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ScienceWeek
COGNITIVE SCIENCE: ON LANGUAGE AND HUMAN INTELLIGENCE
The following points are made by David Premack (Science 2004 303:318):
1) Humans have acquired six symbol systems: two that evolved --the genetic code and spoken language -- and four that we invented: written language, arabic numerals, music notation, and labanotation (a system for coding choreography). Dobzhansky's quip "All species are unique, but humans are uniquest" raises the question: Is it language, the symbol system that evolved only in humans, that makes humans the "uniquest"? Dobzhansky's quip also raises a more fundamental question: What exactly is the nature of human uniqueness?
2) The grammar or syntax of human language is certainly unique. Like an onion or Russian doll, it is recursive: One instance of an item is embedded in another instance of the same item. Recursion makes it possible for the words in a sentence to be widely separated and yet dependent on one another. "If-then" is a classic example. In the sentence "If Jack does not turn up the thermostat in his house this winter, then Madge and I are not coming over," "if" and "then" are dependent on each other even though they are separated by a variable number of words (1-3). Are animals capable of such recursion? Fitch and Hauser (4) have reported that tamarin monkeys are not capable of recursion. Although the monkeys learned a nonrecursive grammar, they failed to learn a grammar that is recursive. Humans readily learn both.
3) The lack of recursion in tamarins may help to explain why animals did not evolve recursive language, but it leaves open the question of why they did not evolve nonrecursive language. Recursion is not, of course, the only preexisting faculty on which the evolution of language depends.
4) A laboratory chimpanzee does not call to attract the attention of its trainer; instead, it pounds on a resonant surface. Similarly, when chimpanzees become separated in the compound, they do not call to one another, as humans would, but search silently until they see one another and then rush together. If, as the evidence suggests, vocalization in the chimpanzee is largely non-voluntary (reflexive), speech could not have evolved. But then why don't chimpanzees sign to each other? The chimpanzee has voluntary control of its hands. However, sign language depends on the face as well as the hands, and facial expression in the chimpanzee is evidently as reflexive as vocalization. Facial expressions play linguistic roles in signing, such as denoting the boundaries of clauses. A signer processes emotional facial expression in the right hemisphere, but linguistic facial expression in the left hemisphere (5). This does not mean, of course, that chimpanzees could not have evolved a language based on pounding on resonant surfaces, arranging stones on the ground, and so on. But it does suggest that they could not have evolved one that is like either speech or sign. (Of course, speech and sign "travel" with the speaker in a way that stones and resonant surfaces do not.)
5) What are the factors that distinguish human intelligence? A major distinctive feature of human intelligence is flexibility. Animals, by contrast, are specialists. Bees are adept at sending messages through their dances, beavers at building dams, the nuthatch at remembering the location of thousands of caches of acorns it has buried. But each of these species is imprisoned by its adaptation; none can duplicate the achievement of the other. The nuthatch cannot build dams; bees do not have an uncanny memory for hidden caches of food; beavers cannot send messages. Humans, by contrast, could duplicate all these achievements and endlessly more. Why? Is recursive language the key to human flexibility?
6) Human intelligence and evolution are the only flexible processes on Earth capable of producing endless solutions to the problems confronted by living creatures. Did evolution, in producing human intelligence, outstrip itself? Apparently so, for although evolution can do "engineering", changing actual structures and producing new devices, it cannot do science, changing imaginary structures and producing new theories or explanations of the world. Clearly, language and recursion are not the sole contributors to human uniqueness.
References (abridged):
1. N. Chomsky, Syntactic Structures (Mouton, The Hague, 1957)
2. S. Pinker, The Language Instinct (Morrow, New York, 1994)
3. M. D. Hauser, N. Chomsky, W. T. Fitch, Science 298, 1569 (2002)
4. W. T. Fitch, M. D. Hauser, Science 303, 377 (2004)
5. E. L. Newport, T. Supulla, in The MIT Encyclopedia of the Cognitive Sciences, R. Wilson, F. Keil, Eds. (MIT Press, Cambridge, MA, 1999), pp. 758-760
Science http://www.sciencemag.org
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NEUROBIOLOGY: HUMAN LANGUAGE
The following points are made by A.R. McIntosh and N.J. Lobaugh (Science 2003 301:322):
1) The human ability to use language is one of the most remarkable features of our species. Through speech and writing we communicate with astonishing complexity. Given this unique skill, we might expect that the human brain has developed certain areas that are specialized for language. Early studies of people with brain damage suggested an apparent division of labor between the brain's two hemispheres (lateralization), the left hemisphere being more important for language than the right.
2) It is not clear which functions are lateralized to the right hemisphere, but they seem to involve visuospatial operations such as identifying the locations of objects in the environment. Although there is consensus on the general division of labor, the nature of the separation is not at all clear. Specifically, it is still a matter of debate whether lateralization depends on a linguistic stimulus or on the nature of the task (linguistic versus visuospatial) that the stimulus elicits. Stephan et al (Science 2003 301:384) present neuroimaging evidence from human subjects suggesting that the functional split between the left and right hemispheres of the brain depends on what must be done with an incoming stimulus (the task), rather than the nature of the stimulus itself.
3) One difficulty in assessing the differential specialization of the two hemispheres is choosing the most appropriate stimulus. Thus, when assessing visuospatial judgment tasks, the tendency is to avoid a linguistic stimulus. Stephan et al. predicted that if lateralization depends on the task rather than the stimulus, then the same stimulus could be used to directly test lateralization of linguistic and visuospatial functions. Brain activity was measured with functional magnetic resonance imaging (fMRI) while subjects performed experimental tasks. Even though words were presented to subjects, the brain regions activated reflected what the person had to do with the word.
Science http://www.sciencemag.org
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ON THE ACQUISITION OF LANGUAGE BY CHILDREN
The following points are made by J.R. Saffran et al (Proc. Nat. Acad. Sci. 2001 98:12874):
1) Before infants can begin to map words onto objects in the world, they must determine which sound sequences are words. To do so, infants must uncover at least some of the units that belong to their native language from a largely continuous stream of sounds in which words are seldom surrounded by pauses. Despite the difficulty of this reverse-engineering problem, infants successfully segment words from fluent speech from approximately 7 months of age.
2) How do infants learn the units of their native language so rapidly? One fruitful approach to answering this question has been to present infants with miniature artificial languages that embody specific aspects of natural language structure. Once an infant has been familiarized with a sample of this language, a new sample, or a sample from a different language, is presented to the infant. Subtle measures of surprise (e.g., duration of looking toward the new sounds) are then used to assess whether the infant perceives the new sample as more of the same or something different. In this fashion, we can ask what the infant extracted from the artificial language, which can lead to insights regarding the learning mechanisms underlying the earliest stages of language acquisition.
3) Syllables that are part of the same word tend to follow one another predictably, whereas syllables that span word boundaries do not. In a series of experiments, it has been found that infants can detect and use the statistical properties of syllable co-occurrence to segment novel words. More specifically, infants do not detect merely how frequently syllable pairs occur, but rather the probabilities with which one syllable predicts another. Thus, infants may find word boundaries by detecting syllable pairs with low transitional probabilities. What makes this finding astonishing is that infants as young as 8 months begin to perform these computations with as little as 2 minutes of exposure. By soaking up the statistical regularities of seemingly meaningless acoustic events, infants are able to rapidly structure linguistic input into relevant and ultimately meaningful units.
Proc. Nat. Acad. Sci. http://www.pnas.org
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GRAMMAR VS. LANGUAGE IN NEUROLINGUISTICS
The following points are made by Massimo Piatelli-Palmarini (Nature 2002 416:129):
1) Two styles of explaining the science of mind and behavior have been competing for as long as anyone cares to remember: empiricist, centering on habit formation, statistical learning, imitation and association; and rationalist, focusing on the projection of internally represented rules. Despite relentless effort, the former has delivered rather meager results, whereas the latter, with its pivotal concept of an internally represented grammar, has produced the solid "conceptual cognitive revolution".
2) For a rationalist cognitive scientist, a grammar is a finite mental object, systematically assigning abstract structures to all the well-formed expressions of a language -- that is, to each member of a set that, for natural languages (such as Chinese or Italian), is infinite and discrete. Infinite, because every speaker of a language can produce and understand an unlimited number of new grammatical sentences. Discrete, because continuous modification of a sentence to change it into another is impossible. No sentence could be halfway between "It's a good car, but they don't sell it" and "It's a good car, but they don't tell it."
3) A grammar capable of generating complex structures for all well-formed sentences of a natural language must have recursive rules, because phrasal constituents can contain other phrasal constituents of the same or higher kinds ("The young doctor's three beautiful sisters" is a noun phrase containing another noun phrase; "The spy who came in from the cold" is a noun phrase containing a sentence). Moreover, structural rules of sentence formation can be applied recursively to embed relative clauses embedding other relative clauses, without limit (as in "This is the cat that killed the rat that ate the malt that lay in the house that Jack built"). Because such grammars are finite, whereas the languages they generate are infinite and contingently shaped by use, it is advantageous, and methodologically cogent, to consider the concept of grammar as primary, and that of language as derived.
4) Since the mid-1950s, powerful formal criteria, derived from analysis of the artificial languages of mathematics and computer programming, have been applied to the study of natural languages to determine principles by which a given class of grammars can generate a given target language. A universal ('Chomsky') hierarchy of grammars (automata) was established: the most powerful class contains as a subclass the immediately less powerful one, and so on. In tune with the dominant empiricist inductivist tradition of the 1950s, the first grammars to be explored at the lowest level in the hierarchy were probabilistic and finite-state. From a very large corpus of ascertained utterances of the language, one can compute the conditional probability that a word (or string of words) will follow another.
Nature http://www.nature.com/nature
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