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Lecture 22

PHIL-UA 73 Lecture 22: Two Visual Systems (when Block was late and forgot his slides)

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New York University
Ned Block

12/5/17 TwoVisualSystems Ned Block 1) Fast a. Feeds tothetopthehead, dorsalvisualsystems 2) Slow a. Feeds toventralvisualsystemin theoccipitallobe andthen intothetemporallobe The most dramatic discoverywas the onefoundbyAndyClark, in which2classesof patientswere discovered: - Patients have onefunctionwithoutthe other (vise-versa) - Rarest/mostunclear patient: D.F. o Something in DFs shower emitted carbon dioxide,which destroyed a part of her brain between thetemporalandoccipital lobesthe lateral occipitalcomplex. Shehad visual-form agnosia.Shecould distinguishcolors,shapes,andtextures,but notshadesorforms.In particular,shewas atchanceonsayingwhethersomething was horizontalorvertical.She could pushacardthrougharotatingslot almostas goodatthis asa normalperson,butshecould notsaywhethertheslotwas horizontalorvertical. Dorsalactivity =good; ventral activity =bad Shecandraw,butcannotrecognizeherowndrawing. - Optic ataxia:peoplewhocandescribewhattheysee, butcannot pickthings upormovethem. Theyare less purethantheotherin thatthe areasthatcontrolmotionaremuchlarger. Youneeda lot ofdamage to Difference - Dorsal has almostno memory, but the ventral system does o If she is starting topostsomething andthencloseshereyes,shecannotrememberwhere things andheractions areduetochance - Dorsalspecialized for fast behavior (ex: dribbling a ball) o Clark: thepurposeof theventralsystem is toplanvisually guidedactions.Buttheexecution of themis done bythedorsalsystem. - Dorsal feeds to peripheral o Some people havetheexperience ofrunningdownthe beach andavoiding thestoneswithout realizing it. Youareseeingthestones unconsciouslyand feetare guidedtomiss them - Illusions of the ventralsystem thatdo not fool the dorsalsystem o Afew oftheseare onesthatfooltheeye,butnotthehand Ex 1)Group ofcircles one of thecircles looks biggertotheeyebecausetherearea group ofsmallercirclesaroundit. However,thehand is notfooledandtheperson reachesforthe bigger one (notsurrounded bycircles) Ex2)Seriesof lights thesubjects handwas below them,andwhenone of thelights lit up,subjectwassupposedtomove his handtothelocationand touch it. Intervention:while thesubjectshandwas inmotion,theresearcher changedthelight. Thesubjectsfoundthemselvesreachingfortherightone, but onlyseeingthechangeof color aftertheirhandwasalreadyattheright spotCanrevealthatsomething elseis guiding theiractions otherthan theirconsciousbrain.As ifsomeoneelsewasdirectingthem.Unconscious vision of thechangecausedachange of trajectory;therefore action is largely guidedbyunconsciousvisualsystems. Weallhaveazombiewithin us asignificantcomponentof ourmentallife is going onwithoutanycontactwiththementallifethatyouthinkof as yours Commonsenseconception of ourselves isseriouslywrong Split-brainphenomenon - Brain has twohemispheres o Visualaction occurs in both;thehemispherescommunicate byaband offiberscalled the corpus callosum. Insomepatients,thatbridgehas beencut.Peoplewhoappeartohave differentcenters ofconsciousness inthe differenthemispheres. o In anexperiment,asplit-brained patient hastwodifferentitems infrontof him --onein each visualsystem(leftandright). The left hemispherecontrolsspeech,soitcontrols voiceand whatwesayaboutwhats onthe rightside. The left handis controlled bytherightsideandwill picksomethingthattherighthemisphere isseeing. Therefore,if there is achicken ontheleft andashovelontheright, hewillsayshovel,butreachforthechicken. Confabulation:theshovelforshoveling outthechickenshit - This experimentsuggests that ourconcept otheself reallystartstocrumblewhenwefindoutnew things aboutthemind and brain.Webegin towonderif thenotion of thesel
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