DEVELOPMENTAL PLASTICITY IN NEURAL CIRCUITS FOR A LEARNED BEHAVIOR

The neural substrate underlying learned vocal behavior in songbirds pr ovides a textbook illustration of anatomical localization of function for a complex learned behavior in vertebrates. The song-control system has become an important model for studying neural systems related to learning, behavior, and development. The song system of zebra finches is characterized by a heightened capacity for both neural and behavior al change during development and has taught us valuable information re garding sensitive periods, rearrangement of synaptic connections, topo graphic specificity, cell death and neurogenesis, experience-dependent neural plasticity and sexual differentiation. The song system differs in some interesting ways from some well-studied mammalian model syste ms and thus offers fresh perspectives on specific theoretical issues. In this highly selective review, we concentrate on two major questions : What are the developmental changes in the song system responsible fo r song learning and the restriction of learning to a sensitive period, and what factors explain the highly sexually dimorphic development of this system? We discuss the important role of sex steroid hormones an d of neurotrophins in creating a male-typical neural song circuit (whi ch can learn to produce complex vocalizations) instead of a reduced, f emale-typical song circuit that does not produce learned song.