Evolution of Sound Source Localization Circuits in the Nonmammalian Vertebrate Brainstem.

The earliest vertebrate ears likely subserved a gravistatic function for orientation in the aquatic environment. However, in addition to detecting acceleration created by the animal's own movements, the otolithic end organs that detect linear acceleration would have responded to particle movement created by external sources. The potential to identify and localize these external sources may have been a major selection force in the evolution of the early vertebrate ear and in the processing of sound in the central nervous system.