Auditory Brain Stem Processing in Reptiles and Amphibians: Roles of Coupled Ears

Comparative approaches to the auditory system have yielded great insight into the evolution of sound localization circuits, particularly within the nonmammalian tetrapods. The fossil record demonstrates multiple appearances of tympanic hearing, and examination of the auditory brain stem of various groups can reveal the organizing effects of the ear across taxa. If the peripheral structures have a strongly organizing influence on the neural structures, then homologous neural structures should be observed only in groups with a homologous tympanic ear. Therefore, the central auditory systems of anurans (frogs), reptiles (including birds), and mammals should all be more similar within each group than among the groups. Although there is large variation in the peripheral auditory system, there is evidence that auditory brain stem nuclei in tetrapods are homologous and have similar functions among and within these groups. It appears that the more pronounced changes in processing are related to detecting airborne sound, the addition of high-frequency hearing, and the extent of acoustic coupling of the ears. This chapter focuses on the similarities and differences in peripheral structures as well as the anatomy and physiology of auditory brain stem nuclei.