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c o r t e x 5 8 ( 2 0 1 4 ) 3 0 1 e3 2 4Available online at www.sciencedirect.comScienceDirectJournal homepage: www.elsevier.com/locate/cortexSpecial section: ReviewNeuroplasticity as a function of second languagelearning: Anatomical changes in the human brainPing Li*, Jennifer Legault and Kaitlyn A. LitcofskyCenter for Brain, Behavior, and Cognition, Center for Language Science, Department of Psychology,Pennsylvania State University, United Statesarticle infoabstractArticle history:The brain has an extraordinary ability to functionally and physically change or reconfigureReceived 18 January 2014its structure in response to environmental stimulus, cognitive demand, or behavioralReviewed 31 March 2014experience. This property, known as neuroplasticity, has been examined extensively inRevised 22 April 2014many domains. But how does neuroplasticity occur in the brain as a function of an in-Accepted 1 May 2014dividual's experience with a second language? It is not until recently that we have gainedAction editor Costanza Papagnosome understanding of this question by examining the anatomical changes as well asPublished online 17 May 2014functional neural patterns that are induced by the learning and use of multiple languages. Inthis article we review emerging evidence regarding how structural neuroplasticity occurs inKeywords:the brain as a result of one's bilingual experience. Our review aims at identifying the pro-Neuroplasticitycesses and mechanisms that drive experience-dependent anatomical changes, and inte-DTIgrating structural imaging evidence with current knowledge of functional neural plasticityVBMof language and other cognitive skills. The evidence reviewed so far portrays a picture that isSecond language acquisitionhighly consistent with structural neuroplasticity observed for other domains: second lan-Bilingualismguage experience-induced brain changes, including increased gray matter (GM) density andwhite matter (WM) integrity, can be found in children, young adults, and the elderly; canoccur rapidly with short-term language learning or training; and are sensitive to age, age ofacquisition, proficiency or performance level, language-specific characteristics, and individual differences. We conclude with a theoretical perspective on neuroplasticity in language and bilingualism, and point to future directions for research. 2014 Elsevier Ltd. All rights reserved.1.IntroductionMore than half of the world's population are actively learningor speaking a second language in addition to their nativetongue (Grosjean & Li, 2013). What impact does experiencewith a second language have on the human brain? Traditionally, both folk wisdom and scientific evidence point to thedecreasing plasticity of the adult brain in acquiring a newlanguage, especially given the arguments of the so-called“critical period hypothesis” (Kennedy & Norman, 2005;* Corresponding author. 452 Moore Bldg., Department of Psychology & Center for Brain, Behavior, and Cognition, Pennsylvania StateUniversity, University Park, United States.E-mail addresses: pul8@psu.edu, pingpsu@gmail.com (P. 0010-9452/ 2014 Elsevier Ltd. All rights reserved.
302c o r t e x 5 8 ( 2 0 1 4 ) 3 0 1 e3 2 4Lenneberg, 1967). Recent scientific evidence, however, haschallenged this view. In particular, cognitive and brain studiesof bilingual language acquisition, along with studies ofmemory, attention, and perception, have demonstratedcontinued neuroplasticity for language learning in the adultbrain that has never been previously imagined (see Abutalebi& Green, 2007; Hernandez, 2013; Li, 2014, for reviews). Thestudy of neuroplasticity of language learning in adulthood,along with the understanding of neural correlates of languageprocessing and representation, has made significant progressin the last decade thanks to rapid advances in neuroimagingtechnologies (see reviews in Hickok, 2009; Poeppel, Emmorey, nen, 2012; Price, 2000, 2010; Richardson &Hickok, & PylkkaPrice, 2009; Rodriguez-Fornells Cunillera, Mestres-Misse, &de Diego-Balaguer, 2009).Experience-dependent neural changes can result frommany aspects of environmental input, cognitive demand, orbehavioral experience, but the intensity and frequency oflanguage use may be particularly powerful in bringing aboutsuch changes in the brain (see Bates, 1999 for an earlier synthesis; see Bialystok & Barac, 2013 for a recent discussion).Many people are born bilingual in our increasingly moreconnected and multilingual world, while many others arelearning a new language later in life due to travel, business, orimmigration. Globalization, widespread use of digital technology, and increased cross-cultural communication providefurther impetus to the rapid rise of bilingualism and multilingual societies. The study of the bilingual brain originallyarose from neuroscientists' interest in understanding how thesame brain supports and represents two or more languages.Since the mid-to-late 1990s, a large number of neurocognitivestudies, using neuroimaging methods such as functionalmagnetic resonance imaging (fMRI), positron emission tomography (PET), and electroencephalography/event-relatedpotential (EEG/ERP), have revealed specific functional brainpatterns in the learning of a second language (L2) (see reviews n-Galle s,in Abutalebi, Cappa, & Perani, 2005; Costa & Sebastia2014; Hernandez, 2013; Indefrey, 2006; Li & Tokowicz, 2012;van Hell & Tokowicz, 2010). These studies indicate that incontrast to predictions of the critical period hypothesis, L2learning, even if it occurs late in adulthood, lead to bothbehavioral and neural changes that may approximate thepatterns of native or first language (L1).Even more surprising is that the neural patterns of L2experience are often, if not always, accompanied byanatomical changes in brain structure. Such anatomicalchanges can occur in the form of, for example, increased graymatter (GM) density, increased cortical thickness (CT), orenhanced white matter (WM) integrity. A number of recentstudies have thus begun to examine the structural oranatomical changes induced by L2 experience on the brain.Given the significant anatomical changes that have been reported for memory, attention, and other cognitive domains(see Section 3.2), it is important that we consider theanatomical substrates of second language learning. In thisarticle, we provide an overview and synthesis of the relevantstudies, and identify key variables and mechanisms underlying language experience related structural neuroplasticity. Weaim at not only reviewing the emerging literature, but alsoidentifying the common principles that drive brain changes inorder to integrate our knowledge of the structure-functionbehavior relationships.2.Anatomical correlates of second languagelearningResearch in bilingualism and second language1 has generatedmuch enthusiasm lately in the study of the mind and the brain(see Diamond, 2010). What has brought bilingualism to thespotlight? There may be several reasons but one key line ofresearch behind the current enthusiasm is the neurocognitiveimpact that the learning and use of multiple languages mayhave on the brain (see reviews in Bialystok, 2009; Costa & n-Galle s, 2014; Hernandez, 2013; Li, 2014). The bilinSebastiagual brain is a highly adaptive system, and it responds tomultiple language experiences flexibly and reflects the adaptive dynamics as both functional and anatomical brainchanges. In this section we review the major evidence that hasaccumulated in the last decade on how the learning of L2, orbilingual experience more generally, may bring aboutanatomical changes in the brain.Functional neuroimaging methods, especially fMRI, haveplayed a key role in the study of bilingualism and secondlanguage acquisition (see Abutalebi, Cappa, & Perani, 2005;Grosjean & Li, 2013, Chapter 10; Hernandez, 2013; Indefrey,2006; Li & Tokowicz, 2012; for reviews). While functionalneuroimaging has led to a significant understanding of thebilingual brain, the use of structural imaging techniques hasonly begun recently in the study of bilingualism and secondlanguage. As we will discuss below, structural imagingmethods allow us to measure brain changes in anatomicalstructure and may offer broader implications for understanding the bilingual brain, particularly with regard to theirability to identify causal links between experience and neuroplasticity through training. Let us first briefly review thethree major measures and the methodologies with which wecan identify learning-induced or experience-dependentchanges in the brain's anatomical structures.2.1.Measures of anatomical changesNeurons are organized within the brain to form GM andWM. GM consists primarily of neuronal cell bodies,whereas WM consists of axons and support cells (e.g., gliacells). Bundles of axons form the so-called fiber tracts thatconnect different cortical regions within the same hemisphere (through association tracts), between hemispheres(through commissures, e.g., the corpus callosum (CC)), orbetween cortical and subcortical structures (projectiontracts). The brain is filled with cerebrospinal fluid (CSF),which also runs through the ventricles of the brain.Measures of anatomical changes focus mainly on changesin GM and WM.1Many people learn or speak a third or fourth language. Herewe use “bilingualism” or “second language” as a generic and inclusive term to cover situations of two or more than twolanguages.
c o r t e x 5 8 ( 2 0 1 4 ) 3 0 1 e3 2 42.1.1.GM densityGM density or volume has been one of the most commonmeasures of anatomical brain changes. Although it is notentirely clear what exactly an increase in GM volume entailsat a microstructural level, it is generally believed that it reflects an aggregate measure of the changes in cell size of bothneurons and glial cells, neurogenesis associated with bothneurons and glial cells, and possible changes in the intracortical axonal architecture including synaptogenesis (May &Gaser, 2006; Zatorre, Fields, & Johansen-Berg, 2012). Thus,GM density does not directly translate to the density of neurons, or other simple measure of the brain morphology. Toidentify GM density, researchers rely on voxel-basedmorphometry (VBM), an analytic method that extracts GMinformation from structural MRI scans (see Ashburner &Friston, 2000; Mechelli, Price, Friston, & Ashburner, 2005).VBM typically involves the normalization of each brain scan toa standard stereotactic space (e.g., MNI space), delineation ofgray versus WM versus CSF, and a voxel-by-voxel analysis ofthe tissue concentration. VBM identifies the local tissueenvironment after correction for macroscopic anatomicaldifferences across participants.2.1.2.Cortical thicknessCT, also based on structural MRI scans, measures the thickness of GM (Fischl & Dale, 2000; Kim et al., 2005; Lerch & Evans,2005). Unlike GM density or volume, CT is a direct measure ofcortical morphology. In this technique, voxels are firstsegmented into GM, WM, or CSF. The boundaries between GMand WM, and between GM the pia mater are then delineatedeither manually or through automated procedures. Finally,the thickness between these surfaces is measured using avariety of methods, each determining the distance betweennodes on each surface for the entirety of the cortex examined.CT provides sub-millimeter accuracy and takes into accountthe folding of the cortical surface. Structurally there may be aninverse relationship between CT and GM due to the corticalfolding patterns: thicker cortical regions are less convolutedand therefore have less GM density (see Chung, Dalton, Shen,Evans, & Davidson, 2006). It is relatively insensitive to differences in MRI scanners and parameters, but is less accurate forareas where the GM/WM boundary is less clear, such as inprimary sensory areas that contain more myelination.2.1.3.WM integrityWM integrity refers to a measure based on data from diffusiontensor imaging (DTI), a technique that examines the diffusionof water molecules in the brain. DTI compares the degree ofdiffusivity of neurons along the axon, referred to as axialdiffusivity (AD) along with the radial diffusivity (RD) that isperpendicular to the axon diameter (Filler, 2009). Anothermeasure, the mean diffusivity (MD), is used to measurediffusion within a voxel, regardless of orientation, and iscalculated by averaging the eigenvalues (Alexander, Lee,Lazar, & Field, 2007). Lower MD values often correspond togreater WM integrity. By far the most commonly used value tocalculate the magnitude of diffusion is the fractional anisotropy (FA), a normalized standard diffusivity value between0 and 1 calculated from the eigenvectors of the diffusiontensor (Assaf & Pasternak, 2008). FA has been a yardstick of303WM integrity in the literature, where a value of 0 indicates anisotropic environment as is seen in the ventricles of the brain,and a value of .2e1 an anisotropic environment as is seen inWM tracts (Kunimatsu et al., 2004). The higher the FA value,the more integrity the WM has (contrasting the interpretationof the MD value). In addition, a high FA value, when coincidingwith a low RD value, could suggest increased myelination.A sizable number of studies in the last decade have usedthe above three methods to examine anatomical changes inthe brain as a function of bilingual or L2 experience (seeRichardson & Price, 2009 for a review of monolingual studiesusing these methods). Table 1 presents a summary of thesestudies and Table 2 an overview of the different regions andtracts of interest modulated by bilingual experience. Fig. 1presents a direct comparison between bilinguals and monolinguals in the areas where anatomical changes have beenobserved, and how these changes correlate with behavioraltasks or variables. We discuss the details of these studies inthe sections that follow.2.2.Structural brain changes induced by bilingualexperience in children and adultsOne of the pioneering studies using VBM to examine GMdensity in bilingual learners was Mechelli et al. (2004). In thisstudy, bilinguals were participants who had learned a European language before the age of 5 (early bilinguals) or betweenthe ages of 10 and 15 (late bilinguals). In general, the bilingualsshowed greater GM density in the left inferior parietal lobule(IPL)2 than did monolinguals, but the effect was greater in theearly bilinguals than in the late bilinguals. The IPL has beenpreviously implicated in functional imaging studies as animportant area for phonological working memory, lexicallearning, and semantic integration (Baddeley, 2003; Della Rosaet al., 2013; Mechelli et al., 2004;). The expansion of this areamight be particularly related to the bilingual's acquisition andprocessing of a larger vocabulary due to the L2 (see Richardson& Price, 2009). More important, Mechelli et al. also showedthat the extent of GM density increases was positively associated with the proficiency of the learner in the L2 (moreproficient, more GM), and negatively correlated with thelearner's age of L2 acquisition (the earlier the learning, themore the GM).Other studies have since replicated Mechelli et al.'s findingand confirmed the role of the IPL and adjacent regions in thetemporo-parietal cortex for bilingualism, showing that bilinguals, in general, have greater GM density than monolinguals in this brain area (e.g., Della Rosa et al., 2013; Groganet al., 2012; see discussion in Section 2.3). More specifically,IPL, including the posterior supramarginal gyrus (SMG), hasbeen implicated to play an important role for vocabularyknowledge in general, for both L1 and L2: Lee et al. (2007) founda significant positive correlation between monolingual vocabulary size and GM volume in the bilateral IPL and posteriorSMG. Additionally, Xiang et al. (2012) showed how variability instructural pathways was related to language abilities as2See Appendix for all abbreviations of the brain regions used inthis paper. We follow accepted conventions in the literature formost if not all acronyms.
304c o r t e x 5 8 ( 2 0 1 4 ) 3 0 1 e3 2 4Table 1 e Studies of structural changes associated with language experience and short-term training.StudyMeanAoAMRImethods23.426.6 5N/Aer-fMRIVBM23 Chinese BI (12 L1Cantonese, L2 English,11 two Chinesedialects)22 Italian MOCummine &12 Chinese-English BIBoliek, 2013 11 English MO62.261.918.9N/AVBMBI MO: L ATP24.228.5 5N/ADTIMO BI: R IFOF,R superior ATR,bilateral inferior ATRDella Rosaet al., 20139.9EarlyVBM37.928.4NotgivenDTI37.027.4Varied, VBMLate n 13 Spanish-Basque BIGarcía-Pentoet al., 201413 Spanish MO24.129.1.5N/ADTINetworkanalysisGoldet al., 201320 BI63 MOEnglish one languagefor all63.964.4 10N/AVBMDTIGroganet al., 201230 BI31 MLEnglish is non-nativelanguage for all26.726.98.2VBMEnglish¼ 6.4Hosodaet al., 2013137 Japanese-Englishlearners24.011VBMDTI24 Japanese-Englishlearners, (TG)20 Japanese-Englishlearners, (CG)Trainingb: 4-monthlaboratory trainingon L2 vocabulary20.120.11111VBMDTIAbutalebiet al., 2012Groups17 German-Italian BI14 Italian MOAbutalebiet al., 2014Elmeret al., 2011Elmeret al., 201415 MLLongitudinal design,scanned twice,separated by 1 year12 SI12 ControlsLanguage not reported12 SI12 MLVaried languagesAgeGM/WM differenceseeControls SI: R IPL,dorsal R CN,among othersML SI: L SMG,bilateral IFGpt,L IFGop, among othersBrain-behaviorcorrelationsaFlanker; Language SwitchingBI:(þ) GM density ACC & functionalconflict effect( ) GM density ACC & behavioralconflict effectL1 & L2 picture naming(þ) GM volume in L ATP & L2picture namingWord reading:BI: ( ) R ITG, L EC, L CN, etc. & RT toinconsistent words( ) L SFG, R CB, R STG, L lateralsulcus, etc. & RT to consistent wordsMO:( ) L SFG & R IFG, R SN,L IPL, L POS & RT to inconsistent words( ) cingulate sulcus, etc. & RT toconsistent wordsAttentional Network Task (ANT)Change over time: (þ) IPL &multilingual talent interactioneSI: ( ) R IFGop, L IFGpt, bilateralCN, middle-anterior cingulategyrus & cumulative number ofinterpretation practice hourseBI MO: Moreconnected sub-network:L insula, STG, IFGop,IFGpt, SMG, medial SFGMore connectedsub-network: L SOG,L STP, L AG, L SPG, R SFGHigher global efficiencyin both sub-networksMO BI: Higher globalefficiency in whole networkeGM:WM: FA: MO BI: ILF/IFOF,fornix, portions of CCRD: MO BI: ILF/IFOF,fornix, portions of CCML BI: R posterior SMGPhonemic fluency; Lexical decisiontask (LDT)BI: (þ) L IFGop & phonemicfluency, LDT( ) L IFGop & AoAeEnglish Vocabulary Test (EVT);National Adult Reading Test (NART)GM: (þ) IFGop, CN, STG/SMG,ACC (all bilateral) & EVTWM: (þ) Connectivity ofR IFGop-CN, etc. & EVT(þ) FA in R IFGop, R ILF, R AF & EVTGM: TG CG: R IFGopTest of English for InternationalWM: TG CG: R IFGopCommunication (TOEIC); EVT; NARTIFGop-caudate & RGM & WM:dorsal pathway(þ) R IFGop & TOEICconnectivity(þ) R IFGop-caudate & TOEIC
305c o r t e x 5 8 ( 2 0 1 4 ) 3 0 1 e3 2 4Table 1 e (continued )StudyKleinet al., 2013Kwoket al., 2011Luket al., 2011Mårtenssonet al., 2012Mechelliet al., 2004Mohadeset al., 2012Pliatsikaset al., 2013Groups12 BISIM25 BIE29 BIL22 MO19 MOTraining: 2-hourlaboratory trainingon new color words14 BI English L1 or L214 English MO14 SI TG17 CGAll native SwedishTraining: 3-monthintensive languagecourse focused onvocabulary25 BIE25 BIL25 MOAll native English22 Italian-English BI15 BISIM15 BISE10 MOL1: French or Dutch17 Greek learnersof English22 English , BIL MO: L IFGBIL BIE, BISIM, MO: R IFGBIE MO: R IFGVBMGM: V2/370.5acrossgroups19.920.6 11N/ADTIrs-FCN/AN/AGM/WM differencesBI MO: CC stretching tobilateral SLF, R IFOF, &uncinate fasciculusCTSI TG CG: L MFG, L IFG,HP volume L STG, R HPNot given 510-15N/AVBMNot given 2e34VBMBI MO: L IPLBIE BIL: L IPL, R IPLe9.39.79.6 3 3N/ADTIBISIM BISE MO: IFOFMO BISE BISIM: AC-OL27.524.57.7N/AVBMBI MO: bilateral CBVBMBI MO: HG volume;HG volume GM volume L H
Special section: Review Neuroplasticity as a function of second language learning: Anatomical changes in the human brain Ping Li*, Jennifer Legault and Kaitlyn A. Litcofsky Center for Brain, Behavior, and Cognition, Center for Language Science, Department of Psychology, Pennsylvania State University, United States article info Article history: Received 18 January 2014 Reviewed 31 March 2014 .
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