HOW CHILDREN ACQUIRE LANGUAGE: THE MOTOR THEORY ACCOUNT
HOW CHILDREN ACQUIRE LANGUAGE: THE MOTOR THEORY ACCOUNTPatricia Kuhl, in her Nature review article 37, surveys the research carried out over many years into all aspects of theacquisition of language by children. She recognizes that ‘the mystery’ is not yet solved; although substantive progresshas been made on some aspects of infants’ speech development, notably of the phonology of the parent language. Howfar in fact has the extensive research program into child language taken us and how plausible and helpful so far aretheories of child language acquisition? Over the last few decades research into child language acquisition has beenrevolutionized by the use of ingenious new techniques which allow one to investigate what in fact infants (that ischildren not yet able to speak) can perceive when exposed to a stream of speech sound, the discriminations they canmake between different speech sounds, different speech sound sequences and different words. Infants’ perception ofspeech develops a good way ahead of their capacity to produce speech sounds, no doubt a reflection of the longer time ittakes for the motor capacity for speech to mature. . However on the central features of the mystery, the extraordinarilyrapid acquisition of lexicon and complex syntactic structures, little solid progress has been made.ProblemsAs Saffran, Senghas, and Trueswell40 strikingly put it: "You must discover the internal structure of a system thatcontains tens of thousands of units, all generated from a small set of materials. These units, in turn, can be assembledinto an infinite number of combinations. Although only a subset of those combinations is correct, the subset itself is forall practical purposes infinite. Somehow you must converge on the structure of this system to use it to communicate.And you are a very young child. This system is human language. The units are words, the materials are the small set ofsounds from which they are constructed, and the combinations are the sentences into which they can be assembled.Given the complexity of this system, it seems improbable that mere children could discover its underlying structure anduse it to communicate. Yet most do so with eagerness and ease, all within the first few years of life."More specifically,"Children learn implicitly. By 18 months of age, 75% of typically developing children understand about 150 words andcan successfully produce 50 words."1. How infants learn language with such remarkable speed remains a mystery.2. How do infants acquire and produce the speech sounds (phonemes) of the community language?3. How do infants find words in the stream of speech?4. How do infants link words to perceived objects or action, that is, discover the meanings of words? "The mechanismthat controls the interface between language and social cognition remains a mystery."5. "Why do we not learn new languages as easily at 50 as at 5?"6. "Why have computers not cracked the human linguistic code?"(quoted extracts from Patricia Kuhl)The Motor Theory Account of Child Language Acquisition1. Finding the phonemesOn the motor theory [Note 1], each speech-sound is the product of an articulatory gesture [Note 2]. Articulatory gesturesare the exapted products of innate motor programs which evolved in mammals for the generation of a set of specific armmovements or postures. An infant is sensitized to speech-sounds which generate neural motor programs which matchthe innate set of motor programs for arm postures and movements. This makes it possible for very young infants todiscriminate categorically between heard speech-sounds (as demonstrated by research using HAS (high amplitudesucking) or head-turning paradigms. The Motor Theory of Speech Perception (Alvin Liberman164 and colleagues)indicated how heard speech accesses motor programs required for the production of the heard speech (a cross-modaltransfer). A child becomes able to produce specific speech-sounds as the motor organization of the articulatory systemmatures in close association with muscular and neural motor development for bodily action generally (including controlof arm postures and movements). The infant’s early ability to discriminate speech-sounds more extensive than those inthe phonology of the parent language is narrowed down by exposure over months to the more limited range of speechsounds found in the parent language (as described by Patricia Kuhl and other researchers).1
2. Finding the Words"The word falls, one is tempted to explain, into a mould of my mind long prepared for it" (Wittgenstein181 ).Similarly, Chomsky7 said: "language acquisition commonly proceeds on course even without any concern on the part ofthe human models; the precision of phonetic detail a child acquires cannot possibly be the result of training. The speedand precision of vocabulary growth has to be explained by a biological endowment for language; the child somehow hasthe concepts available before experience with language and basically learns labels for already existingconcepts." (emphasis added). What can be the link between the mould and the word, between the already existingconcept and the word?What account can the motor theory give of the acquisition of words by children, not only the recognition, discriminationof the individual word, but also of the link which the child establishes between a word and the object or action to whichit refers; how does the child acquire the meanings of words? First of all, the child has to be able to pick out theindividual word of the parent language from the stream of speech-sound to which it is exposed. To approach this,consider: what is a word? A sequence of speech-sounds combined into a unity, separable from the flow of heard speech,formed from the phonemes which constitute the phonology of the ambient language. At the same time, a word is theproduct of the combination of articulatory gestures (by the speaker), of motor patterns formed from specific positionsand movements of the speaker’ s articulatory system, from the respiratory apparatus, to the larynx, the mouth, tongueand lips. A word is a neural motor program which can be instantiated to produce a specific patterning of sound in timeand which can be heard as integrated (like a music theme) by the hearer, by the child. An adult can perceive a spokenword as a familiar sound pattern but to the child the sound pattern of the word will be novel, unknown. So there must besome other process which enables a child to latch on to the novel, unknown word. This involves the neuralrepresentations of perceived objects and actions.3. Finding the objects and actions (to which words are to be attached)The ability of humans to recognize a nearly unlimited number of unique visual objects must be based on a robust andefficient learning mechanism that extracts complex visual features from the environment. The basic building blocks ofadult spatial vision are in place, if not fully mature, during the first few months of life. Studies often reveal impressiveperceptual skills in infants, despite physical limitations and a lack of world experience. Eye-related motor controlimproves dramatically over the first few months of life. Infants eventually show their perception of objects by reachingfor them, and at four months old, they can grasp objects. By four to six months of age, they can estimate an object’ sdistance, orientation, and size, as shown by appropriate grasps for the objects. In general, children seem to categorizeobjects based on shape rather than size or texture; shape is important for children. The implication is that shapegeneralizations can facilitate learning of categories for objects. Object segregation requires that infants both perceive (orunderstand) that the items in question are objects, and distinguish between these objects based upon their perceptualdifferences. Infants as young as 2 months of age can perceive a moving object as unified. Infants younger than 4.5months are capable of using featural cues to discriminate between objects, or other test items. Considerably youngerinfants, 2 months in fact, have some notion of "objectness".4. Attaching the words to the objects or actionsIn the brain, what might be the neural ‘mould’ or ‘concept pattern’ into which the word can fall, or fit like a key into alock? The infant’ s task is to find the appropriate words in the ambient language to fit what it already knows. Before theinfant knows words, it will be familiar with many objects, actions, sounds, colours etc. in the external world as well asin terms of its own body, states, feelings, emotions, patterns of its own action. For the infant to acquire the appropriateword for an object, the neural representations of these known objects must be linked with the neural representations ofthe appropriate words.Things which are to be attached to words are the result of many different forms of perception. This is reflected in thedifferent categories of words found in the lexicon: nouns, verbs, adjectives, adverbs, prepositions and conjunctions, theclosed set of function words (in English – in other languages inflections, affixes etc). Nouns and verbs may be concreteor abstract, refer to external perception or internal perceptions. Adjectives include colours, shapes and sizes – and thereare touches, tastes, smells, sounds (besides speech sounds), haptic (touch) experiences.To see how neural representations of words can become linked to neural representations of objects, it is necessary totake account of what research has discovered or suggested about the neural representation of objects. To start withvisual objects. Vision is by far the most important source for the infant’ s growing knowledge of the world. It isremarkably rapid; humans can recognize an object within a fraction of a second, even if there are no clues about whatkind of object it might be. Vision is the most richly represented sensory modality in the cortex. Experiments withmonkeys have revealed at least thirty separate visual cortical areas, occupying about one-half of the total cortex andrepresentation of vision in the human cortex is at least as extensive. Not surprisingly, infants concentrate on shapes;words for visual objects are first acquired;.2
There is a large body of research into the neural representation in the cortex of visual objects. "We are beginning tomake progress in identifying the distributed cortical networks associated with semantic object representations, and thenetworks underlying our ability to retrieve, select and operate upon them" (Martin and Chao133 ). A common feature ofall concrete objects is their physical form. The shape of an object is correlated with perceiving objects in many differentrecognition tasks. Neuroimaging studies have shown that regions in the cortex involved in representing and perceivingobjects represent object shape rather than contours or other low-level features. Common neural representation isobserved when objects have the same shape but different contours, but not when contours are identical and theperceived shape is different. (Ishai,Ungerleider, Martin and Haxby60 ; Ungerleider and Haxby69 ; Kourtzi andKanwisher128 ; Welchman, Deubelius and Kourtzi151 ).Visual perception of objects is a motoric process. The moulds into which, in the language development of the infant, theappropriate words of the ambient language will fall, or be fitted, are formed by the neural representations of the shapesof the concrete objects extracted from the infant’ s stream of visual experience by the remarkable motor processes of theeye. The eye scans the object by a rapid succession of movements (motor commands for the eye muscles producemovement of the eye up or down, from side to side and obliquely). Eye movements are composed of saccades andfixations; a saccade is a rapid movement of the eye to foveate (the fovea is the central and most sensitive part of theretina) one salient feature after another; a fixation is a pause in the movement of the foveated eye when the finer detailof the object at the point at which the movement is halted is to be obtained. The pattern of the actions of the eye is theresult of a complicated neural system, heavily researched but not yet fully described or explained. The outcome of thescanning process is a network of movements and halts, more easily illustrated than described in the work of the Russianphysiologist Yarbus70 (as continued by Noton and Stark65 ). See the famous illustration (recorded by Yarbus) ofmovements of the eye in scanning a photograph of a bust of Nefertiti. It is from the motor record of the scanning of theobject that the shape of the object is derived and finds its neural representation in the dorsal and ventral streams of thebrain’ s cortical visual system. This is how the ‘mould’ or first outline of the conceptual store for an object is formedinto which the neural representation of the appropriate word for the object is to ‘fall’ or be fitted.NEFERTITI5. The recognition of the appropriate word for an objectHow is the appropriate word formed and found to be appropriate, in the sense of fitting into orbeing directly associated with the neural mould constituted by the representation of the visual objectin the cortex? Consider what the situation for the infant or young child is. The child has beensurrounded from birth, enveloped from birth in the normal case, in a stream of speech sound; thechild has been able to distinguish and be responsive to speech sound as distinct from other sound;speech sound has often occurred at the same time as the child is placed amongst objects to whichwords are eventually found to refer. This process can be taking place anywhere in the world, in anylanguage area. In each different language area a child will be exposed to a different set of wordsfound to be appropriate for the visual objects which the child has already acquired. How can allthese words in thousands of different languages, a multitude of different words for the same visualobject, be appropriate? This is to be dealt with later; for the time being the question is to beconsidered for a single language, the ambient language of the developing infant. The question ishow the particular word in the language for a particular visual object, the word which eventuallygoes to fit the ‘mould’ or neural store already developed by the infant for the object, is in some wayspecially appropriate and recognised as such by the infant.3
The question how a word is appropriate to its meaning leads back to the way in which wordsemerged for particular objects in the history of any given language. How does a word emerge? Atsome stage in the history of a human group a new word emerged for the particular object; the newword came into general use and survived because it seemed the fittest word for the object tomembers of the originating group. In what sense could a word be fit for the object to which itreferred? In what sense could the word be said to match the object? Let us consider the perceptionof any object. For many objects, on seeing them, we can, without saying a word, perform a handand arm movement to indicate the object. For example, we can indicate a circle by forming orperforming a circle with our arms. For a tree, we can, with some accuracy, even indicate the kind oftree by using our hands and arms. We can indicate other objects by pointing. to them, to our head,our foot, our ear, our eye and so on. Homely visual objects, a bowl, a cup, a plate, can be indicatedby miming the particular shape. Other items can be indicated by the appropriate contour, a step, anedge, a hill. For many objects we can perform actions to indicate the objects to other persons in ourgroup. Objects are represented by patterns of action for which we have acquired the neuralrepresentations needed to perform the actions. Once we have in our neural store (motor memory)the pattern of action representing the object, we can, by a universally available process of motorequivalence[Note 3], transfer this bodily pattern of action to the articulatory system; an externallyperceivable bodily gesture becomes an articulatory gesture, producing a sequence of speech sounds,a unified word, equivalent to the action. We can do this because, as considered in the section aboveon phonemes, speech sounds are evolutionarily derived from bodily action, from innate programsfor elementary movements and postures of the arms. This does not mean or require that at any pointin the history of a human group there must have been a developed gestural language as a precursorto spoken language. An appropriate word for an object can be generated simply by imagining howan object might be physically represented or by concentrating on the visual perception of the objectand transferring this imagined or visual motor pattern to form an articulatory gesture and soconstitute an appropriate word.We now have arrived at a set of speech sounds forming a word which matches the object, a wordwhich is appropriate for the object to which it refers. How does an infant acquire the word for avisual object on hearing it? From the motor theory of speech perception, the hearing of a word byan adult is perceived in terms of the motor program required to produce the word; the word is crossmodally transformed into the articulatory motor program for producing the word. Similarly for theinfant the word is cross-modally transformed (from auditory to motor). There are well-knownexamples of cross-modal transformation by infants, notably the transformation which must takeplace when an infant reproduces in its own face the facial expression of the experimenter. See thefamiliar illustration of a very young child doing this:from Meltzoff and Moore94, 954
The motor program so generated by the infant on hearing the word matches the motor program already acquired as aneural representation for the object from the motoric visual processes involved in perceiving the object. In the same wayas we recognise a visual object by reacquainting ourselves with the motor shape of the object, so the infant ‘recognizes’the motor structure of the word for the particular object in the ambient language. The two fit together, are associatedtogether neurally. This is how the infant acquires the word and its meaning. The word ‘falls into the mould’ constitutedby the neural representation of the object, or, in Chomsky’ s terms, the infant acquires the appropriate ‘label’ for theobject, a label which is not random or arbitrary but designed to match the motor shape of the object to which it refers.6. Action word acquisition by childrenThe next large class of words acquired by young children are action words. These are usually classified as verbs butmany nouns are also in effect action words, most obviously nouns formed from verbs. There is evidence(Pulvermüller143) that action nouns and verbs are stored together in the cortex. A principal section of action wordsrelate to the child’ s own bodily activities, eating, drinking, biting, walking, talking, chewing, touching, giving, taking,pushing, pulling, climbing, grasping, and these activities as seen in the behaviour of other children and adults. Theformation of words for these actions in the ambient languages in many cases seems especially transparent, particularlyfor words involving movements of the mouth and face. Saying the word ‘spit’ almost amounts in itself to the action ofspitting; the same seems true also for ‘bite’ , ‘chew’ , ‘gnaw’ , ‘snarl’ , ‘sniff’ . The infant will have acquired the neuralrepresentations for most actions at an early stage in its development, no doubt with ‘suck’ and ‘cry" as primordial(innate) actions. How did the words for actions in any language emerge as early items in the lexicon acquired in agroup? The pattern of action generated the articulatory gesture for the action and within a small group (probably a closefamily group) the word was recognised as being appropriate for the act
The Motor Theory Account of Child Language Acquisition 1. Finding the phonemes On the motor theory [Note 1], each speech-sound is the product of an articulatory gesture [Note 2]. Articulatory gestures are the exapted products of innate motor programs which evolved in mammals for the generation of a set of specific arm movements or postures. An infant is sensitized to speech-sounds which .
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Le genou de Lucy. Odile Jacob. 1999. Coppens Y. Pré-textes. L’homme préhistorique en morceaux. Eds Odile Jacob. 2011. Costentin J., Delaveau P. Café, thé, chocolat, les bons effets sur le cerveau et pour le corps. Editions Odile Jacob. 2010. Crawford M., Marsh D. The driving force : food in human evolution and the future.
Le genou de Lucy. Odile Jacob. 1999. Coppens Y. Pré-textes. L’homme préhistorique en morceaux. Eds Odile Jacob. 2011. Costentin J., Delaveau P. Café, thé, chocolat, les bons effets sur le cerveau et pour le corps. Editions Odile Jacob. 2010. 3 Crawford M., Marsh D. The driving force : food in human evolution and the future.
