Evolutionary trends in hearing in nonmammalian vertebrates

The central auditory system is organized similarly in all vertebrates and may reflect the organizational pattern common to the octaval system. In fishes, multiple endorgans project to all of the octaval nuclei, and there is no single nucleus dedicated to auditory processing. In contrast, distinct auditory pathways characterize all tetrapods. Middle ears developed independently in the different tetrapod lineages and resulted in both increased sensitivity to airborne sound and to sensitivity to a wider range of frequencies than their aquatic ancestors. The increase in frequency response was accompanied by differentiation and expansion of the central auditory system. Physiological studies of all vertebrate groups reveal similar mechanisms to encode sound. Phase locking, frequency tuning, the emergence of binaural comparisons, feature detection, and convergence of sensory modalities are found throughout. The central auditory systems in modern vertebrates are marked by many examples of similar solutions to the problem of how to process auditory stimuli.