Effects of early second language acquisition on the cortical language network in multilinguals

Research on language acquisition has shown that language development depends on language experience during early childhood. Early acquisition of one as opposed to two languages should therefore influence the development of language capabilities. Various psycholinguistic studies have provided support for this idea. The impact of early second language acquisition on the establishment of the cortical network, however, remains elusive. In the present study we used functional magnetic resonance imaging (fMRI) to investigate this aspect. Language related brain activity was assessed in two groups of multilinguals with different ages of second language acquisition: “early multilinguals” had acquired a second language (L2) in early childhood (before the age of three years); “late multilinguals” learned a second language (L2) in early adolescence or later (after the age of nine years). Of particular interest was the previously unexplained question of whether the brain representation of languages learned as adolescents or adults is influenced by early second language acquisition. With this perspective in mind, all early and late multilinguals included in the study had also learned a third language (L3) after the age of nine years, so that not only early, but also late learned languages could be compared between both groups. All multilinguals were tested in their three languages (L1, L2 and L3) by fMRI. The brain activity was measured during a language production task in which the subjects had to report the happenings of the previous day. To reduce influences not specifically related to language processes such as variable attention or valuation of the described experiences, the subjects were tested twice, on different days. The average brain activity during processing of L1, of L2 and of L3 has been statistically assessed for the groups of early and late multilinguals separately, including both tested runs. The comparison of the early acquired languages between both groups revealed striking differences in language related neuronal activity. Indeed, the analysis of the data shows that early multilinguals in both of their early acquired languages (L1 and L2) use cortical language regions more than late multilinguals in their L1. Particularly, increased neuronal activity was observed in regions of the left frontal lobe i.e. Broca’s area and the adjacent prefrontal cortex. It should be noted that the early acquisition of a second language (L2) determines not only its own cortical representation, but also that of the first acquired language (L1). The cortical region which is most influenced by the effect of early second language acquisition i.e. the prefrontal cortex, plays a crucial role during early procedural language acquisition. Here, higher activation could indicate the engagement of functions related to the resolution of cross-linguistic interference. Indeed, left prefrontal regions support processes necessary for a selection between competitive alternatives of a motor plan. In early multilinguals, specific cross-linguistic interference between the early acquired languages seemed to manifest itself during the preparation for the speech action. Further evaluation of the data provided more evidence for the involvement of cortical functions to resolve cross-linguistic interference between two early acquired languages. This is apparent from the comparison of the language-related activation pattern in left perisylvian regions between early and late multilinguals, particularly in Broca’s area and in the supramarginal gyrus. The neuronal network connecting these brain regions supports sensory-motor integration of phonological information, i.e. the function of the “phonological loop”. The evaluation led to the discovery of a pronounced motor load of the activated network in early multilinguals. It is known that such a functional adjustment is necessary to resolve interference of sensoryrelated language information at the phonological level. Thus, the comparison of early acquired languages between early and late multilinguals could indicate, that early second language acquisition influences the function of the cortical language network to resolve various aspects of cross-linguistic interference. Since the changes in brain activity could be demonstrated in adults, the results of the present study indicate a persistent adaptation of brain language functions to the early presence of two languages. The comparison of the later-learned L3 between both groups showed principally the same result as the comparison of the early acquired languages: in early multilinguals, functions of language networks supporting resolution of language interference seem to be more established than in late multilinguals. This is the first study which has described an influence of early second language acquisition on the organisation of the cortical language network of subsequent late learned languages. An effect of early second language acquisition on the use of the language network by later-learned languages could however not be described. Indeed, later-learned languages of both groups showed similar differences to the early acquired languages. By demonstrating that exposure to a second language during early childhood not only manifests in the cortical language network of early but also of late acquired languages, this study extends the current view of the importance of early language acquisition for the establishment of the cortical language network.