Anne-Marie Adams and Catherine Willis
The central problem in accounting for the acquisition of language is to explain how a child is able to master the extremely complex series of rules that prescribe the possible combinations of linguistic elements making up the language to which the child is exposed. The response from the nativist tradition has been to propose that the linguistically described rule system is so complex that learning it presents an impossible task and hence at least some description of these rules must be innately given (e.g. Chomsky, 1965, 1986). The child is therefore characterised as a passive recipient of language experience and acquirer of linguistic knowledge. Constructivist approaches to language acquisition offer alternative accounts which propose a more active role for the child (see Messer, 1994 for an overview). Such approaches attribute significance to factors external to the child, for example aspects of the linguistic experience such as the surface form of the language, including its prosodic and phonological nature, the social context and the pragmatic functions of communication. Factors internal to the child are also considered to be instrumental in language acquisition, for example the individual’s relative sensitivity to the physical form of the language, thereby emphasising skills such as phonological perception and prosodic awareness (e.g. Vihman, 1996), and their preferred manner of relating language to experience (e.g. Nelson, 1973).
This emphasis on the child’s experience of the language input and the role of features internal to the child in learning a language, compels a description of the underlying cognitive mechanisms and resources that these imply. Such a description, together with a statement of how these fa c t o r s m a y i m p i n g e o n l a n g u a g e l e a r n i n g , wo u l d c o n s t i t u t e a n information-processing account of language development. To date, however, very little research investigating language development has focused on describing the putative underlying cognitive processes.
The present chapter considers whether working memory, specifically the fractionated account originally proposed by Baddeley and Hitch (1974) and later developed by Baddeley (1986), offers a valid account of the
cognitive processes which may underlie language acquisition. The chapter begins by presenting evidence of an association between the phonological component of the fractionated model of working memory (henceforth PWM) and children’s language development, and introduces the proposal that this association reflects the role of PWM in the long-term learning of phonological representations. The concordance between this proposed role and some constructivist models of language acquisition is highlighted. We then consider possible alternative accounts of the association, including the converse view that the association reflects the influence of mechanisms of the language system on verbal short-term memory task performance. A further possible account, that the association between PWM and language may reflect limitations in a more general working memory resource is assessed using evidence from a competing (typically North American) tradition of working memory research. Finally, the efficacy of the fractionated model is evaluated through the central questions of this volume.
PHONOLOGICAL MEMORY AND THE
ACQUISITION OF LINGUISTIC KNOWLEDGE
The association between PWM and language development
The model of short-term memory proposed by Baddeley and Hitch in 1974 was postulated as a working memory—a short-term memory that is heavily involved in everyday cognitive functioning. The model comprises three distinct components: the visuo-spatial sketchpad, the phonological loop and the central executive (see chapter 1). Very early in the endeavour to determine the areas of everyday cognitive functioning in which working memory was involved, researchers examined the intuitive proposal that the phonological component of working memory (PWM) was implicated in processing speech. Although work with adults indicated that PWM was not involved in skilled language processing (e.g. Shallice, 1988 for a review), recent studies have shown a strong and reliable association between PWM and language skills in children. Indeed much evidence has been accrued to support the contention that PWM may play an important role in learning language (see Baddeley, Gathercole & Papagno, 1998 for a review).
Illustrative studies that have investigated the association between PWM and language development including both vocabulary and morpho-syntactic knowledge in children are presented next.
Children’s phonological memory skills are associated with their vocabulary knowledge, such that children who perform better on PWM tasks also tend to have better receptive and expressive vocabularies (Adams & Gathercole, 1995, 2000; Gathercole & Adams, 1993, 1994;
Gathercole, Hitch, Service, & Martin, 1997). Children grouped on the basis of their PWM performance also demonstrate differences in their knowledge of morpho-syntactic constructions, again evident in both their
WM and Language processing 81 comprehension and production of language. Willis (1997) showed that four-year-old children with better PWM understood a wider range of syntactic constructions than did children with poorer PWM. Groups of children aged between three and five years similarly classified as having either relatively good or poor PWM, could also be distinguished in terms of their morphosyntactic profiles (Adams & Gathercole, 1995, 2000). The speech of children with relatively good PWM comprised longer utterances, in terms of the mean number of morphemes per utterance, and included a wider range of syntactic constructions than did the speech of children classified as having relatively poor PWM skills. There is also some evidence, albeit inconclusive, that this association may not extend to the visuo-spatial component of the fractionated model of working memory (Adams & Gathercole, 2000).
Despite the wide variation in language abilities seen in these studies, none of the children were identified as having problems with language development—all had language skills that were appropriate for their age.
Similar associations between PWM and language development are, however, also evident in children with disordered language development (e.g. Gathercole & Baddeley, 1990; Gillam, Cowan & Day, 1995;
Montgomery, 1996). Although beset by problems of the heterogeneity of the samples and issues of the appropriate comparison group, the PWM deficit of such children has proved to be a robust finding. It may also be that this language/memory association persists in a broader range of developmental conditions (see chapter 6, this volume).
The role of PWM in the long-term learning of phonological representations
What mechanisms might underpin this association between PWM and language development? Baddeley et al. (1998) concluded that this relationship reflected differences in the quality of temporary phonological representations that could be maintained in working memory, since the construction of long-term phonological representations depended on such temporary storage. While accepting that an exact account of the mechanism by which such long-term knowledge may be consolidated remains unspecified, they drew comparisons with the Brown and Hulme (1996) computational model of verbal short-term memory whereby both computational and evolutionary advantage may be gained by postulating a temporary representational system which is able to represent experiences rapidly so that they can be compared to prior knowledge. This affords not only the ability to identify novel stimuli which diverge from long-term representations, but also an efficient means of learning the consistencies in the environment without the need to consolidate every aspect of each experience.
How might this mechanism be influential in morpho-syntactic development? Just as PWM may be required to consolidate long-term
phonological representations of new lexical items in vocabulary acquisition, similar processes may be available which detail the phonological form of syntactic constructions. Speidel (1989, 1993; Speidel & Herreshoff, 1989) proposed that variations in the efficiency of interdependent articulatory and phonological memory processes governed the child’s ability to imitate and create long-term memory phonological representations of morpho-syntactic constructions produced by adults. These representations are then used as templates for the syntactic structure of intended utterances. Language development was proposed to reflect both the quantity and the quality of this corpus of grammatical forms. By this account, children with better PWM would be able to imitate, create and commit to long-term memory, lengthier and more exact representations of the speech they hear. The evidence that children who have better PWM skills not only understand a greater number of syntactic constructions in comprehension tasks, but also include a wider range of syntactic constructions in their spontaneous speech is entirely consistent with this view.
How long-term phonological knowledge may affect language development
PWM may therefore be involved in the construction of long-term phonological representations which underpin the acquisition of both lexical and morpho-syntactic forms. However, learning the purported complex series of abstract rules that dictate the permissible forms of linguistic elements is often presented as a qualitatively different task.
Nevertheless, some constructivist theories of language acquisition do argue that the child’s ability to attend to the surface structure of the language they hear, and perhaps more importantly to retain this phonological information, plays an instrumental role in ‘bootstrapping’ the child to syntactic competence. A sample of such work from a range of theoretical perspectives on language acquisition is presented next.
Construction grammar
Rather than proposing that language merely reflects abstract knowledge of the rules of formal linguistics, construction grammar (e.g. Goldberg, 1995) proposes that all the elements of linguistic form are represented as diverse items (constructions) within a single lexicon. Thus the lexicon contains not only lexical items as commonly proposed (words) but also b ound morphemes and phrase structures. A corollary of this assertion is that a single mechanism is postulated to underlie the acquisition of both words and grammar (for empirical evidence consistent with this view see Marchman & Bates, 1994). Thus an ability to retain the phonological form of the language input may underpin not only lexical development but morpho-syntactic knowledge in the form of constructions too.
WM and Language processing 83 Connectionist learning
Connectionist accounts of language acquisition also propose that children’s progression towards performance which more accurately reflects adult linguistic patterns may be explicable in terms of general principles of learning without the need to stipulate prior (i.e. innate) knowledge of linguistic categories or rules. For example the network of Plunkett and Marchman (1993) accounted for both lexical and grammatical development within a single learning mechanism. These findings can also be interpreted as attributing a central role in language development to the acquisition of a body of knowledge about the surface form of language. In the Plunkett and Marchman (1993) model, the characteristic shift from early competence with both regular and irregular forms of the past tense to overgeneralisation of the suffix with overextensions to irregular forms, was dependent on the size of the lexicon the system had acquired. This pattern was explained in terms of a ‘critical mass’ hypothesis—when and only when the data set is sufficiently large to extract general patterns, does a pattern of language use (overgeneralisation) appear which suggests the application of linguistic rules. Language acquisition may therefore be explicable in terms of the general principles of a gradual learning mechanism which results in long-term memory representations of the input from which linguistic competance may be derived. The hypothesised role of PWM in the acquisition of long-term phonological knowledge should perhaps be considered as one account of this learning mechanism.
Distributional analysis of language
The importance of experience with and knowledge of the surface form of language is emphasised in yet another perspective on language acquisition.
There is increasing evidence that it may be possible to derive linguistic knowledge from a statistical analysis of the language input. Although previously believed to be a prohibitively complex task, Saffran, Newport and Aslin (1996) demonstrated that when learning an artificial language, adults were able to take account of the differences in transitional probabilities between sequences of sounds within words compared to those w h i c h s p a n n e d wo r d b o u n d a r i e s . A d u l t s w e r e t h e r e f o r e a b l e t o discriminate sequences of sounds which formed words even when presented with only acoustic input and no other cues to word boundaries.
Saffran et al. (1996) proposed that similar distributional analytic techniques may be available to infants, who certainly do prefer to listen to the prosodic and syntactic patterns of their native language over those of other languages (Jusczyk, Friederici, Wessels, Svenkerud, & Jusczyk, 1993), and that these skills would aid the acquisition of certain aspects of morphology. Thus distributional analyses of language may also be interpreted as evidence that sensitivity to and the ability to exploit the
phonological form of language, which allows one to derive long-term knowledge (even if implicit) about its consistencies and regularities, may be an important feature of language acquisition.
Unanalysed holistic phrases
Naturalistic assessments of child language have provided empirical evidence that for some children their utterances may simply be the repetition of unanalysed holistic phrases, and that this may be one route to gaining linguistic competence (Pine & Lieven, 1993; Lieven, Pine &
Baldwin, 1997). Pine and Lieven (1993) proposed that some of the phrases children produced were merely imitated and therefore unanalysed (i.e.
they had not been constructed using syntactic rules), since there was little evidence of their productive use across a range of lexical items. Lieven et al. (1997) suggested that the syntactic constructions represented by such phrases were only gradually generalised to other appropriate lexical items.
In their account of language acquisition, therefore, grammatical development evolves from a corpus of rote-learned phrases. It is entirely possible that PWM may be required to construct and retain long-term representations of such unanalysed phrases.
To summarise, the phonological component of the fractionated model of working memory is indeed a potentially viable account of how a corpus of grammatical constructions, perhaps evident in the holistic phrases identified in children’s speech, may be learned. Phonological working memory may also serve as the mechanism which determines the rate of acquisition or representational quality of phonological knowledge that is crucial in distributional analyses of language and hypothesised in connectionist models of the language acquisition process.
COMPETING INTERPRETATIONS OF THE
ASSOCIATION BETWEEN PWM AND LANGUAGE The influence of long-term knowledge on PWM
Thus far our interpretation of the association between PWM and language has been that a child with an impaired ability to retain phonological information in short-term memory is likely to be at a disadvantage when trying to learn new words and syntactic constructions. A plausible alternative, however, is that poor language skills impair performance in verbal short-term memory tasks (e.g. van der Lely & Howard, 1993;
Howard & van der Lely, 1995). In support of this position performance on PWM tasks has indeed been shown to be influenced by long-term knowledge at both the lexical (Hulme, Maughan & Brown, 1991) and the sub-lexical (Gathercole, 1995) level. This suggests that aspects of PWM
WM and Language processing 85 tasks other than their mnemonic component should be considered when analysing the relationship between these tasks and language development.
For example, the language difficulties of children with disordered language development may originate in phonological processing difficulties (e.g. the ability to perceive, combine or articulate phonemes) and be a direct cause of their poor PWM performance (Snowling, Chiat &
Hulme, 1991).
Investigation of this position from within the fractionated working memory tradition, whilst not discounting the importance of phonological p r o c e s s i n g s k i l l s a n d l o n g - t e r m k n ow l e d g e i n s u p p o r t i n g P W M performance, has generally concluded that a substantial proportion of the PWM/ language association rests on the ability to maintain phonological representations in short-term memory. This work has been conducted with children both with disordered (Edwards & Lahey, 1998; Gillam, Cowan &
Marler, 1998) and non-disordered language development (Gathercole et al., 1997; Gathercole, Hitch, Service, Adams & Martin, 1999).
To illustrate the point one such study will be described in more detail.
Gathercole and Pickering (1999) employed a technique devised to obtain separate estimates of the extent to which the serial recall of words and non-words of both high- and low-probability phoneme combinations reflected (a) the capacity of the phonological store, and (b) the contribution of long-term knowledge of lexical items and the phonotactic probabilities of the language (Gathercole, Frankish, Pickering & Peaker, 1999). Children grouped in terms of their vocabulary knowledge (either relatively good or relatively poor) were assessed on these serial recall tasks. The extent to which performance on these tasks varied as a function of vocabulary ability was assessed. The groups differed only on recall performance that could be attributed to the capacity of the phonological store, not to that attributed to long-term lexical or phonotactic knowledge. Together the evidence suggests that the association between PWM and vocabulary knowledge, for the most part, reflects individual differences in PWM that may influence vocabulary development, rather than the beneficial effects of the commensurately greater lexical and phonotactic knowledge on non-word recall.
Commonality of processes
A more radical interpretation of the relationship between language development and the ability to perceive, produce and retain phonological information is that the association arises because processing and storage required in PWM tasks are those same operations that principally exist to process speech. Thus rather than asking how a separate PWM system might support language development, the question can be reversed to ask what are the mnemonic capabilities of the speech system and what purpose in language comprehension and production might these serve?
Researchers working within the fractionated tradition have not remained impervious to the implications of the influence of other forms of knowledge on the contents of the phonological store. Indeed the influence of long-term knowledge on the retention capacity of the phonological store is accepted (Baddeley & Logie, 1999) and attempts have been made to adapt the model to account for such findings. For example Gathercole and Martin (1996) proposed that the ability to store phonological information for short periods of time emerges directly as a byproduct of the speech input processing system. The integrity of the fractionated model, however, requires that the temporary storage of such information remains functionally separate from the processing of long-term phonological information as accomplished by the central executive.
Models of verbal short-term memory (STM) have been proposed, however, which account for such performance on the basis of the processing and storage capabilities of the language system (Barnard, 1985, 1999; Monsell, 1984; Saffran & Martin, 1990). For example within Barnard’s Interacting Cognitive Subsystems (ICS) model, the individual subsystems that underlie the processing of speech include implicational, propositional, morphonolexical and articulatory subsystems (crudely, translating ideas into meanings represented as sounds and then to speech).
Each subsystem can represent, store and recode information. This model d o e s n o t p o s t u l a t e s t o r a g e c o m p o n e n t s ( t h e p h o n o l o g i c a l l o o p ) independently of the system which processes such information, nor a separate limited-capacity processing resource (the central executive), since capacity limitations within the ICS model result from the efficiency of the functioning and the interactions between the subsystems (see also chapter 12, this volume).
Currently the evidence of merely an association between language development and performance in verbal STM tasks is insufficient to decide between these two accounts. Children who perform better in verbal STM tasks may also demonstrate better language skills either because they have a larger PWM which allows them to create better long-term memory (LTM) phonological representations that are advantageous in spoken language production, or because the representational and ‘memory record’
or re-representational processes of the cognitive subsystems involved in language are more efficient. These two models have previously been judged to be highly compatible (Barnard, 1999; Shah & Miyake, 1999);
however, one factor on which they can be discriminated is the need to postulate functionally separate systems for storage and controlled processing. Since this aspect of the fractionated model is directly relevant to an alternative account of the nature of the relationship between working memory and language, it will be considered after this account has been introduced.
WM and Language processing 87 AN ALTERNATIVE ACCOUNT OF THE WORKING
MEMORY/LANGUAGE ASSOCIATION: WORKING MEMORY CONSTRAINTS ON LINGUISTIC PROCESSING
The preceding sections considered how PWM might be involved in learning the phonological form of various aspects of the language from which knowledge of linguistic structure might be derived. However, other research suggests a rather different role that working memory might play in language development. In such accounts limited resources are presumed available to process all aspects of language (e.g. semantic, syntactic and phonological). These limitations extend beyond the storage function of PWM and suggest constraints in more general cognitive processing resources. The association between PWM and language may therefore represent only one feature of a wider link between working memory and language development.
Resource limitations in children’s speech processing
Within developmental psycholinguistics, researchers (e.g. Crystal, 1987;
Gerken, 1991; Valian, 1991) have postulated that children’s speech incompletely represents ‘legal’ syntactic requirements because the accumulated complexity of the target utterance, in terms of message formulation, syntax assembly and lexical selection, exceeds the child’s currently available processing resources. Similar accounts of resource limitations constraining children’s speech have been developed within cognitive psychology. Bock (1982) proposed a model of adult speech production in which the utterance content was constructed in five ‘arenas’
(referential, semantic, phonological, phonetic and motor-assembly).
Although the resources available to construct the utterance were considered to be limited it was regarded that, in skilled speakers, processes other than those that took place in the referential arena (deciding what to say), for example accessing the relevant syntactic and phonological information, were relatively automatic processes. These processes were therefore not demanding of the limited cognitive resources. Whilst not primarily intended as a developmental account, Bock did propose that lower level processes such as word production might only achieve automatisation with development and thus during childhood such phonological processing would make demands on the limited resources available to process speech. Since the entirety of the resources was limited, directing some of these resources to lower level processing would deprive resources needed to construct, for example, referential aspects of the utterance.
A similar account of an interaction between the requirement to actively process phonological information and to construct other aspects of the utterance is outlined in Speidel’s model of language development
presented earlier. In this model too, constructing an utterance is proposed to require cognitive resources whose totality is finite. Assembling an utterance using a stored template of a syntactic construction, however, is suggested to be less demanding of these resources than constructing an utterance from scratch. The child with a more comprehensive storehouse of the phonological form of syntactic constructions may be able to apply these in their speech, thereby freeing resources to incorporate increased complexity in other aspects of the utterance.
Levelt (1992) proposed that automatically accessible long-term phonological representations (lexemes) may be an important part of skilled speech output. However, an unfamiliar phonological form, such as a foreign word or a non-word, may not have an automatically accessible long-term phonological representation. Evidence from both neuropsychology (Bub, Black, Howell & Kertesz, 1987; Caramazza, Miceli & Villa, 1986) and connectionist models of speech (Hartley & Houghton, 1996) indicates what may happen if these long-term representations are not automatically available. Specifically, producing unfamiliar phonological forms in the absence of automatically accessible long-term phonological representations requires the formation of a temporary phonological representation on which the articulatory specifications for output can be based. The developmental analogue is that since children may have fewer and less-completely specified LTM phonological representations of lexical items, PWM may be required to maintain this information and that this limits the complexity of the utterances that they can produce. Such evidence of trade-offs in processing complexity has indeed been found in the speech production skills of very young children. For example Streim and Chapman (1987) noted that increased lexical availability (prior naming) was associated with longer and more fluent utterances and Nelson and Bauer (1991) showed that the phonetic complexity of words was reduced when they were included in more complex utterances.
To summarise, it has been proposed that in the absence of automatically accessible knowledge of the phonological form of lexical items or syntactic constructions, resources from a limited pool, otherwise used to formulate other aspects of the utterance, must be employed to construct a phonological specification during speech production. We have already noted that it seems entirely plausible that PWM may play a role in the acquisition of such long-term phonological representations and thus that children with better PWM skills might be expected to have a wider range of lexical and morpho-syntactic knowledge on which to draw. However, these ‘processing’ accounts maintain that it is the availability of such knowledge which affects the resources available to compose all other aspects of the utterance, that underpins the association between PWM and language. The resources postulated to constrain language processing are t h e r e f o r e n o t p r e - d e s i g n a t e d t o t h e p r o c e s s i n g o f p h o n o l o g i c a l information, but support every aspect of message construction. Can such
WM and Language processing 89 interactions between the resources required to create and maintain specifically phonological representations and the resources for processing and integrating other forms of information be accommodated within the fractionated model? Within the original formulation (Baddeley & Hitch, 1974) there were flexible boundaries between storage in the slave systems and the processing resources of the central executive. More recently, however, the evolution of the concept of the central executive has seen storage and processing encapsulated within the specific systems (Baddeley
& Logie, 1999). Models of working memory which do not maintain such a clear distinction have, however, been proposed.
Integrated accounts of working memory
Interactions or resource trade-offs between the storage and processing of information are the central feature of an alternative tradition of working memory research, often referred to as ‘unitary’ and here termed ‘integrated’
models of working memory (see Miyake & Shah, 1999 for an overview).
Integrated accounts of working memory can be most easily contrasted with the fractionated model in that modality-specific storage systems, functionally separate from the processing resources of the central executive are not postulated in such models. Rather, working memory is proposed to reflect the totality of resources that are available to support both the processing and short-term storage of information (e.g. Daneman &
Carpenter, 1980; Just & Carpenter, 1992; although see Towse & Houston-Price, chapter 11, this volume, for difficulties with this view). The paradigm used to assess integrated working memory is the complex span task and although various versions of this task exist, all share the central premise that material must be simultaneously processed and stored.
The computational CC READER model (Just & Carpenter, 1992) was designed to simulate experimental data from a variety of populations which demonstrated associations between individual differences in complex span and language comprehension (Carpenter, Miyake & Just, 1994). In this model both storage (e.g. maintaining lexical items until they can be integrated with later-occurring related items) and processing (e.g. assigning thematic roles) are supported by a single common resource ‘activation’.
Since the model operates in parallel, such that processing is conducted at different levels of language analysis simultaneously (e.g. syntactic, semantic and referential), and since both storage and processing are demanding of activation resources, capacity constraints are highly influential in determining comprehension abilities.
Whilst not strictly an integrated model, since working memory capacity is a unitary attentional resource, the model of Engle and his colleagues (Engle, Kane & Tuholski, 1999) nevertheless also employs the complex span task as an index of working memory. Working memory, in Engle’s model, comprises both long-term memory representations activated above a certain threshold