No abstract

There is a notion that the dolphin’s spatial hearing operates similarly to that of humans, even though the dolphin’s ability to localize a source in the vertical plane is practically the same as in the horizontal one. Binaural phenomena are used to explain localization in the dolphin; however, there are no interaural differences in the vertical plane. Given the dolphin’s very short echolocation click and high time resolution, one would find it difficult to describe the dolphin’s hearing in terms of binaural phenomenon. The dolphin’s sonar system performs as a monaural rather than a binaural system [G. Zaslavskiy and V. Ryabov, in Marine Mammal Sensory System (1991)]. In fact, very directional sonar click transmission and reception seem to account well for the dolphin’s spatial hearing. In the present study, the way in which dolphins and fur seals respond to a sum of the signals from two sources will be discussed. There are different hearing phenomena in the human associated with the binaural perception of the sound coming from two different directions. Some of these phenomena were found in the fur seal but none of them was present in the dolphin.

Binaural phenomenon is known to be responsible for sound localization in humans. A similar mechanism is thought to provide the dolphin with the ability to localize targets and sound source. At the same time, the dolphin possesses quite good directional hearing, let alone very directional click transmission, which is absent in humans. The directional hearing in the dolphin, at least in the horizontal plane, appears to be consistent with two ears’ reception. Does it signify that the binaural phenomenon coexists in dolphins with directional hearing? As far as humans are concerned, the spatial hearing does not involve any substantial hearing directionality. The directionality seems to negate the main advantage of the binaural hearing, that is instantaneous determination of a sound source direction, and complicate the use of binaural cues. It was found that the lesser intensity of high-frequency clicks the smaller an angle between their sources should be for the dolphin to hear them simultaneously. The directivity pattern of the dolphin’s auditory system was measured as a dependence of the threshold angle between the sources on the click’s intensity. At high frequencies, the dolphin’s auditory system performed as a directional system rather than an omnidirectional binaural system known for humans.

The bottlenose dolphin auditory time resolution of around 300 µs assessed using auditory evoked potentials (AEP) methods is generally believed to be in full agreement with behavioral measurements. In this paper we reassess some behavioral results which are believed to support AEP methods in light of numerous behavioral experiments indicative of the bottlenose dolphin time resolution as high as 20-30 µs. When behavioral results are evaluated according to the time resolution definition as a threshold interval between identical acoustic events, they all point to the bottlenose dolphin time resolution much higher than the AEP method limit of around 300 µs. Physiologically assessed modulation rate transfer functions (MTF) are compared to a bottlenose dolphin's perception of periodicity of a gated noise. The bottlenose dolphin appeared capable of perceiving periodicity of noise envelope as high as 15-20 kHz. The auditory temporal analysis of brief signals in bottlenose dolphins seems to be inaccessible by AEP methods.

In order to measure range accuracy in the dolphin, a range difference discrimination experiment should be performed. However, when two targets are presented at different distances simultaneously, even at an angle as big as 40–60=B0, one dolphin’s click can cause echoes from both targets. As a result, the targets can be distinguished by the difference in the echo arrival time without measuring the target distance. The best way to prevent the dolphin from simultaneous targets insonifying by each click turned out to be successive submerging of the targets in water at different distances from the dolphin. Nevertheless, it proved to be impossible to force the dolphin to measure the distance from itself to the targets. Two dolphins solved the task by measuring the time interval between the target echo and echo from a reference object; that could be the pool walls or even a nylon net. There was no consistent change in the threshold range difference with the targets’ range, whereas it regularly decreased when the targets were moved closer to the reference object. The dolphins preferred to measure the relative target position even if the reference object was at a much greater distance than the targets.