In this issue of Applied Psycholinguistics, we present a first annual Keynote Article. The article itself, in this case by Harald Clahsen and Claudia Felser, is followed by the Commentaries of several other researchers. The authors then reply to the Commentaries. It is our hope that this format will provide our readers with an in-depth discussion of central and topical issues in psycholinguistic research. We are grateful to Drs. Clahsen and Felser for writing the initial keynote article as well as to the group of researchers who provided their thoughtful and rigorous commentary.

The ability to process the linguistic input in real time is crucial for successfully acquiring a language, and yet little is known about how language learners comprehend or produce language in real time. Against this background, we have conducted a detailed study of grammatical processing in language learners using experimental psycholinguistic techniques and comparing different populations (mature native speakers, child first language [L1] and adult second language [L2] learners) as well as different domains of language (morphology and syntax). This article presents an overview of the results from this project and of other previous studies, with the aim of explaining how grammatical processing in language learners differs from that of mature native speakers. For child L1 processing, we will argue for a continuity hypothesis claiming that the child's parsing mechanism is basically the same as that of mature speakers and does not change over time. Instead, empirical differences between child and mature speaker's processing can be explained by other factors such as the child's limited working memory capacity and by less efficient lexical retrieval. In nonnative (adult L2) language processing, some striking differences to native speakers were observed in the domain of sentence processing. Adult learners are guided by lexical–semantic cues during parsing in the same way as native speakers, but less so by syntactic information. We suggest that the observed L1/L2 differences can be explained by assuming that the syntactic representations adult L2 learners compute during comprehension are shallower and less detailed than those of native speakers.

Clahsen and Felser's article (CF) is an important contribution to the field of psycholinguistics in several respects. First, it draws attention to the importance of a better understanding of the processing mechanisms utilized by child and adult language learners. Differences in these mechanisms may be responsible for the final outcome of the acquisition process. Second, the article provides an excellent summary of current first language (L1) and second language (L2) research on processing. A variety of studies, ranging from morphological off-line investigations to on-line research on syntactic development, are reviewed in a concise and accurate manner; the cross-linguistic dimension of the article makes both the review and the argument even more comprehensive and convincing. Third, based on their own experimental results, and the results from other studies, the authors propose a novel account of L2 processing, the so-called shallow syntax hypothesis (SSH).

Clahsen and Felser (CF) deserve praise for their superlative synthesis of literature relating to grammatical processing, as well as for their original contributions to this area of research. CF “explore the idea that there might be fundamental differences between child L1 and adult L2 processing.” The researchers present evidence that adult second language (L2) processing is often less automatic and less efficient than first language (L1) processing. Qualitative differences are suggested as well. Adult L2 processing may be restricted to shallow computations, whereas L1 processing typically involves detailed representations. These conclusions are reached in large part by comparing highly proficient L2 learners with natives on various neurological and behavioral dimensions of processing. I propose that additional comparisons might be carried out that involve an understudied population: learners whose L2 is their dominant language.

Clahsen and Felser (CF) analyze the performance of monolingual children and adult second language (L2) learners in off-line and on-line tasks and compare their performance with that of adult monolinguals. They conclude that child first language (L1) processing is basically the same as adult L1 processing (the contiguity assumption), with differences in performance being due to cognitive developmental limitations. They argue that differences in L2 performance, however, are more qualitative and not explained by shortage of working memory (WM) resources, differences in processing speed, transfer of L1 processing routines, or incomplete acquisition of the target grammar. They propose a shallow structure hypothesis (SSH) to explain the differences reported in sentence processing. According to this, the syntactic representations computed by L2 learners during comprehension are shallower and less detailed than those computed by native speakers and involve more direct form-function mappings.

Clahsen and Felser (CF) review ground-breaking work comparing selected types of language processing in monolingual children and adults, on the one hand, and in monolingual first language (L1) adults and adult second language (L2) learners, on the other. They argue that children behave essentially like adults, but that adult L2 learners, even high-proficiency ones, do not. Thus, there is a principled difference to be made among types of learners; there is continuity of mechanism and process to be observed in monolingual development but L2 acquisition exhibits certain fundamental differences. In particular, L2 learners construct shallow syntactic structures (essentially failing to compute trace chains) when processing long-distance filler-gap dependencies. According to the shallow structure hypothesis (SSH), learners immediately interpret incoming words in a minimal semantic representation by assigning thematic roles to argument expressions and associating modifiers to their hosts. They are not mapping detailed and complete syntactic representations onto semantic representations.

Ever since the derivational theory of complexity (DTC) apparently bit the dust in the late 1960s, experimental psycholinguistics have been afflicted by a dualism at least as troublesome as the mind/brain dichotomy, namely, the grammar/parser distinction. The idea that mentally represented grammar is something fully dissociated from the human language processor is less than compelling, yet it has implicitly informed much of the last half century's psycholinguistics practice on both sides of the formalist–functionalist divide.

Understanding the mechanisms learners use to process target language input is crucial to developing a complete model of both first language (L1) and second language (L2) acquisition. If adult L2 learners are found to process the target language with mechanisms that differ from those used by child L1 learners and adult native speakers, what implications might this have for the developing grammar? Clahsen and Felser review evidence that appears to point to such differences, generalizing their findings under a shallow structure hypothesis about how adult learners process input in L2.

Clahsen and Felser (CF) have written a fairly comprehensive review of the current literature on on-line second language (L2) processing, presenting data from eye movement, self-paced reading, and event-related potential (ERP) studies with the aim of evidencing possible differences between native language (L1) and L2 processing. The thrust of the article, in regard to adult L2 processing, is apparently an attempt to gather evidence to bolster their argument about “shallow processing” in adult L2 learners. Although the authors provide the reader with a generally good overview of the current literature, their argumentation seems to be flawed at times. Consider, first, the authors' presentation of recent ERP evidence. The authors claim that L2 adult learners may lack automaticity in comparison to native speakers in regard to syntactic processing. This is based upon a delayed N400 response, often found in L2 learners compared to native speakers, as well as by the pattern of anterior negativities to morphosyntactic violations. Later, however, this line of argumentation is seemingly undermined. First, as CF rightly underline, the range of variability in anterior negativities found in L2 learners falls within the range of variation observed in native speakers. As such, variability in this response cannot be taken as a marker of differential processing specific to (shallow) syntactic processing in the second language (see also Frenck-Mestre, 2005; Osterhout et al., 2004). Second, as CF later note, the N400 (as well as P600) is systematically observed in adult L2 learners, and is often highly similar to that found for native speakers. Consider, next, the behavioral evidence cited by CF on adult L2 syntactic processing. The authors cite work on various structures, notably relative clause attachment (which has received a great deal of attention in both monolingual and L2 studies). Concerning this structure, although CF cite studies, which show both clear L1 influence on L2 processing and differential effects as a result of experience with the L1, they favor studies that fail to show such effects and reject Mitchell et al.'s (2000) tuning hypothesis as an explanatory model. (Note Mitchell and colleagues have indeed produced evidence of their own showing limitations of their model.) It is also noteworthy that CF's argumentation about the sensitivity of the measure they used to test for immediate preferences for this structure is not as strong as it could be. Indeed, where they report L2 preferences (for low attachment following thematic prepositions), the literature shows the same systematic preference independent of the language tested (cf. Mitchell et al., 2000). As such, the sensitivity of their measure may not be adequately demonstrated. In sum, although CF provide the reader with an impressive collection of current L2 studies, the viewpoint that they espouse does not seem to be as substantiated as they wish to claim.

In their target article, Clahsen and Felser (CF) review studies that they and others have conducted in recent years to confirm the dual mechanism hypothesis and to extend its application to first and second language (L1 and L2) learners. They interpret the findings as supporting both the dual mechanism hypothesis and the claim that the sentence-level processing of L2 but not L1 learners shows “striking” differences compared to adult native speakers. I argue that an exclusive focus on the representation of linguistic knowledge is insufficient for understanding the real-time processes in morphological production and sentence comprehension.

The article by Clahsen and Felser (CF) on grammatical processing in language learning is a timely and much-needed synthesis of research on this topic. It correctly identifies both morphological processing and syntactic processing as key areas that require attention. This commentary raises two issues: the relationship between the grammar and the parser, and the need for a transition theory in adult second language (L2) learning.

This paper does a fine job of advancing discussion concerning a question that is indeed quite underrepresented in the literature, that is, how language learners comprehend and produce language in real time. The paper is firmly rooted in the dual mechanism approach to language processing and takes as its starting point the assumption that normal adult processing is characterized by two systems, one that is lexically based and one that is essentially combinatorial. The authors cite evidence that both first language (L1) learners and adult native speakers show evidence of dual mechanism processing and that, in particular, children's sentence processing shows early reliance on structure-based interpretation and less ability to employ lexical/pragmatic information in the resolution of language ambiguity. One way to view this preference is that L1 learners might know, broadly speaking, considerably more about their language than they do about the world in which they live. Adult second language (L2) learners might be said to be in exactly the opposite situation. It is therefore hardly surprising that adult L2 speakers rely strongly on lexical/pragmatic cues in sentence processing. In the early stages of adult L2 acquisition, the demands of real-time processing make use of such nonsyntactic inference crucial. The question that strikes me as key is whether, as L2 speakers become more proficient, they are weaned from this reliance such that their processing reflects the interaction between syntactic and lexical processing that is characteristic of adult native speakers. When and if they do, we could say that their L2 processing is, both internally and externally, nativelike.

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.

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.

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.