The Journal of Neuroscience, July 15, 1999, 19(14):6037-6057
Development of Topography within Song Control Circuitry of Zebra
Finches during the Sensitive Period for Song Learning
Soumya
Iyengar,
Sandya S.
Viswanathan, and
Sarah W.
Bottjer
Department of Biology, University of Southern California, Los
Angeles, California 90089-2520
Refinement of topographic maps during sensitive periods of
development is a characteristic feature of diverse sensory and motor
circuits in the nervous system. Within the neural system that controls
vocal learning and behavior in zebra finches, axonal connections of the
cortical nucleus lMAN demonstrate striking functional and morphological
changes during vocal development in juvenile males. These circuits are
uniquely important for song production during the sensitive period for
vocal learning, and the overall size of these brain regions and their
patterns of axonal connectivity undergo dramatic growth and regression
during this time. Axonal connections to and from lMAN are
topographically organized in adult males that have already learned
song. We wondered whether the large-scale changes seen in lMAN
circuitry during the time that vocal behavior is being learned and
refined could be accompanied by the emergence of topographic mapping.
However, results presented herein demonstrate that most of these
song-control circuits show the same broad patterns of axonal
connectivity between subregions of individual nuclei at the onset of
song learning as seen in adult birds. Thus, coarse topographic
organization is not dependent on the types of experience that are
crucial for vocal learning. Furthermore, this maintenance of
topographic organization throughout the period of song learning is
clearly not achieved by maintenance of static axonal arbors. In fact,
because the volumes of song-control nuclei are growing (or regressing),
topography must be maintained by active remodeling of axonal arbors to
adapt to the changes in overall size of postsynaptic targets. A salient exception to this pattern of conserved topography is the projection from lMAN to the motor cortical region RA: this pathway is diffusely organized at the onset of song learning but undergoes substantial refinement during early stages of song learning, suggesting that remodeling of axonal connections within this projection during the
period of vocal learning may signify the production of increasingly refined vocal utterances.
Key words:
topography; sensitive periods; zebra finch; songbird; vocal learning; axon arbors; basal ganglia
Copyright © 1999 Society for Neuroscience 0270-6474/99/19146037-21$05.00/0
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