Neural circuitry of the bilingual mental lexicon: Effect of age of second language acquisition Frédéric Isel a,b,c, * , Annette Baumgaertner d , Johannes Thrän a , Jürgen M. Meisel a , Christian Büchel b a Research Centre on Multilingualism, University of Hamburg, Germany b Dept. for Systems Neuroscience, University Medical Center Hamburg-Eppendorf, Hamburg, Germany c Dept. of Neurophysiology and Pathophysiology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany d University Medical Center Schleswig-Holstein, Kiel, Germany article info Article history: Accepted 20 July 2009 Available online 19 August 2009 Keywords: Mental lexicon Bilingualism AoA fMRI Cross-language priming Neural adaptation abstract Numerous studies have proposed that changes of the human language faculty caused by neural matura- tion can explain the substantial differences in ultimate attainment of grammatical competences between first language (L1) acquirers and second language (L2) learners. However, little evidence on the effect of neural maturation on the attainment of lexical knowledge in L2 is available. The present functional mag- netic resonance study addresses this question via a cross-linguistic neural adaptation paradigm. Age of acquisition (AoA) of L2 was systematically manipulated. Concrete nouns were repeated across language (e.g., French–German, valise suitcase –Koffer suitcase ). Whereas early bilinguals (AoA of L2 < 3 years) showed larger repetition enhancement (RE) effects in the left superior temporal gyrus, the bilateral superior fron- tal gyrus and the right posterior insula, late bilinguals (AoA of L2 > 10 years) showed larger RE effects in the middle portion of the left insula and in the right middle frontal gyrus (MFG). We suggest that, as for grammatical knowledge, the attainment of lexical knowledge in L2 is affected by neural maturation. The present findings lend support to neurocognitive models of bilingual word recognition postulating that, for both early and late bilinguals, the two languages are interconnected at the conceptual level. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction Second-language (L2) acquisition necessitates the acquisition of various types of grammatical (e.g., phonological, morphological, syntactic) and lexical knowledge. Whereas the development of mental representations of such knowledge and the acquisition of language processing skills seem to be mastered easily in child first language (L1) acquisition, adult learners of L2 achieve native-like knowledge and proficiency only rarely, if at all. The Critical Period Hypothesis (CPH) first proposed by Penfield and Roberts (1959) as- sumes the existence of a critical period for acquisition of segmental phonology, inflectional morphology and syntax. Changes of the hu- man language faculty caused by neural maturation can be argued to explain substantial differences in the course of acquisition and in ultimate attainment of grammatical competences between L1 and L2 learners (see also Chomsky, 1975; Lenneberg, 1967). Con- cerning the acquisition of lexical knowledge, the CPH did not make any assumptions. On the contrary, a more recent theory of neuro- linguistic development (Locke, 1997) assumes an optimum biologi- cal moment for the appropriate organisation and use of the mental lexicon. According to the Locke’s theory, pregrammatical children for which the phase of lexical material storage (5–20 months) is af- fected by external factors including the availability of appropriate stimulation have difficulties to perform analytical operations such as analyse and detection of recurrent structural patterns (20– 37 months). In the present study, we addressed the question of whether the neuroanatomical organisation of the bilingual mental lexicon may also be affected by neural maturation. In psycholinguistic models of the bilingual mental lexicon, a central question is to know at what level of representation, i.e., orthographic/phonological, lexi- cal, and/or conceptual level, a bilingual’s two languages are inter- connected? One class of models describes the architecture of the bilingual’s memory at two different levels of representation, which are hierarchically related (Potter, 1979; Snodgrass, 1984). For example, the Revised Hierarchical (RH) model (Kroll & Stewart, 1994) postulates that words are stored in separate lexical memory systems, whereas concepts are stored in an abstract memory sys- tem common to both languages. At the early stage of L2 develop- ment, individuals rely more on L2–L1 lexical level translation; with further L2 development (i.e., highly proficient late bilinguals), stronger links are established between the L2 lexical codes and their appropriate conceptual representations. Another class of 0278-2626/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.bandc.2009.07.008 * Corresponding author. Address: Dept. of Neurophysiology and Pathophysiology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany. Fax: +49 40 7410 57752. E-mail address: f.isel@uke.uni-hamburg.de (F. Isel). Brain and Cognition 72 (2010) 169–180 Contents lists available at ScienceDirect Brain and Cognition journal homepage: www.elsevier.com/locate/b&c