Bilingualism and Brain Lateralization

Bilingualism and Brain Lateralization Polina Gavrilova Brain Lateralization and Neural Networks in Bilinguals In recent years, various studies have been conducted on bilingualism in regard to the neural basis of the first language (L1) and second language (L2) processing. The new technical advances, such as position emission tomography (PET) and functional magnetic resonance imaging (fMRI) are used to determine whether L1 and L2 share a common neural network or whether languages are represented in different areas of the brain (Dehaene et al., 1997; Perani et al., 1998; Liu, Hu, and Peng, 2010). Studies in neuropsychology have shown that for most people language processing takes place in the perisylvian areas of the left hemisphere. Research on bilinguals and polyglots who suffered brain injury revealed that occasionally aphasia affects only one of the languages that were previously acquired. This finding suggests that languages are represented in different parts of the brain (Paradis, 1995, cited in Perani et al., 1 998) and that L2 has reduced leftward lateralization (Albert Perani et al., 1998; Liu, Hu, and Peng, 2010). This paper examines whether L1 and L2 are supported by a common neural system or whether a dedicated cortical area represents each language. Furthermore, this paper identifies neural substrates activated by L1 and L2 during auditory, word production, and picture naming tasks. Dehaene et al. (1997) examined bilinguals (French-English) who acquired L2 after the age of seven. The researchers found that while listening to a task the superior temporal sulcus (STS), superior and middle temporal guri (STG and MTG), temporal pole (TP), and left angular gyrus (AG) were constantly activated in the left hemisphere for L1. STS and TP were also activated in the right hemisphere but it varied across subjects and the activation wasn’t as strong as in the left hemisphere. In addition, the neural pathway didn’t extend to AG. The findings for L2 showed greater inter-subject varia bility than for L1. The results of fMRI found that six subjects activated STS, STG, and MTG in the left temporal lobe for L2. However, the pixels of these activations were dispersed compared to the results for L1. The second language didn’t cause any activation in the left TP and AG. Also, some of the subjects didn’t show any neural activation in the left temporal region, which suggests that L2 is mostly dominated by their right hemisphere. The results also displayed that subjects activated additional resources while listening to L2. These additional sub-regions were the right STG and STS in the right temporal lobe. In addition, results of L2 showed that some subjects activated various networks outside the temporal lobe. Specifically, these subjects used the left inferior frontal gyrus, located in the Broca’s area, the inferior precentral sulcus, and the anterior cingulate. The research shows that L1 consistently activated the temporal lobe, especially stimulati ng the STS, STG, and MTG in the left hemisphere. Some subjects also activated these cerebral regions for L2 but with greater dispersion. Participants had strong leftward lateralization for L1 and inconsistent lateralization patterns for L2 across subjects. These results are consistent with the hypothesis that L1 is represented in the left hemisphere for most people. Furthermore, the study suggests that late bilinguals require additional neural networks for L2. Therefore, some subjects recruited left inferior frontal gyrus, which is responsible for language production to help maintain L2 while processing it during tasks. The anterior cingulate was another additional resource, which is responsible for attention and control. This suggests that L2 is not as autonomic as L1 and subjects needed more resources and attention to process L2 (Pardo et al., 1990; Posner Paulesu, Frith, & Frackowiak, 1993, cited in Dehaene et al., 1997).