Browsing by Subject "Audition"

An understanding of sensory ecology, how animals receive and respond to their environment, can be a powerful tool for the conservation of endangered species because it can allow us to assess the potential success of actions designed to mitigate particular threats. We have a general understanding of how sea turtles perceive and respond to certain visual, magnetic, and chemical cues, but we understand very little about how they perceive and respond to acoustic cues. This dissertation explores the acoustic ecology of sea turtles, focusing on their auditory capabilities, responses to acoustic stimuli and the implications of this knowledge for their conservation. I measured the underwater and aerial hearing sensitivities of juvenile green (Chelonia mydas), hatchling leatherback (Dermochelys coriacea), and hatchling hawksbill (Eretmochelys imbricata) sea turtles by recording auditory evoked potential responses to tonal stimuli. Green turtles detected tonal stimuli between 50 and 1,600 Hz underwater (maximum sensitivity: 200-400 Hz) and 50 and 800 Hz in air (maximum sensitivity: 300-400 Hz), leatherbacks detected tonal stimuli between 50 and 1,200 Hz underwater (maximum sensitivity: 100-400 Hz) and 50 and 1,600 Hz in air (maximum sensitivity: 50-400Hz), and hawksbills detected tonal stimuli between 50 and 1,600 Hz in both media (maximum sensitivity: 200-400 Hz). Sea turtles were more sensitive to aerial than underwater stimuli when audiograms were compared in terms of sound pressure, but they were more sensitive to underwater stimuli when audiograms were compared in terms of sound intensity. I also examined the behavioral responses of loggerhead sea turtle (Caretta caretta) to simulated low frequency acoustic deterrent devices (ADDs) and found that these turtles exhibited a mild, aversive response to these sounds. This finding indicates that low frequency tonal ADDs have the potential to warn sea turtles of the presence of fishing gear and suggest that field tests of ADDs are warranted. Finally, I conducted a comprehensive review of our knowledge of the acoustic ecology of sea turtles, examined the sources of marine anthropogenic sound sea turtles are able to detect, evaluated the potential physiological and behavioral effects of anthropogenic sound, identified data gaps, and made recommendations for future research.

After every eye movement, the brain must realign the visual and auditory reference frames in order to co-locate sights and sounds. Exactly where, when, and how such visual-auditory spatial integrations occur is not fully understood. We recently discovered that the eardrum oscillates beginning a few milliseconds before saccades and continuing until well into ensuing periods of fixation (Gruters et al., 2018)(Gruters, Murphy et al PNAS 2018). Information about at least the horizontal direction and length of saccades appear to be reflected in the phase and magnitude of these eye movement-related eardrum oscillations (EMREO).

Here, we sought to assess the full spatial characteristics of this signal for saccade parameters in both vertical and horizontal dimensions. Concurrently we sought to validate that independent estimations of vertical and horizontal saccade parameter contributions can be linearly combined to predict EMREO waveforms for saccades in all directions – a fundamental assumption of current analyses.

We found that EMREOs depend on both horizontal and vertical saccade components, varying predominantly with eye displacement, but modulated by absolute (initial or final) position as well. In toto, EMREO appear to represent combinations of these saccade parameters such that any saccade corresponds to a specific eardrum oscillation that contains a linear combination of the vertical and horizontal saccade parameters. Regressions in both the time and frequency domain create a fuller picture of the spatial information contained in EMREO. These results demonstrate that detailed information about the relationship between visual and auditory reference frames is present in the earliest stage of the auditory pathway. They also demonstrate that this information is mapped linearly and can therefore be recovered with a small set of basis components.

Future work delving into the relationship between EMREO and the transduction of incoming sounds will be needed to ascertain their effects on the processing of auditory spatial locations in relation to the visual scene. While the frequency and magnitude of EMREO suggest that they may be related to middle ear muscle contractions, the underlying mechanisms that generate them are unknown.