Comodulation masking release in bottlenose dolphins (Tursiops truncatus)

Abstract

The acoustic environment of the bottlenose dolphin often consists of noise where energy across frequency regions is coherently modulated in time (e.g., ambient noise from snapping shrimp). However, most masking studies with dolphins have employed random Gaussian noise for estimating patterns of masked thresholds. The current study demonstrates a pattern of masking where temporally fluctuating comodulated noise produces lower masked thresholds (up to a 17 dB difference) compared to Gaussian noise of the same spectral density level. Noise possessing wide bandwidths, low temporal modulation rates, and across-frequency temporal envelope coherency resulted in lower masked thresholds, a phenomenon known as comodulation masking release. The results are consistent with a model where dolphins compare temporal envelope information across auditory filters to aid in signal detection. Furthermore, results suggest conventional models of masking derived from experiments using random Gaussian noise may not generalize well to environmental noise that dolphins actually encounter.

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

“…Recent marine mammal masking studies have demonstrated that auditory masking is not limited to noise within a single auditory filter if the noise is non-G (Branstetter and Finneran, 2008;Erbe, 2008;Trickey et al, 2011). These findings are consistent with previous studies with humans (Hall et al, 1984) and a few other animal listeners (Klump et al, 2001;Pressnitzer et al, 2001).…”

Estimated communication range and energetic cost of bottlenose dolphin whistles in a tropical habitat

“…However, this model does not apply well to non-Gaussian broadband noise such as that produced by snapping shrimp (Hall et al, 1984). Recent psychophysical experiments have demonstrated masked thresholds in comodulated noise well below masked thresholds in Gaussian noise of equivalent spectral noise density and bandwidth for synthetic maskers (Branstetter and Finneran, 2008). The only experimental study with actual environmental noise profiles (including snapping shrimp noise) seems to indicate a 6 dB threshold decrease for 10 kHz tones (Trickey et al, 2010).…”

Signal-to-noise ratio for source determination and for a comodulated masker in goldfish, Carassius auratus

“…One of the most studied release-from-masking effects is comodulation masking release (CMR) in which temporally fluctuating maskers have been shown to be less effective in masking than unmodulated Gaussian noise at the same level (e.g., Hall et al, 1984). It has been argued that if this CMR effect exists in fishes, the effective signal-to-noise ratio would be smaller than that measured in the usual masked signal detection experiments conducted on fishes to the extent that typical anthropogenic and other noise may be comodulated (e.g., Branstetter and Finneran, 2008). One component of the experiments reported here was aimed at determining the relative masking effectiveness of unmodulated and comodulated noise in goldfish and if it is reduced, how much masking release takes place.…”

“…In general, the predicted masking effectiveness of these more natural maskers would be less than that of white noise maskers if the more natural maskers were effectively comodulated, either due to source characteristics or due to complex transmission pathways. The temporal modulations in environmental noise have generally not been noted, but at least one report has described many of these noises as comodulated (Branstetter and Finneran, 2008) and having a large effect on masking in dolphins (up to 17 dB). Klump and Langemann (1995) have noted, in addition, some of the factors that may cause amplitude fluctuations in the natural terrestrial soundscape.…”