BIRTH, MIGRATION, INCORPORATION, AND DEATH OF VOCAL CONTROL NEURONS IN ADULT SONGBIRDS

Neurogenesis continues in the brain of adult birds. These cells are bo rn in the ventricular zone of the lateral ventricles. Young neurons th en migrate long distances guided, in part, by radial cell processes an d become incorporated throughout most of the telencephalon. In songbir ds, the high vocal center (HVC), which is important for the production of learned song, receives many of its neurons after hatching. HVC neu rons which project to the robust nucleus of the archistriatum to form part of the efferent pathway for song production, and HVC interneurons continue to be added throughout life. In contrast, Area X-projecting HVC cells, thought to be part of a circuit necessary for song learning but not essential for adult song production, are only born in the emb ryo. New neurons in HVC of juvenile and adult birds replace older cell s that die. There is a correlation between seasonal cell turnover rate s (addition and loss) and testosterone levels in adult male canaries. Available evidence suggests that steroid hormones control the recruitm ent and/or survival of new HVC neurons, but not their production. The functions of neuronal replacement in adult birds remain unclear. Howev er, rates of HVC neuron turnover are highest at times of year when can aries modify their songs. Replaceable HVC neurons may participate in t he modification of perceptual memories or motor programs for song prod uction. In contrast, permanent HVC neurons could hold long-lasting son g-related information. The unexpected large-scale production of neuron s in the adult brain holds important clues about brain function and, i n particular, about the neural control of a learned behavior-birdsong. (C) 1997 John Wiley & Sons, Inc.