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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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Ned Block

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Two Visual Systems
Ned Block
1) Fast
a. Feeds to the top the head, dorsal visual systems
2) Slow
a. Feeds to ventral visual system in the occipital lobe and then into the temporal lobe
The most dramatic discovery was the one found by Andy Clark, in which 2 classes of patients were discovered:
- Patients have one function without the other (vise-versa)
- Rarest/most unclear patient: D.F.
o Something in DF’s shower emitted carbon dioxide, which destroyed a part of her brain between
the temporal and occipital lobes the lateral occipital complex.
She had visual-form agnosia. She could distinguish colors, shapes, and textures, but
not shades or forms. In particular, she was at chance on saying whether something
was horizontal or vertical. She could push a card through a rotating slot almost as
good at this as a normal person, but she could not say whether the slot was
horizontal or vertical.
Dorsal activity = good; ventral activity = bad
She can draw, but cannot recognize her own drawing.
- Optic ataxia: people who can describe what they see, but cannot pick things up or move them. They are
less pure than the other in that the areas that control motion are much larger. You need a lot of damage
- Dorsal has almost no memory, but the ventral system does
o If she is starting to post something and then closes her eyes, she cannot remember where
things and her actions are due to chance
- Dorsal specialized for fast behavior (ex: dribbling a ball)
o Clark: “the purpose of the ventral system is to plan visually guided actions. But the execution of
them is done by the dorsal system.”
- Dorsal feeds to peripheral
o Some people have the experience of running down the beach and avoiding the stones without
realizing it. You are seeing the stones unconsciously and feet are guided to miss them
- Illusions of the ventral system that do not fool the dorsal system
o A few of these are ones that fool the eye, but not the hand
Ex 1) Group of circles one of the circles
bigger to the eye because there are a
group of smaller circles around it. However, the hand is not fooled and the person
reaches for the bigger one (not surrounded by circles)
Ex 2) Series of lights the subjects hand was below them, and when one of the lights
lit up, subject was supposed to move his hand to the location and touch it.
Intervention: while the subject’s hand was in motion, the researcher
changed the light. The subjects found themselves reaching for the right one,
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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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