P1: PSA/MBG P2: PSA/ARY/MBG/VKS QC: PSADecember 30, 1998 9:56 Annual Reviews AR076-21Annu. Rev. Neurosci. 1999. 22:567–631Copyrightc!1999 by Annual Reviews. All rights reservedBIRDSONG AND HUMAN SPEECH:Common Themes and MechanismsAllison J. DoupeDepartments of Psychiatry and Physiology and Keck Center for IntegrativeNeuroscience, University of California at San Francisco, San Francisco,California 94143; e-mail: [email protected] K. KuhlDepartment of Speech and Hearing Sciences, University of Washington, Seattle,Washington 98195; e-mail: [email protected] WORDS: perception, vocalization, learning, innate, critical period, auditoryABSTRACTHuman speech and birdsong have numerous parallels. Both humans and song-birds learn their complex vocalizations early in life, exhibiting a strong depen-dence on hearing the adults they will imitate, as well as themselves as they prac-tice, and a waning of this dependence as they mature. Innate predispositions forperceiving and learning the correct sounds exist in both groups, although more ev-idence of innate descriptions of species-specific signals exists in songbirds, wherenumerous species of vocal learners have been compared. Humans also share withsongbirds an early phase of learning that is primarily perceptual, which then servesto guide later vocal production. Both humans and songbirds have evolved a com-plex hierarchy of specialized forebrain areas in which motor and auditory centersinteract closely, and which control the lower vocal motor areas also found in non-learners. In both these vocal learners, however, how auditory feedback of self isprocessed in these brain areas is surprisingly unclear. Finally, humans and song-birds have similar critical periods for vocal learning, with a much greater ability tolearn early in life. In both groups, the capacity for late vocal learning may be de-creased by the act of learning itself, as well as by biological factors such as the hor-mones of puberty. Although some features of birdsong and speech are clearly notanalogous, such as the capacity of language for meaning, abstraction, and flexibleassociations, there are striking similarities in how sensory experience is internal-ized and usedto shape vocal outputs, and how learning is enhanced during a criticalperiod of development. Similar neural mechanisms may therefore be involved.5670147-006X/99/0301-0567$08.00Annu. Rev. Neurosci. 1999.22:567-631. Downloaded from arjournals.annualreviews.orgby University of California - San Diego on 01/04/07. For personal use only. P1: PSA/MBG P2: PSA/ARY/MBG/VKS QC: PSADecember 30, 1998 9:56 Annual Reviews AR076-21568 DOUPE & KUHLINTRODUCTIONExperts in the fields of human speech and birdsong have often commentedon the parallels between the two in terms of communication and its develop-ment (Marler 1970a, Kuhl 1989). Does the acquisition of song in birds provideinsights regarding learning of speech in humans? This review provides a crit-ical assessment of the hypothesis, examining whether the similarities betweenthe two fields go beyond superficial analogy. The often cited commonalitiesprovide the topics of comparison that structure this review.First, learning is critical to both birdsong and speech. Birds do not learn tosing normally, nor infants to speak, if they are not exposed to the communicativesignals of adults of the species. This is an exception among species: Mostanimals do not have to be exposed to the communicative signals of their speciesto be able to reproduce them. The fact that babies and songbirds share thisrequirement has intrigued scientists.Second, vocal learning requires both perception of sound and the capacity toproduce sound. At birth, both human infants and songbirds have been hypoth-esized to have innate perceptual predispositions for the vocal behavior of theirown species. We review the nature of the predispositions in the two cases andthe issue of whether they are similar. Given that innate predispositions exist,another important question is how subsequent experience alters perception andproduction in each case. Moreover, vocal perception and production are tightlyinterwoven in the vocal learning process. We examine what is known about therelationship between perception and production and whether in these differentvocal learners it is similar.In addition, neural substrates of vocal communication in humans and birdshave often been compared. Human brains are asymmetric and language tendsto be organized in the left hemisphere as opposed to the right. Birds are alsooften assumed to show similar hemispheric specialization for song. What arethe real parallels between the neural substrates in the two cases?Finally, critical (sensitive) periods are evidenced in both species. Neitherbirds nor babies appear to learn their communicative signals equally well at allphases of the life cycle. This raises the questions of what causes the changein the ability to learn over time and with experience, and whether the causesare the same in human infants and songbirds. And if the plasticity of the brainis altered over the life cycle, what neural mechanisms control this changingability to learn?The research reviewed here relates to ongoing work in developmental biology,ethology, linguistics, cognitive psychology, and computer science, as well as inneuroscience, and it should be of interest to individuals in many of these fields.What our review reveals is that although the comparisons between birdsongAnnu. Rev. Neurosci. 1999.22:567-631. Downloaded from arjournals.annualreviews.orgby University of California - San Diego on 01/04/07. For personal use only.P1: PSA/MBG P2: PSA/ARY/MBG/VKS QC: PSADecember 30, 1998 9:56 Annual Reviews AR076-21BIRDSONG AND HUMAN SPEECH 569and speech are not simple, there is a surprisingly large number of areas where itis fruitful to compare the two. Going beyond the superficial analogy, however,requires some caveats about what may be comparable and what clearly is not.In the end, understanding both the similarities and differences will provide abroader spectrum in which to view the acquisition of communication in humansand other animals.SPEECH AND BIRDSONG: DEFINITIONSSpeech and Song ProductionBoth birdsong and human speech are complex acoustic signals. Figure 1 showsa spectrographic (frequency vs time) display of a spoken human phrase (“Didyou hit it to Tom?”) and Figure 2 a similar display of songs of two differentsongbird species. In both songbirds and