Cross-Linguistic Differences in the Neural Representation of Human Language: Evidence from Users of Signed Languages.

0.5 MB·English

by  Corina, David P.

HYPOTHESISANDTHEORYARTICLE
published:02January2013
doi:10.3389/fpsyg.2012.00587
Cross-linguistic differences in the neural representation of
human language: evidence from users of signed languages
DavidP.Corina*,LaurelA.Lawyer andDeborahCates
CognitiveNeurolinguisticsLaboratory,CenterforMindandBrain,DepartmentofLinguistics,UniversityofCaliforniaDavis,Davis,CA,USA
Editedby: Studies of deaf individuals who are users of signed languages have provided profound
TamaraSwaab,UniversityofCalifornia insight into the neural representation of human language. Case studies of deaf signers
Davis,USA
who have incurred left- and right-hemisphere damage have shown that left-hemisphere
Reviewedby:
resources are a necessary component of sign language processing.These data suggest
OlafHauk,MedicalResearchCouncil
CognitionandBrainSciencesUnit,UK that,despitefrankdifferencesintheinputandoutputmodalityoflanguage,coreleftperi-
MaireadMacSweeney,University sylvianregionsuniversallyservelinguisticfunction.Neuroimagingstudiesofdeafsigners
CollegeLondon,UK have generally provided support for this claim. However, more fine-tuned studies of lin-
*Correspondence: guisticprocessingindeafsignersarebeginningtoshowevidenceofimportantdifferences
DavidP.Corina,Cognitive
in the representation of signed and spoken languages. In this paper, we provide a criti-
NeurolinguisticsLaboratory,Center
forMindandBrain,Universityof calreviewofthisliteratureandpresentcompellingevidenceforlanguage-specificcortical
CaliforniaDavis,267CousteauPlace, representationsindeafsigners.Thesedatalendsupporttotheclaimthattheneuralrepre-
Davis,CA95618,USA. sentationoflanguagemayshowsubstantivecross-linguisticdifferences.Wediscussthe
e-mail:[email protected]
theoretical implications of these findings with respect to an emerging understanding of
theneurobiologyoflanguage.
Keywords:aphasia,Americansignlanguage,deaf,neurolinguistics
INTRODUCTION WilliamStokoe’simpactonLinguisticsandEducationwasquite
Casestudiesofdeafsigningindividualswithacquiredbraindam- minimal(McBurney,2001),thisworkformedasolidbaseforwhat
age and neuroimaging studies of healthy deaf subjects provide wastobecomeanewfieldofresearch:SignLanguageLinguistics
evidence for the importance of left-hemisphere systems in the (forarecentexposition,seeSandlerandLillo-Martin,2006).When
mediationof signedlanguages.Deaf signers,likehearingspeak- theinitialpsycholinguisticstudiesofsignedlanguagessuchasASL
ers,exhibitlanguagedisturbanceswhenleft-hemispherecortical were published (see for example Klima and Bellugi,1979),they
regions are damaged and appear to be remarkably spared fol- came at a time when there remained numerous misconceptions
lowing right-hemisphere lesions that may nonetheless disrupt about the status of signed languages (many of which unfortu-
non-linguisticvisual-spatialfunctions(Poizneretal.,1987).Neu- natelypersisttoday).Theseearlystudieshelpededucatescientists
roimagingstudiesofdeafsignershavegenerallyprovidedsupport andlaypersonsalikethatsignedlanguageslikeASLarenot“uni-
forthisclaim(forrecentreviewsseeCampbelletal.,2008;Corina versallanguages,”noraretheyprimitiveformsofcommunication
andSpotswood,2012).However,thefactthatemergentlinguistic withlimitedexpressivity,ormanualcodesmadeupbyeducatorsto
formsinsignlanguagescapitalizeupontheaffordancesandcon- representthewordsofaspokenlanguage.Fewpeopleunderstood
straintsofthevisualsystemratherthantheauditorysystem,and, thatsignedlanguagesaretruehumanlanguages,albeitexpressed
further,thattheproductionofsignsrequiresaqualitativelydiffer- inadifferentsignalingmodality.
entsystemofarticulatorycontrol,leavesopenthepossibilitythat Thus,the pioneering findings by Bellugi and colleagues were
there may be divergences and subsequent specializations of the indeednews.Theydiscovered,forexample,thatlanguageacqui-
neural systems that underlie the mediation of signed languages. sitionmilestonesforinfantslearningasignedlanguagefromdeaf
Todate,thegreatmajorityofarticlesdiscussingtheneuralrepre- parents as a native language paralleled native spoken language
sentationforsignedlanguageshavebeenstronglybiasedtoward milestones,thatsignsareusedasabasisformemorycodesusedby
reporting the commonalities between the neural representation deafsigners,andthatsignedlanguages,likespokenlanguages,can
of speechandsign,anditisonlyrecentlythatsomeresearchers be analyzed as compositional forms that exhibit linguistic com-
have begun to call into question the purported close homology plexity and have systematic means for marking morphological
between the neural representations of speech and sign (see for and syntactic properties (see Klima and Bellugi,1979). As neu-
example MacSweeney et al., 2008). To understand this state of ropsychologicalstudiesofdeafsignersbegantoappear,onceagain
affairs,itisimportanttoprovideabitofhistoricalbackground. therewasaperceivedneedtoconvincinglydemonstratethatsign
ItwasnotuntilthepublicationofSignLanguageStructurein languagesweretruehumanlanguages.Emergingdatafromstud-
the1960’swhenWilliamStokoeandcolleaguespresentedthefirst ies of sign language aphasia and neuroimaging further showed
formallinguisticanalysisofAmericanSignLanguage(Stokoeetal., that demonstrably similar brain areas are used during sign and
1965).Priortothistime,theunderstandingof signedlanguages spokenlanguageprocessing.Thereportingandinitialinterpreta-
within the scientific community was quite misinformed. While tionsof thisexcitingbehavioralandneurologicaldatafromsign
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Corinaetal. Neuralrepresentationofsignedlanguages
languagesusedbydeafpersonsledmanytobelievethatlanguage remainsanactiveareaofresearch(seeMcGettiganandScott,2012
modality hadlittle influence onthe core propertiesof linguistic forarecentreview).Inaddition,therearerefreshingdevelopments
function; the processes and neural regions underlying sign and intheeffortstounderstandthecontributionsofbothhemispheres
speechfunctionswereconsideredlargelyisomorphic,atestament inspeechperception(seeforexampleScottetal.,2000;Zatorreand
totheintegrityoftheneurobiologyofthehumancapacityforlan- Belin,2001;BelinandZatorre,2003;Poeppel,2003;BenShalom
guage.Indeed,anyresearchthatsuggestedotherwisewasopenly andPoeppel,2008).Oneofthemostimportantdevelopmentsand
challengedasbeingamethodologicalanomaly(cf.Hickoketal., particularlygermanetothepresentpaperistheacknowledgment
1998).Fortunately,asthemessageaboutthestatusofsignedlan- thatunderstandingtheneuralmechanismsof speechprocessing
guagesaslegitimatehumanlanguagesbecamemorewidespread, requiresanappreciationoftheintimatecouplingbetweenacoustic
researchersbegantoquestiontheloreof theotherwiseneatand perception and the articulatory system that gives rise to speech
seemingly inseparable homologies that had been constructed to sounds (McGettigan and Scott,2012). Given this state of affairs
describetheconvergenceofneuralsystemsmediatingsignedand itiscertainlyworthconsideringwhetherthefrankmodalitydif-
spokenlanguages.Inaddition,thegrowingawarenessthatspoken ferences between speech and signed language lead to qualitative
languagemaycalluponbothleft-andright-hemisphereresources differences in neural systems mediating the alternative forms of
made the idea that human languages may have bi-hemispheric humancommunication.
representation seem less challenging. Given the advances in the The fact that left-hemisphere perisylvian regions appear crit-
scientificdiscourseofthelanguagesciences,itisworthconsider- ical for the fidelity of sign language expression has been well-
ingthepossibilitythatthediversityoflinguisticformsexhibitedby established and is uncontroversial (Poizner et al., 1987; Corina,
theworld’slanguageinfacthaverealprocessingconsequences,and 1998a; Hickok et al., 2002; MacSweeney et al., 2008). However,
theseprocessingdifferenceshaveidentifiableneuralconsequences. an unresolved issue concerns the role of the right-hemisphere
Thestarkcontrastbetweensignandspeechlanguagemodalities in the mediation of ASL and indeed other naturally occurring
providesaprivilegedvantagepointtobeginsuchexplorations. signed languages such as British Signed Language (MacSweeney
In this paper,we explore three specific examples that suggest et al., 2002b). Data from both lesion studies and neuroimag-
qualitativedifferencesinneurofunctionalinstantiationof signed ing indicate that right-hemisphere parietal regions may play a
languages.Wefirstconsiderthepossibilitythathemisphericspe- prominent role in the processing of signed languages.A critical
cializationforsignedlanguagesandspokenlanguagesmaydiffer; questioniswhetherthisrelianceontheright-hemisphererepre-
in particular, we review mounting evidence that unique right- sents domain-specific linguistic mechanisms or domain-general
hemisphereresourcesmayberequiredforcompetencyinASLin visual-spatial processing, including processes related to human
amannerthatisnotobservedforspokenlanguages.Second,we actionunderstanding.
considertheevidencethat,withintheleft-hemisphere,thecortical Initialstudiesreportedthatsignerswithdamagetotheright-
organizationforsignedlanguagesmaydifferinsubstantiveways hemispherehadimpairedvisual-spatialdeficitsbutwell-preserved
from what is commonly observed in spoken languages. Finally languageskills(Poizneretal.,1987).However,asfurtherstudies
weconsiderevidencethatsuggeststhattherequirementsforthe appeared,itbecameclearthatright-hemispheredamageinusers
fluentexpressionofasignedlanguageentailuniquemotoriccon- of signedlanguagesalsodisruptedthemeta-controlof language
straints,andpresentevidenceforsign-language-specificlinguistic use,which resulted in disruptions of discourse abilities (Hickok
disorders.Wethenconsidertheimplicationsofthesedataforour et al., 1999). This finding is similar to those suggesting right-
understandingoftheneurobiologyofhumanlanguage. hemispheredamageinusersofspokenlanguageimpactsso-called
extra-linguisticfunctions,suchastheinterpretationofmetaphors
HEMISPHERICSPECIALIZATIONFORLANGUAGE and humor (Brownell et al., 1990; Kaplan et al., 1990; Rehak
Twoofthemostwidelyestablishedfactsofhumanneuroscience et al.,1992).Yet more interesting are the indications that right-
are that the two cerebral hemispheres exhibit differential spe- hemispherelesionsmayfundamentallydisruptaspectsofsyntactic
cialization for cognitive functions and that the left-hemisphere processinginsignedlanguages.Theinitialreportsofthispossibil-
perisylvianregionsplayapredominantroleinprocessingofspo- ityareinfactstatedinPoizneretal.(1987),whonotethatwhile
kenlanguage.Inexpressingthesedifferences,itwascommonto left-hemispheredamagecommonlyresultsindisturbancesofsyn-
claim that some predefined components of language processing tactic processing of ASL,signers with right-hemisphere damage
(forexample:phonology,semantics,andsyntax)represent“core- also exhibited syntactic comprehension problems. For example,
linguisticfunctions,”whicharesomehowmoreelementalinstatus, subjectsS.M.andG.G.[right-hemispheredamaged(RHD)sub-
while everything else (for example, properties of discourse and jectstestedbyPoizneretal.,1987]performedwellbelowcontrols
pragmatics),aresimply“extra-linguisticfunctions.”Morerecently on two tests of spatial syntax. Indeed,as the authors point out,
we can observe a re-characterization of the neural representa- “rightlesionedsignersdonotshowcomprehensiondeficitsinany
tionofspeechwhichclaimsthelefttemporal-lobeexhibitsmore linguistictest,otherthanthatofspatializedsyntax.”Poizneretal.
domain-specificmechanisms,whiletheright-hemisphereexhibits (1987)speculatedthattheperceptualprocessinginvolvedinthe
domain-generalmechanisms(McGettiganandScott,2012).For- comprehensionofspatializedsyntaxinvolvesbothleft-andright-
tunately,concurrentwiththisnewcharacterizationhasbeenthe hemispheres,andthatcertaincriticalareasof bothhemispheres
developmentofsophisticatedneuroimagingtechniquesandsignal mustberelativelyintactforaccurateperformance.
manipulationswhichcanhelpelucidatetheessenceof linguistic Additional evidence for right-hemisphere contributions for
specificity, and the determination of hemispheric specialization aspects of sign comprehension come from the consideration of
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thewayinwhichsignedlanguagesroutinelycapitalizeuponthe damaged (LHD) BSL signers relative to elderly control subjects
postural properties of the body, the manual articulators, and exhibited deficits on all comprehension tests. RHD signers did
the spatial affordances of the visual system to convey complex notdifferfromcontrolsonsingle-signandsingle-predicateverb
meanings including grammatical roles (such as subject/object), constructionsorsentencesthatrangedinargumentstructureand
prepositionalmeaning,locativerelations,andspeakerviewpoint semanticreversibility.RHDsigners(likeLHDsigners),however,
in ways that may not have direct parallels in spoken languages. wereimpairedontestsoflocativerelationshipsexpressedviaclas-
Forexample,certainclassesofsignformshavebeendescribedas sifierconstructionsandontestsofclassifierplacementorientation
depictive.Thatis,someverbshave,inadditiontotheirusualfunc- androtation.
tionasverbs,theabilitytodepicttheeventtheyencode(Liddell, Oneinterpretationofferedforthispatternofresponsesisthat
2003).AsdescribedinDudis(2004,2007)thecontrastbetweena the comprehension of these classifier constructions requires not
non-depictingagreementverb,suchasGIVE,versusadepicting onlyintactleft-hemisphereresourcesbutintactright-hemisphere
verb,such as HAND-TO,illustrates some of these property dif- visual-spatial processing mechanisms. That is, while both LHD
ferences.ThehandshapeandthemovementintheverbGIVEis and RHD signers show comprehension deficits, the RHD sign-
notconditionedbytheobjectbeinggiven(thoughthedirection ers’difficultiesstemfrommoregeneralvisual-spatialdeficitsthan
ofthemovementmaybeconditionedbygrammaticalstructure); linguistic malfunctions per se. Yet there has been little attempt
incontrast,theverbHAND-TOcanbeusedtodescribeonlythe to specify the exact nature of the visual deficits that give rise to
transfer of objects that can be held between the thumb and the theselanguagecomprehensionproblems.Animportantquestion
fourfingersofthehandshape–apaperdocumentorcreditcard, iswhetherthesegeneralvisual-spatialdeficitsshouldbedeemed
butcertainlynotakitchenblender.Thisisonewaythattheverb’s “extra-linguistic” or, rather, be considered a linguistic property
iconicityconstrainsitsusage.Additionally,thepalm’scontinuously of signedlanguages.Forexample,Atkinsonetal.(2005)suggest
upwardorientationandthepathofthehandcreatedviatheelbow “the deficits stem from the disruption of processes which map
emulatethephysicalmotionofthetransferevent.Dudisfurther non-arbitrarysignlocationonthereal-world’sspatialposition.”
suggeststhatitisnotsolelythemorphophonologicaldifferencesin However,depictiveformsarenotonlyusedtorefertoreal-world
thehandshapethatdifferentiatestheseforms,butrathertheverb’s events,butalsotoimaginaryandnon-presenteventsaswell.In
abilitytoportrayadynamicandvisualrepresentationoftransfer, ASLforexample,onemayusedistinctspatiallocationstoestablish
whichisademonstrationratherthanplaindescription. andcontrasttwotheoreticalideas,usingseparatesigningspaceand
In a similar fashion, spatial prepositional relations between referencestothesespatiallocationstoconveysemanticandcausal
objects such as“on,above,under,”etc.,can be conveyed via the relationships between these theoretical constructs. The broader
depiction of the physical relation itself rather than encoded by pointiswhetheraphasic deficitsshouldbesolelydefinedasthose
a discrete lexical item. For example, an ASL translation of the that have clear homologies to the left-hemisphere maladies evi-
Englishsentence“Thepenrestsonabook”may,inpart,involve denced in spoken languages, or whether the patterns observed
the use of the two hands whereby one hand configuration with inthedisruptionof signedlanguageswillforceustoreconsider
an outstretched finger (representing the pen) is placed on the the conception of linguistic deficits (see also MacSweeney et al.,
back of a flat open hand (representing the book). This config- 2008fordiscussion).Anadditionalavenueof inquirytofurther
urationencodesthespatialmeaning“on”butwithouttheneedfor illuminatetheprocessingrequirementsofspatialformsinsigned
aconventionallexicalprepositioncorrespondingto“on.” languagewillcomefromamoresystematiccomparisonbetween
Manysignedlanguagesexpresslocativerelationshipsandevents theroleofcomprehensionofthesespatialformscomparedwith
inthismannerandhavediscreteinventoriesofhighlyproductive theirproduction(Emmoreyetal.,2004).
grammatical forms, typically referred to as classifiers or classi- Neuroimagingstudiesofdeafsignersraiseadditionalquestions
fierpredicates,whichparticipateinthesedepictiveconstructions. concerningtheroleoftheright-hemisphereinsignlanguagepro-
Whiletheuseof theseformsconstitutesamajorcontrastivelin- cessing. Studies of sentence processing in signed languages have
guistic function in signed languages (see Emmorey, 2003 and repeatedlyreportedleft-hemisphereactivationsthatparallelthose
paperstherein)theirtheoreticalstatusremainsapointofvibrant found for spoken languages. These activation patterns include
discussion and debate (for contrastive views see Liddell, 2003; inferiorfrontalgyrus(includingBroca’sareaandinsula),precen-
SandlerandLillo-Martin,2006). tralsulcus,superior,andmiddletemporalcorticalregions,poste-
Thiscontroversy,however,hasimportantimplicationsforour riorsuperiortemporalsulcus,angulargyrus,andsupramarginal
understandingoftheneurolinguisticsofsignedlanguages,assev- gyrus (e.g.,Neville et al.,1998; Petitto et al.,2000; MacSweeney
eral studies have found differential disruptions in the use and etal.,2002a,2006;Newmanetal.,2002;Lambertzetal.,2005;Sakai
comprehension of sentences that involve“depictive”forms. For etal.,2005).Activationsintheseregionsareoftenfoundinstudies
example, Atkinson et al. (2005) conducted a group study of ofauditorylanguageprocessing(Blumstein,1994;Schlosseretal.,
LHD and RHD signers of British Sign Language (BSL). Their 1998;DavisandJohnsrude,2003).
testsincludedsingle-signandsingle-predicateverbconstructions However,inadditiontothemorefamiliarleft-hemisphereacti-
(e.g.,THROW-DART),simple,andcomplexsentencesthatranged vations,studies of sentence processing in signed languages have
in argument structure and semantic reversibility, locative con- also noted significant right-hemisphere activation. For exam-
structions encoding spatial relationships and lexical preposition ple, activations in right-hemisphere superior temporal, inferior
constructions, and a final test of classifier placement orienta- frontal, and posterior-parietal regions have been reported (e.g.,
tion and rotation. Their findings indicated that left-hemisphere Neville et al., 1998; MacSweeney et al., 2002a, 2006; Newman
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Corinaetal. Neuralrepresentationofsignedlanguages
et al., 2002). The question of whether these patterns of activa- detection in the context of topographic sentences was maximal
tionareuniquetosignhasbeenatopicofdebate(seeforexample: forleft-hemisphereinferiorparietallobule(IPL)inskilledsign-
Corinaetal.,1998;Hickoketal.,1998),asstudiesofauditoryand ers. MacSweeney et al. (2002b) suggest these activation patterns
audio-visual speech have observed right-hemisphere activations mayberelatedtorequirementsforspatialprocessingofhands,as
that appear similar to those reported in signing (e.g., Schlosser studiesofnon-signershaveobservedleftIPLactivationinresponse
et al., 1998; Davis and Johnsrude, 2003). More recent evidence toimageryofhandmovementsandhandposition(Kosslynetal.,
suggeststhattheright-hemispherelateralsuperiortemporalacti- 1998;Gerardinetal.,2000;Hermsdorferetal.,2001).Asecondleft
vationsmaynotbesign-specific.Capeketal.(2004),forexample, parietalregioninthesestudies,BA7,issuggestedtoreflectsimilar
showed that for monolingual speakers of English, audio-visual mechanismsofhandorfingermovement(e.g.,Weeksetal.,2001).
Englishsentenceprocessingelicitednotonlyleft-dominantactiva- Analternativeexplanationsuggeststhatneuralregionsforaction
tioninlanguageregions,butalsobilateralactivationintheanterior processingmayhavebeenmoreprevalentduringthetopographic
and middle lateral sulcus. A direct comparison of audio-visual sentences. Several researchers have observed SPL activation in
speech processing in hearing non-signers and BSL processing responsetohumanactionandactionvocabularymoregenerally
indeaf signersrevealedhighlyoverlappingregionsof activation (Damasioetal.,2001;GrezesandDecety,2001;Hauketal.,2004;
including bilateral activation of the temporal-lobes under each Pulvermülleretal.,2005).
linguisticcondition(MacSweeneyetal.,2002a).Previousstudies Importantly, and similar to the findings reported by New-
haveshownthatright-hemispheresuperiortemporalregionsare man et al. (2002), native deaf signers in the MacSweeney et al.
sensitivetofacial,andespeciallymouth,information(Puceetal., (2002b)studydidshowactivationintherightangulargyrus(BA
1998;Pelphreyetal.,2005).Itiswellknownthatdeafsignersfocus 39).Parietalactivationinthehearingnativesigners,however,was
attention on mouth regions while attending to signs (Muir and modulatedbyaccuracyonthetask,withmoreaccuratesubjects
Richardson,2005).Thus,thesestudiessuggestthatlateraltempo- showinggreateractivation.Thisfindingsuggeststhatproficiency,
ralactivationpatternsarenotsign-specificbutarelikelyindexing rather than age of acquisition,may be a critical determinant of
aspectsoftherecognitionofphysicalhumanforms. right-hemisphere engagement. Importantly, activation in right-
In contrast to the common temporal-lobe physical-form hemisphere temporal-parietal regions was specific to BSL and
regions,neuroimagingdatafurthersuggeststhatrightposterior- was not observed in hearing non-signers watching audio-visual
parietal and posterior-temporal regions may play a special role Englishtranslationsofthesamesentences.
inthemediationofsignedlanguages(Bavelieretal.,1998;New- Further evidence of the differential contributions of cerebral
man et al.,2002). In a series of studies,deaf and hearing native insigncomprehensionisfoundinthestudyreportedby Capek
signers, hearing non-signers, and hearing late learners of ASL etal.(2009).Theseresearchersusedelectrophysiologicalrecord-
viewed sign language sentences contrasted with sign gibberish. ingtoassessthebrainsystemsmediatingsemanticandsyntactic
Deaf and hearing native signers showed significant activation aspectsofASLprocessing.Inthisstudydeafnativesignerswatched
in right-hemisphere posterior-parietal and posterior-temporal ASL sentences that were either grammatically well-formed or
regions,includingahomologoftheposterior-temporalWernicke’s grammaticallyanomalous.Thegrammaticalanomaliesincluded
area.Theseactivationpatternswerenotseeninnon-signers,nor violationsofsemanticexpectanciesandtwodifferentsyntacticvio-
weretheyobservedinhearinglatelearnersofASL.Agroupanaly- lations.Inbothcasesthesyntacticviolationsentailedthemisuse
sisofnativeandlate-learninghearingsignersconfirmedthatthe ofacceptedconventionsofspatialsyntax.Inonecase,“reversed”
right angular gyrus was found to be active only when hearing verb-agreement violations were formed by reversing the direc-
native users of ASL performed the task. When hearing signers tion of the verb such that the verb moved toward the subject
who learned to sign after puberty performed the same task,the insteadoftheobject.Thesecondgrammaticalviolationmadeuse
right angular gyrus activation was not observed. Newman et al. of“unspecified”verb-agreementviolations,whichwerecreatedby
(2002) argued that the activation of this neural region during directingaspatial-agreementverbtowardalocationinspacethat
signlanguageperceptionmightbeaneuralsignatureofsignlan- hadnotbeendefinedpreviouslyasthesubjectorobject.
guage acquisition during the critical period for language. Many As expected, semantically anomalous ASL sentences elicited
researchershavespeculatedthatright-hemisphereparietalactiva- an N400 (300–875ms) relative to control sentences. This N400
tioninsignersisassociatedwiththelinguisticuseofspaceinsign responsewasbroadlydistributedoverposteriorregionsinapat-
language(Bavelieretal.,1998;Newmanetal.,2002),andrecent ternwidelyconsistentwithstudiesofsemanticviolationsinaural-
studieshavesoughttoclarifythecontributionsofspatialprocess- orallanguages.Syntacticviolationselicitedananteriornegativity
inginASLandBSLtoobservedright-hemisphereactivations. followed by a widely distributed P600;however,compelling dif-
MacSweeney et al. (2002b), for example, compared the role ferences in the distribution of this early negativity were noted.
of parietal cortices in an anomaly detection task in BSL. They Reversedverb-agreementviolationselicitedanearlyanteriorneg-
testeddeafandhearingnativesignersinaparadigmthatutilized ativity(140–200ms)that waslargestover thelateralsites of the
BSLsentencecontextsthateithermadeuseof topographicsign- left-hemisphere, i.e., a Left Anterior Negativity similar to what
ing space or did not require topographic mapping.Across both has been found in studies of syntactic violations in spoken lan-
groups, comprehension of topographic BSL sentences recruited guages(Friederici,2002).Incontrast,unspecifiedverb-agreement
theleftparietal[BA40andsuperiorparietallobule(SPL)-BA7] violations,relativetocanonicalsentences,elicitedananteriorneg-
andbilateralposteriormiddletemporalcorticestoagreaterextent ativity(200–360ms)thatwaslargestovertheright lateralfrontal
thandidnon-topographicsentences.Activationduringanomaly sites. As with the reversed verb-agreement violations, sentences
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Corinaetal. Neuralrepresentationofsignedlanguages
containing unspecified verb-agreement anomalies also elicited a requirements for the registration of human movements are not
broadlydistributedP600thatwaslargerovertheleft-hemisphere. required.
Takentogether,theseERPdatasuggestthattherearedistinct A second logical possibility is that the right-hemisphere
brainsystemsmediatingsemanticandsyntacticprocessinginASL, involvementdoesreflectlinguisticprocessing;however,thispro-
as has been observed for spoken languages (Kutas and Hillyard, cessingisnotspecifictosignedlanguages.Itissimplythecasethat
1980, 1984; Neville et al., 1991). Importantly, the data suggests wehaveyettosystematicallyidentifythepropergrammaticalcon-
great similarities in systems mediating semantic processing in structionsinspokenlanguagesthattriggertheinvolvementofthis
spokenandsignedlanguage(seealsoNevilleetal.,1997).Incon- region. For example, perhaps there are languages with complex
trast, the syntactic violations evoked left anterior negativity in case-marking systems which require registration of the spatial-
thecaseofthereversedverb-agreementsentences,andunexpect- temporal ordering of morphemes for determining subject and
edly a right anterior negativity for unspecified verb-agreement objectrelationships;or,perhaps,appreciationof specificspatial-
violations.Capeketal.(2009)suggestthattheunspecifiedverb- prepositionconstructionswhoseprocessingdynamicshaveyetto
agreementviolationslikelyplacedifferentdemandsonthesystem befullyexplored.
involved in processing spatial syntax than the reversed verb- A third possibility is that the right-hemisphere activation
agreementviolations.Theunspecifiedverb-agreementviolations reflects general visual-spatial processes and is used in the per-
refertoaspatiallocationatwhichnoreferenthadpreviouslybeen ceptionofhumanmovementsthatunderliebothsignandspeech.
located.Inthesecases,theviewerisforcedtoeitherpositanewref- Asdiscussedabove,thisaccounthasbeenevokedtoexplainthe
erentwhoseidentityisunknown(andwillperhapsbeintroduced lateral temporal activation seen during the processing of audio-
atalatertimeinthediscourse)orinferthattheintendedreferent visualspeechandsignedlanguage,butfailstoaccountfortheright
is one that was previously placed at a different spatial location. posterior-parietal activations reported in Newman et al. (2002).
Either way, different processing is required compared with the Weconsiderfurtherimplicationsofthesefindingsbelow,butfirst
reversedviolations.Capeketal.(2009,p.8787)state“Theresults we examine the possibility that even within the left-hemisphere
clearlyimplicatedistinguishableneuralsubsystemsinvolvedinthe there may be observable language modality conditioned effects
processingof‘spatialsyntax’inASLdependingontheprocessing thatdifferentiatespokenandsignedlanguages.
demands,andsuggestamorecomplexorganizationfortheneural
basisofsyntaxthanaunitary‘grammaticalprocessing’system.” LEFT-HEMISPHERESPECIALIZATION
Thus,whenonecloselyexaminesthedatafromneuropsycho- Signerswithleft-hemisphereposteriorlesionsevidencefluentsign
logicalcasestudiesofdeafsignerswithleft-andright-hemisphere aphasiawithassociatedcomprehensiondeficits.Thereis,however,
damage,neuroimagingdataofASLandBSLcomprehension,and controversyinregardstoanatomicalregionsthatmaygiveriseto
electrophysiologicalstudiesofsyntacticprocessinginASL,acon- comprehensionproblemsinspokenandsignedlanguages.Inan
sistent story begins to emerge. Specifically, it appears that the effort to examine the role of the temporal lobe inASL compre-
comprehensionofparticulargrammaticalconstructions,includ- hensiondeficits,Hickoketal.(2002)comparedthesignlanguage
ing aspects of syntax and classifier-predicate constructions in comprehension abilities of LHD and RHD signers. Signers with
signedlanguages,requiresright-hemisphereresourcesand,assug- left-hemisphere posterior temporal lobe damage were found to
gestedbytheneuroimagingdata,rightposterior-parietalresources performworsethananyothergroup,exhibitingsignificantimpair-
in particular,in a manner that is not generally reported for the mentsonsingle-signandsentenceperformanceasaccessedbyan
comprehension of spoken languages (see for example Emmorey ASLtranslationof thetokentest(DeRenziandVignolo,1962).
etal.,2002). Whiletheauthorsemphasizetheinvolvementofthedamagedtem-
If weacceptthisclaim,thenthesedataraiseimportanttheo- porallobeinthesecomprehensiondeficits,thoughnotdiscussed,
reticalquestionsforourunderstandingofthebiologicaldetermi- inallcasesthelesionsadditionallyextendedintotheparietallobe.
nationofneuralregionsrequiredforhumanlanguageprocessing. Thusisitunclearwhethersignlanguagecomprehensiondeficits
However, before we accept this claim, we need to be careful in require impairment to the temporal lobe only,the parietal lobe
our assessment. There are at least three logical alternative pos- only,orsomecombinationthereof.
sibilities that may account for appearance of right-hemisphere It is noteworthy that the cases described by Chiarello et al.
resourcesduringaspectsofgrammaticalandclassifierprocessing (1982)andPoizneretal.(1987)andthecasestudydescribedby
in naturally occurring signed languages. First, one may assume Corina et al. (1992), exhibited fluent aphasia with severe com-
thattheright-hemisphereinvolvementisnotspecifically“linguis- prehensiondeficits.Lesionsinthesecasestudiesdidnotoccurin
tic”innature,butratherreflectsgeneralvisual-spatialprocessing. corticalWernicke’sareaproper,butratherinvolvedmorefrontal
Thatistosay,theobservedactivations(andimpairments)simply and inferior parietal areas. In these cases,lesions extended pos-
reflect general visual-spatial resources that are required for pro- teriorlytothesupramarginalgyrus.Thisisinteresting,aslesions
cessingof perceivedhumanmovements.Becausesignlanguages associatedwiththesupramarginalgyrusaloneinusersofspoken
require the registration of such visual-spatial information, and languagedonottypicallyresultinseverespeechcomprehension
right-hemisphere parietal regions normally participate in these deficits. These observations have led some to suggest that sign
functions, it stands to reason that we may observe activation language comprehension may be more dependent than speech
of these regions during sign processing and deficits in process- on left-hemisphere inferior parietal areas, that is, regions asso-
ingwhenthesegeneral-purposeregionsaredamaged.Underthis ciated with somatosensory and visual motor integration (Leis-
view,we don’t observe activations for spoken languages as such chner, 1943; Chiarello et al., 1982; Poizner et al., 1987; Corina,
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Corinaetal. Neuralrepresentationofsignedlanguages
1998a,b), while spoken language comprehension might depend ofmovement.Suchmirroringwasnotseenduringnon-linguistic
moreheavilyonposteriortemporallobeassociationregionswhose movementandwasqualitativelydifferentfrommirrormovements
inputincludesnetworksintimatelyinvolvedwithauditoryspeech effectingdistalmovementsometimesseenincasesof hemipare-
processing. Currently the lack of an adequate number of well- sis(Hickoketal.,1996).Whileacknowledgingthelimitationsof
described case studies limits our ability to revolve this contro- raresinglecasestudies,weholdthatthiscaseisimportantforour
versy. However, as researchers begin to provide more rigorous understandingoftheneurobiologyoflanguageasitindicatesthat
behavioral assessments of sign language comprehension defects themodalityand/orformofahumanlinguisticsystemmayplace
following right and left-hemisphere damage (see for example, unique demands on the neural mediation and implementation
Atkinson et al., 2005), coupled with the ability to attain high- of language. These errors can be taken as evidence for selective
resolutionstructuralimagesof thesesubjects,forinstanceusing language-form-specificlinguisticimpairment.
voxel-based lesion symptom mapping techniques (Bates et al.,
2003),moreobjectiveassessmentsoffunctional-anatomicalrela- CONCLUDINGREMARKS
tionshipsoftheposteriortemporalandinferiorparietallobeswill In summary, we have considered three specific instances where
bepossible. datafromusersofsignedlanguagessuggeststhatcompetenciesin
signlanguageusemayrequireparticipationofneuralresourcesnot
ASLPARAPHASIA typicallyencounteredfortheprocessingofspokenlanguage.First,
Signlanguagebreakdownfollowingleft-hemispheredamageisnot we examined the mounting evidence that the right-hemisphere
haphazard, but affects independently motivated linguistic cate- parietalregionsmayberequiredforsomeaspectsofsigncompre-
gories(e.g.,semantics,phonology,morphosyntax).Thisobserva- hension,especiallyinthecontextofspecializedsyntax,aswellasin
tionprovidessupportforviewingaphasiaasauniqueandspecific theappreciationofclassifierforms.Wethenconsideredevidence
cognitivedeficitratherthanasasubtypeofamoregeneralmotor for the claim that left posterior temporal regions are necessary
orsymbolicdeficit.Anexampleof thesystematicityof parapha- forsigncomprehension.Whileresearchshowsthatlefttemporal
sic errors across signed and spoken languages can be found in regions do impact sign language comprehension,we noted that
Corina (2000). These include examples of both semantic para- research to date has not properly excluded the contributions of
phasiaandformational(phonemic)paraphasias.Theformational leftparietalregionsinsignlanguagecomprehensionimpairment,
paraphasiaselucidatethedifferentialvulnerabilityof thesublex- thusunderminingtheclaimsforisomorphicfunctionsoflefttem-
icalstructures(i.e.,handshape,movement,articulatorylocation) poral lobes in the mediation of all human languages,spoken or
of signedlanguages.Theseerrorsdemonstratehowfunctionally signed.Finally,webrieflyrecountedtheunusualpresentationof
similarlanguagecategories(e.g.,“majorclasssegments”)maybe linguisticimpairmentincaseofsignerR.S.,whoselanguageerrors
selectivelyvulnerabletoimpairment.Whilethesurfacemanifesta- are limited to linguistic productions of two-handed ASL signs,
tionsdiffer,theunderlyingdisruptionmayberelatedtoacommon hereagainprovidingevidencethatspecializedneuralresourcesare
abstractlevelofrepresentation. requiredtoimplementarticulatorydemandsofasignedlanguage
Anunusualcaseofsignparaphasiathatdoesnothaveaclear suchasASL.
mappingtopatternsobservedinspokenlanguageisreportedby There are important theoretical issues raised by these data.
Hickoketal.(1996).Alife-longsigner,R.S.,sufferedaninfarct Principally,thedatasuggestsaconflictbetweenanamodalaccount
totheleftfrontaloperculum.Neurologicalexaminationrevealed oflanguageprocessing,inwhichoneconsidersbrainareasrespon-
an initial expressive aphasia that largely resolved with lingering sibleforlanguagetobeindependentoflanguagemodality,versus
problems of word-finding and frequent phonemic paraphasia. a modality-influenced view of language, whereby one acknowl-
Particularlynoteworthyisthenatureoftheseerrorswhich,incon- edgesthattheremaybepropertiesrequiredfortheexecutionand
trast to the handshape errors previously described,demonstrate understandingoflanguageformsthatvaryasafunctionoflinguis-
thewayinwhichalanguage’smodalitymayuniquelyinfluencethe ticformandstructure.Asstatedpreviously,languageresearchers
formofthelinguisticdeficit–inthiscase,anarticulatoryimpair- are increasingly cognizant of the fact that the neural organiza-
mentwithnoclearparallelstospokenlanguagedisruption.Signed tion reflects the intimate coupling between language perception
languages,unlike spoken languages,require coordinated control andproduction.Inthepresentcase,wehaveconsideredthedif-
ofbothhands.Thepossibilityofdifferentialprogrammingoftwo ferencesandsimilaritiesbetweentheneuralprocessingofspoken
potentiallyindependentarticulatorsmayplacequalitativelydiffer- andsignedlanguages.Somescientistswilllikelyconcludethatthis
entdemandsontheonthelinguisticsystem.Intheabsenceoflimb representsanextremesituationwithatendencytooutrightdis-
apraxia,R.S.exhibitedparaphasiarestrictedtotwo-handedsigns. missthevalidityofthecomparisons.However,ifweacknowledge
Forexample,onsignsthatrequiretwohandstoassumedifferent thatbothsignedandspokenlanguagesaretruemanifestationsof
handshapesand/ortomoveindependently,R.S.wouldincorrectly thecapacityforhumanlanguage,onemustconsiderthepossibility
failtomoveoneofherhands.Inothercases,onsignswherethe thatspecializedandlanguage-specificneuralmechanismsmaybe
appropriate movement of one hand was relative to the position requiredforcompetency.Generally,researcherstendtoavoidthis
oftheother,R.S.mightproduceanincorrectrelationmovement latterpositionasitleadstowardtheproverbiallyslipperyslope.If
inthesensethatthemovementitself wascorrect,butitwasnot weacknowledgelanguage-specificneuralspecialization,mightwe
carriedoutcorrectlywithrespecttothespatialrelationshiptothe furtherexpecttoseecasesoflanguagespecificityacrossthediverse
other hand. In addition,during one-handed signing,she exhib- formsofspokenlanguages?Thebroaderpointiswhetheraphasic
itedmirroringmovementandhandshapeof thedominanthand deficitsshouldbesolelydefinedasthosethathaveclearhomolo-
onthenon-dominanthandthatwassomewhatreducedindegree giestotheleft-hemispheremaladiesthatareevidencedinspoken
FrontiersinPsychology|LanguageSciences January2013|Volume3|Article587|6
Corinaetal. Neuralrepresentationofsignedlanguages
languages,orwhethertheexistenceofsignedlanguageswillforce ACKNOWLEDGMENTS
ustoreconsidertheconceptionoflinguisticdeficitssuchasapha- This work was supported in part by grant NSF SBE-0541953
sia and open the possibility that there may be multiple ways in awardedtoGallaudetUniversity’sScienceofLearningCenteron
whichthehumanbrainmaymanifestlinguisticabilities. VisualLanguageandVisualLearning.
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