Directional cues for sound source segregation in birds, crocodilians, and lizards

Abstract

Sound source segregation depends on neural mechanisms that enhance directionality. The main directional cues are interaural time difference (ITD) and interaural level difference (ILD). Birds, crocodilians, and lizards have a brainstem circuit used for detection of ITDs. In birds and crocodilians, this circuit forms a map of ITD by delay lines and coincidence detection. The physical range of ITDs for these maps are small in animals with small heads, which should make detection of ITDs difficult. Both birds and crocodilians have coupled ears, however, which extend the range of ITDs available as well as enhancing ILD. Lizards have even more strongly coupled ears, extending the ITD range by a factor 3, but ITD and ILD covary, and a large part of the ITD is a constant delay created by filtering by interaural cavities, chiefly producing enhanced lateralization. All lizard auditory nerve fibers show strongly directional responses, and effectively every neuron in the lizard auditory pathway is directional, enhancing the already strong lateralization by simple EI-type neural processing, but with no clear maps of auditory space. Thus, the processing of sound direction in the bird, alligator, and lizard CNS is different, but all three groups have mechanisms for enhancing sound source directionality.