A Review of the CognitiveNeuroscience of Executive Function in BilingualsIntroduction Executive function refersto three main functions including; updating information inthe working memory, inhibiting spontaneous responses and shifting between tasksand mental sets (Miyake et al., 2000). These executive functions involve the frontal lobes and premotor cortex of the brain and although they are moderately correlated,they are clearly separable functions. (Miyake et al., 2000) In recent years there has been a contentious debate about whetherbilinguals have enhanced cognitivecontrol comparedto monolinguals which may give them an advantage in cognitive tasks andexecutive functions. Hernandez etal.
, 2015). This notion is known as the bilingual advantage (Kroll & Bialystok, 2013) and has been supported through results from non-linguisticcognitive control tasks such as the Simon task and Stroop test, (Bialystok, 2009) (Soveri, Rodriguez-Fornells & Laine,2011). Bialystok and colleagues (2004) ). These findings supporting the bilingual advantage have notbeen consistent (examples) and the differences observed betweenmonolingual and bilinguals are more likely due to uncontrolled factors such associoeconomic status (SES), socio-cultural demographics, language-switching andindividual differences which can be difficult to separate from bilingualism (YANG, YANG & LUST, 2011) (Antonet al 2014) (Hernandez, Greene, Vaughn, Francis& Grigorenko, 2015). Presently, no consensus has been reached about which facetis specifically improved in bilinguals executive function however, research by Bialystok et al. (2004), (Bialystok, Craik, Green & Gollan, 2009)and Peal and Lambert (1962) suggest the inhibition and shiftingcomponents of executive function may be advanced since bilinguals have demandsfor language switching and must deal with interfering responses from theirsecond language(Miyake et al., 2000). (Stocco, Yamasaki, Natalenko & Prat,2012).
Bialystok, 2001; Bialystok & Martin, 2004; Carlson & Meltzoff, 2008; Festman, Rodriguez-Fornells, & Münte, 2010). This essay will focus on shifting in bilingualswith an aim of using recentneuroimaging studies to investigate the neural correlates of shifting and will explorethe brain processing differences while shifting between bilinguals andmonolinguals, taking inot account the research limitations. Finally, this essaywill discuss how differences in the brain while shifting may be related to the behavioural findings in bilinguals and monolinguals (Luket al., 2012). Until now, bilinguallanguage processing has been studied primarily at the behavioural level with researchersfocused on which group is better. This essay joins with (Hernandez et al.
, 2015) to advocatethat researchers strive to understand more about cognitive development, executivefunction and language processing in both bilinguals and monolinguals instead ofproving who has the advantage. (Hernandezet al., 2015)Shifting in the bilingual brain Shifting is one of the key executivefunctions, it refers to a person’s ability to shift back and forth betweenseveral tasks or mental sets. Shifting is also commonly referred to as task-switching.Hofmann,Schmeichel & Baddeley, 2012).
Recent neuroimaging studies suggest a neural overlap between languagecontrol and non-verbal cognitive control in bilinguals especially involving thebasal ganglia (De Baene et al., 2015) Abutalebi and Green (2008) Garbin etal., 2010, 2011; Guo, Liu, Misra, & Kroll, 2011; Abutalebi & Green, 2008; Wang, Xue, Chen, Xue, & Dong, 2007; Crinion et al., 2006; Hernandez, Dapretto, Mazziotta, & Bookheimer, 2001; Hernandez et al., 2000). Additionally De Baeneet al.
, 2015) examined the overlap in brain activation in betweenlinguistic switching activities and a closely matched non-linguistic switchingtask in highly proficient Spanish–Basque bilinguals, results from the Fmri stronglysuggests that bilinguals share similar brain circuits for language control andgeneral cognitive control. Using the activation likelihoodestimation method (Luk etal., 2012) examined theneural regions involved in bilingual language switching, their resultshighlighted frontal left lateralized clusters including; middle frontal gyrus(BA 9, 46), midline pre-supplementary motor area (BA 6), left inferior frontalgyrus (BA 44 and 47), left middle temporal gyrus (BA 37), right superiortemporal gyrus (BA 22), right precentral gyrus (BA 6) and bilateral caudate nuclei(Figure 1).(Luk et al., 2012), five of these eight regions were also identifiedby (Green & Abutalebi, 2008). Although (Green & Abutalebi, 2008), and luk highlight the activation ofACC in bilingual language switching, (Velanova, Wheeler & Luna, 2008 suggests that ACC is moreinvolved with error monitoring and detection as less activity is detected duringcorrect trials and more activity during error trials). (Luk et al., 2012.
Overall, theprefrontal cortex and basal ganglia are involved in both linguistic switching and non-linguisticswitching, this neuralcomputation overlap suggests that language-switchingis key in understanding executive function in the bilingual brain and helps us to understand how the shifting occurs in thebrain. (e.g.
, Hernandez,Martinez, & Kohnert, 2000; Rodriguez-Fornellset al., 2005). (Stocco,Yamasaki, Natalenko & Prat, 2012 (basal ganglia involved in shifting (Yehene,Meiran, & Soroker, 2008)(e.
g., Lehtonenet al., 2005; Price,Green, & von Studnitz, 1999))The Conditional RoutingModel In 2010, Stocco and colleagues proposed the conditional routing modelwhich provides a neural framework explaining the enhanced executive function inbilinguals. The conditionalrouting model is based on evidence indicating the neural mechanisms whichcontrol language are connected to the neural mechanisms of the shiftingcomponent in executive function(Stocco,Yamasaki, Natalenko & Prat, 2012), particularly the basal ganglia circuitry(REF)and evidence that this circuit is involved in tasks where bilinguals outperformmonolinguals (REF).(Stocco, Lebiere& Anderson, 2010) (Stocco, Yamasaki, Natalenko & Prat, 2012)Bilinguals’ constantly need to apply and switch between languagesand grammatical rules and according to the conditional routing model the basalganglia circuit is responsible for this selection and switching, in particularthe striatum (Stocco, Yamasaki, Natalenko & Prat, 2012).
The basal ganglia are a set ofdistinct grey matter nuclei located in the middle of the brain which form acomplex circuit that send signals to the frontal lobe(Albin,Young, & Penney, 1989; DeLong,1990) (Stocco,Yamasaki, Natalenko & Prat, 2012). The largest structure within the basal ganglia is thecorpus striatum (referred to as striatum), which includes the caudatenucleus(CN) and putamen (Purves, Augustine &Fitzpatrick, 2001). Thestriatum is the input station of the circuit and connectsto the cortical regions and controls signals to the prefrontal cortex, which isassociated with higher order functions such as shifting(Garbin et al., 2010).The caudatenucleus receives and transmits information to the dorsolateral prefrontalcortex and plays a strong role in cognitive control.
Abutalebi,2008). (Hernandez etal., 2015). When bilinguals engage in shifting activities or languageswitching the striatum routes information to the frontal cortex and (Stocco, Lebiere & Anderson, 2010) proposethat extensive bilingual experiencereinforces the basal ganglia’s ability to manage signals across corticalregions, resulting in bilinguals enhanced performance in executivefunction tasks such as x and y. (Hernandezet al., 2015) (Garbin et al., 2010)The conditional routing model is supported by observationsof patients with selective damage in the basal ganglia caused by diseases suchas Parkinson’s and Huntington’s.
This damage leads to a decline in the shiftingcomponent of executive function, finally resulting in poor abilities to switchefficiently between tasks and mental states (Stocco, Yamasaki, Natalenko , 2012)(REF and images of damaged basal ganglia). Additionally, bilingualpatients with injuries impacting the basal ganglia circuit tend to have uncontrolledlanguage-switching (Fabbro,2001)(Stocco,Yamasaki, Natalenko & Prat, 2012)Differences between the monolingualand bilingual brainAccording to the conditional routing model, bilinguals relyon the basil ganglia for shifting and language switching (Stocco, Yamasaki,Natalenko & Prat, 2012) however, research by Garbin et al. (2010) highlights significant brain network differences betweenmonolingual and bilinguals when task switching. Garbin et al. (2010) found larger oxygen consumption in theright inferior frontal gyrus (IFG) and the anterior cingulate cortex ofmonolingual participants, while bilinguals show a reduced switching cost andactivated the left IFG and striatum while shifting.
Furthermore, the size of the switch costs was negativelyassociated with the activity of the left caudate nucleus in bilinguals,suggesting that the recruitment of the CN results in faster switches (Stocco,Yamasaki, Natalenko & Prat, 2012). Researchby (Vaughn et al., 2015) focusedon differences in brain activity during the Simon task based across a range ofbilinguals, considering age of acquisition and proficiency, results suggest laterage of acquisition predicts greater activity in the left inferior parietallobule, while better proficiency predicts less recruitment of the dorsolateralprefrontal cortex and anterior cingulate cortex. These findings suggest thatthe age of second language acquisition may predict how the brain handles cognitivetasks, this is important as it suggests variations in language experience mayinfluence brain networks, executivefunction and behaviour(Vaughn et al., 2015).Another significant difference between the bilingual and themonolingual brain is that the bilingual brain relies on the left caudatenucleus to switch between tasks, while the monolingual brain does not (Garbin etal., 2010)(Stocco, Yamasaki, Natalenko & Prat, 2012).
Regardless of the bilingual advantage, bilingual’sneural activity during these shifting tasks are different to monolinguals,which indicates that the bilingual brain processes these executive functiontasks differently. These researchers provideinformation about the potential neural network differences between bilingualsand monolinguals while task switching however, further research is needed toinvestigate the neural differences between monolingual and bilinguals acrossthe other executive functions.