title: Time-varying auditory gain control in response to double-pulse stimuli in harbour porpoises is not mediated by a stapedial reflex.
abstract: Echolocating 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.
authors.affiliation: Zoophysiology, Bioscience, Aarhus University, C. F. Moellers Allé 3, Aarhus C 8000, Denmark.
authors.forename: Asger Emil Munch
authors.initials: AEM
authors.lastname: Schrøder
authors.affiliation: Zoophysiology, Bioscience, Aarhus University, C. F. Moellers Allé 3, Aarhus C 8000, Denmark.
authors.forename: Kristian
authors.initials: K
authors.lastname: Beedholm
authors.affiliation: Zoophysiology, Bioscience, Aarhus University, C. F. Moellers Allé 3, Aarhus C 8000, Denmark peter.madsen@bios.au.dk.|Murdoch University Cetacean Research Unit, Murdoch University, South Street, Murdoch, Western Australia 6150, Australia.
authors.forename: Peter Teglberg
authors.identifier: http://orcid.org/0000-0002-5208-5259
authors.initials: PT
authors.lastname: Madsen
keywords: Automatic gain control; Echolocation; Hearing; Masking; Stapedial reflex; Toothed whale