Neural Mechanisms of Song Learning in a Passerine Bird

Abstract

The neural control of vocal learning and behavior in songbirds is vested in a highly localized system of interconnected brain nuclei. In zebra finches (Poephila guttata), these neural circuits undergo profound developmental changes during the time when song behavior is being learned, and both song learning and its neural substrate are sensitive to gonadal hormones such as testosterone and estrogen. Work from our lab has suggested that there may be separate neural circuits that are associated with early versus late stages of vocal learning in zebra finches. Thus, lesioning the song-control circuit that includes the forebrain nucleus IMAN (lateral magnocellular nucleus of the anterior neostriatum) disrupts song behavior only during early stages of vocal learning, whereas making the same lesion during later stages of song development has little or no effect on song behavior. In contrast, lesioning the song-control nucleus HVc (caudal nucleus if the ventral hyperstriatum) produces substantial disruption of song behavior in older juvenile and adult birds, but seems to exert less of a disruptive effect early in the song learning process. Both IMAN and HVc are only two synapses away from the motor neurons that control the vocal muscles, but the circuit containing IMAN regresses during vocal development while the circuit containing HVc grows. Interestingly, neurons are lost from IMAN during early stages of song learning whereas many newly generated neurons are added to HVc throughout the time when song is being learned. This pattern may correlate with the finding that the circuit containing HVc appears to assume increasing control over song behavior as the circuit containing IMAN is relinquishing its role.