Birdsong and human speech: Common themes and mechanisms

Human speech and birdsong have numerous parallels. Both humans and song-bir ds learn their complex vocalizations early in life, exhibiting a strong dep endence on hearing the adults they will imitate, as well as themselves as t hey practice, and a waning of this dependence as they mature. Innate predis positions for perceiving and learning the correct sounds exist in both grou ps, although more evidence of innate descriptions of species-specific signa ls exists in songbirds, where numerous species of vocal learners have been compared. Humans also share with songbirds an early phase of learning that is primarily perceptual, which then serves to guide later vocal production. Both humans and songbirds have evolved a complex hierarchy of specialized forebrain areas in which motor and auditory centers interact closely, and w hich control the lower vocal motor areas also found in nonlearners. In both these vocal learners, however, how auditory feedback of self is processed in these brain areas is surprisingly unclear. Finally, humans and songbirds have similar critical periods for vocal learning, with a much greater abil ity to learn early in life. In both groups, the capacity for late vocal lea rning may be decreased by the act of learning itself, as well as by biologi cal factors such as the hormones of puberty. Although some features of bird song and speech are clearly not analogous, such as the capacity of language for meaning, abstraction, and flexible associations, there are striking si milarities in how sensory experience is internalized and used to shape voca l outputs, and how learning is enhanced during a critical period of develop ment. Similar neural mechanisms may therefore be involved.