Abstract
ABSTRACTEcholocating animals reduce their output level and hearing sensitivity with decreasing echo delays, presumably to stabilize the perceived echo intensity during target approaches. In bats, this variation in hearing sensitivity is formed by a call-induced stapedial reflex that tapers off over time after the call. Here, we test the hypothesis that a similar mechanism exists in toothed whales by subjecting a trained harbour porpoise to a series of double sound pulses varying in delay and frequency, while measuring the magnitudes of the evoked auditory brainstem responses (ABRs). We find that the recovery of the ABR to the second pulse is frequency dependent, and that a stapedial reflex therefore cannot account for the reduced hearing sensitivity at short pulse delays. We propose that toothed whale auditory time-varying gain control during echolocation is not enabled by the middle ear as in bats, but rather by frequency-dependent mechanisms such as forward masking and perhaps higher-order control of efferent feedback to the outer hair cells.
Highlights
Echolocating bats and toothed whales navigate and hunt by emission of powerful sound pulses and subsequent auditory processing of weak returning echoes milliseconds later (Griffin, 1958; Au and Simmons, 2007; Madsen and Surlykke, 2013)
Auditory brainstem response (ABR) studies conducted by subjecting toothed whales to two sound pulses with varying delays have revealed a significant reduction in hearing sensitivity for the second pulse, called the test pulse, following the first pulse, the conditioning pulse (Popov and Supin, 1990; Supin and Popov, 1995; Popov et al, 2001; Supin et al, 2007; Supin and Popov 2015)
We test the hypothesis of a stapedial reflex on a trained porpoise using artificial double pulses, and we show that the auditory brainstem responses (ABRs) resulting from the test pulse is masked only if it has the same frequency as the conditioning pulse
Summary
Echolocating bats and toothed whales navigate and hunt by emission of powerful sound pulses and subsequent auditory processing of weak returning echoes milliseconds later (Griffin, 1958; Au and Simmons, 2007; Madsen and Surlykke, 2013). A similar auditory time varying gain control has been shown for toothed whales both for their own echolocation clicks and echoes, and in experiments with artificial double pulses simulating clicks and echoes (Supin et al, 2005, 2010; Li et al, 2011; Linnenschmidt et al, 2012; Finneran et al, 2013).
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