Meeting Abstract
The experiments described are designed to give insight into how binaural hearing evolved. Modern neuroanatomical tract tracing techniques were used to understand the connections among the auditory nuclei in the brain stem of the Red Ear Slider Turtle. Turtles have brain stem nuclei that are connected in the same pattern as the other reptiles, including birds. These nuclei are nucleus angularis (NA), nucleus magnocellularis (NM), nucleus laminaris (NL), superior olive (SO), and torus semicircularis (TS). The auditory nerve bifurcates to NA and NM. NM projects bilaterally to NL. NL and NA project bilaterally to TS, and SO projects contralaterally to TS. Using an in vitro with attached periphery physiology preparation, neurons were characterized by best frequency response, threshold, phase locking. Best frequencies ranged from 100 to 700 Hz. Thresholds were typically around 70 dB SPL. Phase locking vector strength varied, depending on whether the unit recorded was single or multi. Binaural neurons (NL) were additionally characterized by interaural time difference (ITD) sensitivity and phase interactions. These neurons were shown to have a small range of interaural time difference sensitivity responses. The range of ITD tuning curves measured are within a reasonable physiological range for the turtles’ head sizes—200us or less to the left or right. These data demonstrate that turtles have the neural substrates and information necessary for some navigation based on auditory cues. Given that the sounds must be relatively loud, auditory cues are likely not the primary mode for navigation. Although the evolutionary position of testudines is not yet resolved, close study of testudine anatomy and physiology can provide insight. It is most likely that testudines share their most recent common ancestor with the archosaurs and we hypothesize that testudines likely reflect the ancestral condition of auditory processing for the archosaur clade.
