Chapter 13 The Occipital Lobe Anatomy of the Occipital Lobe Subdivision of the occipital cortex Connections of the Visual Cortex

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1 Chapter 13 The Occipital Lbe Anatmy f the Occipital Lbe - Frm the psterir ple f the cerebral hemisphere - Subdivisin f the ccipital crtex Mnkey s crtex was first divided by Brdmann int 3 regins area 17, 18 & 19 Difficulty in cmparing mnkey and human maps is that the mnkey maps are based n anatmy and cnnectins, whereas the human mas are heavily based n nn-invasive imaging such as fmri Names f sme regins in the human map are based n functinal infrmatin in mnkeys that later prved incrrect Ex. V4 believed t be a clur regin in mnkeys but TE and TEO actually prcess clur Because V4 in humans was named by using the earlier infrmatin abut the mney V4 regin, v4 in humans, alng with v8 are clur regins Fig tpgraphy f the visual crtex f the macaque mnkey Fred Previc the upper and lwer fields may have different functins with upper mre specialized fr visual search and recgnitin and the lwer mre specialized fr visumtr guidance V1 cmplex laminar rganizatin lts f layers Appears t anatmically hmgenus, it can als be shwn t actually be hetergeneus by staining it fr the enzyme cytchrme xidase, which is crucial in making energy available t cells (unexpected) Regins f cytchrme rich areas the blbs, are separated by interblb regins f little cytchrme activity Cells in the blbs take part in clur perceptin and the interblbs have a rle in frm and mtin perceptin Area V2 hetergeneus when stained with cytchrme xidase stripes are revealed instead f blbs Distinct stripes called striate crtex Thin stripes: clur perceptin Thick stripes and pale stripes: mtin perceptin Hetergeneity f functin bserved in area V1 representing clur, frm, and mtin is preserved in area V2, althugh it is rganized in a different way Althugh the relative amunt f clur prcessing certainly varies acrss ccipital regins, with area V4 having clur prcessing as its majr functin The prcessing f clur-related infrmatin des mre than simply allw us t tell red frm green Als enriches ur capacity t detect mtin, depth, and psitin Ex. Absence f significant clur analysis like dgs that see the wrld in black and white The ability t differentiate is an imprtant advantage when having t select edible fruits frm a cmplex scene imprtant when the fruits are partly ccluded by leaves (fairly cmmn) Clur = imprtant fr recgnitin - Cnnectins f the Visual Crtex All the cnnectins between the ccipital areas and frm them t the parietal, tempral, and frntal regins are bewildering, but it is pssible t extract a few simple principles V1 (striate crtex) primary visin area = receives the largest input frm the lateral geniculate nucleus f the thalamus & prjects t all ther ccipital regins First prcessing level f the hierarchy V2 prjects t all ther ccipital regins Secnd prcessing level Three distinct, parallel pathways emerge en rute t the parietal crtex, superir tempral sulcus (STS) and inferir tempral crtex fr further prcessing Drsal stream a rle in the visual guidance f mvement Ventral stream cncerned with bject perceptin (including clur) Middle pathway alng the STS (the STS stream) imprtant fr visuspatial functins and in perceptin f certain types f mvements

2 A Thery f Occipital-Lbe Functin - V1 & V2 hetergeneus & segregate prcessing fr clur, frm and mtin V1 & V2 cntrast frm the functins each area fllws in the hierarchy V1 & V2 flw 3 parallel pathways that cnvey different attributes f visin V1 ges t V4 (clur area) frm blbs V1 ges t V2 then t v5 specialized t detect mtin V1 & V2 t area V3 (Semir Zeki calls dynamic frm ) shape f bject mtin V1 (primary crtex) where visin begins V1 sends majr input t V3,V4 & V5 V1 must functin fr the brain t make sense f what the mre specialized visual areas are prcessing - Peple wh suffer damage in the V4 area nly see shades f gray (lss f clur cgnitin r the ability t think abut clur) - Peple wh suffer damage in the V5 area prduces an inability t perceive bjects in mtin Objects nt mving = they perceive, but when bjects mve, the bject vanish frm the persn s view - Peple wh suffer damage in the V3 area affect frm perceptin If there was damage in bth V4 & V3 eliminates frm perceptin - Peple wh suffer damage in the V1 area act as thugh they are blind, but visual input can still get thrugh t higher levels (partly thrugh small prjectins f the lateral geniculate nucleus t V2 and frm the clliculus t the thalamus (the pulvinar) t the crtex Seem nt t be aware f visual input and can be shwn t retain sme aspects f visin nly by special testing - Visual Functins Beynd the ccipital lbe Daniel Felleman and David van Essen 32 crtical areas ( f a ttal f abut 70 in their scheme) that have visual functins in the mnkey s brain, nly 9 are in the ccipital lbe Ttal surface area = 55% f the whle crtical surface which cmpares 11% and 3% fr the smatsensry and auditry regins Visual prcessing in humans cntinues within multiple visual regins in the parietal, tempral and frntal lbes EBA, extrastriate bdy area; FBA, fusifrm bdy area; FFA fusifrm face area, r mving bdies (STSp) PPA (parahippcampal place area) has a ttally different functin analysis f infrmatin abut the appearance and layut f scenes Table 13.1 (summary f visual regins beynd the ccipital lbe) Ventral stream regins area clearly respnsive t sme degree t all categries f stimuli Difference amng the regins are a matter if degree f activity, nt the presence f activity Drsal stream regins specialized fr mving the eyes (LIP) r fr bject directed grasping (AIP, PRR) Sme neurns appear t be purely visual take part in cnverting visual infrmatin int the necessary crdinates fr actin Visual Actin Is a functin f the parietal visual areas in the drsal stream T direct specific mvements The mvement is guided by visin because peple d nt need t shape their hands cnsciusly as they reach Guiding grasing, varius visual areas guide all kinds f specific mvements, including thse f the eyes, head and the whle bdy Visin fr actin must be sensitive t mvement f the target Actin fr visin Figure 13.6 eye mvements while examining a visual stimulus tp dwn prcess viewer actively searches fr nly part f the target bject and attends selectively t it We scan the visual stimulus with numerus eye mvements These mvements are nt randm but tend t fcus n imprtant r distinct features f the stimulus We make a lt f eye mvements directed t the eye and muth

3 We make mre eye scan twards the left visual field (the right side f the persn s face) than t the right visual field Peple with deficits in actin fr visin are likely t have significant deficits in visual perceptin, althugh such deficits have nt been studied systematically In the dark, when peple are searching fr bjects n a cunter, they als make many eye mvements but when they clse their eyes, the mvement stps Visual recgnitin Ability bth t recgnize bjects and t respnd t visual infrmatin We can bth recgnize specific faces and discriminate and interpret different expressins in thse faces We can recgnize letters r symbls and assign meaning t them Visual space Visual infrmatin cmes frm specific lcatins in space This infrmatin allws us t direct ur mvements t bjects in space and t assign meaning t bjects Objects have lcatin bth relative t an individual (egcentric space) and relative t ne anther (allcentric space) Egcentric visual space is central t the cntrl f yur actins twards bjects Allcentric prperties f bjects are necessary fr yu t cnstruct a memry f spatial lcatin It depends n the identity f particular features f the wrld Different aspects f spatial prcessing prbably take place in bther the parietal and the tempral visual regins, and respective functins are integrated in areas that interact and exchange infrmatin Visual attentin Shape, clur, texture, lcatin, wrds, images We select specific aspects f visual input and attend t them selectively Neurns may respnd selectively t stimuli in particular places r at particular mvement is t be executed Independent mechanisms f attentin are prbably required bth fr the guidance f mvements (parietal lbe) and bject recgnitin (tempral lbe) - Visual pathways beynd the ccipital lbe Simple rganisms can detect light and mve t r frm the light David Milner and Melvyn Gdale D.F., a patient with a selective lesin t area LO was blind but nevertheless shaped her hand apprpriately when asked t reach bjects Her drsal stream was intact, as revealed by the fact that she culd uncnsciusly see lcatin, size, and shape Milner and Gdale nted that patients which drsal stream damage cnsciusly reprt seeing bjects but cannt reach accurately r shape the hand apprpriately when reaching They purpse that the drsal stream be thught f as a set f systems fr the nline visual cntrl f actin Three main lines f evidence 1) predminant characteristic f the neurns in psterir parietal regins is that they are active during a cmbinatin f visual stimulatin and assciated behaviur Ex. Cells may be active nly when a mnkey reaches ut t a particular bject Lking at an bject in the absence f mvement des nt activate the neurns These visual neurns are unique in that they are active nly when the brain acts n visual infrmatin 2) psterir parietal neurns can therefre be characterized as an interface between analysis f the visual wrld and mtr actin taken n it. The demands f actin have imprtant implicatins fr what type f infrmatin must be sent t the parietal crtex infrmatin such as bject shape, mvement, and lcatin

4 Each visual features are likely t be cded separately, at least 3 distinct pathways within the drsal stream run frm area V1 t the parietal crtex ne pathway ges frm area V1 directly t area V5 t the parietal crtex Secnd ges frm area V1 t area V3 and then t parietal regins Third ges frm area V1 t V2 t the parietal crtex All three must be functinally dissciable 3) mst f the visual impairments assciated with lesins t the parietal crtex can be characterized as visumeter r rientatinal Fig summary f the visual prcessing hierarchy The drsal stream which takes part in visual actin guides mvements such as the hand pstures fr grasping a mug r pen The ventral stream which takes part in bject recgnitin, identifies bjects such as mugs and pens in ur visual wrld The drsal and ventral streams exchange infrmatin thrugh plysensry neurns in the STS stream Milner-Gdale mdel is an imprtant theretical advance in understanding hw ur visual brain is rganized The tw distinct visual streams have evlved t use visual infrmatin in tw fundamentally different way: Drsal = guiding mvements Ventral = identifying bjects Third stream f visual prcessing riginates frm structures assciated with bth the parietal and the tempral pathways and flws t a regin f the tempral lbe that is buried in the superir tempral sulcus elabratin f the ventral stream that prvides a perceptual representatin f the actins f thers as the perceptin f spatial relatins amng elements in a scene This mdel can be applied t the auditry and smatsensry systems bth systems als functin t guide mvements and identify stimuli Only cnscius f small amunt f what the brain can d STS = plysensry neurns neurns that are respnsive t bth visual and auditry r bth visual and smatsensry input - Imaging studies f drsal and ventral stream Leslie Ungerleider and James Haxby examined PET scans First task, the subjects indicated which f tw faces were identical with a sample face Secnd task, the subjects were asked t identify which f tw stimuli had a dt (r square) in the same lcatin as in a sample The results: activatin f the tempral regins fr the facial stimuli and activatin f the psterir parietal regin fr the lcatin task Fig imaging visual pathways A summary f results f PET studies illustrates selective activatin f A) crtical regins by tasks f facial recgnitin (circles) and spatial lcatin (squares) and B) areas assciated with perceptin f clur (squares), mtin (circles), and shape (triangles) Limitatin: interpreting the spatial task PET task subjects have t mve their eyes, which activates regins in the drsal stream; s whether the spatial r the mvement cmpnents activate the parietal regin is nt clear Detectin f mtin activates regins in the vicinity f area V5, whereas the detectin f shape activates regins alng the STS and the ventral regin f the tempral lbe Perceptin f clur is assciated with activatin f the regin f the lingual gyrus, which is the lcatin area f V4 Studies f reginal bld flw in nrmal subjects shw results cnsistent with the general ntin f tw visual streams, ne t the parietal lbe and the ther t the tempral lbe

5 Disrders f visual pathways - 2 key elements in the way in which the brain rganizes the visual fields: 1)left half f the retina sends its prjectins t the right side f the brain, whereas the right half f each retina sends its prjectins t the left side f the brain the representatin f each side f the visual wrld seen by each eye is sent t the same place in the area V1 If there is damage t just ne eye = damage must be utside the brain, either in the retina r in the ptic nerve 2) different parts f the visual field are tpgraphically represented in different parts f area V1 (fig ) & specific regin f the v1 prduces a lss f visin in a specific part f the visual wrld - Mncular blindness destructin f the retina r ptic nerve f ne eye causes the lss f sight t that eye - Bitempral hemianpia (medial regin f the ptic chiasm severs the crssing fibers) lss f visin f bth tempral fields. Can rise when a tumr develps in the pituitary gland, which sits next t the chiasm put pressure n the medial part f the chiasm and prduce the lss, r disturbance f lateral visin - Nasal hemianpia lss f ne visin f nasal field - Hmnymus hemianpia ( cmplete cuts f ptic tract, lateral geniculate bdy r area V1) blindness f ne entire visual field (fig ) Lesins f the ccipital lbe ften spare the central, r macular regin f the visual field The mst reasnable explanatins are 1) the macular regin receives a duble vascular supply frm bth the middle and the psterir cerebral arteries, making it mre resilient t large hemispheric lesins (mre likely) 2) the fveal regin f the retina prjects t bth hemispheres S if ne ccipital lbe is destryed the ther receives prjectins frm the fvea - Macular sparing (central visual field helps t differentiate lesins f the ptic tract r thalamus frm crtical lesins because macular sparing ccurs nly after lesins in the visual crtex Des nt always ccur, hwever many peple with visual crtex lesins have a cmplete lss f visin in ne quarter (quadrantanpia) f ne half (hemianpia) f the fvea - quadrantanpia & hemianpia brder between the impaired visual area and the adjacent, intact visual field, r quadrant, is sharp the sharpness is due t the segregatin between the left and the right and the upper and lwer visual fields - sctmas (small lesins f the ccipital lbe) small blind spts in the visual field peple are cnstantly unaware because f nystagmus (cnstant, tiny, invluntary eye mvements) & spntaneus filling in by the visual system - fig cnsequences f lesins in area V1 the shaded areas indicate regins f visual lss A) the effect f a cmplete lesin f area v1 in the left hemisphere is hemianpia affecting right visual field B) a large lesin f the lwer lip f the calcarine fissure prduces a quadrantanpia that affects mst f the upper right visual quadrant C) a smaller lesin f the lwer lip f the calcarine fissure results in a smaller injury, a sctma Disrders f Crtical Functin - Case B.K.: V1 Damage and a Sctma Hemianpic in the left visual field Had a migraine strke Few hrs later, the left lwer field began t return but the left upper quadrant was slw t shw any change Fig scan f BK s brain and map f visual fields A) BK MRI scan, shwing the infarct (the black area) B) Map f BK s visual fields 6 mnths after the strke Subnrmal visin persists in the upper-left quadrant shws a infarct (dead tissue) in the right ccipital area

6 Size f a visual field defect is rutinely measured with perimetry, a standardized methd in which the subject fixates n a black dt in the center f a large, white hemisphere Perfrmance is mapped by indicating by indicating the area f blindness n a schematic map f the visual fields Sizes in the visual field is measured by visual angle, in degrees The area f inability t perceive even a large bright light is measured frm the center, 6 degree upward alng the vertical midline and abut 15 degree laterally alng the hrizntal midline beynd this zne in the left upper quadrant des nt have nrmal visin, hwever because BK is still unable t perceive lessbright lights in the area Furth day: darkness had disappeared and was replaced by visual nise (a scintillating sctma) thrughut much f the field especially area f the sctma Blindsight perceived lcatin withut being able t perceive cntent Ischemia neurns that area especially sensitive t a perid f reduced bld flw Facial recgnitin was slwer than it had been befre the strke, because the infrmatin in left visual field appears t be particularly imprtant fr face recgnitin - Case D.B.: V1 Damage and Blindsight Visual disturbance frm an ccipital lesin DB s right calcarine fissure was remved surgically t excise an angima (cllectin f abnrmal bld vessels that results in abnrmal bld flw) DB therefre has hemianpia based n standard perimetry DB had a feeling that a stimulus was appraching r was smth r jaded DB stresses that he saw nthing He pinted t the accurate lcatin His blindsight cntrasts with his subjective impressin that he saw nthing at all DB can detect sme frms f mvement DB has crtical blindness r blindsight n cnscius awareness f seeing but still is able t reprt the mvement and lcatin f bject that he cannt recgnize - Case J.I., V4 Damage and Lss f clur visin Oliver Sacks & Rbert Wasserman artist wh suddenly became clur blind J.I sees the wrld in black and white Clses his eyes t eat, dreams were in black and white t Clur blind frm the specific damage t the ccipital crtex - Case P.B.: Cnscius clur perceptin in a blind patient Zeki PB was electrcuted and was resuscitated but suffers brain ischemia He was left virtually blind, althugh he can detect the sense f light PB ability t remember clurs = intact Frm fmri: activatin in bth V1 and V2 - Case L.M.: V5 (MT) Damage and the perceptin f mvement Jsef Zihl wman wh has a bilateral psterir injury resulting frm a vascular abnrmality Lss f mvement visin LM fund peple disturbing because she cannt see them mve frm ne place t the ther Imprtant because she shws that the brain must analyze the mvement f frm separately frm the frm itself Thmas Schenk LM is unable t intercept mving bjects by using her hand Used TMS t V5 V5 must play a rle in bth visual streams & similar t V1, a rle in mtin prcessing - Case D.F. Occipital Damage and Visual Agnsia fig 13.14

7 Visual agnsia = Sigmund Freud fr an inability t cmbine individual visual impressins int cmplete patterns the inability t recgnize bjects r their pictrial representatins r the inability t draw r cpy them Gdale and Milner DF: her principal deficit is visual frm agnsia (a severe inability t recgnize line drawings f bjects) DF can recgnize many bjects but she cannt recgnize drawings f them DF can draw frm memry but had difficulties cpying frm line drawings and drawing bjects frm life DF culd reach ut and pst a hand-held card int a slt rtated t different rientatins (fig.13.15) DF culd use visual frm infrmatin t guide mvements t bjects (drsal), but she culd nt use visual infrmatin t recgnize the same bjects (ventral) - Case Vk.: Parietal Damage and Visumtr Guidance Fig grasp patterns Optic ataxia: (damage t the psterir parietal lbe) deficit in visually guided hand mvements such as reaching, that cannt be ascribed t mtr smatsensry, r visual field r acuity deficits VK: wman with bilateral hemrrhages in the ccipitparietal regins described by Lrna Jakbsn VK appears virtually blind, symptms dissipated in a mnth and she was left with disrdered cntrl f her gaze, impairment in visual attentin and ptic ataxia (Balint s syndrme) Vk had gd frm and clur visin and culd recgnize and name bjects; hwever her ability t reach fr bjects was impaired Vk was unable t crdinate reaching and grasping fr bjects that she culd perceive VK culd nt frm the apprpriate hand pstures needed t grasp bjects f different shapes DF and VK : mechanisms underlying the cnscius perceptin f bject frm are dissciable frm the mechanisms cntrlling visually guided mvements t the same bjects - Cases D and T: Higher Level Visual prcesses Ruth Campbell D has a right ccipittempral lesin assciated with a left upper quadrantanpia that extends int the lwer quadrant D had initial difficulties in reading but her language abilities were intact She culd nt recgnize peple s faces & had difficulties identifying handwritings Prspagnsia (facial recgnitin deficit) cannt recgnize even their wn faces in a mirrr D culd nt recgnize faces but she culd make use f the infrmatin in the faces T has a left ccipittempral lesin assciated with a right hemianpia Alexia (difficulty t read) She had difficulty t read, was unable t name clurs even thugh she culd discriminate them T had n difficulty identifying faces but was impaired in lip reading Identifying faces and extracting speech infrmatin frm faces d nt call n the same crtical system - Cnclusin frm case studies Several cnclusins: There are clearly distinct syndrmes f visual disturbance Sme symptms can be taken as evidence f a fundamental dissciatin between visin fr guiding mvements (drsal) and visual recgnitin (ventral) The dissciability f the symptms in the varius patients implies that ut intrspective view f unified visual experience is false The fact that bjects can be seen when they are stuck but nt when they are mving is particularly disturbing (cannt see hw they mved there r figure ut if they mved r just std there) Neurscientists have a least suggestive evidence f an asymmetry in ccipital lbe functins Visual Agnsia - Object Agnsia Is t distinguish tw brad frms: apperceptive agnsia and assciative agnsia

8 - Other agnsias Apperceptive Agnsia apperceptive: Any failure f bject recgnitin in which basic visual functins (acuity, clur, mtin) are preserved inability t develp a percept f the structure f an bject r bjects (DF case) fllws grw bilateral damage t the lateral parts f the ccipital lbes sending utputs t the ventral stream cmmn damage frm carbn mnxide pisning neurnal death multagnsia: unusual symptms if tw shapes are presented tgether, nly ne is perceived Assciative Agnsia The inability t recgnize an bject despite its apparent perceptin Can cpy a drawing rather accurately indicating a cherent percept, but cannt identify it Cnsider: high cgnitive level f prcessing that is assciated with stred infrmatin abut bjects Failure f bject recgnitin is a defect in memry that affects nt nly past knwledge abut the bject but als the acquisitin f new knwledge Prspagnsia Facial agnsia cannt recgnize any previusly knwn faces including their wn as seen in a mirrr r phtgraph May nt accept the fact that they cannt recgnize their wn faces, prbably because they knw wh must be in the mirrr and see themselves Alexia An inability t read cmplementary t facial recgnitin deficits Damage t the left fusifrm and lingual areas Visuspatial Agnsia One disruptive frm is tpgraphical disrientatin (inability t find ne s way arund familiar envirnments such as ne s neighburhd Peple unable t recgnize landmarks that wuld indicate the apprpriate directin in which t travel Visual imagery - Visualizatin is a crucial in prblem slving task such as mental arithmetic, map reading, and mechanical reasning - Marlene Behrmann CK cannt recgnize bjects but he can imagine them and draw them in cnsiderable detail frm memry - Ability implies sme dissciatin between neural system dealing with bject perceptin and that dealing with the generatin f images - Neural structures mediating the perceptin and visualizatin f bjects are unlikely t be cmpletely independent, but a deficit in bject perceptin clearly cannt be due simply t lss f mental representatins - Mental rtatin f bjects might be lcalized t sme regin f the right hemisphere is cntrversial - Mental rtatin likely implicates structures related t the drsal stream - Mental imagery appears t be a tp dwn activatin f a sunset f brain s visual areas