This broad topic is one of our major areas of investigation. We are examining a number of issues and questions in this area.
First, in our theoretical work, we have proposed and are further developing the hypothesis that the learning, knowledge and use of second and additional languages depend on declarative and procedural memory (see especially Ullman, 2001, 2005, 2013, 2015). On this view - the declarative/procedural model of second language - later language learning should depend on the two memory systems, just as first language does. However, given that the memory systems change over the course of childhood, later learned languages should be learned somewhat differently than native languages. In particular, evidence suggests that declarative memory improves during childhood (and plateaus during adolescence and early adulthood), whereas procedural memory may be well-established early in childhood, and then might attenuate (though the evidence is less clear for developmental changes in procedural memory than in declarative memory). For these and other reasons (e.g., pedagogical instruction of later languages that might encourage explicit learning in declarative memory), later learned languages should depend more on declarative memory than native languages. However, this increased dependence on declarative memory should hold only for rule-governed aspects of grammar, which can depend on either memory system. In contrast, lexical (idiosyncratic) knowledge can apparently be learned only in declarative memory, and thus should depend on this system in all language learners. Moreover, procedural memory is not posited to be defunct in adults, but simply less available for learning than during childhood (as compared to declarative memory). Therefore, contrary to strong versions of the critical period hypothesis, we predict that rule-governed aspects of grammar will eventually be learned in procedural memory, and thus should become increasingly native-like (first language-like) in their neurocognition over time and with practice/exposure. Finally, various learner, input, and item level factors should affect the extent to which grammar depends more or less on each of the two memory systems, and how this changes over time. For example, learners with a relative advantage in declarative memory (e.g., due to factors such as sex or genotype) should depend earlier and more on this system for grammar. Along the same lines, learning conditions (e.g., types of language training) that enhance learning in either declarative memory or procedural memory should lead to different patterns of dependence of grammar on the two memory systems (Ullman and Lovelett, 2016). Indeed, techniques that enhance learning in either system, such as spaced repetition, retrieval practice, exercise, or diet, should also enhance language learning in either system (Ullman and Lovelett, 2016).
Second, we perform empirical studies to examine these predictions, as well as related issues. Our studies use a variety of techniques, including various behavioral methods (e.g., the examination of frequency effects), event-related potentials (ERPs), and functional neuroimaging (fMRI). We investigate the learning, processing, representation, and retention of language, in various experimental linguistic paradigms, including with natural languages, mini-languages, artificial languages, and artificial grammars. Within natural languages we work mainly with English, but also examine other languages (e.g., Spanish). One recent and exciting line of research has examined the neurocognitive effects (using behavioral and ERP measures) of explicit (classroom-like) versus implicit (immersion-like) training on the neurocognition of syntax, using an artificial language (Morgan-Short et al., JoCN, 2012). We found that only implicit training led to fully native-like patterns of brain processing. In a follow-up study we tested the learners 5 months later, and found that their brain patterns became more native-like for syntactic processing, and that this held for both the explicitly and implicitly trained groups, though again only the implicit group showed full native-like brain processing (Morgan-Short et al., PLoS ONE, 2012). We hypothesize that this increase in native-like brain processing over time was due to consolidation of syntactic knowledge, possibly in procedural memory. Another recent study found that university language learners of Spanish with classroom and immersion experience were able to attain native-like processing of syntax (Bowden et al., 2013). In another study we examined the effects of multiple factors, including sex (male vs. female), age of arrival, and length of residence on the computation (storage vs. composition) of inflected forms (Babcock et al., 2012). And in a recent fMRI study we investigated the learning and generalization of affixal morphology in an artificial language paradigm (Nevat et al., 2017). We also perform both behavioral and neuroanatomical meta-analyses of second language learning.
Third, we are examining the neurocognitive effects of bilingualism. In two behavioral studies, one with Ingrid Finger in Brazil, and the other with Albert Costa in Barcelona, we are testing whether early bilinguals have behavioral advantages over monolinguals at various tasks probing aspects of language learning, memory, and other cognitive functions. And in an ERP study, Grey et al. (submitted) found that even in the absence of bilingual/monolingual differences in behavioral measures of grammar learning, bilinguals appear to show more native-like brain processing.
Ullman, M.T. and Lovelett, J.T. (2016, online). Implications of the declarative/procedural model for improving second language learning: The role of memory enhancement techniques. Second Language Research.
Ullman, M.T. (2015). The declarative/procedural model: A neurobiologically motivated theory of first and second language. In B. VanPatten & J. Williams (Eds.), Theories in second language acquisition: An introduction (2nd ed.). New York: Routledge. pp. 135-158.
Bowden, H.W., Steinhauer, K., Sanz, C., & Ullman, M.T. (2013). Native-like brain processing of syntax can be attained by university foreign language learners. Neuropsychologia, 51. 2492-2511. (Supplementary Data).
Babcock, L., Stowe, J.C., Maloof, C.J., Brovetto, C., and Ullman, M.T. (2012). The storage and composition of inflected forms in adult-learned second language: A study of the influence of length of residence, age of arrival, sex, and other factors. Bilingualism: Language and Cognition. 15(4). 820-840.
Newman, A.J., Tremblay, A., Nichols, E.S., Neville, H.J., and Ullman, M.T. (2012). The influence of language proficiency on lexical-semantic processing in native and late learners of English: ERP evidence. Journal of Cognitive Neuroscience, 24(5). 1205-1223.
Morgan-Short, K., Steinhauer, K., Sanz, C. and Ullman, M.T. (2012). Explicit and implicit second language training differentially affect the achievement of native-like brain activation patterns. Journal of Cognitive Neuroscience, 24(4). 933-947.
Finger, I., Morgan-Short, M., Grey, S., and Ullman, M.T. (2011). SQUIB: Processamento em L2 apresenta ativação neural semelhante à da L1 após meses de ausência de exposição à lingual. Revista LinguíStica, 7(2). 7-17.
Bowden, H.W., Gelfand, M.P., Sanz, C., Ullman, M.T. (2010). Verbal Inflectional Morphology in L1 and L2 Spanish: A Frequency Effects Study Examining Storage versus Composition. Language Learning, 60(1). 44-87.
Morgan-Short, K., Sanz, C., Steinhauer, K., Ullman, M.T. (2010). Second Language Acquisition of Gender Agreement in Explicit and Implicit Training Conditions: An Event-Related Potential Study. Language Learning, 60(1). 154-193.
Ullman, M. T. (2005). A Cognitive Neuroscience Perspective on Second Language Acquisition: The Declarative/Procedural Model. In C. Sanz (Ed.), Mind and Context in Adult Second Language Acquisition: Methods, Theory, and Practice (pp. 141-178). Washington, DC: Georgetown University Press.