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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 summarizethe differences between humans and otherspecies in a concise phrase, but most positeddifferences have turned out to be overstated.Chimpanzees and gorillas use sticks to fish fortermites; orangutans use sticks for autoeroti-cism. And many of these capacities seem to beculturally mediated; they are transferred fromone primate to the next by illustration andobservation, rather than learned afresh by trialand error1.

The report by Timothy Gentner and col-leagues on page 1204 of this issue2 challengesone more putatively uniquely human adapta-tion: the capacity to recognize complex ‘recur-sive’ structure. Gentner et al. showed that atleast one non-human species, the Europeanstarling (Fig. 1), can be trained to acquire com-plex recursive grammars such as the AnBn

language (in the case of the starling, rattle rattle warble warble; see below).

Recursion, or self-embedding, is withoutquestion a hallmark of human language. Forexample, one can take a phrase such as loveconquers all and embed it in a frame such as X knows Y, yielding, say, Chris knows love con-quers all. The output of that process can thenbe fed back into the X knows Y frame, yielding,say, Terry knows Chris knows love conquers all.Embedding also makes relative clauses possi-ble, as in the bracketed part of the paperback[on the coffee table] is hilarious. Marc Hauser,Noam Chomsky and Tecumseh Fitch3 havespeculated that recursion might be unique tohumans — and perhaps even the only contri-bution to language that is human-specific.Consistent with this, Fitch and Hauser4 foundthat cotton-top tamarin monkeys could notdistinguish the AnBn language from an osten-sibly similar language (AB)n that need not beconstructed recursively.

The AnBn language (see Fig. 1 of the paper2

on page 1204) is generally assumed to berecursive because new sentences can beformed by successive insertion into the frameAXB, for example AB, AABB, AAABBB and soon. Gentner and colleagues2 rewarded Euro-pean starlings for pressing a bar in response to

AnBn strings of starling-generated sounds, suchas rattle rattle warble warble, and withheld thereward for responses to the (AB)n grammar(and vice versa for another group of starlings).Although learning was not instantaneous, nineof eleven birds eventually (after 10,000–50,000trials) learned to discriminate reliably betweenthe two grammars, succeeding where the mon-keys had failed. An extensive series of controlcomparisons strongly suggests that the ulti-mately acquired grammar is robust. Notwith-standing some minor worries5, this is strongevidence that humans are not alone in theircapacity to recognize recursion.

What explains the discrepancy between thestarlings’ success and the tamarins’ apparentfailure? One possibility is methodological:Fitch and Hauser4 tested whether tamarinscould acquire AnBn spontaneously from a rela-tively brief exposure, whereas Gentner et al.2

asked whether starlings could acquire similar

structures from considerably longer expo-sures, enhanced with positive feedback, in amore active task. Only further experimenta-tion can clarify whether tamarins’ apparentinability to recognize the AnBn structure iscontext-specific or genuinely absolute.

If the split between tamarins and starlingsdoes prove robust, further comparative workwill clearly be necessary. Can other varieties of birds that don’t (in contrast to starlings) naturally acquire new songs also acquire self-embedded structures? Are humans aloneamong primates in their capacity to do so?Might the capacity for recursion be generalacross great apes, even if it were absent inmonkeys? An intriguing possibility is that thecapacity to recognize recursion might befound only in species that can acquire new pat-terns of vocalization, for example songbirds,humans and perhaps some cetaceans.

Whether or not tamarins can be prodded

Figure 1 | No bird brain. The European starling, Sturnus vulgaris, which Gentner et al.2 show is capableof recognizing complex grammar.

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into recognizing recursion, the analogoushuman capacity seems robust. Humans arequick to notice recursion and are able to do sowithout explicit reinforcement; perhaps mostimportantly, they can generalize recursivestructures broadly. Starlings have thus far beenshown to be able to extend AnBn only to newsequences of familiar sounds. Humans canclearly go further; once you recognize the pattern in AABB and AAABBB, it is a trivialmatter to extend that pattern to new vocabu-lary (for example, CCCDDD or JJJKKK). Takentogether with the tamarins, there actuallyseems to be a three-way split: some speciesmay generalize recursion only to items thathave already been instantiated in a given pattern; some species can generalize recur-sion freely to newly acquired vocabularies(arguably the essence of human language); andsome species apparently cannot recognizerecursion at all.

The “abstract computational capacity of language”3 may consist not so much of a singleinnovation as a novel evolutionary reconfigu-ration of many (perhaps subtly6 or even qualitatively7 modified) ancestral cognitivecomponents, genetically rejigged into a newwhole. Contemporary research suggests thatthe human brain contains few if any uniqueneuronal types, and few if any genes lack a significant ancestral precedent8. At the sametime, humans show much continuity withtheir non-speaking cousins in dozens of waysthat might contribute to language, includingmechanisms for representing time and space,for analysing sequences, for auditory analy-sis, for inhibiting inappropriate action, and for memory.

None of this challenges Chomsky’s long-held conjecture9 that children are innatelyendowed with a universal grammar — a set ofmental machinery that would lead all humanlanguages to have a similar abstract character.But that shared abstract character may have as much to do with our lineage as vertebratesas with our uniquely human innovations. InCharles Darwin’s immortal words, “through-out nature almost every part of each livingbeing has probably served, in a slightly modi-fied condition” in some ancestor or another. ■

Gary F. Marcus is in the Department ofPsychology, New York University, 6 WashingtonPlace, New York, New York 10012, USA.e-mail: gary.marcus@nyu.edu

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).6. Pinker, S. & Jackendoff, R. Cognition 95, 201–236 (2005).7. Marcus, G. F. The Birth of the Mind: How a Tiny Number of

Genes Creates the Complexities of Human Thought (BasicBooks, New York, 2004).

8. Fisher, S. E. & Marcus, G. F. Nature Rev. Genet. 7, 9–20(2006).

9. Chomsky, N. Language and Mind (Harcourt, Brace & World,New York, 1968).

Since 1995, the ‘stripe wars’ have been ragingin the demesne of high-temperature super-conductivity. This fierce conflict, fought withthe highest-calibre weapons of experimentalphysics, has its antecedent in a hotly contestedclaim about the way electrons behave in thecopper oxide materials notoriously used ashigh-temperature superconductors. Suppos-edly, they form highly organized patternscalled quantum stripes — but only on thepicosecond timescale, so the patterns averageaway over longer periods through the elec-trons’ constant quantum dance.

The dispute has lasted so long only becauseit has proved very hard to nail down such gen-uine quantum behaviour. In this issue, how-ever, Reznik et al. (page 1170)1 present furtherevidence in support of quantum stripes. Theyshow that the collective vibrations of theatomic lattice of certain superconducting cop-per oxides behave in a manner that is hard toexplain — unless one assumes that motionscharacteristic of the presence of quantumstripes are shaking the ion lattice. So is the endof hostilities in sight?

It is an everyday experience that, in a many-body system, collective behaviours emerge thatare utterly unrelated to the behaviours of theobjects that make it up. The evolution of anation’s economy over time, for example, is difficult to predict from the often conflictingmotivations of the constituent human mem-bers. The same principles apply in quantumphysics. But the exact rules that govern quan-tum emergence are poorly understood; uncov-ering them is a core business of modern physics.

Conventional superconductors — thoseoperating only at temperatures very close toabsolute zero — demonstrate only a minimalform of quantum emergence. In such materi-als, interactions between electrons diminish atlow temperature, and the macroscopic electronsystem turns into a near-ideal (non-interact-ing) quantum gas of ‘quasi’-electrons. A smallresidual attractive interaction binds thesequasi-electrons in pairs, which in turn collapseto a single quantum state in the process knownas Bose–Einstein condensation.

Although this theory of superconductivity,known as the Bardeen–Cooper–Schrieffer(BCS) model, gets everything right for con-ventional superconductors, it explains hardlyanything in high-temperature superconduc-tors. The discovery 20 years ago of this unusu-ally sturdy form of superconductivity raisedthe curtain on a drama of wider relevance: the

huge numbers of strongly interacting electronsin the copper oxide layers were plainly show-ing an unknown kind of collective quantumphysics (for an overview, see ref. 2). In 1995, it was discovered3 that small changes in thecrystal structure of high-temperature super-conductors can cause superconductivity todisappear, with a peculiar ‘static stripe phase’taking over (Fig. 1). Here, strong interactionsand quantum motions work together to formpatterns of electrons moving in serried ranks. Domains in which the electrons cometo a complete standstill separate these ‘rivers of charge’ (Fig. 1b).

The existence of static stripes, initially con-tentious, is now generally accepted. But quan-tum stripes are more radical and controversial.Members of the quantum-stripe faction holdthat stripes are, in fact, always present. When amaterial superconducts, the stripes do not dis-appear; rather, a quantum-mechanical super-position of countless disordered stripe statesforms (Fig. 1a), in such a way that the overallstate corresponds to that of a superconductingquantum liquid (for a mathematical proof ofprinciple, see ref. 4).

So how can we nail down quantum stripesexperimentally? Consider Erwin Schrödinger’soft-cited cat hidden in a sealed box (Fig. 1c).Classically, the cat must be either dead or alive,but quantum mechanically it is in a super-position of dead and alive states. In the quan-tum state, the cat fluctuates back and forthbetween alive and dead states. This fluctuationtakes a finite time. So, by taking snapshotsquickly enough, one can see either a dead or a live cat. The same scheme works equally well with superpositions of countless many-electron configurations. Here, every quantumconfiguration takes the role of a classical ‘deador alive’ state (Fig. 1c).

The fluctuation time of the quantum stripesis in the picosecond (10�12 second) range, andthe problem for the experimentalist is how tograb a picture of complicated spatial electronpatterns in so little time. One way to do this isto observe the change in kinetic energy of neu-trons that scatter off the material inelastically.Since 1995, such neutron-scattering experi-ments have added to the body of evidence supporting the case for quantum stripes5. Thedrawback is that these studies relied on infor-mation about the direction of the electronspins that was open to alternative interpreta-tions, including some compatible with the con-ventional BCS picture (see ref. 6 and references

SUPERCONDUCTIVITY

Quantum stripe search Jan Zaanen

Do quantum stripes exist or not? Further indirect evidence for thiscontroversial behaviour of electrons in high-temperature superconductorscomes from measurements of atomic-lattice vibrations.

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