The papers in this Special Issue focus on the use of neuroimaging techniques to answer questions about the neural representation, processing and control of two languages. Neuropsychological data from bilingual aphasics remain vital if we are to establish the neural basis of language (see Paradis, 1995) but lesion-deficit studies alone cannot tell us how neural activity relates to ongoing language processing. Modern neuroimaging methods provide a means to do so. There are two broad classes of such methods: electrophysiological methods allow us to answer questions about when a particular process occurs whereas haemodynamic methods allow us to answer the complementary question of where in the brain such a process is carried out. Before giving a thumb-nail sketch of the papers in this Special Issue, I briefly discuss each class of method.

Theoretical and empirical aspects of the neural bases of the mental lexicon and the mental grammar in first and second language (L1 and L2) are discussed. It is argued that in L1, the learning, representation, and processing of lexicon and grammar depend on two well-studied brain memory systems. According to the declarative/procedural model, lexical memory depends upon declarative memory, which is rooted in temporal lobe structures, and has been implicated in the learning and use of fact and event knowledge. Aspects of grammar are subserved by procedural memory, which is rooted in left frontal/basal-ganglia structures, and has been implicated in the acquisition and expression of motor and cognitive skills and habits. This view is supported by psycholinguistic and neurolinguistic evidence. In contrast, linguistic forms whose grammatical computation depends upon procedural memory in L1 are posited to be largely dependent upon declarative/lexical memory in L2. They may be either memorized or constructed by explicit rules learned in declarative memory. Thus in L2, such linguistic forms should be less dependent on procedural memory, and more dependent on declarative memory, than in L1. Moreover, this shift to declarative memory is expected to increase with increasing age of exposure to L2, and with less experience (practice) with the language, which is predicted to improve the learning of grammatical rules by procedural memory. A retrospective examination of lesion, neuroimaging, and electrophysiological studies investigating the neural bases of L2 is presented. It is argued that the data from these studies support the predictions of the declarative/procedural model.

Sentence comprehension in second language (L2) learners was examined using event-related brain potentials (ERPs). Native Japanese speakers who had learned German as a second language after puberty listened to German sentences which were either correct, semantically incorrect, syntactically incorrect or both semantically and syntactically incorrect. Brain responses were registered while participants listened to these sentences. Grammaticality judgments required after each sentence revealed that their overall performance was not perfect but clearly above chance. When comparing the L2-learners' brain responses to those of native listeners, a variety of differences were observed. Whereas semantically incorrect sentences showed an ERP pattern similar to that of native listeners (a centro-parietal N400-effect), correct sentences elicited a greater positivity in L2-learners than in native listeners, possibly in reflection of greater difficulties in syntactic integration. For sentences containing a phrase structure violation, L2-learners did not show significant modulations of the syntax-related ERP components usually seen in native listeners (i.e. the early anterior negativity and the P600). Furthermore, sentences containing a pure semantic or a combined syntactic and semantic violation elicited a late right anterior-central negativity, an effect which has not been observed for native language processing. The topography of the effect may suggest that these additional processes are based on conceptual-semantic rather than on lexical-semantic aspects.

The current study set out to examine word recognition in early fluent Spanish–English bilinguals using a single word presentation lexical decision task (LDT). Reaction times (RTs) and event-related brain potentials (ERPs) were measured while subjects (16 per language condition) made a lexical decision on words and pseudowords in either Spanish or English. Results show associative priming as measured by RTs, but both associative and categorical priming in the ERPs in both language conditions. The dissociation of RT and ERP effects suggests that the two measures might tap into different underlying processes during semantic priming or reflect different sensitivities towards semantic priming. Furthermore, both RT and ERP measures revealed symmetrical priming in L1 and L2. These data indicate that word recognition in early fluent bilinguals is equivalent for L1 and L2.

Dutch–English bilinguals performed a generalized lexical decision task on triplets of items, responding with “yes” if all three items were correct Dutch and/or English words, and with “no” if one or more of the items was not a word in either language. Sometimes the second item in a triplet was an interlingual homograph whose English meaning was semantically related to the third item of the triplet (e.g., HOUSE – ANGEL – HEAVEN, where ANGEL means “sting” in Dutch). In such cases, the first item was either an exclusively English (HOUSE) or an exclusively Dutch (ZAAK) word. Semantic priming effects were found in on-line response times. Event-related potentials that were recorded simultaneously showed N400 priming effects thought to reflect semantic integration processes. The response time and N400 priming effects were not affected by the language of the first item in the triplets, providing evidence in support of a strong bottom-up role with respect to bilingual word recognition. The results are interpreted in terms of the Bilingual Interactive Activation model, a language nonselective access model assuming bottom-up priority.

A key aspect of higher cognitive function is the ability to switch rapidly and efficiently between alternative modes of response where this is appropriate behaviourally. Such suppression appears to be highly dependent upon the integrity of the prefrontal cortex, yet other cortical areas are likely to be necessary to implement response switching. Language switching in bilingual speakers is a clear example of a task in which response switching is required. Functional brain imaging studies have demonstrated parietal cortex activation during repeated language switching within a translation task. Here we used event-related dense-sensor EEG recording techniques to examine the time course of language switching during a visually cued naming task in which bilingual participants named digits in either their first or second language. Switch-related modulation of ERP components was evident over parietal and frontal cortices, and in the latter case showed an asymmetry across first and second languages. Correspondence with a frontal ERP component found when suppressing manual responding in a Go/No-Go reaction time task may imply that similar inhibitory mechanisms are involved in both response suppression and language switching.

Functional neuroimaging, used in conjunction with experimental cognitive tasks, has been extremely successful in establishing functional specialization as a principle of brain organization in man. Positron emission tomography activation techniques and functional magnetic resonance imaging assess localized increases in cerebral blood flow associated with mental processes. The advent of these noninvasive neuroimaging techniques opened a “new era” in the investigation of language organization in healthy individuals. Neuroimaging studies of bilinguals and polyglots have highlighted the potential role of variables, such as age of acquisition, degree of proficiency, level of exposure, dominance/maternality on functional brain mapping of multiple languages. Consistent results indicate that attained proficiency, and maybe language exposure, are more important than the age of acquisition as a determinant of the cerebral representation of languages in bilinguals/polyglots. Indeed, increasing language proficiency appears to be associated at the neural level with the engagement of a common network within the dedicated language areas. Here we have reviewed the functional imaging literature on bilingualism.

There is no causal account of recovery patterns in bilingual aphasia. We propose that the causal mechanisms can be partially revealed by combining neuropsychological and neuroimaging methods. We briefly review the potentials and limitations associated with functional neuroimaging experiments on normal and neurologically impaired patients and then focus on the different levels of description required to reveal the causal basis of recovery patterns in bilingual aphasics. Finally, we suggest how functional imaging investigations might be meaningfully undertaken with bilingual aphasic patients. We illustrate our argument with respect to five different patterns of recovery and consider the theoretical and practical implications of such research.