Advances in neuroimaging techniques and analytic methods have led to a proliferation of studies investigating the impact of bilingualism on the cognitive and brain systems in humans. Lately, these findings have attracted much interest and debate in the field, leading to a number of recent commentaries and reviews. Here, we contribute to the ongoing discussion by compiling and interpreting the plethora of findings that relate to the structural, functional, and connective changes in the brain that ensue from bilingualism. In doing so, we integrate theoretical models and empirical findings from linguistics, cognitive/developmental psychology, and neuroscience to examine the following issues:
(1) whether the language neural network is different for first (dominant) versus second (nondominant) language processing;
(2) the effects of bilinguals’ executive functioning on the structure and function of the “universal” language neural network;
(3) the differential effects of bilingualism on phonological, lexical-semantic, and syntactic aspects of language processing on the brain; and (4) the effects of age of acquisition and proficiency of the user’s second language in the bilingual brain, and how these have implications for future research in neurolinguistics.
Introduction
It has been estimated that more than half of the world’s population are bilinguals and/or multilinguals [1]. How does this widespread capacity for communicating in two or more languages impact the cognitive and brain systems in humans? For many years, the fields of psychology and neuroscience had limited tools and tended to investigate brain structure and cognitive function separately. However, recent advances in neuroimaging techniques and analytic methods have led to a proliferation of neuroscience findings regarding the impact of bilingualism on the human brain. Here we review these numerous and surprisingly diverse findings in light of current cognitive models, thereby enriching current understandings on the effects of bilingualism through mutual perspectives of linguistics, cognition, and neuroscience. Additionally, in the present review, we overcome the limited perspectives of early work in psychology and neuroscience by spanning the gap between brain structure and cognitive function. Specifically, we systematically examine the structural and functional differences in language networks for domain-general and domain-specific component processes in bilinguals/multilinguals (henceforth referred to as “bilinguals”: in this paper, we do not distinguish between findings pertaining to bilinguals versus multilinguals). We also focus on individual-difference factors including age of acquisition and language proficiency that may differentially impact bilingual brain networks. Throughout this review we argue that bilingual cognition is best understood by taking into consideration both structure and function, as well as factors relevant to language learning.
Historical Perspective
Early neuroscience perspectives on the relationships between brain structure and cognitive function drew two opposite conclusions from then-available cruder forms of investigation. Brain structure was considered to be organized into localized, isolated areas where pockets of activity serve very specialized function, as in the tradition of Gall and Spurzheim [2] and Fodor [3]. The alternative view was that brain structure is relatively homogeneous with distributed forms of representation, as in the tradition of Lashley [4] and Hebb [5]. According to this view, brain structure/architecture was related to function in a more holistic way that supports plasticity, whereby functions associated with damaged areas can be picked up by other undamaged areas. The former perspective was supported by myriad functional neuroimaging studies while the latter perspective was taken up by connectionist investigations.
Driven by new technologies and advances in computing power, contemporary approaches to neuroscience have now been able to draw evidence from more sources than in the past, leading to new perspectives placed between these two extremes. Thus, the new evolution of neuroscience investigations is not structurally bound, like the past lesion studies, nor functionally discrete, like the early neuroimaging studies, but can accommodate both perspectives of local specialization with global coordination across areas as self-organized networks that emerge from the individual’s experiential history. Current neuroscience-based models can therefore point to a developed neural substrate of networks forged by nature and nurture, which instantiate soft-assembled coordinative structures [5] organized to meet the constraints of the current behavioral task.
As a case in point, language is multifaceted, with oral and written forms as well as receptive and expressive modes, but shows evidence of certain universal properties of brain structures and their interconnections that underlie the behavioral aspects of communicating through speech and/or print in multiple languages. While structure gives clues to the architecture of language networks, function relates to the manner in which networks may be assembled within different contexts or as a result of personal experiences. At present, unresolved questions for bilingualism include the degree to which there is anatomical overlap in the neural networks used for and processing in various language domains.
One possibility is that there is a common neurobiological foundation for different languages ,addressed below as a “universal language neural network.” Although brain networks may be highly constrained across languages and routed to the same cortical circuits [5.6], this explanation may not be tenable in the case of bilingualism, given that anatomical overlap between language networks could be sensitive to variables such as language proficiency, age of acquisition, and different scripts. A second possibility is that the spatial organization of the neural networks for acts of reading, listening, and speaking is common across one’s different languages, only to the extent that a high proficiency is reached for the languages in question. This would indicate that the universal neural network is only accessed at the end of the learning process. Alternatively, a third possibility is that the linguistic brain structures established from acquisition are coopted for learning only during a critical window of chronological development; that is, only early age of acquisition or simultaneous bilingualism would allow for the assimilation of a universal neural network across languages.
References
1.European Commission Special Eurobarometer, Europeans and their languages, 2012, http://ec.europa.eu/public_opinion/archives/ebs/ebs_243_en.pdf.
2.F. J. Gall and G. Spurzheim, Recherches sur le Système Nerveux en Général et sur Celui du Cerveau en Particuller; Mémoire Présenté à L'Institut de France, le 14 Mars 1808; Suivi D'observations sur le Rapport qui en a Été Fait à Cette Compagnie par ses Commissaires, Bonset, Amsterdam, The Netherlands, 1967, Reprinted from the 1809 edition.
3.J. A. Fodor, Modularity of the Mind, MIT Press, Cambridge, Mass, USA, 1983.
4.K. S. Lashley, Brain Mechanisms and Intelligence: A Quantitative Study of Injuries to the Brain, Dover Publications, New York, NY, USA, 1929–1963.
5.D. O. Hebb, The Organization of Behavior, Wiley, 1949.
6.G. A. Ojemann, “Cortical stimulation and recording in language,” in Localization and Neuroimaging in Neuropsychology, A. Kertesz, Ed., pp. 35–55, Academic Press, San Diego, Calif, USA, 1994.View at: Google Scholar