Forward masking in a bottlenose dolphin Tursiops truncatus: Dependence on azimuthal positions of the masker and test sources

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.


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The unique hearing abilities of odontocetes (toothed whales, dolphins, and porpoises) 28 have been investigated in many respects [1,2]. Nonetheless, several mechanisms responsible for 29 these unique abilities have not been elucidated, particularly concerning spatial hearing and its 30 relation to temporal analysis in odontocetes. 31 As a result of behavioral and auditory evoked potential (AEP) investigations, a number of 32 characteristics of spatial hearing determined in odontocetes are known. In particular, the acuity 33 of binaural [3-8] and monaural [9,10] receiving beams, the spatial discrimination capability in 34 terms of minimum audible angle [11,12], and the value of interaural intensity difference [9,10,35 13] were determined. These data indicate the capability of odontocetes to acutely localize sound 36 sources. 37 However, real acoustic scenes are typically characterized not only by a variety of sound 38 sources but also by their various temporal interrelations. Therefore, the ability to extract a certain 39 sound signal from numerous background sounds depends on both the spatial distribution of 40 sound sources and their temporal distribution, as well as on the ability of the auditory system to 41 separate sounds based on their spatial and temporal characteristics. 42 In odontocetes, the AEP of the auditory system to paired sound pulses with varying 43 delays between them has been investigated in several studies [14][15][16][17][18][19]. At short delays, the To date, all the data on responses to paired stimuli have been investigated in odontocetes 51 when both stimuli were projected from one and the same source position. Regularly, it was at a 52 position along the head midline. These data did not provide information on the ability to respond 53 to signals after a masker projected from another position. In the present study, we attempt to 54 compensate for this deficiency by studying AEPs to paired stimuli when the azimuthal positions 55 of the first (masker) and the second (test) stimuli were different. To achieve this goal, AEPs were 56 recorded at various combinations of (i) azimuthal masker and test source positions, (ii) masker-57 to-test intervals, and (iii) AEP recording positions (vertex or lateral). A comparison of vertex and 58 lateral AEP recordings was undertaken to discriminate between monaural and binaural processes 59 in the detection of the masked test signals.  After two months of experimentation, she was returned back to the dolphinarium.

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At the Utrish Marine Station, the animal was kept in a round plastic tank 6 m in diameter, 70 1.7 m deep, filled with sea water. During the experiments, the water level in the tank was 71 lowered down to 0.4 m. The animal was laid on a stretcher in such a manner that its dorsal head 72 surface was above the water surface and the rostrum tip was in the center of the tank (Fig 1). The 73 animal was kept in such a position during the experiment, which regularly lasted 2 to 3 h. After 5 74 the experiment, the animal was released, and the water depth in the pool was restored to its 75 normal level of 1.7 m.

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The study design was approved by The Ethics Committee of the Institute of Ecology and 77 Evolution of the Russian Academy of Sciences (№ 31, 30.04.2019).  The sound stimuli were tone pips of 64 kHz carriers. The pip envelope was one cycle of a 85 raised cosine function containing 6 cycles of the carrier (Fig 2a, 1). The stimuli were digitally 86 generated by a standard computer at an update rate of 500 kHz using a custom-made program    To minimize sound reflections from the tank walls, the area around the animal's head and 106 transducers was surrounded by a circular fence of sound-absorbing material (rubber with closed 107 air cavities) (see Fig 1). The fence covered the whole water depth. The radius of the fence was

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The waveform of the acoustic signals that were picked up by the monitoring hydrophone 115 did not exactly reproduce the electronic signal that activated the sound-emitting transducers (Fig   116   2). The acoustic signal was longer than the electronic signal. An initial component of a peak 117 level of 135 dB re. 1 µPa (5.6 Pa acoustic pressure) followed by a few smaller (-6 to -10 dB re. 118 the initial peak) components. During 0.5 ms after the peak, the acoustic pressure fell to 119 approximately -20 dB re. the signal peak. For vertex AEP recording, one of the electrodes was fastened at the dorsal head surface, 126 6-9 cm behind the blowhole. This point has been found to feature the highest amplitude of a 127 noninvasively recorded fast AEP known as the auditory brainstem response, ABR [14]. This 128 electrode was considered active for ABR recording. The other electrode was fastened at the 129 dorsal fin where the ABR amplitude was negligible. This electrode was considered reference.

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Both the electrodes were above the water surface.

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For lateral AEP recording, the active electrode was fastened underwater at the lateral 132 head surface, next to the auditory meatus. For this electrode, the embedding suction cup served 133 both as electrode fastening and as insulation from low-impedance sea water. The reference 134 electrode was positioned at the dorsal fin above the water surface.

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The electrodes were connected to the input of a brain-potential amplifier LP511 (Grass 136 Technologies) that provided amplification by 80 dB within a frequency range of 0.3 to 3 kHz.

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This frequency range was chosen as covering the frequency band of ABR but minimally 138 transferring noise outside this band.

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Amplified EEG signals were analog-to-digital converted and processed by the same 140 acquisition board NI USB-6251 that was used for signal generation. The sampling rate for 141 conversion was 40 kHz. To process the brain potential signals, a custom-made program based on 142 LabVIEW (National Instrument) software was used. The forward-masking processes were investigated by recording AEPs to two successively 146 presented stimuli, the first of which was considered a masker (conditioning stimulus), and the 147 second was considered a test. The peak levels of both the masker and test stimuli were equal, 148 specifically, 135 dB re. 1 µPa.

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To separate the ABRs to the two stimuli, a subtraction procedure was used: the response 150 to the masker alone was subtracted from the response to the pair of stimuli. This procedure 8 151 requires the responses to the masker to be equal in records obtained with the pairs of stimuli 152 (masker + test) and masker alone. To make this equality as precise as possible, the masker + test 153 pairs and maskers only were presented in an interleaving manner (Fig 3): the masker + test, 154 (stimuli 1 and 2 in Fig 3), the masker only (stimulus 3 in Fig 3), masker + test again, etc. With 155 this manner of presentation, responses to the masker in pairs and maskers only were equally 156 subjected to any long-term variation in hearing sensitivity that might occur from long-term 157 hearing adaptation.  The stimuli were presented at a rate of 10/s. For extraction of AEPs from brain-potential 165 noise, 1000 responses to masker + test pairs and 1000 responses to maskers were collected. 166 Thus, each recording trial lasted 200 s.

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AEPs were extracted from brain-potential noise by coherent averaging. AEPs to maskers 168 + test pairs and AEPs to maskers were collected and averaged separately (epochs 4 and 5 in Fig   169   3, respectively). The averaged AEP to the test stimulus was extracted by point-to-point 170 subtracting the averaged response to the masker from the averaged response to the masker + test.  The AEP amplitude was assessed as the largest positive-to-negative span within a 2.5-ms 176 window where the response was expected, specifically, from 2.5 to 5 ms after stimulus. A record 9 177 was considered response-present when this span was 3.5 times as high as the root-mean-square  (Fig 4a). With the use of the criterion described above, this component was taken as   Fig 4c). With the use of the criterion described 217 above, this component was taken as the AEP amplitude. with masker-test delays of 1 ms (a) and 5 ms (b). At a shorter delay of 1 ms (Fig 5a), responses 223 to the two stimuli overlapped one another (a, 1); however, the subtraction procedure extracted a 224 masker-provoked response (a, 2) and a test response (a, 3). At a longer delay of 5 ms (Fig 5b), 225 responses to the masker and test did not overlap (record b, 1); the record b, 2 demonstrates the 11 226 non-delayed masker response and the absence of a delayed test response, whereas the record b, 3 227 demonstrates the delayed test response and the absence of a nondelayed masker response.    The records illustrate a stronger forward-masking effect for the ipsilateral masker source  4b). The shortest delay used between the masker and test was 0.5 ms because at shorter 260 delays, the masker "tail" could overlap the test, thus producing simultaneous masking. At a delay 261 of 0.5 ms or longer, the level of the masker was not higher than -20 dB re. the signal peak (see  were averaged (Fig 8). Records were obtained when the masker and test sources were located 283 symmetrically at opposite sides (a) and when they were at one and the same azimuth, in close 284 vicinity to one another (b). In the presented case, measurements were performed with a test 285 source positioned at azimuths of +30° and -30°; records were averaged.  opposite sides relative to the head axis (Fig 8a) and for both stimuli emitted from one and the 296 same azimuth (Fig 8b).   The N3.7-P4.8 component was picked up from the vertex point that was symmetrical 327 relative to both the right and left ears. Therefore, this component reflected activity provoked from any ear. Its latency was longer than that of the monaural component; this allows us to