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Department of Psychology Dietrich College of Humanities and Social Sciences

1996 Perceptual Cues in Pure Alexia Erica B. Sekuler University of Toronto

Marlene Behrmann Carnegie Mellon University

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This Article is brought to you for free and open access by the Dietrich College of Humanities and Social Sciences at Research Showcase @ CMU. It has been accepted for inclusion in Department of Psychology by an authorized administrator of Research Showcase @ CMU. For more information, please contact [email protected]. COGNITIVENEUROPSYCHOLOGY,1996, 13 (7),941±974

PerceptualCues inP ure Alexia

EricaB .Sekuler Universityo fToronto,Toronto,Canada MarleneBehrmann CarnegieM ellonUniversity,Pittsburgh,USA

This studyprovidesevidencethatpureale xia,orletter-by-letterre ading,m ay beattributedto a generalperceptualde®cit thatextendsbeyonda n orthographicdiso rder.T heperceptualp roblemmaybeun maskedwhen appropriatep erceptualcuesare n otavailableto a idin th ederivationof a n integratedstruc turaldescription.Four pureale xicp atients andeightn on- -damagedcontrols participatedin th is study.In the® rst two experiments, subjects’rea dingab ilities werea ssessed onanamingla tency anda lexicald ecisiontask.Experiment3 replicatedF arahandWallace’s (1991)result sthatthep urea lexiade® cit w asno tspeci®c toorth ography. Experiments 4and5furtherex ploredth enatureo ftheperceptualdisorder usingnonorthographicstim uli. In Experiment4,patientp erformanceona targetdetectiontask wasunaŒecte dbythenu mberofpa rts comprising the objectb utwasim pairedw hentheperceptualc ueo fgoodco ntinuationwas absent. Patientpe rformancea lso declinedw henthep erceptualcu eof symmetryw asnotavailableto a idin th eintegrationofoc cludedobjectpa rts inE xperiment5. O verall,the results implyth atp urea lexiais most likelyto arise fromamorege neral, nonorthographicde ®cit, andthatthenatureo f thedisorderis revealedw henth eperceptualcontextlacksstrongperceptual cues.

Keywords: alexia,perception, perceptualde® cit.

Requests for reprints should be addressed to Marlene Behrmann, Department of Psychology,CarnegieMellon University,Pittsburgh, PA15213±3890,USA(email: behrmann+@ cmu.edu). Theresearch presented here wassupported byN aturalScience and EngineeringResearch Council ofCanadagrant OGP0122706awarded to Marlene Behrmann. WethankMorris Moscovitch, EyalReingold, and Meredyth Danemanfor their helpful suggestionson this project and to Allison B.Sekulerfor comments on themanuscript. Wealsotha nk SandraE . Black for referringso meof thepatients to us. Portions ofthese dataw ere presented attheJune 1993CanadianSociety for Brain, Behaviour, and CognitiveScience andExperimental PsychologySo ciety joint meeting in Toronto,Ontario.

Ó 1996PsychologyPress, anim printof Erlbaum(U K)Taylor &Francis Ltd 942 SEKULERANDBEHRMANN

INTRODUCTION

Purea lexia,orletter-by-letterreading,isa disorderthatresultsfro mbrain damageinp remorbidlylitera tea dults.T hemainfe atureo fthisd isorderis thelosso ftheabilityto rea dquicklya ndeciently.Occasionally,th ese patientsw illna metheindividuallettersoutloudbeforep iecingaword togetherb ut,ev enin th oseca sesw hereth elettersa ren otovertlyn amed, theletter-by-lettermethodofreadingcanbeinferredfro mpatients’ personalaccountsa ndreactiontimemeasures.T hisre adingdisorder manifestsitself a salinearincreasein re actiontimewitha nincreasein th e numbero flettersinth estring,aphenomenonreferredto a s``theword lengtheŒ ect.’ ’Purea lexiaty picallyresu ltsfro mdamagetoth elefta nterior inferioroccipitalcortexa ndinvolvesth eposteriorcerebralartery(B lack& Behrmann,1994),althoughthisn eedn otalwaysb ethecase(H enderson, Friedman,Teng,&Weiner,19 85).Somepatientsa lsosu Œerfro maright homonymoushemianopsiao rquadrantanopsia(u suallysu perior)o raloss ofcolourvisionbuttheseim pairmentsd onotalwaysco-occur(Benson, 1985;Damasio& Damasio,19 83;Greenblatt,1 973;P atterson&Kay, 1982). Thelong-standinga ndwidelyac ceptedv iew ofpurea lexiawa sthatthe readingde®cit wa stheonlym ajorneurobehaviouralimpairmentsuŒered bythesep atients.If an yadditionalaccompanyingd e®citsex isted(fo r example,anomia,colourde®cits),th eyw eremild.Accordingtoth isview , thefailureto reco gnisew ordsisspe ci®c too rthographicitem s( the orthographicview )and,thus,p atientsare o nlyim pairedin th eprocessing ofalphanumericm aterials(e.g .De  jeÁ rine,1892:inBu b,Arguin,& Lecours, 1993;Geschwind,1965;Patterson&Kay,1982;Shallice& SaŒran ,1986; Warrington&Shallice,1 980).T he perceptualview ,ontheotherh and, claimsthatamoreb asica ndinclusivevisualprocessingde®c itu nderlies purea lexia(e.g .Farah,1991,1992;Farah&Wallace,1 991;Friedman& Alexander,1 984;Kinsbourne&Warrington,1962).W hereastheortho- graphicv iew impliesthatthereis a separateareao ftheb raind edicatedto processingvisually-presentedla nguage-relateditem s,a ndth atth issy stem canbeselectivelyim paired,th elatterp erceptualv iew doesn otrequireth e invocationofthisty peo fstructure.A ccordingtoth eperceptualview ,the morew idespreadpe rceptualde®cit u nderlyingpurea lexiais m osto bvious whenp atientsattem pttop rocessm ultipleletter scomprisingwords(i.e.in );however,th isim pairmentcanalsob eobservedu nderstringent testingofgeneralvisualp erceptualabilities.Inco ntrastw itha view ofa reading-dedicatedstru cturalorfunctionala rea,theperceptualview suggeststh atrea ding,whichis a relativelyn ewlya cquiredphylogenetic ability,is``p iggybacking’’onapre-existingvisuoperceptualability(see Farah& Wallace,1 991). PERCEPTIONINPUREALEXIA 943

Evidenceco nsistentwithth eperceptualv iew was®rstpre sentedb y Kinsbournea ndWarrington(1962),whoshowedth atp atientsw ithp ure alexiaw ereu nableto reco gnisem ultiplesha pes,b otho rthographica nd nonorthographic,presentedsim ultaneouslya ndinra pidseq uence. Similarly,FriedmanandAlexander(1 984)demonstratedthattheirp ure alexicp atientwasnotonlyim paireda tidenti®ca tionoflettersbuta lsoat recognitionofvisualobjects.Ev idenceforaperceptualde®cit h asalso beenin ferredfro mthereadingb ehaviourofthesep atients;th eym akea highproportionofvisuale rrorsd uringreading,oftenco nfusingletters, especiallyth oseth atarev isuallysim ilar(Bub& Arguin,1995;H anley& Kay,1992;Karanth,1985).A lthoughsupportfo raperceptualbasisfo r purea lexiais g aininginp opularity,almostallv ersionsofthish ypothesis suŒerfro mtheweaknessth attheyd erivefro mtheobserveda ssociation betweenpu realexia a nda perceptuald e®cit, i.e. th eyre¯ ec tacorrelational relationshipb etweenth eco-occurrenceo faperceptualde®cita ndpure alexia.Oneoftherecentresultsfro mFarahandWallace(1991),however, revealsa mored irectcausalrelationshipb etweenth etwo.Th eauthors presentedthecaseo fpatientTU,whoshowedthehallmarkw ordle ngth eŒect asso ciatedw ithp urea lexiaa ndwasalso im pairedo nvarious nonorthographictask s.Fo rexample,TUhadbelow-normalpe rformance ontimedle tterd etection,objectm atching,andnumberstrin gcompar- isons.F araha ndWallacerea sonedthatifT U’sreadingproblemwas attributableto a dicu ltyin v isualencoding,thenamanipulationknown toa Œectth evisualencodingprocesssh oulde xacerbateth ewordle ngth eŒect. Th eytested th isb ymanipulatingthequalityo ftheletter sinw ord stringsusingapatternm aska ndexaminingtheeŒect o fthedegradation onthewordle ngtheŒ ect in o ralreading.Whereascontrolsubjectsd idn ot showaninteractionbetweenwordlen gthan dvisualqualityin th eir readingreactiontime,TU wasdisproportionatelyaŒ ected b ythevisual qualityo fwords.Theseresu ltsle dFarahandWallaceto co ncludethatthe locuso fTU’simpairmentwasat a stageof perceptual encodingandthat thish addirectconsequencesfor h isrea dingoftheletterstrin gs. Basedonthesea ndotherresu lts,F arah(1 991,1992,19 94)proposeda theorytha thasrenewedin terestin th eperceptualview.Faraha ttributes purea lexiato a selectivefailurein th eperceptualrepresentationo f multipleob jectp artsp riortoreco gnitionofthewholeo bject.S hefu rther suggeststha tpurea lexicpa tientsm ayhavep reservedreco gnitionof objectsth atdonotrequirein dividualpartreco gnition.Farahhypothesises theexistenceo ftwotypeso fstructuraldescriptionswithinth evisual recognitionsystem:oneforrepresentingmultiplep arto bjectsa ndthe otherfo rrepresentingmoreh olistic,``complex’’objects.T heformer involvese xtensivepa rtd ecomposition,yieldingmultiplesim plep arts,a nd thelatterinvolveslessd ecompositionandyieldsfewerb utmorec omplex 944 SEKULERANDBEHRMANN parts.B ecausew ordreco gnitioninvolvesex tensivepartd ecompositionand requiresth erepresentationofnumerousparts,m ildd amagetoth e structuraldescriptionsystemsu bservingmultiple-partp rocessinggivesrise top urealexia .Theobviousclaimm adebythisa ccountistha tthede®cit inp atientsw ithp urea lexiaex tendsbeyondalphanumericp rocessing,w ith theconsequenceth atth esep atientsw illalso b eimpairedinp rocessingall objectsth atrequirede composition. Althoughsomeinstantiationoftheperceptualviewh asg ainedin popularity,notallstu dieso npurea lexiah avetakenanexplicitsta ndon theorthographic±p erceptualdistinction.Manyofthesem oren eutral studiesh avesuggested,forexample,th ata de®cit in letter id enti®ca tionis theprimarym echanismg ivingriseto the w ordlen gthe Œectin p urea lexia (Behrmann&Shallice,1995;B ub&Arguin,1995;K ay&Hanley,1991; Reuter-Lorenz&Brunn,1990),orthatletter-by-letterreadingis a consequenceo fadecreasingleft-to-rightgradientofaccuracyin fea ture representations.N oteth attheevidencefro mallth esestud iesis e ntirely compatiblew ithth eperceptualview.Thus,a fundamentalv isual perceptualdisturbancem aygiveriseto th eletterid enti®ca tionde®cito r mayimpairth eevena ndequaldeploymentofresourcesa crossth espatial array.L ikemostresea rchon pu realexia,thesep apersfo cuspredomi- nantlyon th ereadingde®citan ddonotconsiderm orep rimarype rceptual impairments.Im portantly,however,th eya reen tirelyco nsistentwithth e perceptualview. Thegoalofthepresentstudyisto ex plorefurt hertheextentto w hicha generalperceptuald e®citu nderliespurea lexiaa ndtoch aracterise,a sfar aspossible,th eprecisefo rmo ftheperceptualdisorder.If a general perceptualproblemd oesu nderliepu rea lexia,th enth ede®citsfo undin thesep atientssh ouldn otbelimitedto o rthographicm aterials.A lthough moststu diesu tiliseo rthographicstim uliin testin gpurealex icp atients’ abilities,thereh aveonlyb eeno ccasionalattemptsto in vestigateth isissu e usingnonorthographicstim uli(e.g .Farah& Wallace,1 991;Friedman& Alexander,19 84;Kinsbourne&Warrington,1962).M osto fthese attempts,h owever,h aveb eenstu dieso fsingleca sesa nd,therefore,th e generalisabilityo ftheperceptualaccountremainsunclear.Fu rthermore,in manyofthesed ocumentedca ses,th ebrainle sionsaren otcircumscribed, andinso me,th eye xtendbeyondthearean ecessaryto p roducep ure alexia.Itis co nceivable,th erefore,th atthesecase sexhibitm ore widespreadfunctionald e®cits th anwo uldb efoundind istinctcaseso f purea lexiaa ndthatth iswo ulda ccountfortheco-occurrenceofp ure alexiaan daperceptualde®cit.In th isp aper,we avoidth isp roblemb y investigatingthebehaviouroffourpatientsw ithp urea lexia,som e (althoughnotall)o fwhomhavefa irlyrestri ctedlesio ns,in o rderto a ssess howgeneralisableth eperceptualde®cit m ightbe.In ad ditionto PERCEPTIONINPUREALEXIA 945 establishingthataperceptualdisorderexists in th esep atients,w ealsotest whetherth esep atientsh avea particulardicu ltyin rep resentingm ultiple partso fobjects.If, b yFarah’saccount,th ede®cit in p urea lexiaresu lts fromageneralinabilityto d ecomposestim ulia ndtorep resenttheir numerousparts,th enp atientswith pu realex iash ouldh avedicu ltieso n nonorthographicta sksrequiringpartd ecompositionandrepresentation.

METHOD Subjects Fourexperimentalandeightcontrolsu bjectsp articipatedin a llth e followingexperiments.A nydeviationsfromthisa red escribedun derthe particularexperiment.A llag reedto p articipatein th isresea rch.

E x p e rim e n ta l S u b je c ts Allfo urexperimentalsubjectsw ererig ht-handeda ndwere¯uent Englishsp eakers.M A,a37-year-oldfem ale,su staineda closed-headinjury fromacaraccidentin1 991tha tresultedin a righthomonymous hemianopiaa nddicu ltyrea ding.Atthetimeoftheaccident,M Awas employeda sanaccountantina largebank.Nofocallesio nwasevidenton neuro-imagingalthoughEEGrecordingsshoweda bilateralslowingover thefrontallobesa ndanHMPAOSPECTshowedmildlyd ecreased cerebralperfusionbilaterally.Theabsenceo fafocallesionisn ot surprisinggive ntheaetiologyo fthede®c it,b uttherighthomonymous hemianopiais co nsistentwitha posteriorleft-hemispherele sion.MA scored5 7/60ontheBostonNamingTest,re¯ ectin gnormalp erformance. Thereis n oovertev idenceo faphasiaan d,asidefromsomehesitationsand word-®n dingdicu ltiesinsp ontaneousspeech,he rlanguageis ¯ uent.O n writingsinglew ordstodicta tion(usingasubseto fthelevelso fregularity listsfro mShallice,W arrington,&McCarthy,1983),MAmadeseveral regularisations,in cludingw ritingCAULIFLOWER ® colly¯ower, AISLE ® illea ndSEIZE(disambiguatedth roughcontext) ® seas. These errorsto ex ceptionwordsre¯ectsurfa ced ysgraphia,apatternth atis sometimesseen in a ssociationwithp urealex ia(F riedman&Hadley,1992; Patterson&Kay,1982).Whenb lack-and-whitelin edrawingsofsingle objects(Sn odgrass& Vanderwart,1 980)w ereshownonacomputersc reen andthetimeton ametheobjectswasmeasured,MAmadefewerro rsin objectidenti®c ation,butwasslo werth anm atchedn ormalcontrolsubjects, particularlyfo ritemsofhigh ,relativetolo w,visualcomplexity (Behrmann,personalo bservations).M Awasableto id entifyall sin gle letterso fthealphabetwithouterro rat33msecdurationwhenth eyw ere presentedtoth elefto f®xation. 946 SEKULERANDBEHRMANN

TU,a56-year-oldm ale,h asbeenstu diedp reviouslyb yFarahand Wallace(19 91).InN ovembero f1989,hesustaineda lefto ccipital haemorrhagesecondaryto a ruptureda rterialvenousmalformation,which wasresected.TUshowedanimpairmentinrea ding,arighthomonymous hemianopia,a ndarighthemiparesis.D amagein th elefttem porallobe wasrevealedb ypost-surgeryC Tscan.TUwasableto id entify80 %ofall singlele ttersp resentedto h isin tactleft h emi®eld at 3 3msecduration.TU namedco rrectly22 o utofthelast3 0itemsontheBostonNamingTest (Goodglass,K aplan,&Weintraub,1983)an d3additionalitemswith cueing(Farah&Wallace,1992).Hedid,however,ex hibitm arkeda nomia forfruitsa ndvegetablese venwh enn amefrequencya ndfamiliarityw ere takenin toac count.D espiteth is,therew asnoo vertev idenceo faphasia andTUconversed¯ uentlya ndeŒor tlessly.TUhasa high-school educationandworkeda sarailroadinspectorpriorto his in jury. Followinghissu rgery,heworkedo ccasionallya tasmallfa ctory. DSisa 37-year-oldfe malew hosu Œereda posteriorcerebralartery occlusioninla te1 986.A CTscand onea tthetimerevealedaninfarction ofthelefto ccipitallobe,whichw asprobablym igrainousinorigin .DS suŒeredfromrighthomonymoushemianopiaan darighthemiparesis.T he latterreso lvedso onaftertheincident,andtheformerg raduallyp rogressed intoa quadrantanopsiain h erupperrig htv isual®eld.Herrea dingskills wereim pairedfollo wingtheCVA,butotherla nguageskillsre mained intacta ndtherewa snoevidenceo faphasia.DSidenti®ed c orrectly5 8/60 singleu pper-caselette rsp resentedfo r17msecea chto th elefto f®xation. Asidefromasmallnu mberof erro rso ntheBostonNamingTest,sh e labelledall ite mscorrectly.Neuropsychologicalin vestigationseight monthsafterh erstro kerevealedm ildim pairmentsin a ttention, concentration,andverballearning.D S’swritingis u nimpairedwhen evaluatedo ntasksrequiringhertow ritesin glew ordsto d ictation(even thoseth athaveirregularspelling±so undcorrespondenceso ntheShalliceet al.,1 983list)an donthewritingtasksoftheW esternA phasiaBa ttery. Thisp atientisd escribedin g reaterd etailin Beh rmann,Black,a ndBu b (1990)andBehrmannandShallice(1 995).D Soriginallyw orkedas a nurse.F ollowingherstro ke,shecontinuedh erw orka sahome-maker raisingherch ildren.Morerecen tly,shehasattemptedtolea rnto ty pebut hasfoundthisla boriousandpainstaking.Asinth ecaseo fMA,SDis accurateb utslowerth anco ntrolsubjectsin h ernamingofblack-and-white drawings,p articularlyfo robjectso fhighvisualcomplexity. MWisa 67-year-oldm alem inisterw hosustainedalefto ccipitalCVAin Aprilo f1992thatresultedin slo wedrea dingability.Readingandwriting hadalwaysplayeda centralro lein h islife a nd,atthetimeofthestroke,he wasengagedin w ritinghism emoirs.M Wwasrec eivingtreatmentfor ensuingdepressionduringtestingforthisstud y.MWwasalsoslo werth an PERCEPTIONINPUREALEXIA 947 normalsubjectsin n aminglinedrawingsb utdidn otshowanobvious diŒeren ceforthoseofhighvs.lowcomplexity.Hewasnotobviouslya phasic andwas¯uentandexpressiveinh issp ontaneousspeech.M Wwasab leto identifyall sin gleletter spresentedto left o f®xationat50msecd uration.

C o n tro l S u b je c ts

Controlsu bjectsw ere3 malesan d5femalesa ged3 4±67witha mean ageof50.5(sta ndarde rror[S E]=1.77).Subjectsw ererec ruitedfro mthe volunteerp oolatth eRotmanR esearchIn stitutea tBaycrestG eriatric Centre,T orontoa ndwerem atchedto th epurea lexicp atientson a geand educationlevelsa sfara spossible.M eaneducationlevelwas13.4y ears (SE=0.63).Allsu bjectsw ererig ht-handeda ndhadnormalorcorrected- to-normalvision.Nonehadahistoryofreadingdicu ltyn orofa neurologicald e®c it.

Twoinitialexperimentsw ereca rriedo uttoesta blishth atthefour patientsw erein deedletter -by-letterreaders.In the seex periments, performancew asasse ssedo nanaminglatencyandalexicaldecisiontask forwordsofvaryinglength.Toobtainin formationonnormalperformance ontheseta sks,co ntrolsu bjectsa lsoco mpletedth eexperiments.It w as expectedthatth epurea lexicp atientsw ouldsh owanincreasein resp onse latencyw ithin creasingwordlen gth,whereastheeŒect o fwordlen gtho n normalsubjects’rea ctiontimesw asexpectedtob eminimal.

EXPERIMENT1:READINGLATENCIES Subjects Allsu bjects,with th eexceptionofcontrolsubjectK A,participatedin th is experiment.

M aterials A p p ara tu s AMacintoshS Ewithsta ndardb uilt-in9 Í Í screenwa susedin th e readingassessments.T heexperimentsw erecrea tedu singPsychLab software(Bub&Gum,1989,v ersion0.85).Verbalresponsetim eswere recordedviaa microphoneandverbalresponsere laysystem.

S tim u li

Twolistso f60wordseachw ereco nstructed,andeachlist co ntained equalnumberso f3-,5-,and7-letterwords.A llw ordswerep resentedin 948 SEKULERANDBEHRMANN upper-caseG eneva2 4-pointboldfo ntinb lacko nawhiteb ackground. Thevisualanglessu btendedfo r3-,5-,a nd7-letterwordswere approximately0 .5 Ê verticallya ndapproximately1.5 Ê , 2.4Ê , and 3.6Ê horizontally,resp ectively.W ordfreq uencyw ascontrolledac rossth e3 letterstrin glengthswitha nequalnumberof high -andlow-frequency wordsperlen gth.High-frequencyw ordswereth oseth ato ccurredmore than20tim esperm illion;low-frequencyw ordsappearedlessth an2 0 timesperm illion(KucËera& Francis,1 967).Theentireset o f120w ords hadameanwordfre quencyo f52(S D=70).Halfth ewordswereab stract withth eremaininghalfco ncrete.T hesesa mewordlists ha vebeenu sedin severalstudiesw ithp urealex icp atients(B ehrmanneta l.,1 990;Behrmann &McLeod,1995;Behrmann&Shallice,1 995).

P ro c e d u re Subjectsw erein structedtorea daloudasquicklya ndasa ccuratelya s possiblesin glewo rdsthatappearedo nthecomputerscre en.Individual wordsvaryingrandomlyin strin glengthwe rep resentedin th efollowing sequence:oneachtria l,a ®xationpointappearedin th ecentreo fthe screenfo r1000msec.Th en,500msecfo llowingtheoŒseto fthe®xation point,th etargetw orda ppearedo nthescreenan dremainedth ereun tilth e subjectactivatedthevocal-responsek eyb yreadingthestimulusword aloud.Anintervaloftwosecondsoccurredbetweentrials.Forallsu bjects, wordswerep resentedto th elefto f®xation,correspondingtoth epatients’ intactleft v isual®eld,andthe®nallettero feachwo rda ppearedin th e charactersp aceim mediatelyto th elefto f®xation.Thecomputerreco rded reactiontimesin m sec,an dtheexperimentern oteda nyerrors.P riortoth e experiment,th esubjectsp ractisedo nashortlist o fsixwo rds,n oneof whicha ppearedo nthesubsequentexperimentallists.

Results andDiscussion C o n tro l S u b je c ts

Figure1 showsthemeanrea ctiontimesacro ssth econtrolgroupand foreacho ftheindividualexperimentalsubjects.A nANOVAwitho ne between-subjectfa ctor(group)andonerepeatedm easureswith in-subject factor(wordlen gth)revealedth ecriticalinteractionbetweeng roupand length [F(2,20)=7.3, P < .01],withn oeŒect o flengthfo rthecontrol groupbutasigni®c antwordlen gtheŒ ect fo rthepatients.T hediŒere nce betweenth egroups[ F(1,1 0)=13.4, P < .01]andthediŒeren cea cross thediŒeren tlengths,c ollapseda crossth etwogroups[ F(2,2 0)=7.6, P < .01],werea lsosig ni®ca nt.Th emeanreadinglatenciesfo rthe controlgroupwere68 2,693,and703msecfo r3-,5-,and7-letterw ords PERCEPTIONINPUREALEXIA 949

FIG. 1. Meanreadinglatenci es for control groupa nd individualpatients in msec asa function of stringlength. Slope ism easuredin msec/letter.

respectively.Theslopeofthereactiontimefunctionforreadinglatencies acrossth econtrolgroupwas®vemsec/letter,suggestingminimalchange inrea dingperformancew ithea cha dditionalletter.C ontrolperformance onthista skwa snearp erfect.Aswithth epatientd ata,anyerrorsw ere excludedpriortoa nalysis.A dditionally,anytrialso nwhichth e microphonewasm istriggered(few ertha n1%ofthetrials)w ereex cluded. Becausea nalysesoftheRTsfore achin dividualpatientwillb e conducted,wealsow antedto a ssessth evariabilityin p erformancea cross thecontrolsubjects,a ndthereforep erformedA NOVAsonthedata collectedfromeachindividualco ntrolsubjectwithtria lactingasa randomfactor.O nlyo necontrolsubject,JL,hadsigni®ca ntlyd iŒeren t naminglatenciesfo ranyofthewordlen gths[ F(2,116)=6.02, P < .003] andhisrea dinglatenciessh oweda slopeof10msec/letter.However,JL doesn otshowthesamepatternfo undinp urea lexicp atients,th atof increasingRTasth elengtho fthewordin creases.R ather,h eshowsa largediŒere ncein R Tonlyin th eseven-letterwordco nditionrelativeto 950 SEKULERANDBEHRMANN thethree-a nd®ve-letterco nditions.N oneofthecontrolsubjectsd isplayed resultsind icativeofletter-by-letterreadingandtheouterlim ito ftheRT regressionslope,ca lculatedfro mRTasafunctionofwordlen gth,is 10msecp era dditionalletter.These® ndingsarec onsistentwithp revious datash owingminimaleŒects o fwordlen gtho nresponsela tencyin n ormal readers(B ub& Lewine,1 988).

P u re A le x ic P a tie n ts Ascanbeseen in F ig.1,allp atientssh owasigni®ca ntmonotonic increasein rea dingtimesa swordlen gthin creases.T hepatientsv aryin severity,asin dicatedb ythediŒeren tslopeso f1293,5 41,101,and93msec/ letter.E venth emilderimpairments(1 01msec/letterforDSand9 3msec/ letterfo rMW)aresig ni®can tlyab normalw henco mparedw ithresu lts fromthecontrolsubjectsin th isstu dyandinp reviousstudies(Bu b& Lewine,1 988).T hepresenceo fthewordlen gthe Œect,ch aracteristico f purea lexia,inea cho fthefourpatientsd emonstratesth atthesep atients mayallbe class i®ed as letter -by-letterreaders.

EXPERIMENT2:LEXICALDECISION Tocon®rmfu rtherth atthesep atientsa retru lyle tter-by-letterreaders, theyp articipatedin th elexicaldecisionexperiment,ataskty picallyu sedto revealthewordlen gtheŒ ect inp urea lexia.

Subjects Allsu bjects,with th eexceptionofpatientTUandcontrolsubjectMS, participatedin th ise xperiment.

M aterials A p p ara tu s

Thesameapparatuswasuseda sinthe p reviousexperiment.R esponses werereco rdedusin gtwokeysonthecomputerk eyboard.Subjects respondedu singtwo®ngerso ftheird ominant(right)h and.

S tim u li

Sixtyo fthewordsusedin E xperiment1 wereco mbinedran domlyw ith 60nonwords,a llo fwhichw erecre atedb ychanging1 or2lettersof th e realwords.A lln onwordstrin gswerep ronounceablean dorthographically legal;for h alfth enonwords,th ediŒeren cefro marealwordo ccurredat PERCEPTIONINPUREALEXIA 951 thebeginningofth ewordwh ereastheco nversewa struefortheremaining half.

P ro c e d u re

Subjectsview edin dividualletterstrin gsonacomputerscree n,a ndthe subjectsd ecidedw hethero rnotthestringwasarealEn glishw ord.Atrial consistedo fa®xationp ointthatremainedon th escreenfor o nesecond followedbyablankscreenfo ronesecond.Thenth eletterstringappeared toth eleftof th escreen’scentre,w ithth e®nallettero ccupyingth eposition tothe im mediateleft o f®xation,andthestringremainedvisib leu ntila key-pressresp onsewa smade.Th einter-trialin tervalwaso nesecond. Subjectsresp ondedbypressingeitherth e``,’’or``.’’keyon th ecomputer keyboardfo ra``yes’’or``no’’response.K eyswereco unterbalanceda cross subjects,a ndsubjectsp erformedpra cticetrials to fa miliariseth emselves withth eappropriateresp onses.S ubjectsw ereto ldto co mpleteth etaska s quicklya spossiblew ithoutsacri®cin gaccuracy.

Results andDiscussion Figure2 showsreactiontim esfo rthethreepatientsa ndforthecontrol subjects.A nANOVAwitho nebetween-subjectfa ctor(group)andtwo repeatedm easureswithin-subjectfa ctors(ju dgementÐyes/ no;wordle ngth) withR Tsforc orrectresp onseso nlya sthedependentmeasureshowedth at thepatientsw eresign i®c antlyslo wertha nthecontrolsubjectso verall [F(1,8)=15.5, P < .01]and,collapsedac rossb othg roups,th etimeto make``no’’judgementsw asslo werth anthecorresponding``yes’’ judgements[ F(1, 8) = 9.5, P =.01].Thisju dgementdiŒeren cew asalso exaggeratedasa functiono fwordlen gthac rossb othg roups[ F(2, 16) = 7.2, P < .01].Again,independento fgrouptherew asa signi®ca nt eŒect o flength[ F(2,1 6)=8.5, P < .01],withall sub jectssho wingthe tendencyfo rslowerR Tsaslen gthin creasedbut,p redictablya ndmost importantly,thiseŒ ect w asdisproportionatelyla rgerin th epatientthan thecontrolgro up[ F(2,16)=8.5, P < .01].Co ntrolsubjectso btained groupmeanlexicaldecisionscoresfo rrealwo rdsof714,714,an d836msec for3-,5-,and7-letterwords,resp ectively.Theincreasein w ordle ngth manifesteditselfo nlyb etween5 and7lettersandtheregressionslopewas 31msec.A lthoughthisin creasein R Tisn otalwaysseeninn ormal subjects,m anystu diesd osuggesta slightlin earincreasea crosslo nger strings(e.g.F rederiksen&Kroll,1 976;S eidenberg,Waters,S anders,& Langer,1 984).Theincreasesin R Tlatencyforthe3patientsa rea ll dramaticallya ndsigni®c antlylarg erth anthoseo bservedin th enormal subjects(see F ig.2),withslo peso ntheordero f1019,304,and119fo r 952 SEKULERANDBEHRMANN

FIG. 2. Lexicaldecisiontest: Meanrea ctiontimes for correctresponses to realwordstim uli asafunction of stringleng th for three individualsubjects and control group.

subjectsM A,MW,andDS,respectively.T hisw ordle ngtheŒ ect in lexica l decisioncon®rmsthe®ndingsfromthepreviousexperiment. Takentogether,th eresultso fthenaminglatencya ndlexicaldecision experimentsesta blishth atthefourpatientsa reletter -by-letterreaders;a ll patientssh owthehallmarkin creasein rea ctiontimewithan in creasein wordle ngth.Althoughsomeofthenormalsu bjectsa lsosh owaslight eŒect o fstringlength,theslopeoftheincreaseis co nsiderablysm aller thanthatobservedin a nyofthepatients.H avingestablishedth atthe patientsq ualifya spurea lexicsu bjects,Experiments3± 5 investigate whetherth esep atientsh aveageneralperceptualproblemthatm ay underlieth eirp urealex ia.Theseex perimentsu sem ostlyno northographic stimulifo rwhichrea dingisn otrequired.WhereasExperiment3simply examinestheperformanceo fthesubjectso ntestso fperceptualspeeda nd ¯uency,Experiments4 and5ared esignedmoresp eci®ca llyto ex amine theissueofpartrep resentationanddecompositionofnonorthographic stimuli. PERCEPTIONINPUREALEXIA 953

EXPERIMENT3:PERCEPTUALFLUENCY Themainp urposeo fthisex perimentwasto d eterminewhethera perceptualleveld e®cit in pu realexialim itsp atients’ab ilitiesto p rocess multipleob jects.Su bjectsco mpletedth reetim e-limitedtests in w hichth ey werereq uiredto p rocessm ultiplen onorthographica ndo rthographic stimuli.Th ethreep erceptualspeedtests in cludetheFindingAs,N umber Comparison,andIdenticalPicturestests fro mthe Kito fFactor-Referenced CognitiveT ests developedbyEkstrom,French,andHarman(1976). Successo ntheseta sksreq uiresra pidp rocessingofmultipleo bjects.Ifa perceptualde®c itis in volvedin p urea lexia,th enepatientsw ouldb e expectedtop erformsig ni®ca ntlyw orseth anthecontrolsubjectso nall threetests. FarahandWallace’s(1991)singleca sestu dyshowedth attheirp ure alexicp atientwasimpaireda tbotho rthographica ndnonorthographic portionsofthistest; T Uperformedp oorlyo nallth reesec tionsofthistest, whichim pliesth athisp rocessingdic ultiesw eren otrestrictedto orthographicstim uli.H owever,T Uhasadditionalmemoryd e®cit s(see Farah& Wallace,1 992),soh isp erformancem aynotbetypicalofother purea lexics.T herefore,thesetest swerea dministeredtoa llth epatients, andtheirsc oresco mparedw ithth oseo fthenormalcontrolsubjects. Becauseth edependentmeasuresfo rthetestsd iŒer, th edatafro meach testwa sanalysedsep arately.

Subjects Allfo urpatientsa ndm atchedc ontrolsubjectsto okpartin th is experiment.A llsu bjectsw eretested in dividually.

M aterials Allp artso fthise xperimentwerep erformeda spaper-and-penciltests. T he experimenterk epttim ewitha hand-heldsto pwatch.Alltests w ere administeredan dscoreda ccordingto th estandardisedin structions providedb ytheauthorso fthetests.

F in d in g A s

Stimulia ndPro cedure. Subjectsreceivedlis tso flower-caserea lword letterstrin gsandwerein structedtom arka nywordco ntainingtheletter ``a.’’Subjectsw ereco rrectlyinformedth ateachco lumncontained® ve targetw ordscontainingth eletter``a .’’Ine achof2experimentalblocks, subjectsw ereg iven4 pages,ea cho fwhichcontained5columnsofwords with2 1wordsineac hcolumn.Althoughthepositionofthe``a’’inth e wordsisn otperfectlyco ntrolledfo ritslo cusa tthebeginningorendof 954 SEKULERANDBEHRMANN theword,informalobservationshows thatitd oesv aryacro ssth estring reasonablysy stematically.Subjectsh earda standardset o finstructions describingthetest (E kstrometa l.,1 976).Tes tscoresw ered eterminedb y thenumbero fwordscorrectlymarked,andnopenaltyw asgivenfor incorrectresp onses.Th eexperimentersin formedsu bjectso fthisan dtold themto u sethe m oste cientstra tegya vailable.A ftera practiceset o f16 trials,sub jectsh ad2 minutesto co mpleteeac htestb lock.Breaksweren ot givenb etweenb locksa ndnofeedbackw asprovided.Ina ccordancew ith thestandardisedsco ring,th e®nalsco refo rthissectio nwasthemean numbero fwordscorrectlymarked,averagedacrossth etwoblocks.

N u m b e r C o m p a riso n

Stimulian dProcedure. Pairso fdigitstring s,ran gingfrom2to1 3 digitsin len gth,werep resentedin th iste st.E acho fthe2blocksconsisted of48pairsin 2 columnsonasinglep age.Sixteenp racticetrialsw ereg iven beforeth etwoexperimentalblocks.A llsu bjectsh earda standardset o f instructionsb eforeco mpletingthepracticetria ls.Th eexperimentertold subjectsto m akea markb etweenth epairso fdigitstrin gsthatwere diŒeren tandtoig noreth osep airsth atwereth esame.A gain,thelocusof thediŒeren cefo rthediŒere ntpairsw asn otcontrolled,butdiŒeren ces occurredmoreo ftenat th emiddlea ndendthanatthebeginning.Subjects wereg iveno ne-and-a-halfm inutesto co mpleteea cho fthetwoblocks. Breakswerenotgivenb etweenb locksandnofeedbackw asp rovided.One pointwasgivenfo reachco rrectanswer,a ndonepointdeductedfo reach incorrectan swer.Th eexperimenterin formedsu bjectso fthescoring procedureb eforeth etestb egan.Thedependentmeasurew asthenumber correctminusthenumberincorrect,a ndameanwascalculatedac rossth e two blocks.

I d e n tic a l P ic tu re s

Stimulia ndP rocedure. Eachtria lconsistedof ® veshapesina row, witha cueinth eleftmostp osition,andonetargeta ndthreedistracter shapeso ntheright.S ubjectsm arkedth etargetob ject,wh ichw asthe shapethatmostclo selyresem bledth ecue.S ubjectsco mpletedfour practicetria lsb eforeb eginningthetwoexperimentalblocks.Fo rty-eight experimentaltria lsa ppearedin ea chb lock,foratotalo f96trials.E ach blockco ntained2 columnsof12trialson ea cho f2pages.Th epositionof thetargetwasvarieda crossth edistracterp ositions. Instructionsandscoringfollowedp rocedureso utlinedin th etest manual,an dsubjectsk new thescoringprocedurea headoftime.S ubjects indicatedth eirresp onseb ymakingamarku nderneathth eitemm ostlik e thecue.O nea nda halfm inutesw ereallo wedfor th ecompletionofeach PERCEPTIONINPUREALEXIA 955 oftheb locks.F ollowingtheestablishedscoringprotocol,th edependent measurewa sobtainedb ysubtractingone-quartero fapointforeach incorrecttrial fro mthetotalnumbero fcorrecttrials .

Results andDiscussion Acomparisono fthegroup®ndingsrev ealedthatpatientspe rformed signi®ca ntlym orep oorlyth ann ormalcontrols;th isis ev identfro mthe diŒeren cesin th emeans,a sdisplayedin F ig.3.A one-wayANOVA revealedsign i®can tmaineŒ ects o fgroupforeachofthethreetests: FindingAs:[ F(1,14)=26.73, P < .0001];NumberComparison:[ F(1, 14) = 24.41, P < .0002];a ndPictureId enti®c ation:[ F(1,14)=19.63, P < .0006].Allo fthepatients’in dividualscoresw erelo werth anco ntrols onthesete sts.A sFaraha ndWallaceo riginallyfo und,TU’sscoreso n thesetests w ereim pairedre lativeto co ntrols.T U’soriginalsco reswe re1 5, 7,and27forFinding``A ’’s,N umberC omparison,andPicture Identi®c ation,respectively,whereasthistim etheyw ere10,8 ,and15. Althoughhisp erformancediŒers sligh tlyac rossthe tw ooccasions,h is performancew asatlea stas im paireda tthetimeofourtestingasitw as wheno riginallytested b yFarahandWallace(19 91).Therewe reve ryfew falsep ositiveerrorsm adebythesu bjectsa ndtheirpa tternis ch aracterised morein term sofslowthaninaccuratep erformance. Takentogether,th isseries o fthreetests,tw oofwhichin clude nonorthographicstim uli,sh owsthatallp atientsp erformsig ni®ca ntly morep oorlyth anthecontrolsubjects.T heseresu ltsin dicateth atpure alexiais n otspeci®c torea ding-relateditem sandpurea lexicsh aveslowed orimpairedp rocessingonspeededp erceptualtaskswithno northographic aswella sorthographicm aterials.It m aybethecaseth atth epurealex ic patientsw ereim pairedsp eci®c allyb ecauseo fthetests’tim econstraints, andthattheywo uldpe rformperfectlygive nunlimitedtime.In fa ct,som e researchersh avesuggestedth atp urealexiais a de®cit in th erapid processingofvisualmaterial(Friedman& Alexander,1 984),a ndthis accountcanadequatelyex plainp oorp atientperformancein th eperceptual speedta sks.T hisissu eisa ddressedbyExperiments4 and5,whichd onot relyo nspeededta sksbu tonpatternsofperformancew ithineac hsubject group. Thegroupstudyresultso fExperiment3extendthoseo riginally reportedb yFaraha ndWallace(1991)andshowthatth eeŒect ca nbe generalisedto o therpurea lexicp atients,a ndisn otduesolelyto T U’s memoryd e®cit s.R esultsfro mExperiment3indicateth ataperceptual componentmaycontributeto th ede®cit u nderlyingpurealexia .Further experimentsw erec onductedto iso lateth echaracteristicso fsucha perceptualde®cit. 9 5 6 S EK LRULER A DND B HEH R M ANN

FIG. 3. Raw scores for patients and control subjects on the three perceptual speed tasks in Experiment 3. PERCEPTIONINPUREALEXIA 957

EXPERIMENT4:PARTPROCESSING Thepreviousexperimentdemonstratedth ataperceptualimpairment occursin p urealexia.Next,we exploreth echaracteristicsofthat perceptualimpairment.F arah(1991,1 992,1994)recentlysu ggestedth at themainp robleminpu rea lexiacen tresa roundthepatients’in abilityto decomposea nobjecta ndtore presentthemultiplep artso fsucho bjects. Thecurrentexperimentexplorespurea lexics’ perceptionofobjects containingmultiplep artsb yhavingsubjectsd etectamisorientedtarget ind isplayswithin creasingnumbero fparts.It is a rguedth atpatientsh ave dicu ltyin tegratingseverallettersintoa wholew ordw hilere ading.How doesth econ®gurationofan nonorthographic objectaŒ ect p urea lexics’ abilitiesto in tegrateth eparts? Themethodusedto e xaminemultiple-partp rocessingisad aptedfro m recentworkb yDonnelly,Humphreys,a ndRiddoch(1 991),inw hichth ey exploredtheabilityo fnormalsubjectsto d etecta targetasthenumbero f partsp resentinth edisplayincreasedandasth econ®gu rationoftheobject wasdisruptedbyeliminatinggoodcontinuationo ftheparts(the ir Experiments1 and3).Exampleso ftheirstim uli,u sedin th epresent study,aresho wninF ig.4.Thestimulifro mtheseex perimentsp rovidea goodopportunityto in vestigatesim ultaneouslyh owpatientperformanceis

FIG. 4. Examples ofstimuli fromExperiment4:(a )four parts,go od con® guration;(b) ®ve parts,go od con® guration;(c) sixp arts,go od con® guration;(d)four parts, poorcon® guration; (e) ®vep arts,p oorcon ®guration;(f)sixpa rts, poorco n® guration.All stimuli shownare ``target present’’trials. 958 SEKULERANDBEHRMANN aŒectedb ythenumbero fpartsin a nobjecta ndbytheperceptual characteristicso fthestimuli.In terestingly,theirp atientHJA,whoexhibits visualobjecta gnosia,,a ndalexia,p erformedp oorlyo n suchd isplays,lea dingthemto su ggestth ath ehasa de®cit in g rouping visualfeaturesin p arallelacro ssvisu alfo rms(Humphreyset a l.,1 994). Becauseth eexperimentdescribedhe reis a con¯ationofExperiments1 and3oftheoriginalDonnellye tal.(1 991)ex periments,it is n otpossible toco mpareth edatao btainedh erewith th atpresentedinth eirstu dy,b ut wecomparep erformanceto th ematchedco ntrolsubjects. Thepredictionforthepurea lexicp atients’p erformanceon th istask is thattheirta rgetd etectiontimesw illbe sign i®ca ntlya Œectedbythenumber ofpartsp resentinth edisplay.Thism aymanifestas a maineŒ ect o fparts ortheymayshowaninteractionbetweennumbero fpartsa ndstimulus ``goodness.’’Ifth iso ccurs,th einteractionwillb emoreex aggeratedth an thatobservedin th enormalsubjects;b ecausep artp rocessingisa ssumed tob emorep roblematicfo rthepatients,o nemightexpectth atina situationinw hichth ereis n o®guralgoodnesso rinw hichp erceptualcues arew eaker,the d etectionofamisorientedta rgetw illb edisproportionately dicu ltfo rthepatients.Ex periment4,therefore,ex tendstheresultso f Experiment3 byspeci®ca llyex amininghowcertaingestaltp ropertiesand thenumbero fobjectpartsa Œectpatients’ab ilitiesto p erformsu ccessful integration.

Subjects Allfo urpatientsan deightcontrolsubjectsp articipatedin th isex periment.

M aterials A p p ara tu s

Thisex perimentutilisedthesamecomputera ndsoftwarea sin Experiment2 .

S tim u li

Exampleso fthestimuliare sh owninF ig.4.Stimuliv ariedo nthe followingtwodimensions:th enumbero fpartsan dtheobject’scon®gural goodness.F our,®ve,orsixp artsco mprisedeachobject(seeFig.4 a±c). Inrela tionto w ords,th enumberof p artso fano bjectm aybeanalogous toth enumbero flettersina word,albeitat a somewhatdiŒeren tlevelof complexity.Thepartsh adre gularinter-itemsp acingÐthatis,th eyw ere the``regular’’displaysÐa ndallh adclosure.O ntheseconddimension, objectsw eree itherw ell-o rpoorly-con®gured.Well-con®guredstim uli hadtheadditionalgestaltp ropertyo fgoodcontinuation,w hereasth e PERCEPTIONINPUREALEXIA 959 poorly-con®guredstim ulid idn othavegoodcontinuation.Instim uliw ith goodcon®guration,imaginarystraig htlinesco uldb edrawnfrom adjacentcornersto fo rmco mpleteo bjectssu cha sasquare,a ``house,’’ orahexagon.Tocrea testim uliw ithp oorco n®guration,thecornerso f thegoodcon®gurationstimuliwe reea chro tated15 Ê counter-clockwise. Thism anipulationdisruptedthe in tegrityo fthelarger®gure(S eeF ig. 4d±f).T heg oodco ntinuationdisplayscorrespondtoth eregulardisplays fromDonnellyet a l.(1 991)E xperiment1,an dbothp resenta ndabsent trialsa rein cluded,whereasthepoorc ontinuationdisplaysco rrespondto theregulardisplayso fExperiment3andincludebothta rgetpre sentand absenttrials. Thesubject’staskw astod etectthe p resenceo fatargetin th estimulus. Targetsw erecrea tedb y¯ippingoneparto fanobjectalongitsh orizontal andverticalaxes(se eFig.4).T hisresu ltedin th evertexofthetarget pointinginwardstowardsthecentreo ftheobject.Ontargetabsenttrials allof th everticesfa cedou twards.If th epatient’sabilityto in tegrate objectsis a Œectedbythetypeso fperceptualcuesa vailable,th enth eir responsetim estop oorly-vs.w ell-con®gureditemsshouldb edispropor- tionatelylon gerthancontrols’ responses.S ubjectswe reg iven24 p ractice trialsb eforeth eexperimentbegan.Stimuliw erep resentedin 3 blocksof 144randomisedtria ls,co mposedo f12repetitionsofeachco mbinationof numbero fparts × targetp resence × con®gurationtype,fo ratotalof432 trials.

P ro c e d u re

Theprocedurefo rthisex perimentw asadaptedfro mthatoriginally usedb yDonnellyet a l.S ubjectsd eterminedw hethera targetw aspresent oneachtrial. To a idin th ista sk,theyw ereto ldto a skth emselves,``iso ne cornerp ointingin?’’Theyrespondedbypressingthe``,’’ and``.’’keyson theMacintoshk eyboardusin gtheindexa ndmiddle® ngersof the irrig ht hand.Ifthe sub jectsp erformedE xperiments2 and4inth esamesession, botho fwhichrequiredk eyboardresp onses,the ntheallocationo fthekey- responsem appingswask eptconsistenttoa voidco nfusionandto maximiseco rrectresponses.O therwiseth emappingwasrandomly determinedfo rthepatients,a ndthecontrolsubjectsu sedth esame mappingsasth eirco rrespondingmatchedp atients.O neachtria l,a stimulusappearedat th ecentreof th escreenim mediatelyfo llowingthe oŒseto fa500msec®xationpoint.Th estimulusremainedonthescreen untilth esubjectresponded.Thecomputerreco rdedR Tandaccuracy.T he delayb etweentrialsw as1000msec.S ubjectsd idn otreceivefeedback duringtheexperiment,an dtheyto okbreaksinb etweenblocksif th eyso desired. 960 SEKULERANDBEHRMANN

R esults Thefour-wayinteractionbetweentarget(a bsentvs.p resent),c on®guration (goodvs.p oor),p arts(fou rvs.® vevs.six )andsubjectgroup(p atientsv s. controls)d idn otrea chsig ni®ca nce[ F(2,20)=2.8, P > .05],suggesting thattherew asnodiŒeren tialeŒect o fthesev ariableso nthepatientsv s. controlsubjects.P erformanced idn otdiŒer fo rtargetpresentvs.a bsent trials [F(1,1 0)=1.28, P > .05],nordidth isd iŒer a crossth etwogroups [F(1,10)=0.2, P > .05],andsoth edata,showninFig .5,arec ollapsed acrossth isva riablefo reaseof v iewing. Asagroup,patientsw ereslo werth ancontrolsubjectsb y203msec [F(1,10)=7.6, P < .05].Also,meanRTstop oorcon®g urationdisplays were1 68msecslo werth antog oodcon®guration[ F(1,1 0)=32.9,

FIG. 5. Results fromExperiment4.M eanreactio ntimeforpatient and control group results ontarget presenttrials. Goodcon® guration stimuli hadgood continuation,andp oor con® guration stimuli lacked goodcontinuation. PERCEPTIONINPUREALEXIA 961

P < .001],andthisd iŒeren ceh eld,especiallyfo rabsenttrials[ F(1, 10) = 6.0, P < .05].Them ajorresultis th at,a sisev identfromFig.5, performanceo ntrialsw ith® vepartswa sslowerthanth atwitheith er fourorsixp arts[ F(2,20)=5.9, P < .01],butthish eldeq uallya cross thetwogroups,[ F(1,10)=0.2, P < .05].Thisu nexpectedin creaseis presumablya ttributableto th enonstandardco n®g urationofthe®ve- parttria lsa ndtheaddedd ic ultyo fdiscriminatingbetweenth e elementso ntheabsenttrials.ThisdiŒ eren cefo r®ve-parttria lsis ev en moreo bviousforpoorthangoodcon®gurationtrials[ F(1,10)=3.8, P < .05].Importantly,performanceforpatientsa ndcontrolsis sim ilar andthereis n olinearincreasein R Tasthenumberof p artsin creases. Aninteresting®ndingisth atpatientswe red isproportionatelyslo wer thancontrolsin resp ondingtoo bjectsw ithp oorvs.g oodcon®guration [F(1,10)=4.1, P > .05];whereascontrolsubjectsw ereo nly1 31msec slower,pa tientsw ere2 76msecslo wer.Th ediŒeren ceb etweenp atient andcontrolsubjectresp onsesw asdisproportionatea ndnotaccounted forbyth efactth atth epatientsh adhigher y-interceptsth anth econtrol subjects.

Discussion Thisex perimentcomparesth eperformanceo fthepurealexic p atientsa nd controlsubjectson a perceptualtaskin w hichp erformanceisen hanced whenm ultiplep artso fanobjecta rein tegratedinto a coherentwh ole. Patientswe redisp roportionatelyslo wedin re spondingtop oorcompared withh ighco n®gurationstimuli.M ostim portant,h owever,wa sthat patients’p erformancewa sunaŒectedb ythenumbero fpartsto b e integrated.Theseresultsa rec onsistentwithth etheoryth atthepatients haveageneralperceptualde®cit th atisu nmaskedw henin ternal perceptualcuesare d iminished.Inad dition,theresultsd onotp rovide obvioussupportfo rFarah’smultiplep artre presentationtheory.T his theoryp redictsapositivelyin creasingrelationshipb etweenR Tand numbero fpartso raninteractionbetweenp artsa ndgoodnesso fthe stimulusdisproportionateto th atseenin n ormalsu bjects.T hisex peri- ment’sresultssh owthatsu bjectsresp ondedequallyw ellto stim uliw ith fourandsixp artsreg ardlesso fthecon®gurationtype.Thelacko fa systematicin creasein R Tasthenumbero fpartsin creasedsu ggeststh at thepatientsw erea bleto in tegratep artsin tow holes.R esultsfro mthis experimentindicateth atp atientsa reab leto c onstructwhole® guresfro m partsw henstim ulia ren onorthographic,b utthatth isa bilitym aybe fragilea ndmaybedisruptedb ychangesin o bjectstructureo rinth e absenceo fperceptualcueslik egoodcontinuation. 962 SEKULERANDBEHRMANN

EXPERIMENT5:OBJECTADVANTAGE

Resultsfro mthepreviousexperimentsin dicatethatthepurealexicp atients suŒerfro msomegeneralunderlyingperceptualde®cit th atbecomesmore obviousunderco nditionsinw hichth ereis less sup portfo rperceptual processing,su chas w heng oodcontinuationisa bsentfromth eobject. Patientsw ered isproportionatelyim pairedw henp erformingtasksonitems withoutgoodcontinuation,buttheyw eren otdiŒ eren tiallyaŒ ected b ythe increasein th enumbero fpartso ftheobject.T heabsenceo faparte Œect suggeststh atth epatientsw eren otconductingpart-basedse rialsearchesto performth etask.However,this d oesn otnecessarilyin dicateth atpatients were fully integratingtheobjects’pa rtsto so lvethetask. Onewaytoex plorefu rtherp urea lexicp atients’o bjectpartsy nthesis abilitiesis to e xaminewhetherp atients’fe atureco mparisonperformance bene®ts n ormallyfro mintegratingmultipleelem entsin toa singleob ject.A well-documented® ndingistha tnormalsubjectsa reb ettera bleto co mpare orreporttw ofeatures(orelements)o fadisplaywhenth efeaturescome fromthesameobjectth anwhenth eyc omefromtwodistincto bjects (Duncan,1984).Ifp urea lexicp atientsa rea bleto in tegrateo bjectfea tures, theysh ouldsh owthesamesingle-objectsu perioritya sdon ormalsubjects. However,if p atientsh avedicu ltyrep resentingtheindividualelements,a s suggestedb yFarah,th entheym aynotnormallyb ene®t inco mparingtwo featuresfro mthesameobjectv s.tw ofeaturesfromtwodiŒeren tobjects. Experiment5 usesth islo gicto d eterminewhetherth epatientsw erea bleto integrateo bjecte lementsin tow holeo bjects.T hisexp erimentalsofu rther exploredtheissueofhowperceptualcuesa Œectp atients’ab ilitiesto integrateo bjects. Thestimuliin th epresentstudywe rea daptedfromBehrmann,Zemel, andMozer(su bmitted).In th isex periment,su bjectsm adesame/diŒeren t judgementso fthenumbero fbumpsthata ppeareda ttwooffourpossible endsofastimulus.T hebumpgroupsareco nsideredto b eelementso fthe objects.E xampleso f``same’’and``diŒeren t’’trialsa resh ownforeacho f thethreetest co nditionsinFig. 6 ,withth e``same’’and``diŒeren t’’inth e ®rsta ndsecondcolumns,resp ectively.Stimulia1 a nda2areexa mpleso f thesingleo bject(n onoccluded)condition,inw hichb umpsarelo catedo na single,continuousbar.S timulib 1andb2areex amplesfro mthetwo- objectcondition.Inth isco ndition,thebumpswerelo catedo ntheen dsof twodiŒeren tbars.S timulic1 a ndc2re presentamoreco mplicatedsin gle object(occluded)conditioninw hichth etwodisparateb ars,a lbeitfro m thesameobject,a respa tiallyd iscontinuous. Behrmannetal.’sresultsin dicateth atn ormalsubjectsare fa sterat makingjudgementsin th esingle-objectco ndition(a)th aninth etwo- objectcondition(b),replicatingtheadvantageforsingleo vertw oobjects PERCEPTIONINPUREALEXIA 963

FIG. 6. Examples of stimuli fromExperiment 5:(a1)±(c1) ``sam e’’trials; (a2)±(c2 )``diŒeren t’’ trials; (a1)±(a 2)sing le nonoccluded condition; (b1)± (b2)two-object condition; (c1)± (c2) single occluded condition. 964 SEKULERANDBEHRMANN originallyd emonstratedb yDuncan(1984).Interestingly,normalsubjects shownodiŒeren cein d ecisiontimeforbumpsontheendsofsingleo bjects whenth eya reo ccluded(c) co mparedto wh enth eyare o ntheendsofa singleco ntinuousobject(a).S ubjects’sim ilarRTsforjudgementsin th e singleo ccludeda ndsin glen onoccludedco nditionsledB ehrmannetal.to concludeth atnormalsubjectsp erceivedb othth esingleo ccludeda nd singlen onoccludedo bjectsa suni®ed w hole® gures(se ealsoS ekuler& Palmer,1 992).Thepredictionisth atifp atientsp erformlik econtrol subjects,R Ttom akesame/diŒere ntjudgementsw illb efasterfo rstimuli a1anda2comparedw ithb 1andb2.Moreover,th ise xperiment investigatedwh etherp urea lexicp atientsw oulda lsointeg rateth eoccluded objectfeaturesin tou ni®ed w holes(wh atis ca lledth e``singleo bject advantage’’)andshowtheadvantageinreac tiontimeforstimulic1 a ndc2 (asina 1anda2)o verstim ulib 1andb2. Thesestim ulialso pro videda nopportunityto in vestigateth eeŒects o f diminishedp erceptualcueso npatientperformancein in tegratingobject partso rfeatures.P reviousresearchh asfoundthatperceptualcuessucha s symmetryca naidin co mpletionofoccludedo bjects(S ekuler,19 94; Sekuler,P almer,& Flynn,1994).Inth isex periment,pe rceptualcuesw ere manipulatedb yvaryingthepresenceo rabsenceo fsymmetryin sa mevs. diŒeren ttrials,re spectively.Thepresenceo foccludedstim ulip rovidesa situationinw hichsy mmetryca nbeutilisedtoin tegratesp atially discontinuousobjectpa rts.Th erefore,E xperiment5directlya ddresses theissueo fwhetherp urea lexicpa tientsare a bleto in tegrateo bject elementswh ilea lsoin vestigatingtheeŒect o fperceptualcueso nthe patients’a bilitiesto p erformthe integ ration.

Subjects Allfo urpatientsan deightcontrolsubjectsp articipatedin th isex periment.

M aterials A p p ara tu s

Thise xperimentusedth esameapparatusasin Ex periments1, 2 ,and4 .

S tim u li Subjectsm adesame±d iŒeren tjudgementso nthenumberofbumps locateda ttheendoftwooverlappingbarswith o nebarp artiallyo ccluding theother(see F ig.6).Thestimuliw erecon®guredso tha ttheb umps appearede ithera tthetwoendsofo nesingleo bject,orontheendsoftwo diŒeren tobjects.T hebumpsappeareda ttwoo fthebarends,a ndeach groupofbumpscontainedeithertw oorthreebumps.S ubjectsd ecided PERCEPTIONINPUREALEXIA 965 whetherth ebarsco ntainedth esamenumbero fbumpsonthetwoends (e.g.twoandtwoinC olumn1)oradiŒeren tnumberof b umps(e.g.two andthreein C olumn2inF ig.6).Therew ereth reee xperimentalconditions inth isex periment:a singlen onoccludedo bject,tw oobjects,a ndasingle occludedo bject(see Fig .6).In the sin glen onoccludedcondition(a),bump pairsw erelo catedo ntheoppositeen dsofthetopoverlappingbar;th is barwasn otoccludeda ndthereforeclea rlyco ntinuous.In th etwo-object condition(b),bumpgroupsa ppearedo ntwoseparateb ars.In th esingle occludedc ondition(c),bumpgroupsappeareda ttheoppositeen dsofa barthatwaspartiallyo ccludedb ytheoverlyingb ar.T hisco ndition requiredin tegrationo fthetwoendsofthebarintoa singlere presentation. Itp rovidesa perceptualmiddleg roundbetweenth etwootherstim ulus groups,w hichclea rlyd elineatespro cessingwithino neorbetweentw o items. Thestimulisu btended®vede greeso fvisualangle.Thedista ncebetween thebumpsinth esingleo ccludedan dnonoccludedco nditionswassix degreeso fvisuala ngle.In th etwo-objectcondition,th ebumpswereth ree degreeso fvisualanglea parta ttheclosestp ointandsixd egreeso fvisual anglea parta tthefa rthestpo intbetweenth etwobumpgroups.Su bjects wereg ivena seto f24practicetria ls.In ha lfo fthetrials,th e``top’’(single) barwasorientedin o nedirection,andinth eotherh alf,it w aso rientedin theopposited irection(formorede tails,se eBehrmanneta l.,su bmitted). Experimentaltrialsw erem ixeda ndrandomlyp resentedin 3 blocksof96 trialsfo ratotalo f288experimentaltrials.T herew ere96trialso feacho f the3conditions(occluded,nonoccluded,andtwoitems).H alfo feach conditionwere``same’’trialsan dhalf``d iŒeren t’’trials.W ithinea chb lock therew asafulld istributionofcondition,judgementtype,andorientation ofthe``top’’bar.

P ro c e d u re

Atthebeginningofea chtria l,a ®xationpointappearedcen trallya nd remainedonthescreenforonesecond.A ftera 500msecin terval,th e stimulusappearedon th escreenlefto f®xationandremainedo nuntilth e subjectmadeakeypressresp onse.A responsew asmadebypressingeither the``,’’or``.’’key,whichrep resentedeith er``same’’or``diŒeren t.’’ Responsem appingswereco unterbalanceda crosssu bjects.A llsu bjects usedthe in dexan dmiddle® ngerso ftherighthandtoresp ond.Theinter- trialintervalw ason esecond,measuredfro mthetimearesponsew as made.S ubjectsw ereen couragedtotak ebreaksinb etweenblo cks, althoughmostde clined.RTandaccuracyw erem easured,butonlyR T wasanalyseda sthedependentmeasureb ecausea ccuracyw asnearceiling forbothth epatienta ndcontrolsubjectg roups. 966 SEKULERANDBEHRMANN

R esults Groupdataw eresu bjectedtoa repeatedm easuresA NOVA,withsu bject groupasthebetween-subjectsfa ctorandjudgement(``same’’vs. ``diŒeren t’’)andcondition(singlen onoccludedvs.sin gleo ccludedv s.tw o objects)a sthewithin-subjectvariables.GroupmeansaredisplayedinFig. 7 asafunctionofconditionandjudgement.Bec auseth erew ereo nlya few patientsa ndthepowero ftheanalysisw asweak,w ealsora naseparate ANOVAwithinth epatientgrouptoex aminethe®ndings.T ukeyp osth oc tests (P < .05)w erep erformedto a idin th einterpretationoftheresults. Asigni®ca ntbutsmalleŒ ect ofconditionwasfoundw hensu bjectg roup wascollapsed[ F(2,20)=10.2, P < .001],andthish elde quallya crossth e two groups [F(2,2 0)=1.3, P > .10].Therewasnosigni®ca ntdiŒere ncein RTsforsingleo ccludedandnonoccludedtrialsb utbothofthesewerefaster thanthetwo-objectco ndition.Theseresu ltsrep licateth osefou ndin Behrmanneta l.’sstudyw ithyo ungnormalsubjects,w hichfo unda diŒeren ceo fapproximately4 0msecbetweensingle(o ccludeda ndnon- occluded)andtwo-objecttria lsw henco llapsedacrossju dgementtype.The patterno fresponsestothethreeco nditionsin``same’’and``diŒeren t’’trials diŒered eq uallyfo rpatientsa ndcontrols;``d iŒeren t’’trialsa reslo werth an ``same’ ’trialsb y82msec[ F(1,10)=25.7, P < .001].Themajorresultis that,relativetocontrols,p atientswerem uchslower,sp eci®ca llyo ndiŒeren t occludedtria ls.A lthoughthis d idn otquiterea chsta tisticalsigni®ca ncein thegroupanalysis[ F(2,2 0)=2.6, P > .05],whichisn otsurprisingg iven thesmalln umbero fsubjects,the p atientgroup,whenc onsidereda lone, revealsn odiŒeren ceb etweeno ccludeda ndnonoccludedo n``same’’trials butaseparationbetweenth emo n``diŒeren t’’trials.F inally,overall, patientsw ereslow erth anc ontrolsubjectsb y392msec[ F(1,10)=40.9, P < .0001],butwered isproportionatelyslo werthanc ontrolsfo rthe ``same’ ’vs.``d iŒeren t’’judgements;rela tivetoc ontrols,th eyw ere3 22msec and464msecslowerat``sa me’’vs.``d iŒeren t’’trials,re spectively,as revealedb ythejudgement × groupinteraction[ F(1,10)=11.5, P < .01].

Discussion On``same’’trials(i.e .withsym metricalends),b othp atientsa nd controlssh oweda signi®c antsingleo bjecta dvantageinb othth eoccluded andnonoccludedconditions,rela tivetoth etwo-objectco ndition.On ``diŒeren t’’trials(i.e. w itha symmetricalen ds),co ntrolsu bjectsa lso showedth issam epattern;responsesto th etwo-objectco nditionw ere signi®ca ntlylon gerth antoeith ero fthetwosingle-objectc onditions. However,th epatients’``d iŒeren t’’responsed eviatesfro mthisp attern.A signi®ca ntadvantageisstill fo undinth esinglen onoccludedo bject conditionrelativetoth etwo-objectco ndition,implyingthattheyare a ble P RER C PEP TI NON NIN P RUR E A L EXIA 9 6 7 FIG. 7. Group mean reaction time for Experim ent 5 plotted by subject group and judgem ent as a function of condition. 968 SEKULERANDBEHRMANN tofo rma uni®ed pe rceptofthisco ntinuousobjectb utresponsetim esto ``diŒeren t’’singleo ccludedobjectsn owtakeaslongasju dgementso ntwo diŒeren tobjects.O neinterpretationoftheseto f®ndingsisth atpatients area bleto fo rma uni®ed p erceptbutonlywh enthe® gureis co ntinuous orsymmetrical.W henth eoccludedo bjectis a symmetrical,h owever, patientsa reim paireda tintegratingtheelements,su chtha ttheoccluded objectsare tre atedn odiŒeren tfromthetwo-objectd isplays.N ormal subjects,o ntheotherh and,donotrelyo ncueslikesymmetrya ndshow theobjecta dvantageevenontheasymmetric,d iŒeren ttrials. Althoughtheyd orelativelyw ell,p urea lexicpa tientsd onotperform completelyn ormallyo nthisn onorthographictest. T heproblem,however, isn otobviouslyo neofintegratingcomponentparts(as th eya reab leto d o soin the ``sa me’’trials);ra ther,p atientsha daspeci®c dicu ltyin integratingano ccludedo bjectwhenth enumbero favailablepe rceptual cueswasreduced.Thepurealexicsco uldfo rmu ni®ed w holesfrompartso r featureso fsomenonorthographico bjects,b utth eyweremorerelia nton perceptualcues,su chassymmetry,toin tegratetheobjectssuc cessfully.T his additionaldependenceo nperceptualcuesto a idin p rocessingthestimulus isco nsistentwithth eviewthatageneralperceptualproblemunderliesp ure alexia.Theproblemd oesn otseemtob eoneo fintegratingpartsp erse, a s thepatientsa lsosho wthenormala dvantageforsingleo ccludedo vertw o objectsfo rthe``same’’and``diŒeren t’’trials,re¯ ectin gtheira bilityto integratethe e lementsin toa coherentpercept.R ather,th edicu lty manifestsitse lfu nderimpoverishedperceptualconditionsinw hichth ereis lesssu pportfro morganisationalcuesfo rrepresentingthedisplay.

GENERALDISCUSSION Thegoalofthisstu dywastod eterminewhetherpa tientswith p urealexia, orletter-by-letterreading,havegeneralperceptualdic ultiesex tending beyondtheird icu ltieswithw ordrecognitionand,ifso,whatthenatureof thesed icu ltiesm ightbe.F ourpatientsw ithp urea lexiaa ndeig ht matched,normalcontrolsubjectsp articipatedin sev eralexperiments conductedto in vestigateth eseissu es.N oneofthefo urpurealexic su bjects wasaphasic,th eirla nguageco mprehensionandexpressionwaswell preserved,andsowereth eirsin gleletter id enti®ca tionabilities.Experiments 1and2utilisedread inglatenciesan dlexicald ecisionscoresto d etermine whetherth epatientsw ereletter -by-letterreaders.T he®ndingsfromthese twostudiesw ereco nsistentinre vealingthatallfo ursubjectssh owedth e hallmarkw ordlen gtheŒ ect, re¯ ectin gthelinearincreasein R Twitha n increaseinthelengtho ftheletterstrin g.Experiment3assessedwhetherthe purea lexics’d e®cits ex tendedbeyondorthographybyexaminingtheir perceptual¯uencyo nbotho rthographica ndn onorthographicm aterial. PERCEPTIONINPUREALEXIA 969

Thisstu dyservedasareplicationofFarahandWallace(1991)witha larger patientsample.T he®naltwoexperimentsu tilisedn onorthographicstim uli toclarif ythenatureof the p erceptualde®cit fo undinE xperiment3,and evaluatedw hetherthede®cit is in th erepresentationofpartso fanobject,as hasbeensu ggestedb yFarah(1991,1992,1994)orwhethersomealternative perceptualimpairmentmightexplainthe d ata. Threem ain® ndingsemergedfromthepresentstudy,indicatingthatpure alexicp atientsm ayh avea morew idespreadperceptualbasis,ex tending beyondaspeci®c orthographicd isorder.F irst,in E xperiment3 ,patient performancew asim pairedrela tivetoth econtrolsubjectsonthenonortho- graphic,as we lla sontheorthographic,tests.S econd,inEx periment4, patientsrespondeddisproportionatelyslo wertopoorvs.goodcon®guration trials.In th atexperiment,patients’resp onsesw ereslo wedb ytheabsenceof goodcontinuationinno northographicstim uli.T hird,Experiment5showed thatpatientshaddicu ltyin in tegratingtwohalvesofasingleo bjectwhen symmetryw asabsentandtwodisparateelementshadtob eintegratedinthe presenceofanoccluder.T hisla st® ndingisd icu ltto e xplainth rough damagetoa northographic-speci®c mechanism.Allo fthese® ndingsa rgue forageneralp erceptualdisorderunderlyingpurealexia,andtheyalsorev eal characteristicso fthatperceptualdisorder. Recently,Fa rahhasp utfortha theoreticalac countofpurea lexiab ased ongeneralperceptualdisturbances.S heproposesth atpurea lexiais d ueto adisruptionin a subsystemresp onsiblefo rrepresentingmultiplep arts (Farah,1991,1992,1994).Herth eoryp redictsth atthepatientsw ouldb e impaireda ttasksrequiringseveralpartsto b erepresentedsim ultaneously. Theresultso fExperiment4arein consistentwithth isp rediction.Patients donotshowthepredictedlin earincreasein R Tasth enumbero fpartso f theobjectin creases.R esultsfro mExperiment5alsoco ntradictF arah’s theory.Theysh owth atpatients are ableto rep resentmultiplep arts.In ``same’ ’trials,p urea lexicsre spondedfa stertosin gleo ccludedth an multiple(tw o)partsju dgements,a sdidth enormalsubjects.Th epatients needto b eableto rep resentsimultaneouslyth emultiplep artsof th esingle occludedstim uliin o rderto ga inth issin gleitem a dvantage.B othresu lts discon®rmth eplausibilityo fFarah’smultiplep artsthe ory 1 .

1Onerecent studyha scast additionaldoubt on Farah’shypothesis. Rumiati, Humphreys, Riddoch,andBateman(1994)hav epresented acase of apatientwith object agnosiabut no signsof prosopagnosiao rpure alexia.This patient istherefo re able to readandrecognise faces, butisim paired atrecogn ition of both picturesandrealob jects. The existence ofthis patient violates thefoundationofFarah’shypothesis,thattw oseparate recognition mechanisms exist. Farahclaimsthatone recognition subsystem dealsw ith multiple parts and one with complex parts,bu tin this schemeit is impossible to havean ycombinationsof damagethatwould produce apatient with ade® cit in object recognition alone. The presentation of this patient causes seriousrecons ideration of Farah’stheory. 970 SEKULERANDBEHRMANN

Otherp erceptualtheoriesof p urea lexiaa lsofail to p redicto ur®ndings completely.Forexample,K insbourneandWarrington(1962)claimedth at purea lexiaw asamildfo rmo fsimultanagnosia,whichlim itsp erceptionto asingleo bjectatatime.T heoretically,thesimultanagnosicv isualsystem requiresarefractoryp eriodafterea cho bjectorletteris p rocessed,andthis encouragesletter -by-letterreading.Thesimultanagnosiath eoryis o nly ableto a ccountforsomeofourresu lts.F orexample,K insbourneand Warrington’stheoryd oesnotaccuratelyp redictth eresultso fExperiment 4.Apatientunableto p erceivemoretha noneobjecta tatimewouldsh ow increasinglatenciesas the n umbero felementso ftheobjectin creased. Resultsfro mthise xperimentshowthatset siz edidn othaveamonotonic eŒect o nRTlatenciesfo rthepatientgroup,nordidset siz einteractw ith whethero rnottheob jectha dgoodorpoorcon®guration. FriedmanandAlexander(1 984)proposedy eta notherperceptualtheory butthattoocannotfu llyacc ountforthe p resentresults.T heyclaimth at purealex iaresu ltsfrom a ninabilityto identify visualinputrapidlya nd automatically.FriedmanandAlexander’stheoryp redictsth at,in Experiment4 ,patientsw ouldresp ondnod iŒeren tlyto g oodandpoor con®gurationitemsinco mparisontoco ntrols.T hepatientsw oulda lsob e expectedtop erformn ormallyo nExperiment5becauseid enti®c ationof theitemsisn otrequiredto p erformth attask.Finally,inn eithero fthese latterex perimentsw eresu bjectsu ndera nytimepressurefo rrapido r speedyperceptualprocessing. Noneofth esep erceptual-leveltheoriesa rea bleto a ccountforallo fthe current®ndings.O urresultsd oclearlyin dicate,h owever,th atthesefo ur patientsa relette r-by-letterreaders,all o fwhomalsoh avepe rceptual de®cits. T hese® ndingslea dtotw omajorconclusions.F irst,th eresults indicateth atthede®cit u nderlyingpurea lexiais n otspeci®c to orthography.Ourresults,th erefore,d onotsupportth eexistenceofa functionallya nd/orstructurallydistin ctv isualsystemsp eci®c topr ocessing language-relateditem s.T hisco nclusionmaybeunsurprisingasreadingis arelativelyre centlyp hylogeneticallya cquiredskilla ndislik elyto ex ploit existingvisualp rocessingsystemsratherth anrelyingondedicated,newly acquiredvisuala bilities.Second,theperceptuald e®cit ap pearswh encu es areu navailableto a idth eperceptualprocessingofthestimuli.F or example,inEx periment4,patientsare d iŒeren tiallyim pairedin ta rget detectiono nlyo npoorcon®gurationdisplays.W heng oodcontinuation,a salientperceptualcue,isp resent,p erformanceis n otqualitativelyd iŒeren t fromthenormalsubjects.A similar®ndingarisesinE xperiment5. Patientssh owtheex pectedsin gle-objecta dvantagewithsin gleo ccluded stimuli,in dicatingthatp atientsa rea bleto integ rateth eoccludedb arin to asingleo bjectw hensy mmetryis p resent(i.e.in th esamecondition). However,w henth eperceptualcueofsymmetryis a bsent,in th ediŒeren t PERCEPTIONINPUREALEXIA 971 condition,thepatientsn olongersh owtheadvantageforthesingle occludeditem rela tivetoth etwo-objectco ndition. Takentogether,th ese® ndingssuggestth atpatientsw ithp urea lexiad o haveageneralp erceptualde®cit th atmanifestsitself u nderconditionsin whichth eperceptualdemandsareg reateran dinw hichth ereis less supportfo rorganisingorparsingthestimulus.Inm oste veryday situations,th erea rea numbero fperceptualcuesp resentthatthepatients canuseto su pportp erceptualp rocessing.Thism ayexplainw hypure alexicp atientsd onotexhibita nobviousdebilitatingperceptualde®cit.It iso nlyu nderm oreco ntrolledan drigoroustestingconditionsthatthe de®cit m aybeuncovered.Thenotionofp erceptualsupportin a display maythenex plainwh yitis th atth emajord e®c itd emonstratedby th ese patientse mergesd uringreading.Whenp rocessingaletterstrin g,therea re noobviousaidso rcuesfo rthefo rmulationofacoherentpercept.C ues sucha ssymmetryan d®guralgoodnessare o fnodirectb ene®t inw ord processing.F urthermore,in E nglish,thereare few in trinsicp erceptualcu es tod irectthecombinationofletterg roupsintosp eci®c phonemes,sy llables, orwords.T heabsenceo fexplicitcu esmakesw ordp rocessinga particularlyd icu ltsitu ationforpurea lexicp atients,h ighlightingtheir de®cit an dbringingitto th efore.In deed,theabsenceo fsalientcuesm ay alsoe xplainth eobservedim pairmentinletter p rocessing,aspe rceptual supportis a lsola rgelya bsentundertheseco nditions.E xistingtheorieso f letter-by-letterreading,sucha sthoseth atfocusonthede®cits in letter identi®c ationandprocessing(Behrmann&Shallice,1 995;see a lsoBu b& Arguin,1995;Kay&Hanley,19 91;Reuter-Lorenz&Brunn,1990),are thereforeco nsistentwiththe cla imsmadehere.It isim portanttore cognise thatthede®cit u nderlyingpurea lexiam ayindeedb eanimpairmentin letterp rocessingbutth atthism ay,intu rn,beattributedtoa more fundamentalperceptualproblem.Wedoacknowledge,h owever,th atth e patternw ehavedocumenteda crossth efourpatientsm aynotapplyto a ll patientsw ithp urealex ia,althoughthefactth atitex istsac rossa llfo ur suggestsso megeneralisability.Nevertheless,th eheterogeneitya mongst thesep atientsis w ellk nown(H oward,1991;Price& Humphreys,1992).In keepingwitho ur®ndings,w eproposetha tstringenttestingofthe perceptualproblemsino therpu realexicp atientsis lik elyto u ncovera general,m orew idespread,perceptualde®cit. Beforeco ncluding,thereis o nealternativepossibilitytha tshouldb e raiseda ndthatconcernstheproblemofcausation.Wehavep rovided evidencethatthepurealexic p atientsa lsoh aveaperceptualproblemb ut wehaven otdemonstratedthatitis th eperceptualproblemp erse th at givesrise to th ereadingde®cit; i.e. co rrelationisn otcausation.Thisissu e wasnotedb yFarahan dWallace(1 991)andtheya ddressedit b yshowing thattherew asasigni®ca ntinteractionbetweenw ordlen gtha ndthevisual 972 SEKULERANDBEHRMANN qualityof th estimulusinT U’sreading.The®ndingthatincreasingthe perceptualburdenslo wsreadingtimessu ggeststh atthe p erceptualde®cit isc ausallyrela tedto the rea dingproblem.Ifth eperceptualde®cit is manifestp articularlyu nderta xingconditions(andwehavearguedth at readingiso nesuchco ndition),thenwe mightalsoex pectto see th ese patientsp erformp oorlyo nobjectid enti®ca tionunderd icu ltc onditions. Asdiscussedin o urdescriptionofthepatients,w ehaveo bservedth at thesep atientsare a ccurateb utsigni®can tlyslo wedin the irob ject identi®c ation,especiallyw henth eobjectsa reo fhighvisualcomplexity. Documentingthissy stematicallyis a nongoingfocusofresearchin o ur laboratory. Inco nclusion,theresultso fthisstudyareco mpatiblewith th eview that ageneralperceptualde®c itu nderliesp urealexia .However,p revious perceptualtheories,in cludingFarah’srecentmultiplep artrep resentation theory,dono taccuratelyp redicto ur®ndings.Fo rexample,co ntraryto Farah’stheory,ourpatientsw ereab leto rep resentm ultiplep artso fan object.A lthoughw econcurwithh erv iew thatpurea lexicp atientsh avea de®cit ex tendingbeyondorthography,we suggestth attheperceptual impairmentisu nmaskedin situ ationswherefew in trinsicp erceptualcues existto a idin th eintegrationofmultiplep artso fanobject,su cha sin reading.Thereforeth efunctionalde®c itu nderlyingpurea lexiam aybe relatedtoth emechanismresp onsiblefo rstimulusprocessinginth e absenceo fstrongperceptualcues.If th isis th ecase,th ena dding additionalperceptualgroupingcuesm aya meliorateo ratleastred uce patients’rea dingdicu lties.Th ism aybea ccomplishedb ygroupingletters throughuseo fsimilarcolour,ca se,or sp acing.Furtherresea rchw illb e requiredto in vestigateth isp ossibility.

Manuscript received2June 1995 Revised manuscript received March19 96 Manuscript accepted 11A pril 1996

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