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Evidence for Low-Frequency Sound Localization in the American Alligator (Alligator mississippiensis)

Lookup NU author(s): Cindy Carr, Dr Christian Brandt


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Initially, acoustical cues for localization are processed by filtering of sound waves by the head, generating a range of interaural time and level differences. Unlike frequency, which is mapped along the sensory epithelium, the location of a sound is primarily determined through computation of binaural information. In mammals and birds, these computations primarily occur at the level of brainstem nuclei. Using directional masking of auditory brainstem responses (ABRs), we show that responses of a crocodilian, Alligator mississippiensis, are strongly directional at 1000 Hz. Masked ABRs (mABRs), the response to a short broad-band stimulus minus the response to the same stimulus in the presence of a pure tone masker (Christensen-Dalsgaard et al., Biol Lett 2010), were elicited in awake restrained animals, using a stimulus emitted from a speaker placed lateral to the animal‘s head and a second speaker emitting a masker tone at eight different radial positions around the animal. Depending on the animal, the mABR varied by as much as 5 - 20 dB with position. Masking was greatest when the speakers were positioned on opposite sides of the animal‘s head. Repeated measurements done on anesthetized animals suggest that directionality cues are also present at frequencies slightly lower than 1000 Hz. Observed directional sensitivity to 1000 Hz and lower frequency tones cannot be accounted by head shadowing because the tone wavelength is much larger than the head diameter of the juvenile animals studied. Furthermore, the effect of masking was observed during the initial part of the mABR, suggesting directional sensitivity at the level of the auditory nerve. CT scans and dissections demonstrate that the contralateral middle ear (tympanic) cavities of the alligator are continuous and interconnected both superior and inferior to the braincase.Interaural coupling has been shown to generate directional pressure-difference receiver effects in other species (e.g. Christensen-Dalsgaard et al., JARO 2008), and we hypothesize that alligator ears may also be acoustically coupled.

Publication metadata

Author(s): Bierman H, Carr C, Brandt C, Young B, Christensen-Dalsgaard J

Publication type: Conference Proceedings (inc. Abstract)

Publication status: Published

Conference Name: Socity for Neuroscience Annual Meeting

Year of Conference: 2011