Evgeniya V. Sysueva

Short-latency auditory evoked potentials (AEPs) to paired sound pulses (the leading stimulus (masker) and the delayed stimulus (test)) were recorded non-invasively in a bottlenose dolphin Tursiops truncatus. The stimuli were played through transducers contacting the left and right acoustic windows at the lower jaw. Two types of stimulation were used: monaurally (the both stimuli played through one and the same transducer) and dichotically (the stimuli played through different transducers, contacting the left and right acoustic window). Thе masker and test stimuli were equal in level and duration characteristics. The inter-stimulus delay varied from 0.15 to 10 ms. At the monaural stimulation, the suppression of the test stimulus was constant at interstimulus intervals from 0.15 to 0.5 ms; at longer intervals, the test response recovered. At the dichotic stimulation, the deepest suppression of the test response appeared at an interval of 0.5 ms; the test response recovered at both shorter and longer intervals. The complete recovery appeared at intervals as short as 0.15 ms and as long as 2 ms. Implications of the found regularities for the preceding effect and biosonar is discussed. [Work supported by RSF (project No. 22-25-00025).]

Short-latency auditory-evoked potentials (AEPs) were recorded non-invasively in the bottlenose dolphin Tursiops truncatus. The stimuli were two sound clicks that were played either monaurally (both clicks to one and the same acoustic window) or dichotically (the leading stimulus (masker) to one acoustic window and the delayed stimulus (test) to the other window). The ratio of the levels of the two stimuli was 0, 10, or 20 dB (at 10 and 20 dB, the leading stimulus was of a higher level). The inter-stimulus intervals (ISIs) varied from 0.15 to 10 ms. The test response magnitude was assessed by correlation analysis as a percentage of the control (non-masked) response. At monaural stimulation, the test response was of a constant magnitude (5–6% of the control) at ISIs of 0.15–0.3 ms and recovered at longer ISIs. At dichotic stimulation, the deepest suppression of the test response occurred at ISIs of 0.5–0.7 ms. The response was slightly suppressed at short ISIs (0.15–0.3 ms) and recovered at ISIs longer than 0.5–0.7 ms. The relation of parameters of the forward masking to echolocation in dolphins is discussed.

Short-latency auditory brainstem evoked potentials (ABR) to paired sound pulses (the conditioning and test stimuli) were recorded non-invasively in a bottlenose dolphin Tursiops truncatus. The stimuli were played through transducers contacting the left and right acoustic windows at the lower jaw. Two manners of stimulation were used: monaural (the both stimuli played through one and the same transducer) and dichotic (the conditioning and test stimuli played through different transducers, contacting the left and right acoustic window). Thе conditioning and test stimuli were equal in level and duration. The inter-stimulus delay varied from 0.15 to 10 ms. At the monaural stimulation, the suppression of the test stimulus was constant at interstimulus intervals from 0.15 to 0.5 ms; at longer intervals, the test response recovered. At the dichotic stimulation, the deepest suppression of the test response appeared at an interval of 0.5 ms; the test response recovered at both shorter and longer intervals. The complete recovery appeared at intervals as short as 0.15 ms and as long as 2 ms. Implications of the found regularities for the preceding effect and biosonar is discussed.

“Acoustic window” in the mandibular of dolphins: evaluation of the bone thickness “Акустическое окно” на нижней челюсти дельфина: оценка толщины кости

In a beluga whale, the positions of sound receiving areas on the head surface were determined by comparing the acoustic delays from different sound source positions. For this investigation, auditory evoked potentials (AEPs) in response to short tone pips were recorded. Latencies of the first AEP wave that presumably reflected the activity of the auditory nerve were measured at different sound source azimuths. For AEPs of equal amplitudes, the difference in AEP latencies was attributed to the difference in the acoustic delays. These delay differences were used to compute the azimuths of sound receiving points. Measurements were conducted at frequencies from 22.5 to 90 kHz in half-octave steps. At all stimulus frequencies, the receiving points were located 24–38 cm caudal of the melon tip, which is near a proximal part of the lower jaw. Thus, the results indicated the latero-mandibular acoustic window. Possible causes for not finding a lateral or ventro-mandibular window are discussed.

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.

The frequency resolving power of hearing (FRP) of the beluga whale Delphinapterus leucaswas studied as dependent on influence of lasting low-intensity sounds (of the ultrasonic range from –20 to +10...