A circuit for detection of interaural time differences in the nucleus laminaris of turtles

Abstract

The physiological hearing range of turtles is about 50-1,000 Hz, as determined by cochlear microphonics (Wever and Vernon, 1956a). These low frequencies can constrain sound localization, particularly in Red-eared Slider Turtles, which are freshwater turtles with small heads and isolated middle ears. To determine if these turtles were sensitive to interaural time differences (ITDs), we investigated the connections and physiology of their auditory brainstem nuclei. Tract tracing experiments showed that the VIII nerve bifurcated to terminate in the first order nucleus magnocellularis and nucleus angularis, and the nucleus magnocellularis projected bilaterally to nucleus laminaris. Since the nucleus laminaris received inputs from each side, we developed an isolated head preparation to examine responses to binaural auditory stimulation. Magnocellularis and laminaris units responded to frequencies from 100-600 Hz, and phase locked reliably to the auditory stimulus. Responses from the nucleus laminaris were binaural, and sensitive to ITD. Measures of characteristic delay revealed best ITDs around ± 200µs, and nucleus laminaris neurons typically had characteristic phases close to 0, consistent with binaural excitation. Thus, turtles encode ITDs within their physiological range, and their auditory brainstem nuclei have similar connections and cell types to other reptiles.