Using nonword repetition tasks as an experimental approach with both adults and children has become quite common in the past 10 to 15 years for studying lexical learning and phonological processing (e.g., Bailey & Hahn, 2001; Gathercole, Frankish, Pickering & Peaker, 1998; Munson, Edwards, & Beckman, 2005; Storkel, 2001; Vitevich & Luce, 2005). In her Keynote, Gathercole (2006) indicates that “The ability to repeat multisyllabic nonwords…probably represents the most effective predictor of language learning ability that is currently known” and that “nonword repetition…may also hold the key to understanding developmental disorders of language learning.” Her Keynote reviews many of the findings from a variety of nonword repetition studies with typically developing children and children with specific language impairment (SLI) or other language-related disorders. Despite the substantial benefits and interesting findings that nonword test paradigms have provided in addressing different issues, a number of questions and precautions should be kept in mind regarding such approaches. Researchers who routinely employ these procedures are likely to be well aware of these issues, and in fact, Gathercole points out some of the limitations. For those less familiar with nonword repetition tasks, it seems advisable to reinforce the need for a certain amount of caution in evaluating the results of such approaches.

In 1990 Gathercole and Baddeley proposed a strong hypothesis that has generated a wealth of research in the field of language development and disorder. The hypothesis was that phonological memory, as indexed by nonword repetition, is causally related to vocabulary development. Support for the hypothesis came from an impressive range of longitudinal, correlational, and laboratory training studies, and from studies of specific language impairment (SLI). However, more recently, Gathercole, Tiffany, Briscoe, Thorn, and The ALSPAC Team (2005), directly tested the causal hypothesis by following a cohort of children from age 5 to 8 years. Contrary to prediction, children with poor nonword repetition abilities at age 5 had normal vocabulary at the age of 8.

We join other responders in thanking Clahsen and Felser (CF) for pulling together these observations about the development of language processing. We are especially impressed by the generality and inclusiveness of CF's treatment of development in L1 and L2. Because most of their specifics concerned comprehension processes, our contribution will emphasize the added value of an appeal to production processes. In particular, we will articulate the value of applying existing production models to developmental phenomena. Language development can be interpreted in revealing ways through the lens of adult sentence production models. These models specify how lexical, syntactic, morphological, and phonological knowledge are integrated in real time as we produce sentences. They are performance models, but they go well beyond general measures of working memory and general notions of limited capacity and resources.

In their Keynote Article, Clahsen and Felser (CF) provide a detailed summary and comparison of grammatical processing in adult first language (L1) speakers, child L1 speakers, and second language (L2) speakers. CF conclude that child and adult L1 processing makes use of a continuous parsing mechanism, and that any differences found in processing can be explained by factors such as limited working memory capacity and incomplete lexical knowledge. The authors then suggest that the existing differences between L1 (both adult and child) and L2 processing provide evidence that parsing mechanisms are qualitatively different between these groups. They posit that this qualitative difference between L1 and L2 is due to L2 speakers having shallower and less detailed syntactic representations than L1 speakers. This commentary focuses on discussing this shallow structures account and considers what this means for L2 processing.

Gathercole's (2006) comprehensive and interesting Keynote Article on the nature of the relations between nonword repetition and word learning highlights the complex number of interacting factors that affect this relation through development. In this Commentary we focus on the impact of higher level cognition, particularly linguistic representations on lower level functions such as attention and processing, as well as higher level functions such as memory. In addition, we note the importance of distinguishing children with specific language impairment (SLI) who do and do not have phonological deficit when testing for memory impairments. We argue that further detailed investigations are warranted at the linguistic levels of cognitive processing alongside memory tasks that tap different components of language and nonlinguistic memory. Such studies would help tease apart the complex, and probably bidirectional relations between attention, memory, and linguistic representations. Moreover, we propose that this investigative strategy crucially needs to take a longitudinal developmental perspective if we are to understand the developmental trajectory.

The work of Susan Gathercole and others on nonword repetition has increased general interest in the relationship between memory and language, and has provided a fertile theoretical framework for researchers to explore how the language system makes use of the phonological loop, a component in Baddeley's (1986) working memory model. Gathercole (2006) integrated a number of findings from a variety of research methodologies and populations to support this theoretical framework. She also discussed how this framework might be applied to increase our understanding of language disorders characterized by word learning problems. Synthesizing the findings from many and diverse areas of study to formulate a coherent theory is a challenge, but this challenge must be undertaken if we hope to significantly advance our understanding of human behavior, including the differences in processing that are found across the lifespan and across individuals. I admire Gathercole for undertaking this challenge (and the additional challenge of applying the theory to language disorders) in the area of word learning.

Clahsen and Felser (CF) offer a novel explanation for the qualitative differences in language processing often observed between adult first language (L1) speakers and second language (L2) learners. They argue that, although L2 learners are successful in drawing on lexical, morphological, and pragmatic sources of information, they underutilize syntactic structure, which results in shallower and less detailed processing than that of native speakers.

The proposal by Clahsen and Felser (CF) has the potential of marking a turning point in second language (L2) acquisition research. Contrary to much L2 research to date, it suggests that some of the differences between native and (advanced) nonnative speakers may be at the level of grammatical processing, rather than grammatical representations. Accounting for L2 speakers' divergent behavior does therefore not necessarily involve positing “representational deficits”: L2 speakers can, and indeed do, attain target representations of the L2, but may compute incomplete (“shallow”) syntactic parses in comprehension. Such shallow processing is often accompanied by reliance (or overreliance) on lexical, semantic, and pragmatic information, which can lead to seemingly trouble-free comprehension in ordinary communication. It is only when speakers are faced with sentence ambiguities, which impose a greater than normal processing load, that the differences between adult L2 language acquirers and child first language (L1) acquirers become apparent: both child and adult language learners have difficulty in integrating structural and nonstructural information in on-line comprehension, but although children prioritize structural information, adult learners privilege nonstructural lexical–semantic information. CF's proposal opens up new perspectives on the nature of ultimate attainment in adult L2 acquisition; at the same time it raises some questions that, in my view, are of crucial importance for future research.

Because words represent the building blocks upon which the facility to produce and comprehend language at all levels is based, the capacity of a child to learn words has immense impact on his or her developing abilities to communicate and engage properly with the outside world. Both the Keynote Article and the Commentaries in this issue demonstrate that this capacity to acquire vocabulary is neither singular nor simple. Children may fail to learn new words in as rapid and efficient manner as their peers for many reasons: they may, for example, have inadequate environmental experience of either the spoken and printed form of the language (Huttenlocher, Haight, Bryk, Seltzer, & Lyons, 1991), or they may have poor abilities to produce the sound contrasts of the language (Mirak & Rescorla, 1998). The focus of the present discussion lies somewhere in between these extremes of influence, in the intervening perceptual and cognitive processes that constitute the speech processing and word learning system. Here, too, complexities abound. The developing language system is characterized by dependencies between the multiple processes involved in processing and learning language (Bishop, 1997), rarely evincing the dramatic dissociations in adults with acquired language disorders that have served cognitive neuropsychology so well in its bid to identify a modular structure of the language system. Weaknesses in perceptual analysis of the sound structure of the language, in the storage of the resulting mental representations, and in the availability of existing representations that can support the processing of new words, often coexist within an individual. As a consequence, it can be extremely difficult to tease apart the developmental underpinnings of language acquisition. In this article, I argue that real progress toward understanding vocabulary acquisition requires a substantial and systematic body of research evidence designed to provide strong empirical challenges to existing (and new) hypotheses and theories. Whereas mere description is unlikely to lead to major advances, systematic experimental analysis and the specification of detailed theoretical accounts should result in a more complete understanding of the complexities and constraints of new word learning.

Clahsen and Felser (CF) present a thought-provoking article that is likely to have a strong impact on the field, in particular, on developmental psycholinguistics and second language (L2) acquisition research. Unlike the majority of previous work on language acquisition that focused on “competence,” that is, the knowledge basis underlying grammar, CF emphasize the need to approach language acquisition with psycholinguistic measures of processing. Based primarily on behavioral and electrophysiological on-line data, they argue that language acquisition in early first language (L1) and late L2 follows different patterns.

In this article, the authors lay out an impressive body of evidence that supports two main claims. First, they favor the continuity hypothesis, according to which children's parsing mechanisms are essentially the same as adults'. Parsing strategies change little over time, and those changes that occur are attributed to differences in lexical processing efficiency and working memory capacity. Second, they suggest that there are substantial differences in the parsing strategies adopted by native speakers and adult learners of second languages.

Clahsen and Felser (CF) have written a beautiful and important paper. I applaud their integrative empirical approach, and believe that their theoretical account is largely correct, if not in some of its specific claims, at least in its broader assumptions. CF directly compare their shallow structure hypothesis (SSH) with a model that my colleagues and I have proposed for aspects of the neurocognition of first and second language: the “declarative/procedural” (DP) model. Although some of CF's discussion accurately depicts the DP model and its relation to the data, they also make a few critical errors.Here, I first summarize the DP model in both first language (L1) and adult-learned second language (L2), in order to be able to contrast it with the SSH, and then address the relevant problems in CF. For further details on the DP model and L1, see Ullman (2001a, 2001c, 2004) and Ullman et al. (1997). For the model as it applies to L2, see Ullman (2001b, 2005).

The core idea that we argued for in the target article was that grammatical processing in a second language (L2) is fundamentally different from grammatical processing in one's native (first) language (L1). Our major source of evidence for this claim comes from experimental psycholinguistic studies investigating morphological and syntactic processing in child and adult native speakers, and nonnative speakers who acquired their L2 after childhood and for whom their L1 is the dominant language. With respect to child L1 processing, we argued for a continuity of parsing hypothesis claiming that the child's structural parser is basically the same as that of mature speakers and does not change over time. Adult L2 learners, in contrast, were seen to underuse syntactic information during sentence processing and to rely more on lexical–semantic cues to interpretation. To account for the observed L1/L2 differences in processing, we proposed the shallow structure hypothesis (SSH) according to which the representations adult L2 learners compute during processing contain less syntactic detail than those of child and adult native speakers.