Persistent representation of juvenile experience in the adult songbird brain

Juveniles sometimes learn behaviors that they cease to express as adults. Whether the adult brain retains a record of experiences associated with behaviors performed transiently during development remains unclear. We addressed this issue by studying neural representations of song in swamp sparrows, a species in which juveniles learn and practice many more songs than they retain in their adult vocal repertoire. We exposed juvenile swamp sparrows to a suite of tutor songs and confirmed that, although many tutor songs were imitated during development, not all copied songs were retained into adulthood. We then recorded extracellularly in the sensorimotor nucleus HVC in anesthetized sparrows to assess neuronal responsiveness to songs in the adult repertoire, tutor songs, and novel songs. Individual HVC neurons almost always responded to songs in the adult repertoire and commonly responded even more strongly to a tutor song. Effective tutor songs were not simply those that were acoustically similar to songs in the adult repertoire. Moreover, the strength of tutor song responses was unrelated to the number of times that the bird sang copies of those songs in juvenile or adult life. Notably, several neurons responded most strongly to a tutor song performed only rarely and transiently during juvenile life, or even to a tutor song for which we could find no evidence of ever having been copied. Thus, HVC neurons representing songs in the adult repertoire also appear to retain a lasting record of certain tutor songs, including those imitated only transiently.