Anatomical and Physiological Characterization of the Turtle Brain Stem Auditory Circuit

Abstract

The goal of this dissertation is to add to understanding of the evolution of hearing by studying the testudine taxon. This dissertation focuses on central auditory processing in the context of evolution. The experiments described are designed to give insight into how binaural hearing evolved.

Follow the findings of Christensen-Dalsgaard and colleagues (2012) that an amphibious turtle had lower hearing thresholds under water than in air and that this difference is conferred by resonance of the middle ear cavity, I examined middle ear cavities across families of Testudines. I found that middle ear cavity structure and function is shared by all testudines (Willis, et al., 2013).

Modern neuroanatomical tract tracing techniques were used to understand the connections among the auditory nuclei in the brain stem of the turtle. Turtles have brain stem nuclei that are connected in the same pattern as the other reptiles, including birds. These nuclei are nucleus angularis, nucleus magnocellularis, nucleus laminaris, superior olive, and torus semicircularis. Details of neuron structure were also examined and quantified.

Finally, I developed an isolated head preparation that enables in vivo-like physiological recording. As proof of principle, neurons were characterized by best frequency response, threshold, phase locking. Additionally, binaurally responsive neurons were found, which have a range of interaural time difference sensitivity responses.

Although the evolutionary position of testudines is not yet resolved, it is most likely that testudines share their most recent common ancestor with the archosaurs. I hypothesize that testudines likely reflect the ancestral condition of auditory processing for the archosaur clade.

All experiments described in this dissertation were performed according to the guidelines approved by the Marine Biological Laboratory (Woods Hole, MA, USA), the University of Maryland Institutional Animal Care and Use Committees (IACUC) and the Danish National Animal Experimentation Board (Dyreforsøgstilsynet).