Adaptive axonal remodeling in the midbrain auditory space map

The auditory space map in the external nucleus of the inferior colliculus ( ICX) of barn owls is highly plastic, especially during early life. When juv enile owls are reared with prismatic spectacles (prisms) that displace the visual field laterally, the auditory spatial tuning of neurons in the ICX a djusts adaptively to match the visual displacement. In the present study, w e show that this functional plasticity is accompanied by axonal remodeling. The ICX receives auditory input from the central nucleus of the inferior co lliculus (ICC) via topographic axonal projections. We used the anterograde tracer biocytin to study experience-dependent changes in the spatial patter n of axons projecting from the ICC to the ICX. The projection fields in nor mal adults were sparser and more restricted than those in normal juveniles. The projection fields in prism-reared adults were denser and broader than those in normal adults and contained substantially more bouton-laden axons that were appropriately positioned in the ICX to convey adaptive auditory s patial information. Quantitative comparison of results from juvenile and pr ism-reared owls indicated that prism experience led to topographically appr opriate axonal sprouting and synaptogenesis. We conclude that this elaborat ion of axons represents the formation of an adaptive neuronal circuit. The density of axons and boutons in the normal projection zone was preserve d in prism-reared owls. The coexistence of two different circuits encoding alternative maps of space may underlie the ability of prism-reared owls to readapt to normal conditions as adults.