Vol 440 | 27 April 2006 NEWS & VIEWS LANGUAGE Startling starlings Gary F. Marcus Recursion , once thought to be the unique province of human language , now seems to be within the ken of a common songbird -- perhaps providing insight into the origins of language . 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 error1 . The report by Timothy Gentner and colleagues on page 1204 of this issue2 challenges one more putatively uniquely human adaptation : the capacity to recognize complex ` recursive ' structure . Gentner et al. showed that at least one non-human species , the European starling -LRB- Fig. 1 -RRB- , can be trained to acquire complex recursive grammars such as the AnBn language -LRB- in the case of the starling , rattle rattle warble warble ; see below -RRB- . 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 -LSB- on the coffee table -RSB- is hilarious . Marc Hauser , Noam Chomsky and Tecumseh Fitch3 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 Hauser4 found that cotton-top tamarin monkeys could not distinguish the AnBn language from an ostensibly similar language -LRB- AB -RRB- n that need not be constructed recursively . The AnBn language -LRB- see Fig. 1 of the paper2 on page 1204 -RRB- 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 colleagues2 rewarded European starlings for pressing a bar in response to D. D. BALECKAITIS Figure 1 | No bird brain . The European starling , Sturnus vulgaris , which Gentner et al. 2 show is capable of recognizing complex grammar . AnBn strings of starling-generated sounds , such as rattle rattle warble warble , and withheld the reward for responses to the -LRB- AB -RRB- n grammar -LRB- and vice versa for another group of starlings -RRB- . Although learning was not instantaneous , nine of eleven birds eventually -LRB- after 10,000 50,000 trials -RRB- 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 worries5 , this is strong evidence that humans are not alone in their capacity to recognize recursion . What explains the discrepancy between the starlings ' success and the tamarins ' apparent failure ? One possibility is methodological : Fitch and Hauser4 tested whether tamarins could acquire AnBn spontaneously from a relatively brief exposure , whereas Gentner et al. 2 asked whether starlings could acquire similar 2006 Nature Publishing Group 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 . If the split between tamarins and starlings does prove robust , further comparative work will clearly be necessary . Can other varieties of birds that do n't -LRB- in contrast to starlings -RRB- naturally acquire new songs also acquire self-embedded structures ? Are humans alone among primates in their capacity to do so ? Might the capacity for recursion be general across great apes , even if it were absent in monkeys ? An intriguing possibility is that the capacity to recognize recursion might be found only in species that can acquire new patterns of vocalization , for example songbirds , humans and perhaps some cetaceans . Whether or not tamarins can be prodded 1117 NEWS & VIEWS NATURE | Vol 440 | 27 April 2006 into recognizing recursion , the analogous human capacity seems robust . Humans are quick to notice recursion and are able to do so without explicit reinforcement ; perhaps most importantly , they can generalize recursive structures broadly . Starlings have thus far been shown to be able to extend AnBn only to new sequences of familiar sounds . Humans can clearly go further ; once you recognize the pattern in AABB and AAABBB , it is a trivial matter to extend that pattern to new vocabulary -LRB- for example , CCCDDD or JJJKKK -RRB- . Taken together with the tamarins , there actually seems to be a three-way split : some species may generalize recursion only to items that have already been instantiated in a given pattern ; some species can generalize recursion freely to newly acquired vocabularies -LRB- arguably the essence of human language -RRB- ; and some species apparently can not recognize recursion at all . The `` abstract computational capacity of language `` 3 may consist not so much of a single innovation as a novel evolutionary reconfiguration of many -LRB- perhaps subtly6 or even qualitatively7 modified -RRB- ancestral cognitive components , genetically rejigged into a new whole . Contemporary research suggests that the human brain contains few if any unique neuronal types , and few if any genes lack a significant ancestral precedent8 . At the same time , humans show much continuity with their non-speaking cousins in dozens of ways that might contribute to language , including mechanisms for representing time and space , for analysing sequences , for auditory analysis , for inhibiting inappropriate action , and for memory . None of this challenges Chomsky 's longheld conjecture9 that children are innately endowed with a universal grammar -- a set of mental machinery that would lead all human languages to have a similar abstract character . But that shared abstract character may have as much to do with our lineage as vertebrates as with our uniquely human innovations . In Charles Darwin 's immortal words , `` throughout nature almost every part of each living being has probably served , in a slightly modified condition '' in some ancestor or another . Gary F. Marcus is in the Department of Psychology , New York University , 6 Washington Place , New York , New York 10012 , USA . e-mail : gary.marcus@nyu.edu 1 . Whiten , A. Nature 437 , 52 55 -LRB- 2005 -RRB- . 2 . Genter , T. Q. , Fenn , K. M. , Margoliash , D. & Nusbaum , H. C. Nature 440 , 1204 1207 -LRB- 2006 -RRB- . 3 . Hauser , M. D. , Chomsky , N. & Fitch , W. T. Science 298 , 1569 1579 -LRB- 2002 -RRB- . 4 . Fitch , W. T. & Hauser , M. D. Science 303 , 377 380 -LRB- 2004 -RRB- . 5 . Kochanski , G. Science 303 , 377 380 -LRB- 2004 -RRB- . 6 . Pinker , S. & Jackendoff , R. Cognition 95 , 201 236 -LRB- 2005 -RRB- . 7 . Marcus , G. F . The Birth of the Mind : How a Tiny Number of Genes Creates the Complexities of Human Thought -LRB- Basic Books , New York , 2004 -RRB- . 8 . Fisher , S. E. & Marcus , G. F. Nature Rev. Genet . 7 , 9 20 -LRB- 2006 -RRB- . 9 . Chomsky , N. Language and Mind -LRB- Harcourt , Brace & World , New York , 1968 -RRB- . SUPERCONDUCTIVITY Quantum stripe search Jan Zaanen Do quantum stripes exist or not ? Further indirect evidence for this controversial behaviour of electrons in high-temperature superconductors comes from measurements of atomic-lattice vibrations . Since 1995 , the ` stripe wars ' have been raging in the demesne of high-temperature superconductivity . This fierce conflict , fought with the highest-calibre weapons of experimental physics , has its antecedent in a hotly contested claim about the way electrons behave in the copper oxide materials notoriously used as high-temperature superconductors . Supposedly , they form highly organized patterns called quantum stripes -- but only on the picosecond timescale , so the patterns average away over longer periods through the electrons ' constant quantum dance . The dispute has lasted so long only because it has proved very hard to nail down such genuine quantum behaviour . In this issue , however , Reznik et al. -LRB- page 1170 -RRB- 1 present further evidence in support of quantum stripes . They show that the collective vibrations of the atomic lattice of certain superconducting copper oxides behave in a manner that is hard to explain -- unless one assumes that motions characteristic of the presence of quantum stripes are shaking the ion lattice . So is the end of hostilities in sight ? It is an everyday experience that , in a manybody system , collective behaviours emerge that are utterly unrelated to the behaviours of the objects that make it up . The evolution of a nation 's economy over time , for example , is difficult to predict from the often conflicting motivations of the constituent human members . The same principles apply in quantum physics . But the exact rules that govern quantum emergence are poorly understood ; uncovering them is a core business of modern physics . Conventional superconductors -- those operating only at temperatures very close to absolute zero -- demonstrate only a minimal form of quantum emergence . In such materials , interactions between electrons diminish at low temperature , and the macroscopic electron system turns into a near-ideal -LRB- non-interacting -RRB- quantum gas of ` quasi ' - electrons . A small residual attractive interaction binds these quasi-electrons in pairs , which in turn collapse to a single quantum state in the process known as Bose Einstein condensation . Although this theory of superconductivity , known as the Bardeen Cooper Schrieffer -LRB- BCS -RRB- model , gets everything right for conventional superconductors , it explains hardly anything in high-temperature superconductors . The discovery 20 years ago of this unusually sturdy form of superconductivity raised the curtain on a drama of wider relevance : the 2006 Nature Publishing Group huge numbers of strongly interacting electrons in the copper oxide layers were plainly showing an unknown kind of collective quantum physics -LRB- for an overview , see ref . 2 -RRB- . In 1995 , it was discovered3 that small changes in the crystal structure of high-temperature superconductors can cause superconductivity to disappear , with a peculiar ` static stripe phase ' taking over -LRB- Fig. 1 -RRB- . Here , strong interactions and quantum motions work together to form patterns of electrons moving in serried ranks . Domains in which the electrons come to a complete standstill separate these ` rivers of charge ' -LRB- Fig. 1b -RRB- . The existence of static stripes , initially contentious , is now generally accepted . But quantum stripes are more radical and controversial . Members of the quantum-stripe faction hold that stripes are , in fact , always present . When a material superconducts , the stripes do not disappear ; rather , a quantum-mechanical superposition of countless disordered stripe states forms -LRB- Fig. 1a -RRB- , in such a way that the overall state corresponds to that of a superconducting quantum liquid -LRB- for a mathematical proof of principle , see ref . 4 -RRB- . So how can we nail down quantum stripes experimentally ? Consider Erwin Schr dinger 's oft-cited cat hidden in a sealed box -LRB- Fig. 1c -RRB- . Classically , the cat must be either dead or alive , but quantum mechanically it is in a superposition of dead and alive states . In the quantum state , the cat fluctuates back and forth between alive and dead states . This fluctuation takes a finite time . So , by taking snapshots quickly enough , one can see either a dead or a live cat . The same scheme works equally well with superpositions of countless manyelectron configurations . Here , every quantum configuration takes the role of a classical ` dead or alive ' state -LRB- Fig. 1c -RRB- . The fluctuation time of the quantum stripes is in the picosecond -LRB- 10 12 second -RRB- range , and the problem for the experimentalist is how to grab a picture of complicated spatial electron patterns in so little time . One way to do this is to observe the change in kinetic energy of neutrons that scatter off the material inelastically . Since 1995 , such neutron-scattering experiments have added to the body of evidence supporting the case for quantum stripes5 . The drawback is that these studies relied on information about the direction of the electron spins that was open to alternative interpretations , including some compatible with the conventional BCS picture -LRB- see ref . 6 and references 1118

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