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Empirical ArticlesStudies of Neural Processing in Deaf Signers: Toward aNeurocognitive Model of Language Processing in the DeafDavid P. CorinaUniversity of Washington

The ability to comprehend and produce language stands as adefining characteristic of human cognition and enables thetransfer of knowledge and culture within human society. Aproper characterization of the human capacity for languageis required for the development of interventions that may beused to assist those individuals who have failed to achieve, orwho have lost competence in, language behaviors. For signedlanguages, models of competent language use are lacking.Thi s lack of knowledge hampers the development of effectiveassessment measures for deaf children who may be experi-encing learning problems beyond those confronting the nor-mal deaf child. I discuss two research avenues that have be-gun to provide a window into the neural systems involved insign language processing: studies of language disruptions inadult deaf signers who have suffered brain injury, and studiesof functional brain imaging in normal deaf signers. This re-search provides a basis for the development of a comprehen-sive neurocognitive model of sign language processing.

The ability to comprehend and produce languagestands as a defining characteristic of human cognitionand enables the transfer of knowledge and culturewithin human society. A proper characterization of thehuman capacity for language is required for the devel-opment of interventions that may be used to assistthose individuals who have failed to achieve, or whoTh ii work ws supported in part by grant from the University of W ash-ington, RRF-13401 m d a grant from NID CD R29 DC03099-01A1awarded to the author. I thank my collaborator! on the fMRI studies, Drs.Helen N eville and D aphne Bavelier. I thank Connie Schachtel for edito-rial assistance. Figure 2: copyright Dr. Ursula Bellugi, the Salk Institute,La Jolla, California 92037. Figures 3 and 4: copyright DT. David Corina,all righo reserved, University of Washington, 98195. Correspondenceshould be sent to David P. Corina, Department of Psychology, Un iveni tyof Washington, Seattle WA 98195-351525.Copyright O 1998 Oxford University Press. CCC 1081-4159

have lost competence in , a full ran ge of language beh av-iors (e.g., effective interpersonal communication, read-ing, writing, etc.). Cognitive psychologists have madegreat strides in understanding the functional and neu-ral mechanisms underlying the use of spoken language.Th ese studies have led to a wide range of effective edu -cational and clinical programs for enhancing languagebehaviors (Tallal, Miller, Bedi, Byma, Wang, Nagarian,Schreiner, Jenkins, & Merzenich, 1996; Merzenich,Jenkins, Johnston, Schreiner, Miller, & Tallal, 1996).

For signed languages however, models of compe-tent language use are lacking. This lack of knowledgehampers the development of effective assessment mea-sures for deaf children who may be experiencing learn-ing problems beyond those confronting the normaldeaf child. Moreover, our lack of knowledge stiflesthe development of effective intervention strategies forthese children. The development of a comprehensiveneurocognitive model of sign language processing iscrucial if we are to properly serve the needs of the deafsigning community. Development of such a modelwould also benefit basic science, providing insight intohow altered sensory experience affects the d evelopm entof neural systems underlying cognitive functions. Fi-nally, this model will benefit cognitive scientists inter-ested in functional models of human language.

Within the last decade there has been a monumen-tal increase in our knowledge of cognitive processingin deaf signing individuals (see, for example, Hanson,1990; Hanson, Lichtenstein, 1990; Neville, Mills,& Lawson, 1992; Parasnis & Samar, 1985, Marschark,

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36 Journal of Deaf Studies and Deaf Education 3:1 Winter 19981993a, 1993b; Emmorey, Corina, & Bellugi 1995; Em-morey, Kosslyn & Bellugi, 1993). In addition, the bodyof literature exploring online processing of sign lan-guage is growing (Emmorey, 1993; Emmorey & Corina1990; Mayberry & Eichen , 1991; Mayberry & Fischer,1989). Advances in sign language linguistics have pro-vided a basis of comparison for cross-language andcross-modality studies (Perlmutter, 1993; Corina, 1990;Corina & Sandier, 1993). Taken together these be hav-ioral studies provide a foundation for the developmentof functional models of cognitive and language pro-cessing in deaf signers. An equally important goal isto understand the neural systems that underlie thesefunctional characterizations. Research in this directionis required in order to develop a comprehensive neuro -cognitive model of sign language processing. In this ar-ticle I discuss two research avenues that have begun toprovide a window into the neural systems involved insign language processing: studies of language disrup-tions in adult deaf signers who have suffered brain in-jury, and studies of functional brain imaging in norm aldeaf signers.

Sign Language AphasiaIn the 1800s noted neurologist Hughlings Jacksonbroached the issue of neural control of signed lan-guage. In a much quoted musing, Jackson said: "Nodoubt by disease of some part of his brain the deaf-mute might lose his natural system of signs" (Jackson,1878). Since this time researchers have looked to casestudies of deaf signing individuals to answer two broadquestions: first, whether left hemisphere structuresmediate the signed languages of deaf individuals, andsecond, whether deaf individuals show complementaryhemispheric specialization for language and nonlan-guage visuospatial skills. More recent investigationshave focused on the question of intrahemispheric spe-cialization for sign language systems. Important gener-alizations regarding these two questions are beginningto emerge from this complicated literature.

Of the case studies of brain-damaged deaf and/orsigning individuals rep orted to date, roughly 20 involveleft-hemisphere damage, while 8 involve right hemi-sphere damage (see Corina, in press, for a recent re-view). These case studies vary greatly in their ability to

explain underlying brain processes involved in signing.Man y of the early case studies were ha mp ered by a lackof understanding of the relationships among systems ofcomm unication used by deaf individuals. For example,several of the early studies discussed disruptions offingersp elling and only briefly me ntioned or assessedsign language use. Anatomical localization of lesionswas often lacking or confounded by the existence ofmultiple infarcts. Rarely were etiologies of deafness oraudiological reports presented. Despite these limita-tions, with careful reading general patterns do emerge.More recently, well-documented case studies havestarted to provide a clearer picture of the neural sys-tems involved in language processing in users of signlanguages.

One conclusion that can be drawn from the signaphasia literature is that right-handed deaf signers, likehearing persons, exhibit language disturbances whencritical left hemisphere areas are damaged. Of 16 lefthemisphere cases reviewed by Corina (in press), 12provide sufficient detail to implicate left hemispherestructures in sign language disturbances. Five of thesecases provide neuro-radiological or autopsy reports toconfirm left hemisphere involvement and provide com-pelling language assessment- to implicate aphasic lan-guage disturbance.

In hearing individuals, severe language compre-hension deficits are associated with left hemisphereposterior lesions, especially posterior temporal lesions.Similar patterns have been observed in users of signedlanguages. For example, in the case of WL , reported byCorina, Kritchevsky, and Bellugi (1992), the subjecthad damage to posterior temporal structures and evi-denced marked comprehension deficits. WL showed agradation of impairment across tasks, with somedifficulty in single sign recognition, moderate impair-ment in following commands, and severe problemswith complex ideational material. In contrast, a righthemisphere-damaged signer, SM, who also suffered alarge right hemisphere lesion with parietal extension,showed only mild impairment on only the most diffi-cult of comprehension tests (see Figure 1).

In users of spoken languages, impairment in lan-guage production with preserved com prehension is as-sociated with left hemisphere anterior lesions. The exe-cution of speech movements, for example, involves the

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38 Journal of Deaf Studies and Deaf Education 3:1 Winter 1998area classically associated with speech comprehensiondisturbance.

Research also suggests that the equivalent of a signlanguage B roca's aphasia may involve cortical areas thatdiffer from those associated with Broca's aphasia inspoken language. The argument is based upon the ob-servation that cases of Broca's-like sign aphasia areconsistently reported with accompanying agraphia(i.e., written language impairment), as well as withfingerspeUing disturbances. In contrast, spoken lan-guage Broca's aphasia may or may not co-occur withagraphia (Levine & Sweet, 1982). This dissociation hasnot yet been observed in signers. Based on this obser-vation, Corina (in press) speculates that nonfluentsigning aphasias require involvement of classic Broca'sarea and encroachment upon cortical and subcorticalmotor areas of the precentral gyrus involved in handand arm representations. Th us , while at a general levelthe anterior/posterior and nonfluent/fluent dichoto-mies hold for spoken and signed languages, there aresome indications that within-hemisphere reorganiza-tion may be present in the deaf. Data suggest possiblesubtle differences in the cortical organization of signand speech that must be further validated with bothlesion studies and functional in vivo imaging studies.

Hem ispheric SpecializationCases of right hem isphere-damaged signers provide anopportunity to assess hemispheric specialization fornon linguistic v isuospatial function. All of these cases todate report moderate to severe degrees of visuospatialimpairment in signers with right hemisphere damage(Poizner e t al. 1987; Corina, Kritchevsky, & Bellugi,1996; Corina, Bellugi, Kritchevsky, O'Grady-Batch,& Norman, 1990; Kegl & Poizner, 1991). In contrast,none of the left hemisphere-damaged signers testedon visuospatial tests showed significant impairment.Thus, damage to critical left hemisphere structuresproduces sign language aphasia in deaf signers. Right(but not left) hemisphere lesions produce visuospatialimpairments in deaf signers. Taken together, thesefindings suggest that deaf signers show complementaryspecialization for both language and nonlanguageskills. A recent group level comparison yields a similarconclusion (Hickock, Bellugi, & Klima, 1996). Thesestudies demonstrate that development of hemispheric

specialization is not dependen t upon exposure to ora l/aural language.

Neurolinguistics of Sign Language AphasiaStudies of lifelong signers who have incurred braindamage have established the importance of the lefthemisphere in the mediation of sign language. An im-portant question then is what is the manifestation ofthese sign language impairments? Spoken languagebreakdown following left hemisphere damage is nothaphazard, but affects independently motivated lin-guistic categories. There is now ample evidence thatsign language also breaks down in a linguistically sig-nificant fashion. The best documented work concernsimpairments in sign production. Below I present de-scriptions of sign language phonemic paraphasias andimpairments of lexical and inflectional morphology.

Spoken language phonemic paraphasias arise fromthe substitution or omission of sublexical phonologicalcomponents (Blumstein, 1973). In American SignLanguage (ASL), sublexical structure refers to the for-mational elements that comprise a sign form: hand-shape, location, movement, and orientation. In signedlanguages, paraphasic errors result from substitutionswithin these parameters. For example, Poizner et al.'s(1987) subject KL produced paraphasic signing errorsin which substitutions were found in all four major pa-rameters. For example, the sign ENJOY, which re-quires a circular movement of the hand, was articulatedwith an incorrect up and down m ovement, indicating asubstitution in the formational parameter of move-ment. Substitutions in the parameters of orientation,handshape, and location were also reported.

In principle, selectional errors could occur amongany of the four sublexical parameters (and they do).However, the most frequently reported errors are thoseaffecting th e han dshap e param eter. Corina et al. (1992)describe in some detail the phonemic errors producedby WL , errors which almo st exclusively involved h and -shape specifications. For example, WL produced thesign TOOTHBRUSH with the Y handshape ratherthan the required G handshape, and produced the signSCREWD RIVER with an A handshape rather than therequired H handshape (see Figure 2) . Based upon a lin-guistic analysis of these err ors, C orina et al. (1992) havepresented evidence that these handshape substitutions

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"FINE" "SCREWDRIVER1

" W H I T E " "TOOTHBRUSH"Fi gu re 2 WL 's Handshape-specific errors. Examples of sign language phonem ic paraphasias in patient WL. Most ofWL's errors consisted of incorrect selection of handshapes with correct place of articulation and movement. Copyright, Dr.Ursula Bellugi, The Salic Institute, La Jolla, California. Reprinted with permission.

are phonemic in nature , ra ther than phonet ic misar t ic-ulations. Finally, phonological paraphasias in signaphasia do not compromise the syllabic integrity of asign (Brentari , Poizner, & Kegl, 1995). For example, weobserve subst i tut ions of movements ra ther than omis-sions (the latter would violate syllable well-formednessin ASL) .

Morphological and syntactic errors. A common e r ro r pa t -tern in spoken language aphasia is the substitution andomission of bound and free morphemes. Because lan-guages differ in the degree to which they use morphol-ogy to mark obligatory grammatical distinctions (e.g.,

case and gender, subject and object agreement, etc),patterns of impairment may be more striking in somelanguages than in others (Bates, Wulfeck, &MacWhin-ney, 1991; M enn & Obler, 1990). ASL is a highly in -flected language; in addition to temporal and adver-bial inflections, ASL has a class of verbs that inflectfor person and number agreement. Morphosyntacticagreement distinguishing grammatical subject and o b-ject requires directional movement trajectories. In theabsence of grammatical movement trajectories, a verbsign will be produced in an uninflected "cita tion "form. Poizner et al. (1987) have investigated morpho-syntactic impairments in their patients. Poizner's pa-

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40 Journal of Deaf Studies and Deaf Education 3:1 Winter 1998tient GD consistently omitted required inflectionalmorphemes in her spontaneous signing and insteadproduced uninflected "citation" verb forms. Poizner'spatient PD produced both omissions in inflectionalmorphology and inconsistent verb agreement substitu-tions. That is, PD failed to maintain consistent verbmovement trajectories to spatial locations, as is re-quired by syntactic and d iscourse conventions. GD hada large left hemisphere lesion that involved most of theconvexity of the left frontal lobe, including Broca'sarea. PD had a subcortical lesion in the left hemi-sphere, with anterior focus deep to Broca's area andposterior extension into the white matter in the left pa-rietal lobe. The general pattern of omissions versussubstitutions in signers GD and PD is consistent withprofiles of agrammatic and paragrammatic impair-ment, respectively, reported for users of spoken lan-guage.

Th e use of facial morphology has recendy been in-vestigated as w ell. Facial expressions serve a dual func-tion in ASL, conveying both affective and linguisticinformation. Separate classes of facial expressions sub-serve these two distinct functions. Differential impair-ment in these two classes of facial expressions havebeen reported (Zein, Say, Bellugi, Corina, & Reilly,1993). Recent recognition and production studies of fi-cial expression in normal deaf signers indicate bilateralmediation of facial expressions. This pattern contrastswith the strong right hemisphere advantage shown byhearing persons, for whom facial expressions servemainly affective purposes (Corina, 1989). In summary,produ ction of sign language morphology is vulnerablefollowing left hemisphere damage in deaf signers.

Taken together, these results demo nstrate that lan-guage abilities in deaf signers break down in linguisti-cally significant ways. Thes e finding s provide evidencethat language impairments following stroke in deafsigners are aphasic in nature and do not reflect a gen-eral problem in symbolic conceptualization or motorbehavior.

Right Hemisphere and LanguageWhile the left hemisphere plays a crucial role in coreaspects of language comprehension and production,there is a growing awareness that both hemispheres arerequired for language usage in a social context. For

hearing persons, right hemisphere impairments dis-rupt the meta-control of language use, as evidenced bydisruptions of discourse abilities (Brownell, Simpson,Bihrle, & Potter, 1990; Kaplan, Brown ell, Jacobs, &Gardn er, 1990; Rehak, Kaplan, W eylman, Kelly, Brow-nell, & Gardn er, 1992). Th ere is growing evidence thatthe right hemisphere plays a crucial role in the dis-course abilities of deaf signers. Analysis of language usein right hemisphere-lesioned subject JH (Corina et al.,1992; Corina et al., 1996) revealed occasional non se-quiturs and abnormal attention to details, which arecharacteristic of the discourse of hearing patients withright hemisphere lesions (Delis, Robertson, & Balliet,1983). Subject DN, reported in Poizner and Kegl(1992), showed another pattern of discourse disrup-tion. While DN was successful at spatial indexingwithin a given sentence, several researchers noted thatshe was inconsistent across sentences1 (Emmorey et al.,1995; Poizner & Kegl, 1992). That is, she did not con-sistently use the same spatial index from sentence tosentence. In order to salvage intelligibility, DN used acompensatory strategy in which she restated die nounphrase in each sentence, resulting in an overly repeti-tive discourse style. An English equivalent might in-volve noun repetitions such as: "Tom went to die store;Tom looked for eggs; Tom paid for eggs; Tom wenthome" versus "Tom went to the store; he looked foreggs; he paid for them and then went home."

The cases of JH and DN suggest diat right hemi-sphere lesions in signers can differentially disrupt dis-course co ntent (as in die case of JH ) and discourse co -hesion (as in the case of DN). Lesion site differssignificantly in these two p atients; JH 's stroke involvedcentral portions of the frontal, parietal, and temporallobes, whereas D N's lesion was predom inandy medialand involved the upper part of the occipital lobe andsuperior parietal lobule. Whether similar lesions inhearing individuals would result in differential dis-course patterns awaits furdier study.

In the cases of impaired discourse production, itis reasonable to suppose that general right hemispherecognitive deficits manifest themselves in asp ects of lan-guage use. For example, an attentional deficit mightmanifest itself as a perseveration of topics of conver-sation, while a spatial memory impairment could re-sult in an inability to keep track of references acrossstretches of discourse. For users of A SL, d iese types of

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Studies of Neural Processing in Deaf Signers 41deficits might manifest themselves as impairments inlanguage continuity and spatial reference. It seems rea-sonable, then, to entertain the possibility that righthemisphere damage does not disrupt linguistic func-tion per se, but rather impairs the execution and pro-cessing of linguistic information in sign language, inwhich spatial information plays a particularly salientrole. However, the issues become more complicatedwhen we consider the syntactic aspects of ASL.

Disturbances in syntactic processing in ASL havebeen reported in patients with both left hemisphere andright hemisphere damage. In the case of left hemi-sphere damage, problems in the production and com-prehension of spatialized syntax have been noted. Forexample, patient GD evidenced production problemssuch as omitting n ecessary inflections, whereas subjectPD was inconsistent in his use of spatial referencing inthe service of grammatical relations. Subjects PD, WL,and KL all demonstrated problems in the comprehen-sion of syntactic relationships expressed via the spatialsyntactic system. More surprising, however, is thefinding that some signers with right hemispheredamage also exhibited problems. Two of the righthemisphere-damaged subjects, tested by Poizner et al.(1987), showed performance well below controls ontwo tests of spatial syntax (see description of SM andGG). Indeed, as pointed out in Poizner et al. (1987),"Right lesioned signers do not show comprehensiondeficits in any linguistic test, other than that of spa-tialized syntax." Poizner et al. (1987) speculated thatthe perceptual processing involved in the comprehen-sion of spatialized syntax involves both left and righthemispheres; certain critical areas must be relativelyintact for accurate performance. Recent evidence fromfMRI studies of deaf signers attests to the importanceof both left and right hemisphere function in normaldeaf signers.

Functional Neuroimaging StudiesFunctional neuroimaging studies provide another ave-nue for investigating the underpinnings of languageprocessing in the deaf. Functional neuroimaging,broadly defined, is the measurement of brain activityduring task performance. Common techniques thathave been successfully used to study language and cog -nitive function include Positron Emission Tomography

(PE T) (e.g., Fox, Raichle, M intun , & D ence 1988; Pet-ersen, Fox, Posner, Mintun, & Raichel, 1988; Habib,Demonet, & Frackowiak, 1996), Event-related poten-tials (ERP) (e.g., Osterhout, 1994), regional cerebralbloodflow(rCBF ), Magnetoenchelagram (M EG ) (e.g.,Lounasmaa, H amalainen, H an, & Salmelin, 1996) andFunctional M agnetic Resonance Imaging (fMRI) (C o-hen, N oll, & Schneider 1993; Binder, 1995). One ad-vantage of these techniques is that they provide infor-mation about brain function from awake human beingswhile they are performing controlled experimentaltasks.

Neville et al. have used ERP methodology to studylanguage processing in hearing and deaf subjects. TheERP technique provides millisecond temporal resolu-tion and thus is highly suitable for studying the rapidon-line processing involved in language behavior. TheERP technique, however, is less well suited for estab-lishing the anatomical location of neural areas mediat-ing these behaviors. One im portant question addressedin these studies concerns w hether the various subcom-ponents of linguistic structure (semantics, syntax, andphonology) have identifiable signatures in the event-related brain potentials. Research from written lan-guage paradigms shows evidence for differential pro-cessing of "open" versus "closed" class items (Neville,Mills, & Lawson, 1992). Closed-class items are thosegrammatical formatives that convey structural or re-lational aspects of sentence meaning (for example,articles and auxiliaries in English), while open-classelements (such as nouns, verbs, and adjectives) makereference to specific objects and events. In normalhearing adults the ERP response to meaning-bearing,open-class words is characterized by a prominent nega-tive component maximal around 350 msec after wordonset. This component was larger over posterior brainregions of both hemispheres. In contrast, ERP re-sponses to closed-class words displayed a prominentnegative poten tial at 280 msec that was localized to th eanterior and temporal regions of the left hemisphere.

When this same series of studies was conducted w ithdeaf native signers processing English, ERP responsesto open-class words w ere virtually identical to those ob-served in the normal hearing subjects. However, theresponses to the closed-class words were markedlydifferent. Deaf subjects (for whom English was a sec-ond language) lacked the negative potential over ante-

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42 Journal of Deaf Studies and Deaf Education 3:1 Winter 1998rior regions of the left hemisphere, and no lateralizedasymmetry was observed between the hemispheres(Neville, Mills, & Lawson, 1992). These results suggestthat the neural systems that mediate syntactic pro-cessing between the hemispheres are more vulnerableto variability of the natu re and timing of early languageexperience than are the neural systems linked to lexi-cal-semantic processing. Importantly, a follow up study(Neville, Coffey, Lawson, Fischer, Emmorey, & Bellugi,1996) showed that for native deaf signers processingASL sentences, the characteristic differences betweenopen-class and closed-class grammatical elem ents wereobserved (Neville et al., 1996). Specifically, native deafsigners showed differential specialization of anteriorand posterior cortical regions for aspects of grammati-cal and semantic processing, respectively. These resultssuggest similarities in the organization of the neuralsystems that mediate formal languages, independ ent ofthe modality through which language is acquired.

Recently, functional imaging techniques have beenused to examine sign language representation in thebrain. T hese studies, while providing good spatial reso-lution, are less well suited for establishing the temporaltime-co urse of processing involved in language behav-iors. A series of studies by Soderfeldt (1994) and Sod-erfeldt, Ronnberg, and Risberg (1994) used rCBF andPET to look at functional representations of sign.Studies of hearing bilingual speaker-signers revealedbilateral posterior temporal activation for both spokenlanguage comprehension and sign comprehension. In aseparate study with native deaf signers, a similar bilat-eral pattern was found, but with increased right hemi-sphere parietal-occipital activation. Soderfeldt (1994)suggested that this increased activity reflected thegreater spatial processing required for perception of asigned language. Hickock et al. (1995) reported on anfMRI production study of covert naming and word(sign) generation. Analogous tasks with spoken lan-guage have shown significant activation in Broca's andWernicke's areas in hearing individuals. Two nativedeaf signers showed activation in Brodmann areas 44/45 (Broca's area) and posterior area 22 (Wernicke'sarea) predominantly on the left, although right hemi-sphere homologues did show significant activation.Additional sites of activation included premotor areas,cerebellum, and prefrontal cortex. These authors con-

cluded that the similar patterns of left perisylvian acti-vation in deaf and hearing subjects indicates that neu-ral organization for language processing is modalityindependent. Hickock, Bellugi, and Klima (1996) con-trasted activation for an ASL rhyming test with a work-ing memory task. Previous investigations of rhymingversus verbal working memory tasks in spoken lan-guage have identified activation in the inferior frontalgyrus, predominantly on the left for rhyming tests (see,Zatorre, M eyer, Gjedde, & Evans, 1996) and implicatedorsolateral prefrontal cortex for verbal working mem-ory (Awh, Jonides, Smith, Schumacher, Koeppe, andKatz, 1996; Cohen, Forman, Braver, Casey, Servan-Schreiber, & Noll, 1994; Fiez, Raife, Balota, Schwartz,Raichle, & Pertersen, 1996). Hickock et al. (1996) re-port patterns of activation from a single subject thatsuggest differences in the distribution of activation forrhyming and m emory tasks. This finding suggests thatsubregions within classical Broca's area may contributedifferentially to different language-related tasks.

fMRI Study of ASL Sentence ProcessingOne of the most comprehensive fMRI projects involv-ing deaf signers has recently been reported by Corina,Bavelier, and Neville (Bavelier, Corina, Clark, Jezzard,Padmanhaban, Prinster, Kami, Rauschecker, Turner,& Neville, in press; Neville, Bavelier, Corina, Raus-checker, Kami, Lalwani, Braun, Clark, Jezzard, &Turner, in press). These studies examined neural acti-vation patterns as hearing and deaf subjects read En-glish words or viewed signed ASL sentences. Thesestudies provide further evidence for common neuralareas in the left hemisphere that subserve languageprocessing regardless of language modality. Thesestudies also revealed language-specific activation in theright hemisphere of native hearing and deaf signers.(An overview of the procedure and data analysis can befound in Bavelier et al., in press).

Three groups of subjects were tested: hearing indi-viduals who did not know sign language, native deafsigners, and normally hearing individuals bom to deafparents for whom ASL was a first language. Table 1illustrates the subject characteristics for these threegroups.

Each population was imaged2 while processing sen-

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Table 1 Subject characteristicsHearing Deaf Hearing native signers

English exposure BirthEnglish proficiency NativeASL exposure NoneAS L proficiency No neHearing NormalMean age 26Handedness RightNu mbe r of subjects 16

School age BirthModerate NativeBirth BirthNative NativeProfound deafness Norm al23 35Right Right23 16, 18'

'16 subjects participated in the English condition and 18 in the ASL co ndition.

Table 2 Performance on fMRI task (percentage correct)

EnglishSentencesConsonant strings

ASLSentencesNonsigns

Hearing

85525651

Deaf

85569262

Hearingnative signers

80559260

tences in both English and ASL. The English stimuliwere presented in cycles defined as 32-second blocksof sentences that alternated with 32-second blocks ofconsonant strings. The ASL stimuli consisted of 32-second blocks of videotape of a native deaf signerproducing sentences in ASL. These alternated with32-second blocks in which the signer made nonsigngestures that were physically similar to ASL signs.Four cycles of alternations were presented in each offour runs, two for each language. At the end of eachrun, subjects indicated whether or not specific senten-ces and nonword/nonsign strings had been presented.The behavioral data indicated that subjects were at-tending to the stimuli and were better at recognizingsentences than nonsense strings. Hearing subjects whodid not know ASL performed at chance in recognizingASL sentences and nonsense signs. All subjects per-formed equally well on simple declarative English sen-tences, and deaf and hearing native signers performedequally well on A SL sentences (see Table 2).

We examined the effects of language type for thesethree groups of subjects in four language areas typi-cally associated with language pro cessing: Broca's area,Wernicke's area, the angular gyrus, and the dorsolat-eral precentral cortex (DPLC) (see Figure 3). Broca's

DPLC Angular Gyrus

Broca's Wernickc'Figure 3 Left hemisphere regions of interest evaluatedfor functional activity. Right hemisphere homologues arethe equivalent right hemisphere anatomical areas.

area is involved in language production abilities; Wer-nicke's area is considered important in language com-prehension abilities; classically, the angular gyrus hasbeen implicated in reading; and, as previously stated,the DP LC has been implicated in working memory be-havior (Awh et al., 1996; Cohen et al., 1994; Fiez etal., 1996).fMRI results: English. When hearing subjects read En-glish sentences, we observed robust activation in allfour language areas3 in the left hemisphere. In contrast,within the right hemisphere none of the homologouslanguage areas were reliably active (see Figu re 4 , up perleft panel). Thus, the data from neurologically intactusers of spoken language are consistent with the ubiq-uitous left hemisphere asymmetry described by over acentury of language research.

An analogous pattern of activation was observedwhen hearing signing subjects read English sentences(see Figure 4, upper middle panel). Based on the analy-


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44 Journal of Deaf Studies and Deaf Education 3:1 Winter 1998

HearingEnglish

Hearing Native Signers Deaf

HearingASL

Hearing Native Signers Deaf

p< .025 I KF ig ur e 4 Com posite represen tations of functional activation in response to linguistic stimuli in three subject groups: hearingnonsigners, hearing native signers, and deaf native signers.

sis of these four language areas, a clear left hem ispherelateralization emerges, a pattern that is comparable tothat observed for h earing nonsigners reading English.

In contrast, for the deaf signing subjects readingEnglish, a very different pattern emerged. Deaf sub-jects did not display left hemisphere dominance whenreading English. The only left hemisphere languagearea reliably active was Wernicke's area. Interestingly,we observed reliable right hemisphere activation in theposterior temporal and parietal areas, the right hemi-sphere homologues of Wernicke's area, and the angulargyrus (see Figure 4, upper right panel).fMRI results: ASL. As might be expected, hearing sub-jects who did not know ASL did not show any reliableactivation in response to the difference between mean-ingful and no nmean ingful signs. This lack of activationis consistent with the behavioral data (see Figure 4,lower left panel).

Deaf subjects processing ASL displayed significantleft hem ispher e activation for the ir native language (seeFigure 4, lower right panel). Significant activation is

observed within Broca's area, Wernicke's area, theD PL C, and the angular gyrus. Note that this activationpattern is similar to that observed in hearing subjectsprocessing English. This result suggests that acquisi-tion of a spoken language is not a prerequisite for es-tablishment of language systems within the left hemi-sphere. Interestingly, the processing of ASL sentencesin deaf subjects also strongly recruited right hemi-sphere structures. Reliable activation was observed inhomologous right hemisphere structures includingBroca's area, Wernicke's area, the DPLC, and the an-gular gyrus. This bilateral pattern of activation differssignificantly from that observed when hearing subjectsread English sentences.

Hearing native signers processing ASL displayedleft hemisphere activation similar to that of the deafsigners. All four classic language areas were highly a c-tive (i.e. Broca's area, Wernicke's area, the DPLC, andthe angular gyrus). Interestingly, these same subjectsalso showed right hemisphere activation similar to thatof the deaf subjects. However, the hearing native sign-ers' right hemisphere activation was primarily limited

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Studies of Neural Processing in Deaf Signers 45to posterior temporal and parietal sites (e.g., the righthemisphere homologues of Wernicke's area and the an-gular gyrus) (see Figure 4, lower middle panel). Thefindings of right hemisphere activation for ASL pro-cessing by deaf and hearing native signers are particu-larly important and suggest that these effects are notattributable to hearing loss per se, but may reflect theprocessing demands imposed by ASL.

Discuss ionSeveral important findings emerge from these studies.We have observed (1) differences in brain activationbetween deaf and hearing subjects reading English,(2) consistent left hemisphere activation for subjectsprocessing a native language (either English or ASL),and (3) robust right hemisphere parietal activation inhearing and deaf signers processing A SL.

As noted, hearing nonsigning subjects displayedrobust activation within standard language areas of theleft hemisphere when reading English sentences. Incontrast, for deaf subjects, left hemisphere activationwas limited to Wernicke's area. There are several as-pects of these deaf subjects' varied experiences withEnglish that might account for these differences. Onepossibility is that the bilingual sta tus of the deaf signershas contributed to this pattern. Specifically, learningASL as a first language may have altered neural repre -sentations for English. This hypothesis can be dis-counted, however, by considering the data from thehearing native signers, all of whom also learned ASLas a native language. These subjects, like hearing n on-signers, displayed the expected left hemisphere domi-nance while reading English. Thus, the lack of lefthemisphere activity and the presence of right hemi-sphere activation when deaf subjects read English wasprobably not due to the acquisition of ASL as a firstlanguage, since hearing native signers did not displaythis pattern. Another possibility is that deaf individu-als' acquisition of English does not benefit to the samedegree from the sound-based strategies available tohearing individuals, A third and related possibility isthat many deaf individuals do not obtain full gram mat-ical competence in English. However, it should benoted that on tests of English grammaticality (Line-barger, Scha rtz, & Saffran, 1983), our subject pool

scored reasonably well (mean 80% , range 6% -2 6% er-rors). These results suggest that the cortical organiza-tion for written English is highly specific to the lan-guage experience of the subjects. For native hearingsigners, who are also fluent in English, the cortical p at-terns of activation for written English shared the mainfeatures of those observed in monolingual hearing su b-jects. However, in deaf subjects, the cortical patterns ofactivation were more similar to those observed for ASL(see below). These results suggest that the processingof English in deaf subjects may build on the same sys-tems as those that mediate ASL processing, while inhearing subjects, the processing systems for writtenEnglish are most probably guided by those that medi-ate spoken English processing.

An important finding is the consistent left hemi-sphere activation observed for subjects processing theirnative language (either English or A SL). D eaf subjects,when processing ASL, displayed significant left hemi-sphere activation. Activation within Broca's area, Wer-nicke's area, the DPLC, and the angular gyrus wassimilar to that observed in hearing subjects processingEnglish. This result suggests that acquisition of a spo-ken language is not necessary for establishing languagesystems within the left hemisphere. Robust left hemi-sphere activation of Broca's area, DPLC, and Wer-nicke's area was also observed in native hearin g sign ers.Hence, in every subject (hearing or deaf) processingtheir native language (English or ASL), these lefthemisphere structures were recruited. These resultsimply that there are strong biological constraints thatrender these particular brain areas well-suited for theprocessing of linguistic information, independent ofthe struc ture or the m odality of the language.

Finally, a surprising finding was the extent of righthemisphere activation observed while deaf signers pro-cessed ASL. One question that arises is whether thisactivation is attributable to auditory deprivation or tothe acquisition of a language that depends on visual/spatial contrasts. Fortunately, data from hearing nativesigners provide insight into this question. Hearing na-tive signers processing ASL displayed right hemi-sphere parietal activation similar to that of the deafsubjects. The se data suggest that the activation of rightparietal structures is not a consequence of deafness perse, but may be a neural signature of the linguistic pro -

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46 Journal of Deaf Studies and Deaf Education 3:1 Winter 1998cessing of ASL. The right hemisphere activation thathas been observed may be a response to certain mod-ality-specific characteristics of signed languagesnamely, the fact that linguistic and visuospatial infor-mation temporally coincide. Finally, it is of interest tonote that differences between hearing native signersand deaf native signers were observed in right hemi-sphere frontal areas (Broca's homologue and theDPLC). It is possible that these frontal activations re-flect processing areas that have become specialized as aresult of auditory deprivation. For example, the righthemisphere D PL C has been implicated in visuospatialworking memory (Awh et al., 1996). Whether the re-cruitment of these frontal areas reflects a dependenceupon visual processing in profoundly deaf personsawaits further study.

Taken together, these studies provide evidence forthe role of the left hemisphere in processing early-acquired, fully grammatical linguistic systems. More-over, these studies support the claim that commonneural areas in the left hemisphere subserve languageprocessing, regardless of language modality. Thesestudies also indicate that sign language processingrequires participation of the right hemisphere to agreater extent than does the processing of written En-glish. The co-occurrence of visuospatial and linguisticinformation may result in the recruitment or mainte-nance of these areas in the language system for nativedeaf and hearing signers. Additionally, these studiessuggest that deaf signers' processing of written Englishis markedly different from that of hearing individuals.These differences suggest that deaf individuals may bemaking use of ASL-based neural systems in the encod-ing of written English.

This study of sentence processing across popula-tions is an attempt to explore neural correlates ofsigned and written language processing. However, theresearch discussed above provides only a first glimpseat the neural systems involved in these language behav-iors. Many more q uestions remain to be answered. Forexample, it will be important to determine the pro-cessing contributions of the common left hemispherelanguage areas that are activated in signers and users ofspoken languages. In addition, we need to explore therole of the right hemisphere areas active during signcomprehension. A first step in this direction will be to

examine patterns of neural activation for specific sub-components of language and nonlanguage behaviors(e.g., processing of phonology versus syntax, pro-cessing of linguistic visuospatial pro perties versus no n-linguistic visuospatial processes) using tasks specifi-cally designed for these purposes. Additional work isneeded to explore language representations in popu-lations of normative signers and late learners of signlanguage to further explore the roles that language ex-perience and biological endowments play in languageprocessing.

ConclusionThe development of a comprehensive neurocognitivemodel of sign language processing is crucial if we areto properly serve the educational and th erapeu tic needsof the deaf signing comm unity. Studies of cognitive andlanguage processing in the deaf are beginning to pro-vide a foundation for development of a functionalmodel of ASL processing. An equally imp ortant goal isto understand the neural systems that underlie thesebehavioral functions. This article has reviewed recentfinding s from studies of language abilities in adult deafsigners who have suffered brain injury and studies offunctional imaging in deaf signers. These studies havebegun to specify the neural machinery required forcompetent use of sign language. The combination ofbehavioral and neural imaging techniques provides thenecessary groundwork for the development of a com-prehensive neurocognitive model of sign language pro-cessing.

Not e s1. It should be noted that the characterization of "within-

sentential co-reference" as the province of syntax and "between -sentential co-reference" as the province of discourse is a sim-plification of the complexity of language structure. I thank ananonymous reviewer for this comment.

2. Gradient-echo echo-planar (EPI) images were obtainedusing a 4T whole body M R system, fitted with a removable z-axis head gradient coil. Eight para-sagittal images, positionedfrom the lateral surface of the brain to a depth of 40 mm, werecollected (TR = 4 sec, TE = 20 mns, resolution 2.5 X 2.5 X5 mm, 64 times-points per image). For each subject, only onehemisphere was imaged in a given session since a 20 cm diametertransmit-receive radio-frequency surface coil was used. In addi-tion, at the beginning and end of each run, high resolutiongradient-echo GRASS reference scans corresponding to the EPI

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Studies of Neural Processing in Deaf Signers 47slices were obtained (T R = 20 ms, TE = 10 ms, flip angle 15degrees). These reference scans give good gray/white/CSF con-trast and permitted identification of activated areas in relation tosulcal anatomy.

3. Data were analyzed by performing correlations, voxel byvoxel, between the MR signal time series and a sine wave thatmodeled the alternations between sentences and nonwords/non-signs. Th is correlation m ap was thresholded to retain only voxelswhose activity over time correlated with the stimulus alternation(r 2: 0.5; p = 3.1 105). For each s ubject, ana tomical regionswere delineated according to sulcal anatomy (Rademacher ct al.,1992); active voxels were classified according to these anatomicalregions. Averages across subjects were claculated for each ofthese anatomical regions. Activation measurements were madeon the following two variables for each region and dataset: (a) themean percent change of the activation for active voxels in a re-gion, and (b) the mean spatial extent of the activation in the re-gion. For each population and hemisphere, activation within aregion was assessed by MANOVA (BMDP statistical software)on these variables (see Bavelier et al., 1997, in press, for furthe rdiscussion of these procedures). Unless otherwise noted, sig-nificance levels are specified at/> < .025.

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