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., 1998) and that L2 has reduced leftward lateralization (Albert & Obler, 1978, cited in Dehaene et al., 1997). Various studies that examined bilinguals and their language processing have shown that L2 in comparison to L1 doesn’t consistently activate the same neural networks across subjects. The inconsistency between participants could be attributed to the age of acquisition and proficiency level of L2 (Dehaene et al., 1997; 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 variability 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.

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The research shows that L1 consistently activated the temporal lobe, especially stimulating 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 & Dehaene, 1994; Paulesu, Frith, & Frackowiak, 1993, cited in Dehaene et al., 1997).
Perani et al. (1998) studied cortical responses by evaluating bilinguals with high proficiency, late acquisition (HPLA) and high proficiency, early acquisitions (HPEA) and comparing their results with low proficiency, late acquisition (LPLA) study (Perani et al., 1996). Similar to previous studies, L1 of the LPLA bilinguals activated the left hemisphere, including perisylvian areas and temporal lobes and L2 activated different networks across subjects (Perani et al., 1996; Dehaene et al., 1997, cited in Perani et al., 1998). On the other hand, the results demonstrated that balanced bilinguals, HPLA and HPEA, activated similar networks while listening to stories in their native and acquired languages. HPLA subjects activated left hemisphere in the temporal pole, the STS, MTG and hippocampal structures for L1, which is consistent with previous results. However, L2 activated similar neural pathways, which suggests that when L2 is acquired to a high proficiency the speakers activate the same areas of the brain for both languages. HPEA subjects activated temporal poles, hippocampal structures and lingual gyrus for both, L1 and L2, which is similar to the results of HPLA speakers. These results show that once the proficiency level of L2 increase, the speakers recruit less networks to maintain L2 and the activation foci between languages doesn’t vary as it does with unbalanced bilinguals (e.g., LPLA). Furthermore, the results showed that the temporal lobes were consistently activated during tasks. Previous studies showed that the temporal poles get activated during tasks that require listening, reading, or speaking (Mazoyer et al., 1993; Perani et al., 1996, cited in Perani et al., 1998). Therefore, the authors suggest that the temporal poles are responsible for processing at the sentence level rather than unconnected word level.
In another study, Liu, Hu, and Peng (2010) examined Chinese-English bilinguals using word production and picture naming tasks. The results showed that there was increased activation for L2 in the left inferior frontal gyrus (IFG), bilateral supplementary motor area (SMA), left precentral gyrus, Brodman’s area (BA) and bilateral basal ganglia, including the putamen, globys pallidus, and caudate, and bilateral cerebella.The bilateral SMA, left precentral gurys, and the cerebella functions are related to motor processing for word production; therefore, activation in these regions might be related to phonological and articulatory processing in language production. The researchers also found that L2 activated Brodman’s area of BA44/45/48; the BA44 and BA 45 are known are Broca’s area, which is responsible for motor planning and articulation, as well as phonological processing. Activations in these areas suggest that L2 is less autonomic and requires more neural pathways to maintain and control language production for L2 (Braun et al., 2001, cited in Liu, Hu, and Peng, 2010). L2 also activated regions of basal ganglia, which is related to motor behavior and cognition functions (Graybiel, 2000) and regulates planning and execution of actions, and speech motor control. The activations in these areas could be attributed to the fact that unbalanced bilinguals try to reduce interference from a more dominant L1 (Elsinger et al., 2006; Alm, 2004, cited in Liu, Hu, and Peng, 2010). Interestingly, the authors found that L1 activated the right putamen and right globus pallidus of the right basal ganglia. The dissociation between L1 and L2, which activated the left basal ganglia, suggests that different regions of basal ganglia are responsible for different levels of speech execution( Jueptner and Weiller, 1998, cited in the study). The difference between activation of basal ganglia could also be attributed to the fact that Chinese and English use different phonological systems and language scripts, which might activate different parts of basal ganglia (Liu et al., 2006, cited in Liu, Hu, and Peng, 2010). The results also showed overlapping between neural pathways for L1 and L2. Both languages activated the left IFG, which is associated with semantics and phonology, posterior perisylvian area which is responsible for linguistic functions and the cingulate gyrus for cognition and motor control.
The literature review and the present studies concur that L1 has a consistent neural pathway within the left hemisphere and L2 has a more varied cerebral activation patterns. The differences between L1 and L2 are being attributed to the language proficiency of L2 Dehaene et al., 1997; Perani et al., 1998; Liu, Hu, and Peng, 2010). Nonetheless, L1 and L2 also activate common neural system, which differed from one study to another due to the tasks, languages involved, and the level of L2 acquisition. The results of the studies suggest that L2 is less autonomic than L1 and requires more resources to maintain the L2, however as the proficiency of L2 increases the need to activate varied neural pathways decreases, as L2 becomes competent to L1. It’s important to study about the way languages are represented in peoples’ brain as these studies will contribute to our understanding of brain plasticity, language acquisition and neurological diseases, such as aphasia in bilinguals. Also, new studies using advanced technologies will help to clarify agreed upon hypothesis of language lateralization and representation in the human brain.
References
Dehaene, S., Dupoux E., Mehler, J., Cohen, L., Paulesu, E., Perani, D., et al. (1997). Anatomical variability in the cortical representation of first and second language. Neuroreport, 8, 3809–15.
Liu, H., Hu, Z., Guo, T., Peng, D. (2010). Speaking words in two languages with one brain: neural overlap and dissociation. Brain Research, 1316, 75-82.
Perani, D., Paulesu, E., Galles, N.S., Dupoux E, Dehaene S, Bettinardi V, et al.(1998). The bilingual brain: proficiency and age of acquisition of the second language. Brain, 121, 1841–52.