ToappearinK.Cain,D.Compton,&R.Parrila(Eds.),Theoriesofreadingdevelopment.JohnBenjamins

Dyslexiaandwordreadingproblems

RaunoParrila&AthanassiosProtopapas

1.Introduction

Developmentaldyslexiaisthemostcommonlearningdisabilityinchildren,

withprevalenceestimatesvaryingbetween3%and20%ofallschoolage

children(seee.g.,Shaywitz,1996;Snowling,2013).Itisacknowledgedtoaffect

childrenacrosslanguages,writingsystems,andeducationalapproachesto

readinginstruction.Developmentaldyslexiaisalsothemostwidelystudied

behaviourallydefineddevelopmentaldisorder,witharapidlyexpanding

evidencebaseonassociatedgenetics,neuralfunctioning,cognitiveskills,and

environmentalinfluences.Inthischapter,weprovideanoverviewofwidely

availablecognitivetheoriesofdevelopmentaldyslexia.Ourreviewisbyno

meansexhaustiveintermsoftheoriesincludedortheevidenceforandagainst

eachofthem—notevenabook-lengthtreatment(see,e.g.,Elliot&Grigorenko,

2014)couldachievethat.Wehope,however,thatwecoverthemaintheories

andreferencesdrivingthecognitiveresearchondyslexiaatthemoment.

Further,welimitourdiscussionofdevelopmentaldyslexiatoalphabetic

orthographies,andmainlytoEuropeanalphabeticorthographiesthathavebeen

studiedmostextensively.Wemakenoclaimsabouttheuniversalityofideas

presented(seeMcBride-Chang,thisvolume;McCardle,Miller,Lee,&Tzeng,

2011;Nag,thisvolume;Share,2008,forcross-linguisticissuesindyslexiaand

readingresearch)butacknowledgethatasthetheoreticalmodelsof

developmentaldyslexiadevelopandmovefromsingle-deficitmodelsto

multiple-deficitandhybridmodels,theirpotentialforaccommodatingspecific

featuresofdifferentwritingsystemslikelyimproves.

2

Below,wewillfirstdefinedevelopmentaldyslexiaandthenreviewwhat

wecallthesingle-causetheoriesofdyslexiathathavetraditionallydominated

thefield.Wethenadvancetomorerecentdouble-andmultiple-deficitdeficit

models,andconcludewithadiscussionconsideringtheplaceofdevelopment

andoftheindividualwithdyslexiaindevelopmentaldyslexiaresearch.

2.DevelopmentalDyslexiaDefined

Wedefinedevelopmentaldyslexiaasapersistentandunexpecteddifficulty

indevelopingage-andexperience-appropriatewordreadingskills.Forus,word

readingskillsincludebothaccuracyandefficiency,definedascorrectlyread

wordsperunitoftime.Difficultyincludesbothperformancethatfallsatthelow

endofthedistributionforanygivenageandeducationallevelaswellas

performancethatmaynotbeclinicallylowbutcanbemaintainedonlywith

extraordinaryeffort.Wetakeitforgrantedthatwordreadingabilitiesare

continuouslydistributedinapopulationandourdefinitionpotentiallyidentifies

asdyslexicsthoseindividualswhoconsistentlyfallatthelowerendofthe

distributiononsomewordreadingtest(s).Thecut-offbetweendyslexicsand

non-dyslexicsisarbitraryandsignifiesnoqualitativedifferencesbetweenthose

ondifferentsidesoftheborder.

Byfocusingonwordreadingproblems,wedistinguishbetweendyslexia

andreadingdisabilityandarguethatdyslexiaisasubsetofthelatter.According

toICD-10,forexample,specificreadingdisabilityrequiresimpairmentinreading

comprehension,wordrecognition,oralreading,orintasksthatrequirereading.

Whiledyslexiafrequentlyleadstooralreadingandreadingcomprehension

problems,wesuggestthatdyslexiaispresentwhentheprimaryreadingproblem

3

isatthelevelofwordsandtheadditionalproblemsareeithercomorbidor

secondarytothewordreadingdifficulty.

Bypersistentwemeanthatthedifficultyhastobepresentoversome

periodoftimeandnoteasilyremediedbyanalternativeinstructionalmethod.

Forexample,ifsomechildrenfailtolearntoreadwordsingrade1withone

instructionalmethodbutthenmakeclearprogresswithadifferentinstructional

method,theywouldnotqualifyasdyslexics(seee.g.,Vellutinoetal.,1996).

Instead,wewouldcallthem“teachingdisabled”(followingTunmer&Greaney,

2010).Note,however,thatpersistencedoesnotnecessarilyrequireearlyonset

(seee.g.,Catts,Compton,Tomblin&Bridges,2012;Torppa,Eklund,vanBergen

&Lyytinen,2015).

Finally,ourdefinitionincludestheelementofunexpectedness.Despite

potentialproblemsinoperationalization,thisisnecessaryfordistinguishing

dyslexiafromwordreadingdifficultiesingeneral.Unexpectednessrequiresthat

wecanestablishreasonableexpectationsnotsimplybasedonage.Onesuch

basiscouldbeorallanguagecomprehension(cf.Tunmer&Greaney,2010)but

thisisnottypicallyincludedindefinitionsofdyslexiaandmaybeproblematicin

thatlanguageskillsandwordreadingareintertwined(e.g.,Nation&Snowling,

1998;Ricketts,Nation&Bishop,2007).Instead,manywidelyadopted

definitionsofdyslexia,suchasthoseinDSM-5andinICD-10,thatincludethe

ideaofunexpectednessstatethatpoorgeneralcognitiveability,sensory

perceptionproblems,orinadequateeducationalopportunitiesmustberuledout

aspossiblecausesofpoorreadingbeforeadiagnosisofspecificlearningor

readingdisordercanbeascertained(seealsoInternationalDyslexiaAssociation,

2002).

4

Contrarytomanydefinitionsofdyslexia,wehavenoinclusioncriteria.The

mostcommoninclusioncriterionisanassociatedphonologicalprocessing

deficit.Weacknowledgethatmostindividualswithdyslexiawillexhibita

phonologicalprocessingdeficit.However,aphonologicaldeficitdoesnotseemto

beanecessaryconditionfordyslexiaandmanyindividualswithconsiderable

wordreadingproblemsdonotexhibitdepressedphonologicalawarenessscores

(e.g.,Georgiou,Parrila,Cui,&Papadopoulos,2013;Penningtonetal.,2012;

Torppaetal.,2013;vanBergen,Bishop,vanZuijen&deJong,2015).Thus,it

seemsproblematictoconfinetheuseoftheterm–andtheaccesstoadditional

resourcesandaccommodationsthatmaycomewithit–toonlythosewith

phonologicalprocessingdeficits,nomatterhowlargeaproportiontheyformof

thetotaldevelopmentaldyslexiapopulation.

3.Single-CauseTheoriesofDevelopmentalDyslexia

Atraditionalandmostcommontheoreticalapproachtodevelopmental

dyslexiahasbeentopositaspecificdeficitinsomecognitiveorperceptual

processtoaccountforwordreadingdifficulties.Thedeficitistypicallyobserved

onasignaturenonreadingtask(oranarrowsetoftasks)thatismeanttoexpose

somecrucialunderlyingweakness.Inmostcases,thepositeddeficitismeantto

accountforimpairmentsinlearningtoreadratherthanfordysfunctioninthe

cognitivemechanismofmaturereading;therefore,suchproposalsarebest

viewedasdevelopmental,ratherthanneuropsychological,accountsofreading

difficulties.IntheterminologyofCastlesandColtheart(2004),thesetheories

concerndistalcausesratherthanproximalcauses,necessitatingadditional

theoreticalsteps(andempiricaldemonstrations)tolinkthemwithobserved

readingperformance.Thefollowingdiscussionconsiderssomeimportant

5

aspectsofasubsetofinfluentialapproaches.Furtherinformationcanbefoundin

recentreviewsbyRamusandAhissar(2012)andElliotandGrigorenko(2014).

3.1.PhonologicalDeficits

Thecurrentlydominatingtheoryofdevelopmentaldyslexiapositsa

“phonologicaldeficit”atthecoreoftheproblemforalloralargemajorityof

childrenwithdifficultieslearningtoreadwords(Bishop&Snowling,2004;

Ramusetal.,2003;Vellutino,Fletcher,Snowling,&Scanlon,2004).The

phonologicalfamilyofapproachestoreadingdifficultiesisempiricallybasedon

asetoftasksknownasphonologicalawarenesstasksinwhichchildrenare

askedtosegment,blend,delete,orotherwisemanipulatephonemesinoraltasks

notdirectlyassociatedwithreading.Aspecificcausallinkbetweendeficitsin

phonologicalawarenessandwordreadingdifficultieshasbeendifficultto

demonstrateconclusivelyduetointeractionsbetweenreadingandphonological

awarenessskills,suchastheimplicationoforthographicprocessingin

phonologicalawarenesstasks(Castles&Coltheart,2004).However,thisshould

notdetractfromthefactthatpoorperformanceinphonologicalawarenesstasks

isconcurrentlyandlongitudinallyassociatedwithdyslexiaacrosslanguages

(Ziegler&Goswami,2005).

Inmostphonologicaldeficittheories,phonologicalawarenessisassumed

tobecausallyrelatedtowordreadingbecausebeingabletodeliberately

individuateandidentifyphonemesinaspokenwordisaprerequisiteto

consciouslylinkinggraphemestothosephonemes.However,additional

theoreticalstepsarerequiredtoexplainwhyreadingdifficultiespersistpastthe

initialstagesorwhyphonologicalawarenesspredictsadvancedorthographic

6

knowledgeasmeasuredbywordrecognitionandspellingtests.Moreover,

difficultyinphonemeindividuationandidentification,impedinggrapho-

phonemicdecoding,wouldbeexpectedtocausemajordifficultiesinreading

unfamiliarletterstrings,suchasnovelwordsandpseudowords.Indeed,poor

pseudoworddecodinghaslongbeenconsideredahallmarkofdyslexia.Itis,

however,increasinglyacknowledgedthatwordrecognitionisthemostseverely

affecteddomain,withlargereffectsizesbetweentypicalandpoorreadersthan

pseudoworddecoding(VandenBroeck&Geudens,2012).

Alternativeapproachestoacausallinkbetweenphonologicalawareness

tasksandreadingdevelopmentfocusonphonologicalrepresentations,assuming

thattherepresentationsofphonemesareimpaired,somehowimproperlyor

insufficientlyspecified(Perfetti,1992;Ramus,2003;Snowling,2000).Poor

phonemicrepresentationsaccountforpoorphonologicalawarenessandgrapho-

phonemicdecodingbecausephonemicrepresentationsareneededbothfor

consciousmanipulationandforefficientmappingtographemes.

However,de-emphasizingtheroleofawarenessoverlooksthefactthat

tasksinwhichpoorreadersexhibitpoorperformanceareconsistentlythosein

whichphonologicalrepresentationsmustbeexplicitlymanipulated.Thereis

littleevidencethatspeechperceptionorproductionareaffected,aswouldbe

expectedifphonemicrepresentationswereimpaired(Ramus&Ahissar,2012).

Perceptionstudieshavereportedinconsistentfindings,includingsomewhat

poorer(e.g.,Rosen&Manganari,2001),nodifferent(e.g.,Hazan,Messaoud-

Galusi,Rosen,Nouwens,&Shakespeare,2009),orenhanced(e.g.,Serniclaes,Van

Heghe,Mousty,Carré,&Sprenger-Charolles,2004)discriminationofspeech

sounds.Theonlineuptakeofacousticinformationinmatchinglexicalcandidates

7

alsoappearsnormal(Magnusonetal.,2011).Giventhesechallengestothe

representationaccount,adeficitinphonologicalaccess,ratherthan

representation,hasbeenproposed(Boetsetal.,2013;Ramus&Szenkovits,

2008,2009).Accordingtothisaccount,phonemesareproperlyspecifiedbut

theyarenotefficientlyaccessibleforoperationssuchasthoserequiredfor

explicitphonologicalawarenesstasksandformappingbetweenvisualand

phonologicalcodes.Itremainstobeseenhowthisideamightaccountforthe

observeddeficitsoncemoldedintospecifictheoreticalhypotheseswith

associatedempiricalimplications.

Thedominanceofthe“phonologicaldeficit”intheorizingaboutreading

difficultiesremainsundisputedandmostindividualswithdevelopmental

dyslexiaperformpoorlyinthesignaturetasks;atthesametime,manychildren

withphonologicaldeficitsdevelopintoadequatereaders.Othersingle-cause

approachestendtoacknowledgethesefactsandfallintotwocategories:Inthe

first,complementaryoralternativedomainsofimpairmentarepositedto

explainwordreadingproblemsofdifferenttypes.Inthesecondcategory,the

phonologicaldeficititselfisattributedtoamoregeneralorlower-level

dysfunction.

3.2.RapidNamingDeficit

Asecondmajorbranchoftheorizingisbasedon“rapidautomatized

naming”(RAN)tasks(Denckla&Rudel,1976;Norton&Wolf,2012;Wolf&

Bowers,1999;Wolf,Bowers&Biddle,2000).Inthesetasks,participantsare

shownanarrayofsymbols(letters,digits,colorpatches,orobjects)andaskedto

namethemaloudsequentiallyasfastaspossible.RANtaskshavebeendescribed

as“anearly,simplerapproximationofthereadingprocess,”including“rapid,

8

serialprocessingandintegrationofattentional,perceptual,conceptual,lexical,

andmotoricsubprocesses”(Wolfetal.,2000,p.393).Thetimetogothroughthe

entirearraydifferentiatespoorreadersfromgoodreadersandismoderatelyto

stronglycorrelatedwithwordreadingfluency,bothconcurrentlyand

longitudinally,acrosslanguagesandoveralargeagerange(seereviewinKirby,

Georgiou,Martinussen,&Parrila,2010).LowperformanceinRANtasksis

termeda“namingdeficit”andisconsideredcausaltoreadingperformanceasan

additionalcriticalfactornotsubsumedunderphonologicalprocessing.

RANtasksarecommonlythoughttoexpose“rateproblems”becauseof

theirmultiple“processingspeedrequirements”(Wolf&Bowers,1999),thereby

constitutingapreferredpredictorforreadingfluency.Statedthisway,itsounds

likeamethodvarianceissue,withatimedpredictoraccountingfortime-limited

measures,butRANalsopredictsreadingaccuracy(Kirbyetal.,2010;Parrila,

Kirby&McQuarrie,2004).Despitepronouncementsregardingtaskcomplexity,

inpracticethetheoreticalemphasishasbeenplacedonageneral“processing

speed”constructthataffectscognitivecomponentsrequiredforsymbol

processing.However,namingsinglesymbolsdisplayedindividuallyisnota

strongpredictorofreadingperformanceorreadingdifficulty(Jones,Branigan,&

Kelly,2009;Zoccolottietal.,2013;Zoccolotti,DeLuca&Spinelli,2015).The

arraypresentationformat,withmultiplestimulidisplayedsimultaneously,is

criticaltothepredictivepowerofthetask(Georgiouetal.,2013).Therefore

processesinvolvedinnamingindividualsymbolscannotaccountfortheRAN-

readingrelationship,regardlessoftheirspeedrequirements.Instead,itmustbe

theefficiencyinsequentiallynaminganarrayofsymbolsthatbringsout

9

varianceuniquelyrelatedtoreading(Gordon&Hoedemaker,inpress;

Protopapas,Altani,&Georgiou,2013).

Atthemoment,the“namingdeficit”theoryconcernsprimarilyan

assessmentissueratherthanacoherentexplanatoryapproachtoreading

development.PerformanceonRANtaskshelpsidentifychildrenwithreading

difficultythatcannotbeattributedtophonologicaldeficits.Moreover,children

withlowRANperformanceinadditiontopoorphonologicalawarenesstendto

bethepoorestreaders(Kirby,Parrila&Pfeiffer,2003;Torppaetal.,2013)who

likelybenefitleastfromtraditionalintervention(Kirbyetal.,2010).

Thewell-documentedrelationshipsbetweenRANandreadingremain

largelyunexplained(seeGeorgiou&Parrila,2012,forareview).Although

currentresearcheffortsfocusonthecrucialaspectofthetaskformat(deJong,

2011;vandenBoer,Georgiou,&deJong,2016),itisnottheoreticallynecessary

thatthedifficultiesarisedirectlyfromthecognitivecomponentsresponsiblefor

processingsequences(Zoccolottietal.,2015).Perhapsarelativelyminor

difficultyinnamingindividualsymbolsbecomesexacerbatedduetothe

relentlessrequirementsforrapidintegrationamongcognitiveprocesseswhen

goingthroughasequenceofsymbols.Accesstophonologicalrepresentationsor

visualsymbolidentificationcannotprovidetheexplanationbecausearticulation

ofthesymbolnamesisnecessaryforthecrucialindividualdifferencestoemerge

(Georgiouetal.,2013).Yet,articulationratesarenottheanswereitherbecause

silentintervalsbetweensymbols(“pausetimes”)withintheRANtaskarealso

correlatedwithreading(Georgiou,Aro,Liao,&Parrila,2015).WhileRAN

performancehasbeenactivelystudiedforyearsacrossmultiplelaboratoriesand

languages,atpresentthe“namingdeficit”approachremainsaplaceholderfor

10

thefutureidentificationofputativecognitiveandneuralprocessesunderlying

efficientwordandtextreading.

3.3.Auditoryprocessing

Thelackofdirectevidenceforimpairedphonologicalrepresentations

notwithstanding,anumberofresearchprogramshavesoughttoaccountforthe

deficientphonologicalrepresentationsbyaddressingeithergeneralauditory

processingorspeech-specificprocessingunderlyingphoneticperception.Eachof

theseapproacheshasbeenbasedonasignaturemeasure,oranarrowsetof

measures,inwhichsignificantdifferencesareoftenfoundbetweengroupswith

developmentaldyslexiaandgroupsoftypicallydevelopingreaders.Forexample,

“rapidauditoryprocessing”(Gaab,Gabrieli,Deutsch,Tallal&Temple,2007;

Tallal,1980)isassessedwitha“repetitiontest,”inwhichtwobriefstimuliare

presentedinrapidsuccessionandtheparticipantmustreporttheminthe

correctsequence;“temporalsampling”(Goswami,2011,2015)isassessedwitha

“risetimeperceptiontest,”inwhichstimulidifferinginonsetabruptnessmustbe

distinguished;an“allophonicmodeofspeechperception”(Noordenbos,Segers,

Serniclaes,&Verhoeven,2013;Serniclaesetal.,2004)isassessedwith

categoricalperceptiontasksincludingidentificationanddiscriminationof

syntheticspeechsyllables;andsoon.

Atthemoment,robustnessandinterpretationoftheinitialresultsremains

controversial(seeProtopapas,2014,andRamus&Ahissar,2012,fordiscussion

andreferences).First,thepurporteddeficitsdonotreliablyemergeinevery

study;failurestoreplicateandpartialreplicationsabound.Second,despitethe

significantgroupdifferences,whenindividualperformanceisexamineditis

invariablyfoundthatamajorityofparticipantsinthereadingimpairedgroup

11

performwithintherangeofperformanceofthetypicallydevelopinggroup.This

isunlikephonologicalawareness,lowperformanceinwhichistypically

observedforthemajorityofreadingimpairedindividuals.Moreover,besides

groupdifferencesinthetargettasks,differencesaresystematicallyobservedin

tasksthat,accordingtothetheorybeingtested,shouldnotbeaffected.Because

thetasksposecomplexcognitiverequirementsforsuccessfulperformance,it

remainsplausiblethatperformancedifferencesmaybeattributableto

perceptualorcognitiveaspectsofcarryingoutthetaskotherthanthe

hypothesizedauditoryprocessingrequirements.

Moreover,foracausalinterpretationofdifferencesinauditoryprocessing,

thehypothesizeddeficitsmustdemonstrablyprecedeandpredictmediating

deficits,whichinturnmustprecedeandpredictreadingdifficulties.Precedence

canonlybeestablishedinlongitudinalstudiesbeginningatpre-readingages

(Boetsetal.,2011),agoalnoteasytoachieveinpractice.Theexisting

comparisonstoage-matchedcontrolgroupsconfoundperformancewithall

kindsofexperienceandexpertiseassociatedwithreading.Farfromsolvingthis

problem,thealternativereading-levelmatchdesignsconfoundgroupwithage

andcanonlyrevealdevelopmentalanddistributionalaspectsofthemeasures

ratherthantheoreticallyimportantdifferencesamongindividualchildren(Van

denBroeck&Geudens,2012).Asitisbecomingclearthatphonological

representationsmaynotbeimpairedinthesenseoriginallythought,itremains

tobedeterminedwhetherandhowlowperformanceinvariouspsychophysical

tasksmaybeinvolvedintheformationofandaccesstophonological

representationsorotherwiseinlearningtoread.

12

3.4.Visualattention

Anentirelydifferentsetofalternativeapproachestoexplaining

developmentaldyslexiahavefocusedonvisual-spatialattention(Vidyasagar&

Pammer,2010).Oneproposalisbasedona“letterspan”task,inwhichasetof

fivelettersisflashedbrieflyonthescreenandtheparticipantisaskedtoreport

eitherallofthelettersorasingleletterinapositioncuedaftertheir

disappearance.Itishypothesizedthatthistaskassessesthenumberofvisual

elementsthatcanbeprocessedsimultaneously,asrequiredforefficientreading.

A“visualattentionspan”deficitispositedascomplementarytothephonological

deficitapproachoritcanexistindependentlyandaccountforreadingproblems

intheabsenceofphonologicaldeficits(Bosse,Tainturier,&Valdois,2007;

Zoubrinetzky,Bielle,&Valdois,2014).

Theuseoflettersasstimuliinthecriticaltaskadmitsalternative

interpretationsbesidesvisualattention.Forexample,uptakeofvisualletter

informationmaybelimitedduetoinsufficientreadingexperience,orinefficient

graphophonemicconnectionsmayslowdownactivationofphonologicalcodes

forthelettersthroughfeedbackloops.Diminishedeffectshavebeenreported

withstimuliotherthanlettersordigits(Ziegler,Pech-Georgel,Dufau,&Grainger,

2010),thoughsubsequentvisualcategorizationdata(Lobier,Zoubrinetzky,&

Valdois,2012)andneuroimagingdata(Lobier,Peyrin,Pichat,LeBas,&Valdois,

2014)wereunaffectedbystimulustype.Still,thecausaldirectionalityofthe

visualattentionspanremainstobeindependentlyverifiedbecausereading

practicemayconceivablyaffectvisualattentionalefficiencyandmulti-element

processingofnon-alphanumericstimuliaswell(Dehaeneetal.,2010).

13

Amorerecentproposalisbasedonaspatialcueingtask,inwhichthe

locationofasymbolonthescreenisbrieflycuedpriortoitsappearance.Italian

childrenwithdyslexiawerefoundtobenefitlessfromcorrectcueingthan

typicallydevelopingreaders(Facoettietal.,2006,2010).Inaprospective

longitudinalstudy,theperformanceofpreschoolersonthistaskwasfoundto

predicttheirreadingperformanceingrades1and2.Thishasbeeninterpreted

asevidenceforafundamentaldeficitinorientingvisualattention,termed

“sluggishvisualattention,”theorizedtounderlieletterandwordrecognition

(Franceschini,Gori,Ruffino,Pedrolli,&Facoetti,2012).However,thisproposalis

notspecificallyrelatedtoletterandwordidentificationperformance.Rather,the

linktowordrecognitionisthroughphonologicaldecoding,viaamultimodal

attentionmechanismthatmediatesefficientorthographic–phonologicalbinding

(Gori&Facoetti,2015).

3.5.Discussionofsingle-deficittheories

Mostofthesingle-deficittheoriesofdyslexiaofferlittlemorethan

observationofanassociationbetweenpoorreadingperformanceandlow

performanceinasignaturenonreadingtask(orinanarrowsetoftasks).The

mostsuccessfulamongthem,thephonologicaldeficitandthenamingspeed

deficittheories,includeevidenceforlongitudinalassociationsandfindingsthat

aremorerobustacrossstudies.However,muchmorewillbeneededbeforea

“theoryofdyslexia”canbeproclaimed.Wehighlightheretwoobstaclesevident

incurrenttheorizingaboutdyslexia,namely,(a)understandingtask

performanceand(b)constructingcausaltheoriesofreadingdevelopmentthat

involvethenecessarytheoreticalconstructstoconnectreadingwiththe

signaturetasks.

14

Signaturetaskperformanceisoftentakentoindexaspecifictarget

construct,ignoringothercognitiveandperceptualrequirementsforsuccessful

performance.Forexample,performanceonphonologicalawarenesstasksmay

betakentoindexthequalityofphonologicalrepresentationseventhough

successfultaskperformancealsorequiresaccurateperception,retentionin

short-termandworkingmemory,manipulation,andformationandexecutionof

anarticulatoryresponse.Weaknessesinanyofthesestepsorintheirintegration

willimpairtaskperformancewithoutnecessarilyinvolvingpoorqualityofthe

phonologicalrepresentations.Similarly,performanceinrapidnamingtasks,

risetimeperceptiontasks,letterreporttasks,etc.,willnecessarilyinvolvea

multitudeofperceptualandcognitiveprocessesandrepresentations,anyof

whichmightbeimplicatedinpoorperformance.Importantly,the“weaklink”

neednotbeasinglesteporprocess:perhapstwoormoreelementsmightneed

tobecompromisedbeforeperformancedecrementscanbeobserved.This

possibilitycannotbeaddressedintheabsenceofin-depthtaskanalyses.

Thevalidityofataskindexingaconstructcannotbedeterminedapriori.

Convergentanddivergentvaliditymustbedemonstratedbyreferenceto

additionaltasksthatdoordonotsharethepurportedcriticalrepresentationor

process.Foratheoreticalproposaltostandonsolidground,thecrucial

theoreticalconstructsmustbeproperlyoperationalizedbytheirempirical

indices.Thismustincludeawiderangeoftaskshypothesizedtoinvolvethe

constructinquestion.Crucially,itmustalsoexcludetasksofsimilarformand

comparabledifficultythatdonotinvolvetheconstruct.Intheabsenceofwell-

definedconstructs,theoreticalconnectionsarepositedinavacuum.

15

Second,thetheoreticalconstructsmustbeunambiguouslyimplicatedina

causaltheorythatclearlyshowshowtheyunderliepoorreadingperformance.

Admittedly,itisnotclearhowthiscanbeachievedintheabsenceofawell-

definedtheoryofreadingdevelopmentandreadingperformance.Still,it

behoovestheproponentsofspecifictheoriesofdyslexiatoexplainhowlearning

toreaddependsonthehypothesizedtheoreticalconstructsandtodemonstrate

thedependenceinproperlycontrolledlongitudinalandexperimentalstudies

thatsimultaneouslyassessconvergentanddivergentconstructvalidityandalso

includetasksassessingalternativehypothesesforthesameindividual

differences.Interventionstudiesinparticularcanbeeffectiveiftrainingina

distaldomaincanbeshowntoaffectreadingskill.However,itmustbeclearly

demonstratedthattheeffectarisesspecificallyduetothetheoretically

hypothesizedaspectoftraining.Thisisonlyachievableinthecontextofnot

simplyactivecontrolgroupsbutofawell-matchedcontroltrainingregimethat

differsonlyinthetheoreticallycriticalfeaturefromtheproposedintervention.

4.AcknowledgingHeterogeneity:SubtypesofDyslexia

Oneprincipledapproachtotheheterogeneityofwordreadingdifficulties

hasbeenthroughsubtyping.Thatis,ifasingle-deficittheorycannotaccountfor

allcasesofreadingfailure,perhapsacombinationoftheorieswill.Theremaybe

two(ormore)kindsofproblemspotentiallyimpedingwordreading

development,thereforetwo(ormore)kindsofdevelopmentaldyslexia.Asa

consequence,childrencanstillbe“dyslexic”iftheirwordreadingisbelowpar,

butassignedto“typeA,”“typeB,”ora“combinedA+B”dyslexicgroupdepending

onsomerelevantprofiling.Thecategorizationmaybebasedonmeasuresof

readingperformanceoronothercognitive,linguistic,orperceptualindices,

16

includingthesignaturetasksdiscussedabove.Wereviewherethemost

prominentexamplesofsuchsubtyping.

4.1.Phonologicalvs.SurfaceDyslexia

Perhapsthebest-establishedapproachtosubtypingisbasedona

theoreticaldistinctionoriginallydrawninneuropsychologicalEnglish-speaking

patients(Coltheart,2012).Specifically,itwasobservedthat,followingbrain

damage,certainpatientshavereadingdifficultiesthatmanifestdifferentially

withdifferenttypesofletterstrings:Somepatientshavemoredifficultywith

unfamiliarormade-upwords(pseudowords)whereasotherpatientshavemore

difficultywithfamiliarbutinconsistentwords,thatis,wordsthatare

pronounceddifferentlyfromotherwordswithsimilarspellingpatterns

(Woollams,2014).

Thedistinctionbetweenconsistentandinconsistentwordsisespecially

relevantfortheEnglishorthography,inwhichthereisagreatrangeof

graphophonemicconsistency,includingwordswithpronunciationhardly

licensedbytheirspelling,suchasyachtandrough.Anabsolutedivisionbetween

“regular”and“irregular”wordshasbeenimposedonacontinuumof

consistency,basedonthetheoreticalassumptionofcontent-independent“rules”

forgraphophonemicconversion.Anywordnotfullypronounceablebytherules

istermed“irregular”regardlessofitsrelationstootherwords.Forexample,the

wordpintisdeemedirregularforfailingtoadheretothesamepatternasmint

andhint,eventhoughtheusualmappingsholdforthreeoutofitsfourletters

andpronunciationoftheletteriinthiscontextisconsistentwiththewordpine

(Plaut,McClelland,Seidenberg,&Patterson,1996).Thedevelopmental

17

plausibilityandcross-linguisticrelevanceofthisrule-baseddistinctionremain

controversial.

Thedifferentialpatternsofimpairmentseeninpatientswithacquired

dyslexia,inconjunctionwiththetheoreticalhypothesisofabsolute

graphophonemicrules,haveledtoatwo-prongedapproachtowordreading,

includinga“nonlexical”routeassignedtoapplyingrulesanda“lexical”routeto

recognizefamiliarwords(Coltheart,Rastle,Perry,Langdon,&Ziegler,2001).

Accordingly,twoalternativeroutestofailurecanbeposited:Damagetothe

nonlexicalrouteimpedesgraphophonemicconversionwhereasdamagetothe

lexicalrouteimpedesrecognitionoffamiliarwords.Theeffectsofdamagewould

bemostobviousonpseudowordsandirregularwords,respectively,because

thesecanonlybereadbythecorrespondingroute.Incontrast,regularwords

canbereadcorrectlybyeitherroute,andthereforetheyarenotdiagnostic.

Childrenwithdifficultiesinpseudowordreadingaretermed“phonological”

dyslexicswhereasthosewithdifficultiesinirregularwordreadingaretermed

“surface”dyslexics,consistentwiththeclassificationofneuropsychological

patients(Castles&Coltheart,1993).

Althoughtheoreticallyattractiveinitssimplicity,thisproposalhasmet

withempiricaldifficultyinthatthevastmajorityofchildrenwithreading

difficultyexhibitlowperformancewithallkindsofwordsandpseudowords

(Manis,Seidenberg,Doi,McBride-Chang,&Petersen,1996),raisingconcerns

abouttheparsimonyofpositingsimultaneousimpairmentinbothroutes.The

surfacesubtypehasalsobeenelusiveininvestigationsusingreading-levelmatch

designs(Manisetal.,1999;Stanovich,Siegel,&Gottardo,1997).Asforthe

phonologicalsubtype,ithaslongbeenknownthatthepseudowordreading

18

deficitiscontingentonitemdifficulty,emergingmainlywithcomplex

pseudowordsthatdonotresemblewords(Rack,Snowling,&Olson,1992).

Recentadvancesinunderstandingthepsychometricissuesincomparing

performancedecrementsacrossdomains,suchasbetweenwordsand

pseudowords(VandenBroeck&Geudens,2012),havefurtherunderminedthe

potentialfordefiningsubtypesonthebasisofrelativeperformanceinsuchtasks.

4.2.Double-DeficitTheories

Proponentsofvisualandnamingdeficittheoriesreviewedabovehave

takenaslightlydifferentapproachtosubtyping.Lowperformanceinthe

signaturetask(letterspanorrapidnaming)isattributedtoanunderlyingdeficit

thatimpedesreadingdevelopmentindependentlyofphonologicaldeficits.

Therefore,thevisualornamingdeficitsareseenasalternativeoradditionalto

thephonologicaldeficit,supportingclassificationintofourquadrants:Children

withoutdifficulty,childrenwithaphonologicaldeficit,childrenwitha

visual/namingdeficit,andchildrenwithadoubledeficit(Bosseetal.,2007;Wolf

&Bowers,1999).Thispermitsexplanationofpoorreadingintheabsenceof

phonologicalproblems,whichremainsathornyissueforproponentsofthe

phonologicalcoreapproach.Thedoubledeficitapproachesalsoprovidean

additionaldimensionofseverity,possiblyassociatedwithpoorerresponseto

intervention,insofaraschildrenwithadoubledeficitwouldsufferfrommore

pervasiveandseveredifficulties,whichmayalsobemoredifficulttoameliorate

(Wolfetal.,2000).

Howdeficitsareassociatedwithreadingperformancevariesacrossthe

doubledeficitapproaches.IntheoriginaldoubledeficittheoryofWolfand

Bowers(1999),namingspeeddeficitsimpedethedevelopmentofefficientword

19

recognition,whereasphonologicaldeficitsaffectthedevelopmentofaccurate

decoding;combineddeficitsimpedereadingdevelopmentonbothfronts,

resultinginmoreseveredifficulties.Bosseetal.(2007),inturn,suggestedthat

visualattentiondeficitsimpairvisualwordprocessingmoregenerally,

manifestingthemselvesindecodingaswell,evenintheabsenceoffrank

phonologicaldeficits.Thus,thevisualattentionspanisproposedtoaccountfor

readingproblemspotentiallyincludingthosetypicallyattributedtoimpaired

phonologicalprocessing.Nevertheless,visualattentionspanproblemsare

expectedtoaffectsightwordreadingmoreseverelythanphonologicaldecoding.

Finally,proponentsofvisualattentiondeficitsbasedonspatialcueingtaskshave

claimedthatdeficitsinspatialattentionspecificallyaffectprocessesrelatedto

phonologicaldecoding(Ruffino,Gori,Boccardi,Molteni,&Facoetti,2014).Inthis

approach,subtypesdonotdistinguishamongpatternsofreadingperformance

but,rather,amongpatternsofcognitiveskillsthatunderliesimilardifficultiesin

wordreading.

4.3.DiscussionofSubtypingTheories

Themovefromasinglephonological-coreapproachtowardsencompassing

alternativecognitivesubstratesofwordreadingdifficultiesseemswelcomein

thecontextoftheestablishedcognitiveheterogeneityinthelowendoftheword

readingperformancespectrum.Freedomfromthetyrannyofasinglecausemay

paveapathtowardamorepervasiveacceptanceofamultitudeofpotential

routestowordreadingdifficulties.However,theoriesdefiningsubtypesonthe

basisoflowperformanceinspecificsignaturetasksaremorestronglyrelatedto

single-causetheoriesthantomultipledeficitalternativesdiscussedbelowinthat

theyseektoidentifycircumscribed,distinctcausesforspecificpatternsofword

20

readingdifficulties.Thus,theyalsoinheritproblemsassociatedwithsingle-cause

approaches,includingthosestemmingfromtheheterogeneityofreading

problems,whichmaynotfitentirelywithinanyparticularclassification.For

example,thealternativesreviewedabovewouldalreadyleadonetoexpecta

futuretriple-orquadruple-deficittheoryofdyslexia.

Moreover,subtypingproposalsraiseadditionalconcernsregardingthe

reliabilityandstabilityoftheclassification,inadditiontothevalidityofthe

constructsunderlyingtheclassification.Reliabilityisdifficulttoassess

conclusivelyduetothearbitrarinessofcutoffpointsplacedoncontinuous

performancedistributions.Astaskperformanceisinherentlynoisy,the

reliabilityofclassificationdependsonthereliabilityofthesignaturetasks.Here,

issuesoftheoreticalimportance(e.g.,intraindividualvariability)maybe

dismissedastrivialmeasurementnoise.Stabilityofsubtypinghasbeenfoundto

bemoderateforsurfacevs.phonologicaldyslexia(Manisetal.,1999)andfor

namingspeedvs.phonologicaldeficit(Steacy,Kirby,Parrila,&Compton,2014).

Althoughstudiesaddressingaspecificdistinctionmaynotgeneralizetoother

subtypingapproaches,itbehoovestheproponentsofsubtypestodemonstrate

thereliabilityandstabilityofclassificationoverlargeandrepresentative

populationsacrosslanguages.

5.Multiple-DeficitModelsofDyslexia

Theevidencereviewedabovesuggeststhatsingle-,double-ortriple-cause

theoriesofdevelopmentaldyslexiaareunlikelytoprovidesatisfactory

explanationsofdyslexiaasabehaviorallydefineddevelopmentaldisorder.There

isnowwidespreadconsensusthattheterm“dyslexia”referstothelowendofa

wordreadingdistributionratherthantoadiscretecondition.Ifthereisno

21

discreteconditionthataccountsforthegreatmajorityofchildrenwithword

readingdifficulties,thesearchforspecificcausesisgreatlychallenged.Rather,

thefocusisturningtomultifactorialdevelopmentalpathwaysthatcangiverise

tobrainsthatdifferintheirpropensityforlearningtoread(orlearningmath,or

acquiringanyotherculturalartifactourevolutionhasnotspecificallyequipped

usfor).Individualdifferencesinlanguageandreadingdevelopmentare

increasinglyattributedtoamultitudeofinteractinggenetic,neural,cognitive,

behavioral,andenvironmentalfactors,potentiallyleadingtohighorlowreading

performanceviamultipledevelopmentalpathways.

Insometheoreticalmodels,cognitivemultiplicityisassumedtoreducetoa

singlegeneticorneuralcausefordevelopmentaldyslexia,butthesecallsfor

simplicityfacesizeableempiricalchallenges.Thegeneticstudiesofdyslexia

startedwiththeexpectationofdominantinheritancecontrolledbyasinglegene

(Hallgren,1950).Specificsusceptibilitygenesweresoughtwithsingle-gene

strategies(suchasgeneticlinkage,targetedassociation,andchromosome

translocationordeletion).Instead,multiplelociwithmultiplesusceptibility

geneshavebeenidentified(Kere,2014).Molecularandbehaviorgeneticstudies

ofdyslexianowagreethatthegeneticarchitectureassociatedwithdyslexiais

complex,polygenic(twoormoregenescontributetothephenotype),and

heterogenic(thesamebehavioraloutcomecanbeassociatedwithmultiple

differentcauses;Carrion-Castillo,Franke,&Fisher,2013;Elliot&Grigorenko,

2014).Thereareexamplesoffamilieswheretheinheritancepatternis

consistentwithararemutationofasinglegene(e.g.,deKoveletal.,2004;

Nopola-Hemmietal.,2000).However,allgenesidentifiedinrarefamilialforms

ofdyslexiajointlyexplainatinyfractionofthevarianceinreadingabilitywhen

22

testedwithlargersamples.Instead,mostcasesofdyslexiaareprobablyaffected

byaverylargenumberofgenes,eachwithonlyweakeffects(Carrion-Castilloet

al.,2013),furthercomplicatedbyamultitudeofgene×geneandgene×

environmentinteractions(Bishop,2015;Jablonka&Lamb,2014).

Neurallevelexaminationshavenotfaredmuchbetterinsimplifying

dyslexiatheories.Recentmeta-analysesoffunctionalneuroimagingstudies

(Maisog,Einbinder,Flowers,Turkeltaub,&Eden,2008;Richlan,Kronbichler,&

Wimmer,2009,2011)haveidentifiedmorethanahundredfociofdifferences

betweendyslexicandnormallydevelopingreaders.Theresultsfromthemeta-

analysesweremostlyconsistentwiththetypicalneurophysiologicalaccountof

developmentaldyslexiaforadults(e.g.,Pughetal.,2000)buthighlightedthe

needforrefinementinthedevelopmentalaccount.Inparticular,Richlanetal.

(2011)foundnobrainareastypicallyassociatedwithphonologicalcodingtobe

reliablyunderactivatedacrossstudies.Theirresultsdidnotsupportthe

assumptionthattheprimaryandearlyemergingdysfunctionresidesintheleft

temporo-parietalcortexhousingthedorsalreadingsubsystem.Instead,they

suggestedthatanearlyandlimitedleftoccipito-temporaldysfunctionbecomes

extendedovertimeandisaccompaniedbyalefttemporo-parietaldysfunctionby

adulthood.

Asindividualfunctionalimagingstudiescontinuetoproducewidely

varyingresults,itmaybenecessarytoexaminehowmuchofthisvariabilityis

relatedtoheterogeneity(beyondage)inthedyslexiasamplesandhowmuchis

relatedtovariabilityinimagingandanalysismethodsandthesignaturetasks

usedindifferentlaboratories.Notably,substantialvariabilityisnotlimitedto

functionalimagingbutisalsopresentinstructuralimaging,asdemonstratedin

23

meta-analysesofvoxel-basedmorphometrystudiesofgraymatter

(Linkersdörfer,Lonnemann,Lindberg,Hasselhorn,&Fiebach,2012;Richlan,

Kronbichler,&Wimmer,2013;seereviewinJednorogetal.,2015)andwhite

matter(Vandermosten,Boets,Wouters&Ghesquière,2012)comparingdisabled

andtypicallydevelopingreaders.Asstructureandfunctionofthebrainareboth

alteredbyexperience(Gabrieli,2009;Krafnick,Flowers,Luetje,Napoliello,&

Eden,2014;Simosetal.,2002),therelevantheterogeneityinthedevelopmental

dyslexiasamplesisnotlimitedtothereadingandcognitivemeasuresusually

usedtoidentifythedyslexics.

5.1.ProbabilisticMultipleDeficitModels

Theassumptionthatbehaviorallydefineddevelopmentaldisorderscan

haveasinglecauseatanylevelofanalysishasbeenfurtherchallengedby

multifactorialetiologicalmodels(e.g.,Gottlieb&Halpern,2002;Lyytinenetal.,

1998;Pennington,2006;vanBergen,vanderLeij,&deJong,2014).Inan

influentialpaper,Pennington(2006)reviewedgenetic,neural,cognitive,and

comorbiditystudiesofdevelopmentaldyslexiaandconcludedthatconverging

evidenceprecipitatesamajorreconceptualizationoftheexistingtheoretical

models.Hearguedthatprobabilisticmultipledeficitmodels(PMDM)areneeded

toproviderealisticaccountsofdevelopmentaldisorders,theircomorbidity,and

thenondeterministicrelationshipsbetweendisordersandtheirpresumed

causes.HesuggestedfurtherthatsuchPMDMsmustincludeprotectiveandrisk

factors,multiplelevelsofanalysis,bidirectionalconnectionsbetweenconstructs

withineachlevel(horizontalorintralevelinteractions),andbidirectional

connectionsbetweenlevels(verticalorinterlevelinteractions)toaccountfor

interactionsbetweenprotectiveandriskfactorsfunctioningatdifferentlevelsof

24

analysis(seealso,Ford&Lerner,1992;Gottlieb,1983,1997;Gottlieb,Wahlsten,

&Lickliter,2006;Lyytinenetal.,1998).

Figure1showsasimplifiedversionofPennington’s(2006)PMDMwith

interlevelconnectionsomitted.Theleftsideofthemodelshowsthelevelsof

analysis–etiological,neural,cognitive,andbehavioral–andtherightside

displaysthemechanismsthatunderliehorizontalinteractionsineachlevel.

AccordingtoPennington,theetiologicallevelofanybehaviorallydefined

developmentaldisorder–includingdyslexia–ismultifactorialandinvolves

interactionofmultipleriskandprotectivefactorsthatcanbeeithergeneticor

environmental;thesejointlyandprobabilisticallyinfluencethedevelopmentof

neuralsystemsand,further,thecognitiveprocessestheysupport.Atthe

behaviorallevel,thedisorderisjointlyandprobabilisticallyproducedby

multiplecognitiveriskandprotectivefactors,eachinfluencedbymultiple

etiologicalfactors.Someoftheetiologicalriskandprotectivefactorsinfluence

severaldisorders(causingcomorbidity)whereasothersarespecifictoone

disorder.Nosingleetiological,neural,orcognitivefactorissufficient;a

combinationofseveralmaybenecessarytoproducethebehavioralsymptoms

thatdefinethedisorder.Finally,theliabilitydistributionforanygivendisorder

iscontinuous.Anindividual’spositiononthedistributionisaffectedbyriskand

protectivefactorsatanylevel.

Recently,vanBergenetal.(2014)extendedPennington’sPMDMtoallow

forintergenerationaltransferofriskandprotectivefactors.Toexplicitlyaccount

forparentaleffects,theyproposedtheintergenerationalmultipledeficitmodel

(iMDM;seeFigure2),whichincludesnotonlygenetictransmissionfromparents

tochildrenbutalsopassiveandevocativegene-environmentcorrelationsand

25

culturaltransmissionfromparents.InFigure2,environmentasshapedand

selectedbytheparentsisseparatefromextra-parentalenvironmentandfrom

geneticeffectsintheetiologicallevelofPennington’sformulation.IntheiMDM,

parentalskills(asexpressedintheirphenotype,PT)aretransmittedboth

geneticallyandviathehomeenvironment.Thisextendedhomeenvironmentcan

exertadirecteffect(culturaltransmissioninFigure2)onchildreninthat

parents’cognitivephenotypesimpacttheenvironmentstheycanofferfortheir

children,includingtheprotectiveandriskfactorsintheseenvironments.Ifhome

environmentcorrelateswithparents’andchild’sgenotype,weobservepassive

gene-environmentcorrelation(rGEinFigure2);theseinfluencescould,for

example,includehighlyliterateparentshavingahigherincomeandthusaccess

tobetterschools.EvocativerGEariseswhensomeofthechild’shighlyheritable

characteristics(e.g.,goodphonologicalawareness)elicitaresponsefromthe

environment(e.g.,morerhymingandalliterationgamesandplayingwithletters)

thatfurtherstrengthensthechild’sreadingdevelopment.Sharedenvironmental

confoundcontributestoparent-childresemblancebyaffectingreadingabilityin

bothgenerations.Forexample,accesstoeducation,particularlyforfemales,isa

significantsharedenvironmentalconfoundinmanypartsoftheworld.Another

suchconfoundcouldbeasharedhomelanguagethatisdifferentfromthe

languageofeducation.

Separatingenvironmentfromgenesasetiologicalfactorsallowedvan

Bergenetal.toalsodifferentiatebetweenenvironmentasshapedbyparentsand

extra-parentalenvironmentthatparentshavelessinfluenceover;thiscould

includereadinginstructionmethod,accesstoprintanddigitalmedia,peer

influences,legislationofspecialprovisionsandresourcesintheschoolsfor

26

studentswithdyslexia,andthevalueofliteracyinthesocietyatlarge.Van

Bergenetal.alsodistinguishbetweenactiverGE(e.g.,achildwholearnstoread

easilyismotivatedtoreadmoreandseeksoutopportunitiestodosointhe

environment)andevocativerGE,wherechildren’sgeneticallyinfluencedability

elicitsdifferentialreactionsfromtheenvironment,suchasgoodreadersbeing

givenmoredemandingmaterialstoread.Intheeducationalliterature,asimilar

differenceismadebetweentheactiveandevocativeimpactachildcanhaveon

teachers’behavior(e.g.,Nurmi,2012).

5.2.DiscussionofMultipleDeficitModels

Multipledeficitmodelsprovideaninterestingmeta-theoreticalframework

toadvancedevelopmentaldyslexiatheorizingandresearch.Thesemodelsare

examplesofdynamicordevelopmentalsystemsmodelsthathavealonghistoryin

developmentalembryologyandbiology(seee.g.,Gottlieb,2002)andhave

permeateddevelopmentalsciencesforsometime(seee.g.,Ford&Lerner,1992,

andThelen&Smith,1994,forintroductionstoearlierapproaches,andMolenaar,

Lerner&Newell,2014,fornewerformulations).Whileseveralprominent

authorshaverecentlyacknowledgedthelimitationsoftraditionalmodels(e.g.,

Catts,thisvolume;Snowling&Melby-Lervåg,2016),systemsapproachesin

generalhavehadlittletractionindyslexiaresearch(however,seeMorrison&

O’Connor,thisvolume,foranexampleofasystemsapproachtoreading

development),perhapsbecausesystemsapproachesposeformidableempirical

challengesandtheoreticalquestionstheanswerstowhichpoorlymatchour

dominantresearchtraditionsandpresuppositions..

Whiletherearemultiplefamiliesofdevelopmentalordynamicsystems

theories,thePMDMmodelsasoutlinedbyPennington(2006)andvanBergenet

27

al.(2014)bearaclosetheoreticalresemblancetodevelopmental

psychobiologicalsystemstheories(e.g.,Gottliebetal.,2006).Inthesetheories,

developmentisconceptualizedasasequentialemergenceofnewstructuraland

functionalpropertiesandcompetenciesatalllevelsofanalysisasaconsequence

ofhorizontalandverticalinteractionsamongtheparts(Gottliebetal.,2006).

Thisimpliesthatanycausalexplanationofadevelopmentaloutcome,suchas

dyslexia,mustdescribethedevelopmentalsystemthat,overtime,ledtothe

observedoutcome.Whilewecanstudycomponentsofthesystem,suchasrapid

namingtasksorindividualgenes,inrelativeisolation,individualcomponents

neitherexplainnorcause(normalorabnormal)developmentinanymeaningful

sensewithoutanaccountoftherestofthesystem;suchanaccountmustinclude

theorganismandthephysical,biological,andsocialfactors(“developmental

niche”)thatinteractwithandshapeitovertime.Asthecomponents,or

interactants,inthesedevelopmentalnetworksarethemselveslargelyproducts

ofearlierdevelopment,developmentalexplanationsrequirethatwestudytheir

interactionsoveraperiodoftime.Asaresult,weneedtorethinkbothwhat

constitutesanexplanationandwhatkindsofobservationsarerequiredto

understandthedevelopmentalpathwaystotheobservedoutcomes.

Asdevelopmentalsystemsmodels,PMDMsinherittheideathatan

explanationrequiresunderstandingthedevelopmentalsystemwithallofitsrisk

andprotectivefactors.Thus,theunitofexplanationisnotanindividualbuta

relationalcausalnetwork.Relationalcausalnetworksincludetheideathatno

singleelementorlevelinthedevelopingsystemhascausalprimacy,andthe

functionalsignificanceofanyelementondevelopmentcanonlybeunderstoodin

thecontextofthedevelopmentalsystemofwhichtheyarepart.Ateachlevelof

28

thedevelopmentalsystem,theeffectofanyelementisdependentontherestof

thesystem,makingallfactorspotentiallyinterdependentandmutually

constraining(Gottlieb,1991).

ThePMDMsreviewedaboveimplythiskindofmultidimensionalitywhere

assigningcausalprioritytoanylevelisproblematic.However,theseideasseem

difficulttoreconcilewithcurrenttheorizingthatassignscausalpriorityatthe

geneticlevelandassumesthatgeneticandenvironmentalinfluencesare

additive.Incontrast,thedevelopmentalsystemsviewleadstoconceptualizing

geneticandenvironmentaleffectsasinterdependent.Atleastinprinciple,

genetic(andotherinherited)effectsleadingtoindividualdifferencescannotbe

understoodapartfromdevelopmentoccurringinaspecificenvironmental

context.Complexhumanbehaviors,suchasreading,“areinfluencedbyhundreds

orthousandsofproteinsencodedinhundredsorthousandsofgenesofsmall

effectthatinteractwithoneanother,theenvironment,andtheepigenomein

complexways”(Charney&English,2012,p.30).Whatthenconstitutesan

explanationofdevelopmentanddevelopmentaloutcomeismuchmorecomplex

thanmostdevelopmentalmodelsindevelopmentaldyslexiaresearchcurrently

acknowledge.

Methodologically,weneedtosupplementcurrentnomotheticvariable-

centeredstudieswithidiographicstudiesandperson-centeredanalyses(see

examplesinMolenaaretal.,2014).Nomotheticvariable-centeredstudiesthat

havedrivenmostofthetheorydevelopmentdescribedaboveareinformativeof

generaltendenciesandthecomponentsthatexplanatorymodelsneedtoinclude,

buttheycannotpredicthowtheprocessofdevelopmentunfoldsovertime.We

suggestthatweneedtodevelopandtest“dynamicmechanisticexplanations”

29

(Bechtel&Abrahamsen,2005,2010)ofhowdifferentinteractantsworktogether

inproducingobservableoutcomes.Whilewecurrentlyhavefewsuchmodelsto

buildon(seeGiraud&Ramus,2013,foranexception),thesekindsofmodelsare

notuncommoninotherfieldsofinquiry(see,e.g.,Becheretal.,2014,fora

complexmodelofhoneybeecolonydynamics).Dynamicmechanistic

explanationsrequirelongitudinalandexperimentalstudies,goingbeyond

individualdifferencestoobservationsofdevelopmentalmechanisms,aswellas

computationalmodeling(e.g.,agent-basedmodels,seeRailsback&Grimm,

2011)ofthesemechanismsgroundedinempiricalobservationsandaimingto

understandtheirfunctioningwhereobservationsandexperimentationarenot

possible.Forexample,ifweconceptualizegenetic,neural,andcognitive

interactantssuggestedbyGiraudandRamus(2013)asagentsandmodeltheir

functioningandinteractionswithagent-basedmodels(seee.g.,Railsback&

Grimm,2011;Wilensky&Rand,2015;examplesat

https://ccl.northwestern.edu/netlogo/)thatfirstsimulatewhatwealready

know,wecanthenstarttopositmechanismsandconditionsunderwhichthey

operate.Aswelearnmore,themodelswillgetmorecomplexbyhavingto

includesimulationsofnewempiricalfindingsandtheywillproducenew

hypothesestoexamineempirically(seeBechtel&Abrahamsen,2010,foran

exampleofincreasingmodelcomplexity).Thescopeofthestudiesdoesnothave

tobeanymoreexpansivethanthestudieswealreadyconduct,butwhenweuse

modelsasfirstapproximationsofthedevelopmentalinteractionsbetweenthe

componentsweobserve,theexplicitnessofourtheorieswillincreasebecause

wewillhavetofocusonthemechanismsandnotonlyonassociationsamong

measures.

30

Finally,thenotionofequifinalityhasbecomeanaxiomofdevelopmental

systemstheory(seee.g.,Ford&Lerner,1992;Gottliebetal.,2006).Inthisview,

organismswithdifferentearly—or“initial”—conditionscanreachthesame

endpointandorganismswiththesameinitialconditionscantakedifferent

routesorpathwaystoreachacommonendpoint.Equifinalityisanimportant

principleinpsychologicaldevelopment,buttheconceptisseldomdiscussedin

developmentaldyslexiaresearch.However,ifPMDMmodelsareinterpretedas

developmentalsystemstheories,itfollowsthatdevelopmentisinfluencedby

manyriskandprotectivefactorsthatinteracttoproducethereadingbehavior

weusetodiagnosedyslexia.Suchcomplexprobabilisticnetworksareboundto

producesimilarobservablestateswithdifferentinteractants(e.g.,different

explanationsofwordreadingfailure;Snowling&Melby-Lervåg,2016).Examples

areboundtoproliferatewithincreasedemphasisonperson-centeredand

idiographicmethods.

6.Conclusion

Theoriesofdevelopmentaldyslexiacannotsimplybetheoriesofindividual

differencesinwordreadingdevelopment;instead,theyneedtoprogress

towardsdynamicmechanisticexplanationsofvariousdevelopmentalpathways

toinaccurateorinefficientwordreading.Theformeraregeneralandmeantto

accountprimarilyforassociationsamonggeneralconstructs,effectively

describingtheaveragesituationthatmaynotapplytoanyindividual(seee.g.,

Velicer,Babbin,&Palumbo,2014).However,ifthegoalistodevelopatheoryof

dyslexiaasaspecificconditionratherthanadiagnosticlabel,thenweneeda

muchmorespecifictheory,ofamoreappliednature.Thistheoryshouldaccount

foreachandeverychildthatdeservedlyreceivesthediagnosticlabel“dyslexia”

31

followingextensivetesting,examination,andpossiblyfailedintervention.There

isnoroomforlettingsomechildrenslip.Howeverlargeandheterogeneous,

groupsofchildrenwithdyslexiamustbefullyaccountedforbyanytheory

purportingtobeatheoryofdyslexia.Otherwiseitisnotreallyatheoryof

dyslexiabutmaybeatheoryofsomeofthedifficultiesofsomeofthechildren

whofailtolearntoreadwords.Therequirementforfulldiagnosticcoverage

seemsextremelyunlikelytobesatisfiedbyanyapproachfocusingonsingle

causesorsingle-factorcharacterizations,anditseemsverylikelytoinclude

multiplepathwaystothesamebehavioralcondition.

Theabovediscussionhasindicatedanumberofpotentialinteractantsfrom

thegeneticthroughtheenvironmentallevelthatneedtobeconsideredina

developmentalsystemstheoryofdevelopmentaldyslexia.Theseinteractants

wereidentifiedinvariable-centeredstudiesfocusingonindividualdifferences

because(a)weknowtomeasurethem,and(b)theyco-varysufficientlywiththe

dependentvariableintheexaminedsamples.Wesuspectthattheinteractants

wecurrentlyknowofas“actualdifferencemakers”(Waters,2007)arebuta

smallsubsetofthoseneededforadynamicmechanistictheoryofdevelopmental

dyslexia.Wemayhavealreadyincludedsome“potentialdifferencemakers”

(Waters,2007;seealsoGriffiths&Tabery,2013,andTabery,2014)inour

empiricalstudiesbutfailedtorecognizetheirsignificanceforadevelopmental

theoryofdyslexiabecausetheyeitherdidnotvarysufficientlytoproducethe

statisticalassociation,ortheheterogeneityoftheirexpressioninthesamples

drownedthesignal.However,thereareundoubtedlymorepotentialdifference

makersatalllevelsofanalysisthatareyettobeidentified.Hereiswheresingle-

casestudiesandexploratorycomputermodelingcancaptureextremecasesto

32

enhancetheorydevelopment.Thetrulydevelopmentalscienceofdevelopmental

dyslexiarequiresthatwe“deconstruct”thephenomenonateachlevelintoits

constituents,butalsothatwethenattempttoreconstructthedevelopingsystem

totesthypothesesaboutinteractionsbetweenlevelsandmechanismsofeffect.

Thetheorieswewantaretheonesthatnotonlyexplainwhyachildwith

dyslexiareadsdifferentlyfromanotherchildwithorwithoutdyslexia,butalso

whereinthatdevelopmentalsystemwecanintervenesuccessfully.

33

References

Becher,M.A.,Grimm,V.,Thorbek,P.,Horn,J.,Kennedy,P.J.&Osborne,J.L.

(2014).BEEHAVE:Asystemsmodelofhoneybeecolonydynamicsand

foragingtoexploremultifactorialcausesofcolonyfailure.Journalof

AppliedEcology,51,470-482.

Bechtel,W.&Abrahamsen,A.(2005).Explanation:Amechanisticalternative.

StudiesinHistoryandPhilosophyofBiologicalandBiomedicalSciences,36,

421-441.

Bechtel,W.&Abrahamsen,A.(2010).Dynamicmechanisticexplanation:

computationalmodelingofcircadianrhythmsasanexemplarfor

cognitivescience.StudiesinHistoryandPhilosophyofScience,41,321-333.

Bishop,D.V.M.(2015).Theinterfacebetweengeneticsandpsychology:Lessons

fromdevelopmentaldyslexia.ProceedingsoftheRoyalSocietyB,282:

20143139.

Bishop,D.V.M.,&Snowling,M.J.(2004).Developmentaldyslexiaandspecific

languageimpairment:Sameordifferent?PsychologicalBulletin,130,858–

886.

Boets,B.,deBeeck,H.P.O.,Vandermosten,M.,Scott,S.K.,Gillebert,C.R.,Mantini,

D.,...&Ghesquière,P.(2013).Intactbutlessaccessiblephonetic

representationsinadultswithdyslexia.Science,342,1251–1254.

Boets,B.,Vandermosten,M.,Poelmans,H.,Luts,H.,Wouters,J.,&Ghesquière,P.

(2011).Preschoolimpairmentsinauditoryprocessingandspeech

perceptionuniquelypredictfuturereadingproblems.Researchin

DevelopmentalDisabilities,32,560–570.

34

Bosse,M.L.,Tainturier,M.J.,&Valdois,S.(2007).Developmentaldyslexia:The

visualattentionspandeficithypothesis.Cognition,104,198–230.

Carrion-Castillo,A.,Franke,B.&Fisher,S.E.(2013).Moleculargeneticsof

dyslexia:Anoverview.Dyslexia,19,214-240.

Castles,A.,&Coltheart,M.(1993).Varietiesofdevelopmentaldyslexia.Cognition,

47,149–180.

Castles,A.,&Coltheart,M.(2004).Isthereacausallinkfromphonological

awarenesstosuccessinlearningtoread?Cognition,91,77–111.

Catts,H.W.,Compton,D.,Tomblin,J.B.,&Bridges,M.S.(2012).Prevalenceand

natureoflate-emergingpoorreaders.JournalofEducationalPsychology,

105,166–181.

Charney,E.&English,W.(2012).Candidategenesandpoliticalbehavior.

AmericanPoliticalScienceReview,106(1),1-34.

Coltheart,M.(2012).Dual-routetheoriesofreadingaloud.InJ.S.Adelman(Ed.),

Visualwordrecognition,volume1:Modelsandmethods,orthographyand

phonology(pp.3–27).Hove,UK:PsychologyPress.

Coltheart,M.,Rastle,K.,Perry,C.,Langdon,R.,&Ziegler,J.(2001).DRC:Adual

routecascadedmodelofvisualwordrecognitionandreadingaloud.

PsychologicalReview,108,204–256.

deKovel,C.G.,Hol,F.A.,Heister,J.G.,Willemen,J.J.,Sandkuijl,L.A.,Franke,

B.,…Padberg,G.W.(2004).Genomewidescanidentifiessusceptibility

locusfordyslexiaonXq27inanextendedDutchfamily.JournalofMedical

Genetics,41(9),652–657.

35

Dehaene,S.,Pegado,F.,Braga,L.,Ventura,P.,NunesFilho,G.,Jobert,A.,…Cohen,

L.(2010).Howlearningtoreadchangesthecorticalnetworksforvision

andlanguage.Science330,1359-1364.

deJong,P.F.(2011).Whatdiscreteandserialrapidautomatizednamingcan

revealaboutreading.ScientificStudiesofReading,15,314–337.

Denckla,M.B.,&Rudel,R.G.(1976).Rapid‘automatized’naming(R.A.N.):

Dyslexiadifferentiatedfromotherlearningdisabilities.Neuropsychologia,

14,471-479.

Elliott,J.G.,&Grigorenko,E.L.(2014).Thedyslexiadebate.CambridgeUniversity

Press.

Facoetti,A.,Trussardi,A.N.,Ruffino,M.,Lorusso,M.L.,Cattaneo,C.,Galli,R.,...&

Zorzi,M.(2010).Multisensoryspatialattentiondeficitsarepredictiveof

phonologicaldecodingskillsindevelopmentaldyslexia.Journalof

CognitiveNeuroscience,22,1011–1025.

Facoetti,A.,Zorzi,M.,Cestnick,L.,Lorusso,M.L.,Molteni,M.,Paganoni,P.,...&

Mascetti,G.G.(2006).Therelationshipbetweenvisuo-spatialattention

andnonwordreadingindevelopmentaldyslexia.Cognitive

Neuropsychology,23,841–855.

Ford,D.H.&Lerner,R.M.(1992).Developmentalsystemstheory:Anintegrative

approach.NewsburyPark,CA:Sage.

Franceschini,S.,Gori,S.,Ruffino,M.,Pedrolli,K.,&Facoetti,A.(2012).Acausal

linkbetweenvisualspatialattentionandreadingacquisition.Current

Biology,22,814–819.

Gaab,N.,Gabrieli,J.D.E.,Deutsch,G.K.,Tallal,P.,&Temple,E.(2007).Neural

correlatesofrapidauditoryprocessingaredisruptedinchildrenwith

36

developmentaldyslexiaandamelioratedwithtraining:AnfMRIstudy.

RestorativeNeurologyandNeuroscience,25,295–310.

Gabrieli,J.D.E.(2009).Dyslexia:Anewsynergybetweeneducationand

cognitiveneuroscience.Science,325,280-283.

Georgiou,G.K.,Aro,M.,Liao,C.H.,&Parrila,R.(2015).ThecontributionofRAN

pausetimeandarticulationtimetoreadingacrosslanguages:Evidence

fromamorerepresentativesampleofchildren.ScientificStudiesof

Reading,19,135–144.

Georgiou,G.&Parrila,R.(2012).Rapidnamingandreading.InL.Swanson,K.

Harris&S.Graham(eds.),HandbookofLearningDisabilities(2nded.,pp.169-

185).NewYork:Guilford.

Georgiou,G.K.,Parrila,R.,Cui,Y.,&Papadopoulos,T.C.(2013).Whyisrapid

automatizednamingrelatedtoreading?JournalofExperimentalChild

Psychology,115,218–225.

Giraud,A.-L.&Ramus,F.(2013).Neurogeneticsandauditoryprocessingin

developmentaldyslexia.CurrentOpinioninNeurobiology,23,37-42.

Gordon,P.C.,&Hoedemaker,R.S.(inpress).Effectiveschedulingoflookingand

talkingduringrapidautomatizednaming.JournalofExperimental

Psychology:HumanPerceptionandPerformance.Advanceonline

publication.http://dx.doi.org/10.1037/xhp0000171

Gori,S.,&Facoetti,A.(2015).Howthevisualaspectscanbecrucialinreading

acquisition?Theintriguingcaseofcrowdinganddevelopmentaldyslexia.

JournalofVision,15(1):8,1-20.

Goswami,U.(2011).Atemporalsamplingframeworkfordevelopmental

dyslexia.TrendsinCognitiveSciences,15,3–10.

37

Goswami,U.(2015).Sensorytheoriesofdevelopmentaldyslexia:three

challengesforresearch.NatureReviewsNeuroscience,16,43–54.

Gottlieb,G.(1983).Thepsychobiologicalapproachtodevelopmentalissues.InM.

M.Haith&J.J.Campos(Eds.),Handbookofchildpsychology(4thed.,vol.2,

pp.1-26).NewYork:JohnWiley&Sons.

Gottlieb,G.(1991).Experientialcanalizationofbehavioraldevelopment:Theory.

DevelopmentalPsychology,27,4-13.

Gottlieb,G.(1997).Synthesizingnature-nurture:Prenatalrootsofinstinctive

behavior.Mahwah,NJ:LawrenceErlbaumAssociates.

Gottlieb,G.(2002).Individualdevelopmentandevolution.Mahwah,NJ:Lawrence

ErlbaumAssociates.

Gottlieb,G.&Halpern,C.T.(2002).Arelationalviewofcausalityinnormaland

abnormaldevelopment.DevelopmentandPsychopathology,14,421-435.

Gottlieb,G.,Wahlsten,D.,&Lickliter,R.(2006).Thesignificanceofbiologyfor

humandevelopment:Adevelopmentalpsychobiologicalsystemsview.In

R.M.Lerner&W.Damon(Eds.),Handbookofchildpsychology(6ed.,Vol.

1,pp.210-257).Hoboken,NJ:JohnWiley&Sons.

Griffiths,P.E.&Tabery,J.(2013).Developmentalsystemstheory:Whatdoesit

explain,andhowdoesitexplainit?InR.M.Lerner&J.B.Benson(eds.),

Advancesinchilddevelopmentandbehavior(vol.44,pp.65-94).Waltham,

MA:AcademicPress.

Hallgren,B.(1950).Specificdyslexia(congenitalword-blindness);aclinicaland

geneticstudy.ActaPsychiatricaetNeurologicaScandinavica

Supplementum,65,1-287.

38

Hazan,V.,Messaoud-Galusi,S.,Rosen,S.,Nouwens,S.,&Shakespeare,B.(2009).

Speechperceptionabilitiesofadultswithdyslexia:isthereanyevidence

foratruedeficit?JournalofSpeech,Language,andHearingResearch,52,

1510–1529.

InternationalDyslexiaAssociation(2002).Definitionofdyslexia.RetrievedMay

15,2015,fromhttp://eida.org/definition-of-dyslexia/

Jablonka,E.&Lamb,M.J.(2014).Evolutioninfourdimensions(rev.ed.).

Cambridge,MA:TheMITPress.

Jednorog,K.,Marchewka,A.,Altarelli,I.,MonzalvoLopez,A.K.,vanErmingen-

Marbach,M.,Grande,M.,Grabowska,A.,Heim,S.&Ramus,F.(2015).How

reliablearegraymatterdisruptionsinspecificreadingdisabilityacross

multiplecountriesandLanguages?InsightsfromaLarge-scalevoxel-

basedmorphometrystudy.HumanBrainMapping,36,1741-1754.

Jones,M.W.,Branigan,H.P.,&Kelly,M.L.(2009).Dyslexicandnondyslexic

readingfluency:Rapidautomatizednamingandtheimportanceof

continuouslists.PsychonomicBulletin&Review,16,567–572.

Kere,J.(2014).Themoleculargeneticsandneurobiologyofdevelopmental

dyslexiaasmodelofacomplexphenotype.BiochemicalandBiophysical

ResearchCommunications,452,236–243.

Kirby,J.R.,Georgiou,G.K.,Martinussen,R.,&Parrila,R.(2010).Namingspeed

andreading:Frompredictiontoinstruction.ReadingResearchQuarterly,

45,341–362.

Kirby,J.R.,Parrila,R.K.&Pfeiffer,S.L.(2003).Namingspeedandphonological

awarenessaspredictorsofreadingdevelopment.JournalofEducational

Psychology,95,453-464.

39

Krafnick,A.J.,Flowers,D.L.,Luetje,M.M.,Napoliello,E.M.&Eden,G.F.(2014)

Aninvestigationintotheoriginofanatomicaldifferencesindyslexia.The

JournalofNeuroscience,34,901-908.

Linkersdörfer,J.,Lonnemann,J.,Lindberg,S.,Hasselhorn,M.,&Fiebach,C.J.

(2012).Greymatteralterationsco-localizewithfunctionalabnormalities

indevelopmentaldyslexia:AnALEmeta-analysis.PLoSONE,7(8),e43122.

doi:10.1371/journal.pone.0043122

Lobier,M.A.,Peyrin,C.,Pichat,C.,LeBas,J.F.,&Valdois,S.(2014).Visual

processingofmultipleelementsinthedyslexicbrain:Evidencefora

superiorparietaldysfunction.FrontiersinHumanNeuroscience,8,479.

Lobier,M.,Zoubrinetzky,R.,&Valdois,S.(2012).Thevisualattentionspandeficit

indyslexiaisvisualandnotverbal.Cortex,48,768–773.

Lyytinen,H.,Ahonen,T.,Aro,M.,Aro,T.,Närhi,V.,&Räsänen,P.(1998).Learning

disabilities:Aviewofdevelopmentalneuropsychology.InR.Licht,A.

Bouma,W.Slot,&W.Koops(Eds.),Childneuropsychology:Reading

disabilityandmore.Delft,NL:Eburon.

Magnuson,J.S.,Kukona,A.,Braze,D.,Johns,C.L.,VanDyke,J.A.,Tabor,W.,Mencl,

W.E.,Pugh,K.R.,&Shankweiler,D.(2011).Phonologicalinstabilityin

youngadultpoorreaders:Timecoursemeasuresandcomputational

modeling.InP.McCardle,B.Miller,J.R.Lee,&O.J.L.Tseng(Eds.).Dyslexia

acrosslanguages(pp.184–201).Baltimore,MD:PaulH.Brookes.

Manis,F.R.,Seidenberg,M.S.,Doi,L.M.,McBride-Chang,C.,&Petersen,A.

(1996).Onthebasesoftwosubtypesofdevelopmentdyslexia.Cognition,

58,157–195.

40

Manis,F.R.,Seidenberg,M.S.,Stallings,L.,Joanisse,M.,Bailey,C.,Freedman,L.,...

&Keating,P.(1999).Developmentofdyslexicsubgroups.Annalsof

Dyslexia,49,105–134.

Maisog,J.M.,Einbinder,E.R.,Flowers,D.L.,Turkeltaub,P.E.&Eden,G.F.(2008).

Ameta-analysisoffunctionalneuroimagingstudiesofdyslexia.Annalsof

theNewYorkAcademyofSciences,1145,237–259.

McCardle,P.,Miller,B.,Lee,J.R.,&Tzeng,O.J.L.(2011).Dyslexiaacross

languages:Orthographyandthebrain-gene-behaviorlink.Baltimore,MD:

PaulH.Brookes.

Molenaar,P.C.M.,Lerner,R.M.,&Newell,K.M.(eds.)(2014).Handbookof

developmentalsystemstheoryandmethodology.NewYork:Guilford.

Nation,K.,&Snowling,M.(1998).Semanticprocessingandthedevelopmentof

word-recognitionskills:Evidencefromchildrenwithreading

comprehensiondifficulties.JournalofMemoryandLanguage,39,85–101.

Noordenbos,M.W.,Segers,E.,Serniclaes,W.,&Verhoeven,L.(2013).Neural

evidenceoftheallophonicmodeofspeechperceptioninadultswith

dyslexia.ClinicalNeurophysiology,124,1151–1162.

Nopola-Hemmi,J.,Taipale,M.,Haltia,T.,Lehesjoki,A-E.,Voutilainen,A.&Kere,J.

(2000).Twotranslocationsofchromosome15qassociatedwithdyslexia.

JournalofMedicalGenetics,37,771-775.

Norton,E.S.,&Wolf,M.(2012).Rapidautomatizednaming(RAN)andreading

fluency:Implicationsforunderstandingandtreatmentofreading

disabilities.AnnualReviewofPsychology,63,427–452.

Nurmi,J.-E.(2012)Students’characteristicsandteacher–childrelationshipsin

instruction:Ameta-analysis.EducationalResearchReview,7,177–197.

41

Parrila,R.K.,Kirby,J.R.,McQuarrie,L.(2004).Articulationrate,namingspeed,

verbalshort-termmemory,andphonologicalawareness:Longitudinal

predictorsofearlyreadingdevelopment?ScientificStudiesofReading,8,

3-26.

Perfetti,C.A.(1992).Therepresentationprobleminreadingacquisition.InP.B.

Gough,L.C.Ehri,&R.Treiman(Eds.),Readingacquisition(pp.145–174).

Hillsdale,NJ:LawrenceErlbaum.

Pennington,B.F.(2006).Fromsingletomultipledeficitmodelsofdevelopmental

disorders.Cognition,101,385-413.

Pennington,B.F.,Santerre-Lemon,L.,Rosenberg,J.,McDonald,B.,Boada,R.,

Friend,A.…Olson,R.K.(2012).Individualpredictionofdyslexiabysingle

versusmultipledeficitmodels.JournalofAbnormalPsychology,121,212-

224.

Plaut,D.C.,McClelland,J.L.,Seidenberg,M.S.,&Patterson,K.(1996).

Understandingnormalandimpairedwordreading:Computational

principlesinquasi-regulardomains.PsychologicalReview,103,56–115.

Protopapas,A.(2014)Fromtemporalprocessingtodevelopmentallanguage

disorders:mindthegap.PhilosophicalTransactionsoftheRoyalSocietyB:

BiologicalSciences,369(1634),1-11.

Protopapas,A.,Altani,A.,&Georgiou,G.K.(2013).Developmentofserial

processinginreadingandrapidnaming.JournalofExperimentalChild

Psychology,116,914–929.

Pugh,K.R.,Mencl,W.E.,Jenner,A.R.,Katz,L.,Frost,S.J.,Lee,J.R.,etal.(2000).

Functionalneuroimagingstudiesofreadingandreadingdisability

(developmentaldyslexia).MentalRetardation&Developmental

42

DisabilitiesResearchReviews,6,207–213.

Rack,J.P.,Snowling,M.J.,&Olson,R.K.(1992).Thenonwordreadingdeficitin

developmentaldyslexia:Areview.ReadingResearchQuarterly,27,29–53.

Railsback,S.F.&Grimm,V.(2011).Agent-basedandindividual-basedmodeling:A

practicalintroduction.Princeton,NJ:PrincetonUniversityPress.

Ramus,F.(2003).Developmentaldyslexia:specificphonologicaldeficitor

generalsensorimotordysfunction?CurrentOpinioninNeurobiology,13,

212–218.

Ramus,F.,&Ahissar,M.(2012).Developmentaldyslexia:Thedifficultiesof

interpretingpoorperformance,andtheimportanceofnormal

performance.CognitiveNeuropsychology,29,104–122.

Ramus,F.,Rosen,S.,Dakin,S.C.,Day,B.L.,Castellote,J.M.,White,S.,&Frith,U.

(2003).Theoriesofdevelopmentaldyslexia:insightsfromamultiplecase

studyofdyslexicadults.Brain,126,841–865.

Ramus,F.,&Szenkovits,G.(2008).Whatphonologicaldeficit?TheQuarterly

JournalofExperimentalPsychology,61,129–141.

Ramus,F.,&Szenkovits,G.(2009).Understandingthenatureofthephonological

deficit.InK.Pugh&P.McCardle(Eds.),Howchildrenlearntoread(pp.

153–169).NewYork,NY:PsychologyPress.

Richlan,F.,Kronbichler,M.,&Wimmer,H.(2009).Functionalabnormalitiesin

thedyslexicbrain:Aquantitativemeta-analysisofneuroimagingstudies.

HumanBrainMapping,30,3299–3308.

Richlan,F.,Kronbichler,M.,&Wimmer,H.(2011).Meta-analyzingbrain

dysfunctionsindyslexicchildrenandadults.Neuroimage,56,1735–1742.

Richlan,F.,Kronbichler,M.,&Wimmer,H.(2013).Structuralabnormalitiesinthe

43

dyslexicbrain:Ameta-analysisofvoxel-basedmorphometrystudies.

HumanBrainMapping,34,3055–3065.

Ricketts,J.,Nation,K.,&Bishop,D.V.M.(2007).Vocabularyisimportantforsome,

butnotallreadingskills.ScientificStudiesofReading,11,235–257.

Rosen,S.,&Manganari,E.(2001).Istherearelationshipbetweenspeechand

nonspeechauditoryprocessinginchildrenwithdyslexia?Journalof

Speech,Language,andHearingResearch,44,720–736.

Ruffino,M.,Gori,S.,Boccardi,D.,Molteni,M.,&Facoetti,A.(2014).Spatialand

temporalattentionindevelopmentaldyslexia.FrontiersinHuman

Neuroscience,8,331.

Serniclaes,W.,VanHeghe,S.,Mousty,P.,Carré,R.,&Sprenger-Charolles,L.

(2004).Allophonicmodeofspeechperceptionindyslexia.Journalof

ExperimentalChildPsychology,87,336–361.

Share,D.L.(2008).Ontheanglocentricitiesofcurrentreadingresearchand

practice:Theperilsofoverrelianceonan“outlier”orthography.

PsychologicalBulletin,134,584-615.

Shaywitz,S.E.(1996).Dyslexia.ScientificAmerican,275(5),98-104.

Simos,P.G.,Fletcher,J.M.,Bergman,E.,Breier,J.I.,Foorman,B.R.,Castillo,E.M.,

Davis,R.N.,Fitzgerald,M.,&Papanicolaou,A.C.(2002).Dyslexia-specific

brainactivationprofilebecomesnormalfollowingsuccessfulremedial

training.Neurology,58,1203-1213.

Snowling,M.J.(2000).Dyslexia(2nded.).Oxford,UK:Blackwell.

Snowling,M.J.(2013).Earlyidentificationandinterventionsfordyslexia:A

contemporaryview.JournalofResearchinSpecialEducationalNeeds,13

(1),7-14.

44

Snowling,M.J.&Melby-Lervåg,M.(2016,January4).Orallanguagedeficitsin

familialdyslexia:Ameta-analysisandreview.PsychologicalBulletin.

Advanceonlinepublication.http://dx.doi.org/10.1037/bul0000037.

Stanovich,K.E.,Siegel,L.S.,&Gottardo,A.(1997).Convergingevidencefor

phonologicalandsurfacesubtypesofreadingdisability.Journalof

EducationalPsychology,89,114–127.

Steacy,L.M.,Kirby,J.R.,Parrila,R.,&Compton,D.L.(2014).Classificationof

doubledeficitgroupsacrosstime:Ananalysisofgroupstabilityfrom

kindergartentosecondgrade.ScientificStudiesofReading,18,255–273.

Tabery,J.(2014).Beyondversus:Thestruggletounderstandtheinteraction

betweennatureandnurture.Cambridge,MA:TheMITPress.

Tallal,P.(1980).Auditorytemporalperception,phonics,andreadingdisabilities

inchildren.BrainandLanguage,9,182–198.

Thelen,E.&Smith,L.B.(1994).Adynamicsystemsapproachtothedevelopment

ofcognitionandaction.Cambridge,MA:TheMITPress.

Torppa,M.,Eklund,K.,vanBergen,E.,&Lyytinen,H.(2015).Late-emergingand

resolvingdyslexia:Afollow-upstudyfromage3to14.Journalof

AbnormalChildPsychology,43,1389-1401.

Torppa,M.,Parrila,R.,Niemi,P.,Poikkeus,A.-M.,Lerkkanen,M.-K.,&Nurmi,J.-E.

(2013).ThedoubledeficithypothesisinthetransparentFinnish

orthography:alongitudinalstudyfromkindergartentograde2.Reading

andWriting:AnInterdisciplinaryJournal,26,1353-1380.

Tunmer,W.&Greaney,K.(2010).Definingdyslexia.JournalofLearning

Disabilities,43,229-243.

vanBergen,E.,Bishop,D.V.M.,vanZuijen,T.L.,&deJong,P.F.(2015).Howdoes

45

parentalreadinginfluencechildren’sreading.Astudyofcognitive

mediation.ScientificStudiesofReading,19,325-339.

vanBergen,E.,vanderLeij,A.&deJong,P.F.(2014).Theintergenerational

multipledeficitmodelandthecaseofdyslexia.FrontiersinHuman

Neuroscience,8,346.

vandenBoer,M.,Georgiou,G.K.,&deJong,P.F.(2016).Namingofshortwords

is(almost)thesameasnamingofalphanumericsymbols:Evidencefrom

twoorthographies.JournalofExperimentalChildPsychology,144,152–

165.

VandenBroeck,W.,&Geudens,A.(2012).Oldandnewwaystostudy

characteristicsofreadingdisability:Thecaseofthenonword-reading

deficit.CognitivePsychology,65,414–456.

Vandermosten,M.,Boets,B.,Wouters,J.&Ghesquière,P.(2012).Aqualitative

andquantitativereviewofdiffucsiontensorimagingstudiesinreading

anddyslexia.NeuroscienceandBiobehavioralReviews,36,1532–1552

Velicer,W.F.,Babbin,S.F.,&Palumbo,R.(2014).Idiographicapplications:Issues

ofergodicityandgeneralizability.InP.C.M.Molenaar,R.M.,Lerner&K.

M.Newell(eds.),Handbookofdevelopmentalsystemstheoryand

methodology(pp.425-441).NewYork:Guilford.

Vellutino,F.R.,Scanlon,D.M.,Sipay,E.R.,Small,S.G.,Pratt,A.,Chen,R.S.,&

Denckla,M.S.(1996).Cognitiveprofilesofdifficult-to-remediateand

readilyremediatedpoorreaders:Earlyinterventionasavehiclefor

distinguishingbetweencognitiveandexperientialdeficitsasbasiccauses

ofspecificreadingdisability.JournalofEducationalPsychology,88,601–

638.

46

Vellutino,F.R.,Fletcher,J.M.,Snowling,M.J.,&Scanlon,D.M.(2004).Specific

readingdisability(dyslexia):Whathavewelearnedinthepastfour

decades?JournalofChildPsychologyandPsychiatry,45,2–40.

Vidyasagar,T.R.,&Pammer,K.(2010).Dyslexia:adeficitinvisuo-spatial

attention,notinphonologicalprocessing.TrendsinCognitiveSciences,14,

57–63.

Waters,C.K.(2007).Causesthatmakeadifference.JournalofPhilosophy,104,

551-579.

Wilensky,U.&Rand,W.(2015).Anintroductiontoagent-basedmodeling:

Modelingnatural,socialandengineeredcomplexsystemswithNetLogo.

Cambridge,MA:TheMITPress.

Wolf,M.,&Bowers,P.G.(1999).Thedouble-deficithypothesisforthe

developmentaldyslexias.JournalofEducationalPsychology,91,415–438.

Wolf,M.,Bowers,P.G.,&Biddle,K.(2000).Naming-speedprocesses,timing,and

reading:Aconceptualreview.JournalofLearningDisabilities,33,387–

407.

Woollams,A.M.(2014).Connectionistneuropsychology:Uncoveringultimate

causesofacquireddyslexia.PhilosophicalTransactionsoftheRoyalSociety

B:BiologicalSciences,369,20120398.

Ziegler,J.C.,&Goswami,U.(2005).Readingacquisition,developmentaldyslexia,

andskilledreadingacrosslanguages:Apsycholinguisticgrainsizetheory.

PsychologicalBulletin,131,3–29.

Ziegler,J.C.,Pech-Georgel,C.,Dufau,S.,&Grainger,J.(2010).Rapidprocessingof

letters,digitsandsymbols:whatpurelyvisual-attentionaldeficitin

developmentaldyslexia?DevelopmentalScience,13,F8–F14.

47

Zoccolotti,P.,DeLuca,M.,Lami,L.,Pizzoli,C.,Pontillo,M.,&Spinelli,D.(2013).

Multiplestimuluspresentationyieldslargerdeficitsinchildrenwith

developmentaldyslexia:AstudywithreadingandRAN-typetasks.Child

Neuropsychology,19,639–647.

Zoccolotti,P.,DeLuca,M.,&Spinelli,D.(2015).Discreteversusmultipleword

displays:Are-analysisofstudiescomparingdyslexicandtypically

developingchildren.FrontiersinPsychology,6,01530.

Zoubrinetzky,R.,Bielle,F.,&Valdois,S.(2014).Newinsightsondevelopmental

dyslexiasubtypes:Heterogeneityofmixedreadingprofiles.PloSone,9,

e99337.

48

Figure1.Pennington’s(2006)ProbabilisticMultipleDeficitModel.Notethat

causalconnections(andfeedbackloops)betweenlevelsnotshowninthefigure

butacknowledgedinthetext.

49

Figure2.vanBergen,vanderLeijanddeJong’s(2014)intergenerational

multipledeficitmodel(seevanBergenetal.,2014,fordetaileddescriptionsof

components).