The major goal of our lab is to understand the biological basis of learning. We believe that a comprehensive understanding of the relevant mechanisms for learning will require relating discrete synaptic processes to behavior, ultimately in the awake animal. To this end, we focus on a constellation of interconnected brain nuclei in the songbird that acquire, produce, and maintain its learned vocalizations. The wealth of behavioral data characterizing birdsong learning coupled with the discrete anatomical nature of the song circuitry make songbirds an ideal organism in which to study the cellular mechanisms of learning and memory.
Song learning in birds shares many features with speech learning in humans, including developmental restriction to an early "critical period." During the critical period, the juvenile bird listens to and copies the song of a tutor; after the critical period closes, such copying apparently is impossible. One major goal of our lab is to understand the neuronal regulation of the critical period for song learning, in part because it could yield important insights into the relatively prodigious learning power of the juvenile brain.
An important experimental handle on the critical period is that it is not chronologically invariant, but instead can be regulated in its duration by the amount of tutor song exposure, vocal practice, and the brain's steroidal milieu. One idea that we are testing is that those same factors which alter the pace of song learning modulate the development of the song circuitry. Therefore, we are directly examining synaptic transmission within the song system, by making in vivo and in vitro intracellular recordings from individual neurons within this circuit, an approach that permits detailed analysis of the electrophysiological properties of the song system over the course of development. These studies have resulted in the first synaptic level characterization of this complex circuit, and have also allowed us to study how the pace of neuronal maturation is affected by altered auditory, vocal or hormonal experience. We found that both sex steroids and auditory experience exert powerful effects on synaptic development of the avian song circuitry, suggesting a cellular pathway via which such factors influence the duration of the critical period itself.
A second striking manner in which birdsong resembles speech is the important role for auditory feedback in both song development and maintenance. The dependence of the song motor program on sensory feedback can be revealed by either deafening the bird or exposing it to artificially distorted auditory feedback, manipulations which disrupt song learning in juvenile birds and cause the previously stable song of the adult to deteriorate. The finding that adult deafening triggers song degradation is especially remarkable, given the otherwise high stereotypy of the adult song.
How does auditory information alter the vocal phenotype? One idea is that the same telencephalic circuitry that patterns song also receives auditory signals used to instruct and maintain the appropriate vocal phenotype. Indeed, remarkably selective auditory neurons have been detected in the song control nuclei; these neurons fire almost exclusively to the bird's own song, but not to the songs of other birds, nor to temporally altered versions of the bird's own song.
How does experience alter the brain to enable it to generate such narrow auditory selectivity for a learned vocalization, and do such auditory signals serve as part of an error correction system for song learning and maintenance? To begin to address these questions, we have applied in vivo intracellular methods using one or more electrodes to explore the synaptic mechanisms by which selectivity arises, and measure how it propagates through the song control circuitry. To relate such observations to the bird's behavior, we are beginning to make chronic recordings from identified neurons in the singing and listening songbird. These various approaches provide an experimental foundation for connecting the discrete synaptic properties of the song control circuitry to the learning, production, and perception of birdsong.
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