in the hearing sensitivity under influence of sound stimuli, and little attention has been paid to hearing adaptation in cetaceans. In a Yangtze finless porpoise Neophocaena asiaeorientalis (Cetacea, Phocoenidae), effects of long-lasting sound stimuli (pip trains) on auditory evoked potentials (AEP) were investigated. The stimuli were rhythmic tone pips of either 90 or 128 kHz frequency. Stimulus levels varied from 80 to 120 dB re 1 μPa. During stimulation, the stimulus level was either kept constant the stimulus level changed up and down by 20 or 40 dB every 1000 ms. A stimulus level increase produced quick response amplitude increase with subsequent slower exponential decay; the exponent time constant was 59.2 ± 9.7 ms. A stimulus level decrease produced quick response amplitude suppression with subsequent exponent recovery; the exponent time constant was 153.9 ± 15.9 ms. The response variations were assessed as manifestations of adaptation processes. The finless porpoise is capable of quickly adjusting its sensitivity to the current auditory scene. This flexibility allows for quick sensitivity recovery after exposure to loud noises and is important for the effective use of auditory orientation. Abbreviations: RFR: rate-following response; AEP: auditory evoked potentials; A/D: analogue-to-digital (A/D); ERD: equivalent rectangular duration

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.

The effects of adapting sounds (pip trains or pure tones) on auditory evoked potentials (the rate following response, RFR) were investigated in a beluga whale. During RFR acquisition, adapting signals lasting 128 ms each were alternated with test signals lasting 16 ms each; the test signal levels varied randomly. Adapting signals were trains of cosine-enveloped tone pips or pure tones. Pip rate varied with the envelope cosine cycle maintained at 0.125 of pip intervals and the cosine rise–fall time maintained at 0.0625 of pip intervals. Adapting signals shifted the amplitude-level function upward compared to the baseline (no adapting signal) function. The higher the adapting signal level was, the bigger the shift in the amplitude-level function was. The slower the pips were in the adapting signal, the smaller the adaptation effect was. A train of pips with a 0.0625-ms rise–fall time and 125 dB SPL shifted the function by 35–40 dB, whereas a train of pips with a 1-ms rise–fall time or a pure tone with the same SPL shifted the function by approximately 15 dB. The difference between the “fast” and “slow” adapting signals is supposed to be associated with their abilities to stimulate the auditory system in odontocetes.

The effects of prolonged 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 64 kHz frequency at levels from 80 to 140 dB re 1 μPa. During stimulation, every 1000 ms stimulus level either was kept constant (the steady-state stimulation) or changed up/down by 20 or 40 dB. With such stimulus presentation manner, RFR amplitude varied as follows. (i) After a stimulus level increase, the response amplitude increased quickly and then decayed slowly. The more the level increased, the higher the response amplitude increased. (ii) After a stimulus level decrease, the response amplitude was suppressed and then recovered slowly. The more the level decreased, the stronger was the response suppression. (iii) At the end of the 1000 ms window, the response amplitude approached, but did not reach, the amplitude characteristic of the steady-state stimulation. As a result, both after a sound level increase and decrease, the responses were almost stabilized during an analysis time as short as 1 s. This stabilization is attributed to an adaptation process. RFR decay after initial increase could be approximated by an exponent with a time constant of 59.4 ±1.8 (standard error) ms; RFR recovery after initial decrease could be approximated by an exponent with a time constant of 139.2 ±9.9 ms.

Hearing thresholds were measured and audiograms were obtained in seven belugas, three males and four females, provisionally 2 to 7 years old. The measurements were performed using a transducer located 1 m in front of the head. The stimuli were tone pip trains of carrier frequencies ranging from 11.2 to 128 kHz with a pip rate of 1 kHz. Auditory evoked potentials (the rate following responses) were recorded from the head vertex. In majority of the subjects, audiograms were similar to the typical odontocete audiograms with the lowest thresholds (from 53.7 to 56.4 dB re 1 μPa) at mid-frequency range (from 32 to 64 kHz) and a sharp thresholds rise (up to 79.5 dB re 1 μPa) at high frequencies (90–128 kHz). One beluga (female, 6–7 years old) featured an asymmetric hearing loss within a frequency range from 22.5 to 54 kHz. The reason for the loss is the subject for not defined. The evoked potential audiograms should be included into base screening of odontocete subjects involved in any kind of hearing research. Work supported by the Russian Science Foundation (project # 17-74-20107), the Russian Foundation for Basic Research and The Russian Geographic Society.]

Hearing thresholds were measured and audiograms were obtained in seven belugas, two males and five females, provisionally 2 to 8 years old. The measurements were performed using a transducer located 1 m in front of the head. The stimuli were tone pip trains of carrier frequencies ranging from 11.2 to 128 kHz with a pip rate of 1 kHz. Auditory evoked potentials (the rate following responses) were recorded from the head vertex. In majority of the subjects, audiograms were similar to the typical odontocete audiograms with the lowest thresholds (53.4±2.3 dB re 1 μPa) at mid-frequency range (from 32 to 76 kHz) and a threshold increase (up to 73.8±4.9 dB re 1 μPa) at high frequencies (90-128 kHz). One beluga (female, 6-7 years old) featured an asymmetric hearing loss within a frequency range from 22.5 to 54 kHz. The reason for the loss is the subject for not defined. The evoked potential audiograms should be included into base screening of odontocete subjects involved in any kind of hearing research.