Representing multiple discrimination cues in a computational model of the bottlenose dolphin auditory system

Abstract

A computational model of the dolphin auditory system was developed to describe how multiple discrimination cues may be represented and employed during echolocation discrimination tasks. The model consisted of a bank of gammatone filters followed by half-wave rectification and low pass filtering. The output of the model resembles a spectrogram; however, the model reflects temporal and spectral resolving properties of the dolphin auditory system. Model outputs were organized to represent discrimination cues related to spectral, temporal and intensity information. Two empirical experiments, a phase discrimination experiment [Johnson et al., Animal Sonar Processes and Performance (Plenum, New York, 1988)] and a cylinder wall thickness discrimination tasks [Au and Pawolski, J. Comp. Physiol. A 170, 41-47 (1992)] were then simulated. Model performance was compared to dolphin performance. Although multiple discrimination cues were potentially available to the dolphin, simulation results suggest temporal information was used in the former experiment and spectral information in the latter. This model's representation of sound provides a more accurate approximation to what the dolphin may be hearing compared to conventional spectrograms, time-amplitude, or spectral representations.

Comodulation masking release in bottlenose dolphins (Tursiops truncatus)

“…However, this explanation does not take into account the band widening masking release effect found in experiment I. A more comprehensive explanation for these results is that at higher modulation rates, temporal information will tend to degrade or "smear" due to the low-pass nature of the dolphin's temporal resolution ͑Moore et Dolphin et al, 1995;Branstetter et al, 2007͒. Across-channel comparisons based on increasingly degraded envelope information will result in increasingly degraded signal detection.…”

Decreased echolocation performance following high-frequency hearing loss in the false killer whale (Pseudorca crassidens)

“…The subject probably relies on multiple cues at many frequencies from the target echo to conduct discrimination. Although there may be many cues that odontocetes use for target discrimination (Branstetter et al, 2007;Gaunaurd et al, 1998;Muller et al, 2008), the use of high frequencies is very likely to be the most important, and the link between high-frequency hearing loss and a reduction in discrimination performance cannot be ignored. These data strongly suggest that the high-frequency component of echoes provides a great deal of information for fine-scale discrimination Part of the explanation for a reduction in discrimination abilities may also be attributable to the second part of the odontocete sonar system: the click production mechanism.…”

“…Much of this effort has been based on the demonstrated ability of odontocetes to differentiate small differences between arbitrarily constructed echolocation targets, including differences in target size, shape and materials from which they are constructed (Nachtigall, 1980). The full suite of cues that odontocetes use to discriminate targets is unknown, but it is thought that they may use small differences in the complex structure of target echoes to differentiate targets (Branstetter et al, 2007;Gaunaurd et al, 1998;Muller et al, 2008). The ability of cetaceans to differentiate and recognize the acoustic characteristics of objects using echolocation has an obvious biological benefit.…”

Auditory masking patterns in bottlenose dolphins (Tursiops truncatus) with natural, anthropogenic, and synthesized noise

“…However, if the signal and noise are examined in the time domain in addition to the spectral domain, the mechanism is less elusive. A simple model of the mammalian auditory periphery system consists of a bank of auditory filters (Finneran et al, 2002) followed by half-wave rectification and lowpass filtering (Viemeister, 1979;Berg, 1996;Branstetter et al, 2007). This model is often referred to as a leaky integrator or envelope detector (Viemeister, 1979).…”

“…When detecting a tonal signal in noise, the noise within a band centered on the signal has the most masking effect (Fletcher, 1940). In theory, an energy detector at the output of an auditory filter registers an increase in energy when a signal is present (Branstetter et al, 2007). If the increase in energy in that filter is sufficient, a decision is made that a signal is present.…”