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    • br The subject of the study materials

    2018-10-24


    The subject of the study, materials and experimental procedures The experiments were carried out at the T.I. Vyazemsky Karadag scientific station – Nature Reserve of RAS (Feodosia) in an enclosed concrete 27m×9m×4.5 m-sized pool. An adult Black Sea bottlenose dolphin (Tursiops truncatus) who had not previously participated in acoustic experiments acted as the experimental subject. We employed the technique of instrumental conditioned reflexes with food reinforcement using the ‘go/no-go’ paradigm [14]. On a signal from the trainer (position 5 in Fig. 1), the dolphin was trained to swim to the walkway, where the trainer placed (or did not place) a sound-shielding hood on the rostrum of the animal in the MF area. After that, the dolphin was signaled to swim to start arm 2 suspended at a depth of 1m, and remained at that depth with almost no movement (quasi-stationary), touching the tip of the start arm with its rostrum. After a few seconds the experimenter turned on the auditory tetracycline hcl (shown in Fig. 2) that the dolphin found (or did not). If the stimulus was produced and the dolphin found it, the dolphin left the starting position (go) and pressed its rostrum against signal arm 4 (located near the surface of the water), thus indicating that it had found the stimulus. If the stimulus was not presented, the dolphin remained at the starting position until receiving a signal from the trainer (no-go). In these cases, the dolphin received food reinforcement for correctly solving the problem. If necessary, the trainer removed the shielding hood from the dolphin\'s rostrum for that purpose every time. The cases when a stimulus was produced but the dolphin did not approach the signal arm, or there was no stimulus but the dolphin approached the signal arm (a false alarm) were regarded as errors and were not rewarded. Each threshold value of the sound pressure level (SPL) of the stimulus was determined by at least three sessions. Each session consisted of about 60 tests. Typically, the stimuli with the SPL significantly exceeding the threshold were produced in the first tests of each session. Throughout the session, the SPL of the stimuli gradually decreased (with a step of 3dB) to the threshold values (50% of the level of correct responses). The SPL of a stimulus then increased by 4–6dB, up to the correct reactions of the animal, and the threshold (a decrease in the SPL of the stimulus) was again approached with a step of 3dB steps. The threshold SPL value of the stimuli corresponding to 75% of correct answers was calculated by averaging between the maximum subthreshold and the minimum superthreshold values of their SPL. The latter were obtained by repeatedly crossing the threshold. To reduce the effect of interference of the direct signal and of those reflected by the pool on the results of the experiments, short broadband acoustic pulse signals from a special transmitter (position 3 in Fig. 1) located at a distance of 2m from the start arm, and at a depth of 1m were used as stimuli. The distance between transducer 3 and the wall of the pool was 3m. Stimuli with the energy peaks at frequencies of 30, 60 and 100kHz were formed through a reaction of spherical acoustic transducers made of piezoelectric ceramics, 50, 30 and 20mm in diameter, to rectangular pulses of 17, 9 and 5µs, respectively. A transducer with a diameter of 50mm was used for obtaining pulses with energy peaks at frequencies of 8 or 16kHz. The transducer was excited by a rectangular pulse with the duration of 56 or 36 microseconds through an octave band filter with the central frequency of 8 or 16kHz, respectively. Examples of the stimuli applied are shown in Fig. 2. The waveforms were detected with the emitter and the receiver submerged to a depth of 1m, at a distance of 1m away from each other. The amplitude of the rectangular pulse exciting the emitter was 20V. A calibrated 8103 hydrophone with a precision 2650 amplifier (40dB gain) by the B&K company was used as a receiver. The duration of the stimuli did not exceed three periods of the corresponding frequency of the peak acoustic pulse energy. In this case, the reflections of the stimulus from the pool walls and the water surface did not overlap with the direct stimulus, as they arrived with a sufficient time delay and were significantly attenuated compared to it. For the threshold SPL values of the stimulus, the reflection levels were below the auditory pulse detection threshold of the dolphins, which allowed to perform measurements without special sound-absorbing coatings.