Spatial release from masking in a bottlenose dolphin Tursiops truncatus

Abstract

The impact of maskers on the receiving beam of a bottlenose dolphin, Tursiops truncatus, was investigated using the auditory evoked potential (AEP) method. The test signal was a train of tone pips with a 64 kHz carrier frequency. The stimulus produced AEPs as a sequence of waves replicating the pip rate: the rate following response (RFR). The masker was band-limited noise, with a passband of 45 to 90 kHz and a level of 105 dB re 1 μPa. Masker azimuths were at 0°, ±30°, ±60°, and ±90° relative to the head midline. The receiving beam was evaluated in terms of the RFR threshold dependence on the signal azimuth. The masked thresholds were higher than the baseline thresholds, which appeared mostly as a shift rather than a deformation in the receiving beam. The largest threshold shift appeared when the masker source was located in the most sensitive direction (zero azimuth); at lateral masker source positions, the threshold shift decreased. When the masker source was not at the head midline, the masked thresholds were higher at signal positions ipsilateral to the masker source than at positions contralateral to the masker source. The largest asymmetry was observed at the 30° masker azimuth in conjunction with the ±30° and ±120° signal azimuths; the asymmetries were 5.6 and 8.1 dB, respectively. This masking asymmetry was lower than expected from the previously found interaural intensity difference, which may be explained by the conflict between the test signal and the masker when it appeared at a binaural level of the auditory system.

Mechanisms of auditory masking in marine mammals

“…In real ocean environments, noise sources are typically spatially separated from biologically relevant signals. Under such conditions, spatial release from masking (SRM) can occur (e.g., Au and Moore, 1984 ; Holt and Schusterman, 2007 ; Popov et al 2020 ). In humans, where research on this topic is extensive, the position of sound sources relative to a listener can act as one of the most salient cues to segregate multiple sounds in a complex auditory scene (Bregman 1990 ).…”

“…For the dolphin in Au and Moore ( 1984 ), the noise source was always fixed at 0° azimuth, and for the pinnipeds in Holt and Schusterman ( 2007 ), the tonal source was always fixed at 0° azimuth. Although the focus in the current paper is on behavioral studies, it is important to note that an evoked potential study did measure SRM where the locations of both the signal and noise varied on the horizontal axis (Popov et al 2020 ). In this study, the signal was restricted to one frequency, a 64-kHz tone pip, and the masker was band-pass noise between 40 and 90 kHz.…”

Sperm whales exhibit variation in echolocation tactics with depth and sea state but not naval sonar exposures

“…Besides acoustic and auditory factors, the extent of masking also depends upon sender/receiver location and orientation (Erbe et al, 2016). For example, spatial release from auditory masking can occur when the listener spatially separates the signal and masker by altering its orientation (Bain & Dahlheim, 1994; Holt & Schusterman, 2007; Popov et al, 2020) or increasing its distance (Erbe et al, 2016) relative to the masking source. Importantly, many antimasking tactics require changes in behavior (e.g., call characteristics, animal orientation, and dive depth) and are therefore possible to measure, e.g., using passive acoustic monitoring or animal‐attached sound and movement recording tags.…”