Abstract
Experience-dependent changes in neural connectivity underlie developmental learning and result in life-long changes in behavior. In songbirds axons from the cortical region LMAN(core) (core region of lateral magnocellular nucleus of anterior nidopallium) convey the output of a basal ganglia circuit necessary for song learning to vocal motor cortex [robust nucleus of the arcopallium (RA)]. This axonal projection undergoes remodeling during the sensitive period for learning to achieve topographic organization. To examine how auditory experience instructs the development of connectivity in this pathway, we compared the morphology of individual LMAN(core)→RA axon arbors in normal juvenile songbirds to those raised in white noise. The spatial extent of axon arbors decreased during the first week of vocal learning, even in the absence of normal auditory experience. During the second week of vocal learning axon arbors of normal birds showed a loss of branches and varicosities; in contrast, experience-deprived birds showed no reduction in branches or varicosities and maintained some arbors in the wrong topographic location. Thus both experience-independent and experience-dependent processes are necessary to establish topographic organization in juvenile birds, which may allow birds to modify their vocal output in a directed manner and match their vocalizations to a tutor song. Many LMAN(core) axons of juvenile birds, but not adults, extended branches into dorsal arcopallium (Ad), a region adjacent to RA that is part of a parallel basal ganglia pathway also necessary for vocal learning. This transient projection provides a point of integration between the two basal ganglia pathways, suggesting that these branches convey corollary discharge signals as birds are actively engaged in learning.