Language Switching

The frontal lobes (FLs) are cited as responsible for mediating activation and executive control of other cognitive systems, which include language system as well (Luria, 1973; Stuss

& Benson, 1986). Indeed, clinical studies have shown that individuals with the FL damage, while not aphasic, would experience disoriented and disturbances in their language functions.

The impairment in FL would make individuals unable to appreciate and respond to sentences with inferential or implicit meanings under certain social context as they are only able to focus on the concrete aspects of the information given (McDonald & Pearce, 1996). This finding is in accordance with Fabbro’s proposal (2000) that pathological switching might deal with the pragmatic aspects of the language as patients are unable to code switch effectively in an appropriate social context.

Within the FL, the dorsolateral prefrontal cortex (DLPFC) had been correlated with non-verbal task switching (Meyer et al., 1997). Hernandez and colleagues’ studies (2000, 2001) reported the DLPFC as a key region for controlling language switching and inhibiting the currently non-targeted language. Their studies were conducted by using single- and dual-language picture naming tasks. Spanish-English proficient bilinguals were instructed to name the picture covertly in accordance to the pre-stimulus cue indicating the language to use. The result demonstrated an increased activation of the DLPFC for the switched language condition relative to the non-switching condition, which reflects the executive processing induced by

language switching.

Holtzheimer et al. (2005) provided further support for the involvement of the left DLPFC in language switching. Two bilingual patients suffered from major depression undergo repetitive transcranial magnetic stimulation (rTMS) treatment which applied to the left DLPFC.

After the rTMS treatment, although being able to converse with others fluently in their L1, the patients felt more natural and experienced stronger urge to speak in their less frequently used language (L2) even knowing the interlocutor could not speak that language. The first patient had this strong impulse for at least two hours after the rTMS session, while the second patient only experienced this feeling for two minutes. Therefore, it was proposed that the role of language switching in the brain may have been disrupted when left DLPFC was stimulated repeatedly. Thus, this finding provides further support to previous studies (Fabbro et al., 2000;

Fabbro, 2001b), suggesting that DLPFC may be involved in language switching. However, it is plausible that the rTMS disrupted the suppression of the non-target language rather than the stimulation of the language switching phenomenon (Rodriguez-Fornells et al., 2002). To tackle this question, more direct evidence was required to examine the competition between languages and to determine how bilinguals inhibit the non-target language interference. Rodriguez-Fornells et al. (2002) investigated the underlying neural mechanism of language selection in early bilinguals (Catalan-Spanish) and monolinguals with fMRI. Participants had to inhibit the non-target language (Catalan), while responding only to the words in the target language

(Spanish). It was found that only bilinguals, but not monolinguals, exhibited activation in the left anterior prefrontal region (Brodmann areas 45 and 9) when executing the language inhibition process. This study provided support for the fact that even highly proficient bilinguals require the inhibition mechanisms to allow for the language selection process.

Therefore, it might be possible that the rTMS disrupt the suppression of non-target interference rather than engaging in the language switching process.

The PET (positron emission tomography) technique was used to investigate the underlying neural mechanisms in language switching. Price (1998) conducted a PET test on German-English bilinguals with high proficiency, participants were instructed to read alternated L1 and L2 words that were visually presented. Increased activations were found in the left inferior frontal gyrus and the supramarginal gyri (SMG) in the switched trials than non-switched trials. The supramarginal gyri were suggested by Price (1998) to function as the mapping of orthography into phonology. Thus, while alternating between L1 and L2, higher demands would be placed on the phonological recoding operations. The result of this study is in contrast with Hernandez’s discovery as the DLPFC was not detected in this task. Price (1998) reasoned that the non-activation in the DLPFC might be due to the highly predictability pattern and cue given by the L1 and L2 input in this task. Therefore, it is important to know that the types of language task being assigned would affect the involvement of the systems in modulating language switching Price et al., 1999.

Chee et al. (2003) investigated the effect of word repetition within and across languages in high proficient Mandarin-English bilinguals. The within-languages condition comprises a single language (English-only), whereas the across-languages condition contains word pairs of mixed languages (e.g. English-Mandarin word pairs). Participants were requested to read each word silently and figure out its meaning. The result indicated a greater activation in the left prefrontal, lateral and inferior temporal regions under mixed-language condition. It was further suggested that greater cognitive resources were required while processing the mixed-language condition as more extended left prefrontal activity were observed.

In addition to the cortical areas, subcortical regions also play a role in language switching, including the ACC and the basal ganglia. The involvement of the ACC was reported in Fabbro et al. (2000). The authors demonstrated a case study on a 56-year-old bilingual patient, who was fluent in both Friulian (L1) and Slovenian (L2). The patient had a lesion in the prefrontal lobe (both left and right) and left ACC and experienced pathological compulsive switching between languages. Various studies have also shown that the ACC plays an essential role in error detection and conflict monitoring (Ide & Chiang-shan, 2011), where it receives incoming information, provides responses, monitors the outgoing responses, and implements further actions if there is a violation of expectancy (Luu & Pederson, 2004). When the ACC receives signals, it would further allocate the signals to internal processing or external stimulation to take a prompt action (Seeley et al., 2007).

Crinion et al. (2006) conducted PET and fMRI studies to compare the highly proficient German-English bilinguals and Japanese-English bilinguals. The inclusion of entirely different linguistic families (i.e. German and Japanese) hopes to provide support for the universality of the language mechanism. Participants were instructed to perform relatedness semantic decision task based on the targeted words (either in the same or different language). The result revealed an increased activation in the left caudate for the unrelated semantic words pairs or word pairs that were presented in different languages. It was proposed that the left caudate was responsible for the identification of different languages and a universal role in controlling and monitoring the language in use.

Many neuroimaging studies have shown that the dorsolateral prefrontal cortex, inferior parietal cortex, caudate nucleus (posited in the subcortical structure of the basal ganglia), anterior cingulate cortex (ACC) and bilateral supramarginal gyri were involved in the cognitive control and information processing (Graybiel, 1997; Botvinick et al., 1999; Duncan & Owen, 2000; Middleton & Strick, 2000; Botvinick et al., 2001; Braver et al., 2001; Bunge et al., 2002;

Kerns et al., 2004; McCormick et al., 2006), implicating the involvement of subcortical-cortical network in the voluntary and involuntary language switching. Based on the previous neuroimaging studies on language switching in bilinguals, Abutalebi and Green (2008) proposed a neurological pathway which may explain the complex network at work during language switching. The prefrontal cortex (concerned with executive functions,

decision-making, response selection, response inhibition and working memory) was linked with the ACC (involved in attention, conflict monitoring and error detection) and basal ganglia (dealing with language selection, set switching, language planning and lexical selection) for the function of response inhibition, specifically, to suppress the interference from the non-target language.

It was further proposed that the supramarginal gyrus (SMG) in the inferior parietal cortex (concerned with the maintenance of representation and working memory) dealt with cases of unpredictable language switches. The left SMG biased selection away from the language which is not in use, whereas the right SMG biased selection towards the language in use. Abutalebi and Green (2008) suggested that either a left basal ganglia-left prefrontal cortex network was activated to subserve language planning and/ or the inhibition of a prepotent response by basal ganglia towards the supplementary motor area (SMA) (Sumner et al., 2007).

Finally, research has also shown that the right hemisphere might be involved in pathological switching. Lebrun (1990) investigated the spontaneous switching phenomenon, in which the patients experienced constant uncontrolled switching of languages within several sentences such as using L1 in the first sentence, switching to L2 in the second sentence, and switching to L3 in the third sentence, and so on. A multilingual patient (Italian-French-German) who was diagnosed with Alzheimer’s disease at the age of 57 after exhibiting memory disturbances was found to attain language switching disorder at the same time: he often switched between languages without reasons De Vreese et al., 1988. Lebrun (1990) suggested

that the lesion might localize in the right hemisphere which are also responsible for the regulation of the verbal output in languages. In addition, Martin et al. (1994) reported a case study of a polyglot patient (Indian-English), who had epilepsy in the right temporal lobe and experienced a pathological switching disorder. After anti-epileptic therapy was conducted, the patient attained full recovery from the language switching disorder (Martin et al., 1994). These two cases suggested that the right hemisphere was involved in the language selection and switching between languages.