Barn owls are capable of great accuracy in detecting the interaural time differences (ITDs) that underlie azimuthal sound localization. They compute ITDs in a circuit in nucleus laminaris (NL) that is reorganized with respect to birds like the chicken. The events that lead to the reorganization of the barn owl NL take place during embryonic development, shortly after the cochlear and laminaris nuclei have differentiated morphologically. At first the developing owl’s auditory brainstem exhibits morphology reminiscent of that of the developing chicken. Later, the two systems diverge, and the owl’s brainstem auditory nuclei undergo a secondary morphogenetic phase during which NL dendrites retract, the laminar organization is lost, and synapses are redistributed. These events lead to the restructuring of the ITD coding circuit and the consequent reorganization of the hindbrain map of ITDs and azimuthal space. Key words: avian development; morphogenesis; auditory; laminaris; evolution; interaural time difference The barn owl’s (hereafter referred as owl) auditory system uses interaural time differences (ITDs) and interaural level differences (ILDs) for sound localization. Coding of ITDs relies on coincidence detection by nucleus laminaris (NL) neurons, which are sensitive to binaural ongoing phase disparities (Moiseff and Konishi, 1983). The circuit responsible for the computation of ITDs in birds has been studied extensively in chickens and owl
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