The neural basis of first and second language processing

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Fundamental breakthroughs in the neurosciences, combined with technical innovations for measuring brain activity, are shedding new light on the neural basis of second language (L2) processing, and on its relationship to native language processing (L1). The long-held assumption that L1 and L2 are necessarily represented in different brain regions in bilinguals has not been confirmed. On the contrary, the available evidence indicates that L1 and L2 are processed by the same neural devices. The neural differences in L1 and L2 representations are only related to the specific computational demands, which vary according to the age of acquisition, the degree of mastery and the level of exposure to each language. Finally, the acquisition of L2 could be considered as a dynamic process, requiring additional neural resources in specific circumstances.

Introduction

During the past few years, a large body of neuroimaging and neurophysiological studies has been devoted to the study of the neural organization of language. To date, the results of positron emission tomography (PET), functional magnetic resonance imaging (fMRI) and event-related potential (ERP) studies have not only converged with the findings of clinical aphasiology but have also opened several new perspectives to our understanding of the brain–language relationship.

There has been increasing attention to the role of cortical regions that are outside the traditional left perisylvian areas in language processing [1, 2]. Within these networks, specific frontal, temporal and parietal regions, together with subcortical structures, are differentially involved in specific aspects of linguistic computation, from word-level to sentence processing [3, 4] modern concept of brain functional specialization that emerged as a consequence of the development of sophisticated experimental paradigms in functional neuroimaging and neurophysiology enables the study of language subcomponents, such as phonology, syntax and lexical semantics, together with their computational processing demands. The use of refined experimental tasks, rather than of verbal activities, such as speaking and listening, has played a crucial part in the characterization of linguistically relevant systems [1, 3].

In recent years, functional techniques for measuring brain activity have also shed new light on the neural basis of second language (L2) processing and its relationship to native language processing (L1). A basic issue in this area is whether a L2 learnt later in life can be processed through the same neural mechanisms underlying L1 acquisition and processing. Considering that L1 is acquired implicitly, and mediated, according to many theorists, by innate learning mechanisms triggered during a critical period, it remains an open question as to whether or not the same mechanisms underlie the acquisition of L2.

A recent theory claims that the processing of L2 acquired late in life depends upon different cognitive mechanisms and cerebral structures from L1 [5]. Following this view, grammatical knowledge for L2 is declarative rather than implicit, as is the case for L1 grammar. By contrast, lexical knowledge is represented in the declarative memory system for both L1 and L2. Because implicit and declarative knowledge are mediated by distinct neural systems (a left frontal-basal ganglia circuit for implicit knowledge and left temporal language areas for declarative knowledge), Ullman's differential hypothesis claims that L2 acquisition in adulthood could not depend on the same brain mechanisms that are used to process the native language [5].

Brain imaging offers a unique opportunity to assess directly the representation of L2 subcomponents in the bilingual brain. The results have so far contradicted Ullman's hypothesis [6••, 7, 8•]. In particular, during grammatical tasks in bilinguals the brain structures traditionally associated with grammatical processing (e.g. Broca's regions, basal ganglia) were involved at a comparable level when performing the tasks in both L1 and L2. Additional activation for L2, extending into areas adjacent to the areas subserving L1 grammar, was evident only in bilinguals with low proficiency and/or late acquisition. These results indicate the effects of variables, such as the age of L2 acquisition and proficiency, on the pattern of brain activation in bilinguals.

In the present review we focus on studies that show how several factors, such as the age of acquisition, degree of proficiency and exposure to L2, affect the neural organization of L2.

Section snippets

The neural organization of L2 processing

Since its inception, neuroimaging work on bilinguals has been motivated by the same ‘localizationist’ questions that run through the bilingual aphasia literature: whether multiple languages are represented in overlapping or in separate cerebral systems. For instance, in bilingual aphasics the observation of selective recovery of one language was often interpreted as evidence for differential neural representation of languages [9]. However, there are limitations to the generalization of such

Is age-of-L2 acquisition fundamental for the neural organization of L2?

An ongoing issue in neurobiology concerns the fact that the acquisition of language seems to depend on appropriate input during a biologically based ‘critical period’ [13]. It has also been suggested that L2 learning might be subject to such crucial time-locked constraints [12]. However, L2 can be acquired at any time in life, although L2 proficiency is rarely comparable to L1 if L2 is acquired after the critical periods [12, 13]. The dependence of grammatical processing upon these age effects

Language proficiency and L2 brain organization

As mentioned above, the degree of language proficiency seems to exert a more pervasive influence on the lexical–semantic level of L2. According to psycholinguistics, during the early stages of L2 acquisition there might be a dependency on L1 to mediate access to meaning for L2 lexical items [18]. As L2 proficiency grows, this dependency disappears. Greater levels of proficiency in L2 produce lexical–semantic mental representations that more closely resemble those constructed in L1. According to

The role of exposure and usage of L2

Plasticity after brain damage in the adult human brain has been repeatedly observed in sensory and motor pathways and has also been suggested for language systems during recovery from aphasia [26]. Very little is known about the effect of environmental input in shaping the neural organization of L2.

The study of bilinguals with increasing L2 proficiency might offer a suitable model to test the neural effects of environmental input. For example, the neural differences between low and high

Conclusions

The available evidence supports a dynamic view of the neural basis of L2 processing. The most important contribution of brain imaging studies to the neurobiology of language in bilinguals is the observation of both invariance and plasticity. First, concerning language acquisition, L2 seems to be acquired through the same neural devices responsible for L1 acquisition. Second, regarding L2 processing, the patterns of brain activation associated with tasks that engage specific aspects of

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

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