Visual Neglect - Cog Sci

Visual neglect is a common neurological syndrome in which patients fail to acknowledge stimuli toward the side of space opposite to their unilateral lesion. VisualNeglect Visualneglectis acommonneurologicalsyndromeinwhichpatientsfailtoacknowledgestimuli towardthesideofspaceoppositetotheirunilaterallesion.Thisdisability affectsmanyaspectsoftheirlife.Forexample,afterarightlesion,patients typicallyfailtoeatthefoodlocatedontheleftsideoftheirplate,orto shaveormakeuptheleftsideoftheirface,and,inextremecases,mayno longeracknowledgetheleftsideoftheirbodyastheirown.Inacommon clinicaltestknownaslinecancellation,theyfailtomarklinestowardthe leftofthepage(seefigure1A)despiteoftenbeingabletodetectisolated linespresentedintheirleftvisualfield,demonstratingthattheyarenot simplyblindonthatside. Figure1A.Leftitemsneglectedinacancellation task.B.VisualdisplaysfromtheDriveretal.(1994)experiment.Thecross indicatesthefixationpoint.Left-neglectpatientsperformedbetterforthe bottomconfigurationthanthetopone,eventhoughthegaptobedetectedwas atthesameretinallocation.Thispatternisconsistentwithleft object-centeredneglect.C.Visualreceptivefieldofatypicalmonkeyparietal neuron,forthreedifferenteyepositions.Theretinotopicpositionofthe receptivefieldisinvariantacrosseyepositionsbutthegainoftheresponse changes.(AdaptedfromAndersen,Issick,andSiegel1985.) Thissyndromeisobserved primarilyafterunilaterallesionstotheparieto-occipitaljunction, especiallyintherighthemisphere(Heilman,Watson,andValenstein1985; Bisiach1996).Lesionstotherightfrontalcortexandtovarioussubcortical sitescanalsotriggerneglect-likesymptoms,althoughwithsubtledifferences fromparietalneglect(Heilmanetal.1985;Guarigliaetal.1993).Herewe concentrateonparietalneglect,asitisthemostcommonform,andcanbe relatedtorecentdataontheparietallobefromnonhumanprimates. Twomajoraccountshavebeen proposedforneglect.Sometheoriespositadeficitindirectingattention towardcontralesionalevents(Posneretal.1984;Kinsbourne1987;seeATTENTIONandATTENTIONINTHEHUMAN BRAIN).Forinstance,rightparietalpatients--whosufferfromleft neglect--tendtohaveparticulardifficultyindetectingstimuliintheleft hemifieldiftheirattentionhaspreviouslybeendrawntotherightside (Posneretal.1984).Bycontrast,many"preattentive"aspectsofvision appeartobesparedontheaffectedside(Driver,Baylis,andRafal1992; McGlinchey-Berrothetal.1996;Mattingley,Davis,andDriver1997). Otheraccountsarguethatthe patient'slesionsimplydisruptstheneuronalcodingofcontralesionalspace, atrelativelyhighlevelsofrepresentation(BisiachandLuzzatti1978; Bisiach,Luzzati,andPerani1979;RizzolattiandBerti1990;Halliganand Marshall1991;Karnath,Schenkel,andFischer1991).Thisperspectivehasdrawn supportfromthefindingthatevenmentalIMAGERYcanbe impairedinsomeleft-neglectpatients(BisiachandLuzzatti1978),suchthat theyfailtoreportwhatwouldappearontheirleftwhenretrievingfrommemory theviewofafamiliarvisualscene. Thedichotomybetween attentionalandrepresentationalaccountshasrecentlybeenchallengedby severalauthorsusingneuralnetworkmodelsinwhichattentionaland representationalfunctionsareinterwoven(MozerandBehrmann1990;Cohenet al.1994;PougetandSejnowski1997a).Thisworksuggestsacompromiseview, wherebyneglectresultsfromdamagetocorticalareasthatarelocatedatthe interfacebetweensensoryandmotorsystems,andwhichareresponsibleforboth therepresentationofthepositionofobjectsandtheselectivecontrolof spatialaction,thatistosay,"attention." Framesofreference:In principle,"left"neglectmightrefertotheleftofthevisualfield, ortheleftofthehead,orthetrunk,orevenofthesurroundingenvironment. Todeterminetheframeofreferenceforhemineglect,onecantestpatientsin variouspostures,sothatastimuluslocationchangesinoneframeofreference whileremainingconstantintheothers.Forinstance,onemighttestapatient lookingstraightaheadvs.withthegazedeviatedtwentydegreestotheright, whilekeepingallstimuliatthesamepositionwithrespecttotheRETINA.If neglectwerepurelyretinotopic,theseconditionsshouldnotdiffer,whereasif itwerehead-orbody-centered,performanceshouldchangeaccordingly.Such experimentshavetypicallyrevealedthatneglectaffectsamixtureofframesof referenceconcurrently,ratherthanjustonesingleframe.Thus,the probabilitythatapatientwillneglectaparticularvisualstimulusis typicallyafunctionofitspositioninvariousegocentricframesofreference, suchaseye-,head-ortrunk-centered,aswellasshowinginfluencesfromcues intheenvironment,forexample,asregardsthegravitationalupright(Bisiach, Capitani,andPorta1985;Ladavas1987;Ladavas,Pesce,andProvincial,1989; Calvanio,Petrone,andLevine1987;Farahetal.1990;Karnathetal.1991; BehrmannandMoscovitch1994). Afewexperimentssuggestthat visualneglectcanalsobe"object-centered,"thatis,patientstend toneglecttheleftsideofanobjectregardlessofitspositionororientation (Driveretal.1994;TipperandBehrmann1996).Forexample,Driveretal. (1994)devisedasituationinwhichleft-neglectpatientscoulddetectagapin partofatrianglewhenthisgapwasperceivedtobeontherightsideofan object,butmissedthesamegapwhenitwasseenasbelongingtotheleftside, eventhoughitstillfellatthesamelocationrelativetothepatient(figure 1B).Suchresultsseemconsistentwiththeexistenceofobject-centered representationsintheparietalcortex. Manyotherstudiesclaimtohave foundevidenceforobject-centeredneglect(DriverandHalligan1991;Arguin andBub1993;HalliganandMarshall1994),butaspointedoutbyDriveretal. (1994),theirresultscouldbeexplainedinsteadbywhatwewillcallrelative neglectinstrictlyegocentriccoordinates(seealsoKinsbourne1987;Mozerand Behrmann1990;DesimoneandDuncan1995;andPougetandSejnowski1997afor variationsonthisidea).Whenconfrontedwithtwocompetingobjects,patients mayneglecttheonefarthertotheleftevenifbothfallintheright hemispaceegocentrically,andlikewiseforthesubpartsofasingleobject (DriverandHalligan1991;Driveretal.1992;Driveretal.1994;Halliganand Marshall1994).Thus,itappearsthattherelativepositionofobjects ortheirsubpartsisjustasimportantastheirabsolutepositionwith respecttothepatient.Thisphenomenoncanbeexplainedifthelesioninduces agradientofneglectwithincreasingseverityintheegocentric contralesionaldirection(Kinsbourne1987;Driveretal.1994;Pougetand Sejnowski1997a). Neuralbasis:Therehavebeen severalattemptstorelateneglecttowhatisknownoftheresponseproperties ofparietalneuronsfromsingle-cellrecordingsinmonkeys(MozerandBehrmann 1990;Duhameletal.1992;Anderson1996;Mozer,Halligan,andMarshall1997; PougetandSejnowski1997a;seealsoMODELING NEUROPSYCHOLOGICALDEFICITSandSPATIALPERCEPTION). Suchmodelsgenerallyrelyoncellsintheparietalcortexhavingretinotopic receptivefields,witheachhemispheretendingtooverrepresentthe contralateralvisualfield(see,however,Duhameletal.1992foradifferent approach).Consequently,arightlesionleadstoaneuronalgradientinwhich theleftsideoftheretinaislessstronglyrepresentedthantherightside, producingleftneglect.Insuchmodels,thereisnoparticulardividingmidline suchthatanystimulustotheleftofitisinvariablyneglected.Instead, neglectdependsonlyontherelativepositionofcompetingstimuli,as discussedabove,withobjectsorobjectpartsthatarefarthertowardthe retinalleftthantheircompetitorsbeingneglected.Thesemodelsreadily capturethebehaviorofpatientsintaskssuchaslinebisection,line cancellation,andinsomeoftheparadigmsdiscussedabovethathaverevealed relativeneglect. Parietalneurons,however,do notsimplyrespondtovisualstimulation,butalsointegratesensoryresponses withposturesignalssuchaseyeandheadposition.Andersenandcolleagues haveshownthattheretinotopicreceptivefieldsofparietalcellsaregain-modulated bysuchposturesignals(Andersen,Essick,andSiegel1985;Andersenetal. 1997;seefigure1Cforanexampleinwhichthevisualreceptivefieldofa cellismodulatedbyeyeposition).Theseresponsepropertiescanbemodeledas basisfunctionsoftheinputs,atypeoffunctionwhichis particularlywell-suitedtothecomputationaldemandofsensorimotor transformations(PougetandSejnowski1997b). Asimulatedunilaterallesionin suchabasis-functionrepresentationproducesanimpairmentthatresembles clinicalneglect,inthatthedeficitaffectsamixtureofegocentricframesof referenceasfoundinpatients(PougetandSejnowski1997a).Thisapproachcan alsobegeneralizedtoencompassobject-centeredneglect,asintheDriveret al.(1994)experimentdepictedinfigure1B,byconsideringtheperceived orientationoftheobjectasprovidingasignalanalogoustotheposture signalsintegratedbythebasisfunctions(DeneveandPougetinpress).This basis-functionframeworkcanexplainwhyneglectmaybeinfluencedbystimulus positionrelativetotheretina,head,body,otherobjects,andotherpartsof thesameobject,allatthesametime,withoutrequiringcellsintheparietal cortextohavevisualreceptivefieldsexplicitlydefinedinanysingleoneof theseframesofreference. Neglectremainsafascinating butdisablingdisorder,whichstillposesamajorchallengetorehabilitation. Itsfurtherstudywillhopefullyleadtomoreeffectivetreatments,aswellas revealmoreabouthowthebrainrepresentsspace,andallowsforselective spatialattention. Seealso OBJECT RECOGNITION,HUMANNEUROPSYCHOLOGY PHANTOM LIMB SELF SELF-KNOWLEDGE Additionallinks SAIM:A ModelofVisualAttentionandNeglect Visual neglectasa"disengagefailure" --AlexandrePouget andJonDriver ReferencesandFurtherReadings Anderson,B. (1996).Amathematicalmodeloflinebisectionbehaviourinneglect.Brain 119:841-850. Andersen,R.A., G.K.Essick,andR.M.Siegel.(1985).Encodingofspatiallocationby posteriorparietalneurons.Science230:456-458. Andersen,R.A., L.H.Snyder,D.C.Bradley,andJ.Xing.(1997).Encodingofintentionand spatiallocationintheposteriorparietalcortex.Ann.Rev.Neurosci. 20:303-330. Arguin,M.,andD. N.Bub.(1993).Evidenceforanindependentstimulus-centeredreferenceframe fromacaseofvisualhemi-neglect.Cortex29:349-357. 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