The ear is a complex of sensory organs for equilibrium (vestibular sense) and hearing, sending signals to the brain by branches of the eighth (octaval) cranial nerve. In fishes and aquatic vertebrates, a system called the lateral line detects water-borne mechanical signals, and in many species also has electrosensory organs. The ear and lateral line system were traditionally considered closely related, forming a single octavolateralis system, largely for hearing. In this view, evolutionarily early centers in the brain for analyzing lateral line information later became auditory centers in land vertebrates which lack lateral lines. Recent studies of comparative brain anatomy and physiology have demonstrated the four subsystems do not in fact overlap much in the lower parts of their central neural pathways, nor in their physiological sensitivity. This study will use anatomical techniques to trace those pathways in less and more specialized fish, to higher levels including the important forebrain, which evolves into the cerebral cortex of mammals. Central organization of the auditory pathways in fishes will be compared to the lateral line pathways. Defining these pathways is necessary as a baseline to understand how auditory pathways have been modified during the striking evolution of the ear for hearing in land vertebrates. In addition, these studies will clarify the evolution of the remarkably diverse mechanisms for hearing found among the many species of bony fishes, which represent a complicated history of loss, invention, and re-invention of various receptor classes. Results will be important to sensory science, ichthyology, and evolutionary biology.
