Neurolinguistic Contributions to Understanding the Bilingual Mental Lexicon

RENATA MEUTER

Introduction

Many bilinguals will have had the experience of unintentionally reading something in a language other than the intended one (e.g. MUG to mean mosquito in Dutch rather than a receptacle for a hot drink, as one of the possible intended English meanings), of finding themselves blocked on a word for which many alternatives suggest themselves (but, somewhat annoyingly, not in the right language), of their accent changing when stressed or tired and, occasionally, of starting to speak in a language that is not understood by those around them. These instances where lexical access appears compromised and control over language behavior is reduced hint at the intricate structure of the bilingual lexical architecture and the complexity of the processes by which knowledge is accessed and retrieved. While bilinguals might tend to blame word finding and other language problems on their bilinguality, these difficulties per se are not unique to the bilingual population. However, what is unique, and yet far more common than is appreciated by monolinguals, is the cognitive architecture that subserves bilingual language processing. With bilingualism (and multilingualism) the rule rather than the exception (Grosjean, 1982), this architecture may well be the default structure of the language processing system. As such, it is critical that we understand more fully not only how the processing of more than one language is subserved by the brain, but also how this understanding furthers our knowledge of the cognitive architecture that encapsulates the bilingual mental lexicon.

The neurolinguistic approach to bilingualism focuses on determining the manner in which the two (or more) languages are stored in the brain and how they are differentially (or similarly) processed. The underlying assumption is that the acquisition of more than one language requires at the very least a change to or expansion of the existing lexicon, if not the formation of language-specific components, and this is likely to manifest in some way at the physiological level. There are many sources of information, ranging from data on bilingual aphasic patients (Paradis, 1977, 1985, 1997) to lateralization (Vaid, 1983; see Hull & Vaid, 2006, for a review), recordings of event-related potentials (ERPs) (e.g. Ardal et al., 1990; Phillips et al., 2006), and positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) studies of neurologically intact bilinguals (see Indefrey, 2006; Vaid & Hull, 2002, for reviews). Following the consideration of methodological issues and interpretative limitations that characterize these approaches, the chapter focuses on how the application of these approaches has furthered our understanding of (1) selectivity of bilingual lexical access, (2) distinctions between word types in the bilingual lexicon and (3) control processes that enable language selection.

1. Methodological Issues

Studies focusing on the neurophysiological correlates of bilingual language processing often are more concerned with localization and lateralization than with the consideration of how behavioral and physiological data converge to inform our understanding of the bilingual lexicon. (For reviews of select areas see Abutalebi et al., 2005; Fabbro, 2001b; Hull & Vaid, 2005; Vaid & Hull, 2002.) Important insights have been gleaned from the closer observation of bilingual aphasics (Fabbro, 2001a; Paradis, 2004). In addition, a number of studies have provided PET and hemodynamic evidence, as well as ERP correlates, for lexical organization and access (e.g. De Bleser et al., 2003; Phillips et al., 2006), semantic organization (e.g. Halsband et al., 2002) and issues of control (e.g. Rodriguez-Fornells et al., 2006) in the bilingual lexicon. It is important to be mindful of the strengths and limitations of each approach and how they may be optimally applied.

1.1. Bilingual aphasia

Early forays into the bilingual brain intrigued us with descriptions of the many different recovery patterns observed in multilingual aphasia. For example, some patients demonstrated selective recovery of either the first learned or most familiar language (Pitres, 1895; see also Albert & Obler, 1978; Paradis, 1977, 1997). Others showed pathological switching between languages (Fabbro et al., 2000) and selective, antagonistic recovery patterns (Aglioti et al., 1996; Aglioti & Fabbro, 1993). Inferences drawn from aphasic deficits involve a reverse extrapolation to a(n assumed) premorbid state of intact linguistic functioning. However, often little is known about the patient's language history and consequently a retrospective comparison is potentially flawed. Such information is critically important because age and manner of acquisition, and premorbid language use and proficiency would have affected the patient's performance in each language post-injury.

Those concerns aside, the seductive aspect of this approach is that patterns of language performance (both intact and impaired) suggest selective damage to (or preservation of) one or more language components or processing pathways. Following a modular approach to cognitive architecture (cf. Fodor, 1983; Shallice, 1988), dissociations in performance can be taken to reflect an underlying cognitive architecture characterized by subcomponents that can be impaired selectively. However, it is difficult to determine the source of the cognitive deficit and which task components are adversely affected, because it is unclear whether the cause is a circumscribed lesion or more extensive damage, or whether the deficit results from abnormal functioning elsewhere or even reflects intact residual functioning in the lesioned area (see also Green & Price, 2001).

When testing bilingual aphasics, the degree of similarity between the languages spoken is often ignored and there is a tendency to use concrete items. Yet we know that concrete and abstract items are processed differently (e.g. De Groot & Nas, 1991; Dong et al., 2005). An explicit comparison of bilingual aphasic performance on such items would yield important insights, and so would contrasting the processing of cognates and noncognates, also known to be differently represented...