Graduation Year

2008

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Psychology

Major Professor

Toru Shimizu, Ph.D.

Co-Major Professor

David Mann, Ph.D.

Keywords

Trichechus, Audition, AEP, Head related transfer function, Binaural hearing

Abstract

Three experiments investigated the ability and means by which Florida manatees determine sound source directionality. An eight-choice discrimination paradigm determined the sound localization abilities of two manatees within a 360° array of speakers. Five conditions were tested including a 3,000 and 200 ms, 95 dB, 0.2-24 kHz signal, a 3,000 ms, 80 dB, 18-24 kHz signal, a 3000 ms, 110 dB, 0.2-1.5 kHz signal and a 200 ms, 101 dB, 4 kHz tonal signal. A sixth condition attenuated the level of the 3,000 ms, 95 dB, 0.2-24 kHz signal in 3 dB increments until accuracy reached 75%. Subjects performed above the 12.5% chance level for all broadband frequencies and were able to localize over a large level range. Errors were typically located to either side of the signal source location when presented in the front 180° but were more dispersed when presented from the 135°, 180° and 225° locations.

Front-to-back confusions were few and accuracy was greater when signals originated from the front 180°. Head/body related transfer functions determined how different frequencies were filtered by the manatees' head/torso to create frequency-specific interaural level differences (ILDs). Hydrophones were suspended next to each manatee ear and Fast Fourier transform (FFT) ratios compared received signals with and without the subject's presence. ILD magnitudes were derived for all frequencies, as well as specific 0.2-1.5, 0.2-5, and 18-30 kHz bands of frequencies. ILDs were found for all frequencies as a function of source location, although they were largest with frequencies above 18 kHz and when signals originated at 90° and 270°. Larger ILDs were found when the signals originated behind the subjects as compared to in front of them. Auditory evoked potential (AEP) techniques were used to map manatee sound conduction pathways in-water and in-air using 15 and 24 kHz carriers.

All subjects produced AEPs at each position the transducer was placed, however specific sound conduction pathway(s) were not identified. AEP amplitudes were usually greater with the 24 kHz carrier, however patterns between carriers at identical body positions were highly variable between subjects.

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