ScienceWeek

NEUROSCIENCE: BIRDS AND RECURSION STRUCTURE IN LANGUAGE

The following points are made by Gary F. Marcus (Nature 2006 440:1117):

1) Man the tool-maker. Man the cultural animal. Man the mimic. It's tempting to summarize the differences between humans and other species in a concise phrase, but most posited differences have turned out to be overstated. Chimpanzees and gorillas use sticks to fish for termites; orangutans use sticks for autoeroticism. And many of these capacities seem to be culturally mediated; they are transferred from one primate to the next by illustration and observation, rather than learned afresh by trial and error[1]. New work [2] challenges one more putatively uniquely human adaptation: the capacity to recognize complex "recursive" structure. Gentner et al [2] show that at least one non-human species, the European starling, can be trained to acquire complex recursive grammars such as the AnBn language (in the case of the starling, rattle rattle warble warble).

2) Recursion, or self-embedding, is without question a hallmark of human language. For example, one can take a phrase such as love conquers all and embed it in a frame such as X knows Y, yielding, say, Chris knows love conquers all. The output of that process can then be fed back into the X knows Y frame, yielding, say, Terry knows Chris knows love conquers all. Embedding also makes relative clauses possible, as in the bracketed part of the paperback [on the coffee table] is hilarious. Marc Hauser, Noam Chomsky and Tecumseh Fitch [3] have speculated that recursion might be unique to humans -- and perhaps even the only contribution to language that is human-specific. Consistent with this, Fitch and Hauser [4] found that cotton-top tamarin monkeys could not distinguish the AnBn language from an ostensibly similar language (AB)n that need not be constructed recursively.

3) The AnBn language is generally assumed to be recursive because new sentences can be formed by successive insertion into the frame AXB, for example AB, AABB, AAABBB and so on. Gentner and colleagues[2] rewarded European starlings for pressing a bar in response to AnBn strings of starling-generated sounds, such as rattle rattle warble warble, and withheld the reward for responses to the (AB)n grammar (and vice versa for another group of starlings). Although learning was not instantaneous, nine of eleven birds eventually (after 10,000-50,000 trials) learned to discriminate reliably between the two grammars, succeeding where the monkeys had failed. An extensive series of control comparisons strongly suggests that the ultimately acquired grammar is robust. Notwithstanding some minor worries[5], this is strong evidence that humans are not alone in their capacity to recognize recursion.

4) What explains the discrepancy between the starlings' success and the tamarins' apparent failure? One possibility is methodological: Fitch and Hauser [4] tested whether tamarins could acquire AnBn spontaneously from a relatively brief exposure, whereas Gentner et al [2] asked whether starlings could acquire similar structures from considerably longer exposures, enhanced with positive feedback, in a more active task. Only further experimentation can clarify whether tamarins' apparent inability to recognize the AnBn structure is context-specific or genuinely absolute.

References (abridged):

1. Whiten, A. Nature 437, 52-55 (2005)

2. Genter, T. Q. , Fenn, K. M. , Margoliash, D. & Nusbaum, H. C. Nature 440, 1204-1207 (2006)

3. Hauser, M. D. , Chomsky, N. & Fitch, W. T. Science 298, 1569-1579 (2002)

4. Fitch, W. T. & Hauser, M. D. Science 303, 377-380 (2004)

5. Kochanski, G. Science 303, 377-380 (2004)

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