The Property of Recursion in Birdsong and Human Language
Human speech and birdsong have numerous parallels. Both humans and songbirds learn their complex vocalizations early in life, exhibiting a strong dependence on hearing the adults they will imitate, as well as themselves as they practice, and a waning of this dependence as they mature. Innate predispositions for perceiving and learning the correct sounds exist in both groups, although more evidence of innate descriptions of species-specific signals exists in songbirds, where numerous species of vocal learners have been compared. Both humans and songbirds have evolved a complex hierarchy of specialized forebrain areas in which motor and auditory centers interact closely, which control the lower vocal motor areas also found in non-learners. In both these, vocal learners; however, the processing of auditory feedback in these brain areas is surprisingly unclear.1 The idea builds upon Miyagawa’s conclusion, detailed in his previous work, that there are two “layers” in all human languages: an “expression” layer, which involves the changeable organization of sentences, and a “lexical” layer, which relates to the core content of a sentence. His conclusion is based on earlier work by linguists including Noam Chomsky, Kenneth Hale and Samuel Jay Keyser. Based on an analysis of animal communication, and utilising Miyagawa’s framework, the authors say that birdsong closely resembles the expression layer of human sentences — whereas the communicative waggles of bees, or the short, audible messages of primates, are more like the lexical layer. At some point, between 50,000 years–80,000 years ago, humans may have merged these two types of expression into a uniquely sophisticated form of language.2 To consider the difference between the expression layer and the lexical layer, consider a simple sentence: “Todd saw a condor.” Variations such as, “When did Todd see a condor?” can be easily created. This rearranging of elements takes place in the expression layer, it allows us to add complexity and ask questions. However, the lexical layer remains the same, as it involves the same core elements: the subject, “Todd,” the verb, “to see,” and the object, “condor.” Birdsong lacks a lexical structure. Instead, birds sing learned melodies with what Berwick calls a “holistic” structure; the entire song has one meaning, whether about mating, territory or other things. The Bengalese finch, as the authors note, can loop back to parts of previous melodies, allowing for greater variation and communication of more things; a nightingale may be able to recite from 100–200 different melodies.2
Universal Grammar approach on Recursion:
Hauser, Fitch, and Chomsky (2000) have distinguished between the faculty of language in the broad sense (FLB), which is shared between humans and other species, and the faculty of language in the narrow sense (FLN), which is uniquely human. The minimum characteristic that distinguishes FLN from FLB, they claim, is recursion. Pinker and Jackendoff (2005) refer to this as the “recursion-only claim,” and have argued that there are other, non-recursive aspects of human language that distinguish it from other forms of animal communication. Nevertheless it seems clear that recursion is at least one of the distinguishing attributes of human language, which raises the challenge of showing that some nonhuman species may be capable of producing or parsing recursive sequences.
In this article I examine two recent attempts to demonstrate recursive parsing in nonhuman species, one by Hauser and Fitch (2004) in tamarins, and the other by Gentner, Fenn, Margoliash, and Nusbaum (2006) in starlings (Sturnus vulgaris).3
Recursion is a computational procedure that calls itself, or that calls an equivalent kind of procedure. A distinction can be drawn between tail recursion and center-embedded recursion. In tail recursion,...
References: 1. Allison J. Doupe., & Patricia K. Kuhl Birdsong and human speech: common themes and mechanisms. Annu. Rev. Neurosci. (1999) 22:567–631
3. Michael C. Corballis Recursion, Language, and Starlings. Cognitive Science 31 (2007) 697–704
5. Falk, Dean (1983) & Cerebral cortices of East African early hominids. Science 221:1072–1074
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