M U L T I P L E V I S U A L A R E A S 1: Definition of an ‘area’ of visual cortex 2: Discovery of areas in monkey visual cortex; functional specialisation 3: Use of imaging to chart areas in human visual cortex 4: Why are there multiple areas? A ‘theory’ of vision. Brodmann 1909 cytoarchitectonic map of human cerebral cortex Brodmann area 4 (characterised by very large ‘Betz’ cells in layer 5) [or ‘PRIMARY MOTOR CORTEX’, or ‘AGRANULAR FRONTAL CORTEX’ ] LAYER 1 2 3 5 6 area 18 area 17 ‘granular’ layer 4 area 18 area 17 ‘granular’ layer 4 cytoarchitecture Amunts, Zilles et al (2000) [ref. 1] Brodmann’s cytoarchitectonic map of human cortex 1909 MOTOR (area 4) SPEECH (area 44/45) VISUAL (area 17) = ‘Broca’s area’ Brodmann’s theory: different areas represent ‘organs’ of the brain…(?) LocalisaEon of funcEonal modaliEes, e.g vision, hearing, touch, motor control, speech. * But also (unknown to Brodmann) there are many separate areas specialised for visual sub-‐ modali;es (e.g. colour vision) within cyto-‐architectural areas 18, 19 & 37. 1. Definition of an ‘area’ of visual cortex -‐-‐ architecture -‐-‐ connecEvity -‐ -‐ funcEonal map (e.g. map of reEna, or of other sensory surface) -‐ -‐ specific funcEonal properEes cytoarchitecture layer 1 2 3 4 5 6 CORTICAL ARCHITECTURE myeloarchitecture (area 18) V2 V1 (area 17) cytoarchitecture V2 V1 ‘Stria of Gennari ’ Golgi stain; cell stain; fibre stain myeloarchitecture Cajal; L’Histologie du Système Nerveux CORTICAL ARCHITECTURE Golgi stained corEcal pyramidal cells -‐ as studied by Spanish neuroanatomist Ramon y Cajal (Nobel Laureate 1906), giving rise to the ‘neuron doctrine’. VISUAL PATHWAYS ‘Primary visual cortex’ is defined as the cor;cal field in receipt of the op;c radia;on. PRE-‐CHIASMAL temporal hemi-retina nasal hemi-retina only nasal fibres cross over = ‘decussation’ optic nerve (= axons of retinal ganglion cells) optic chiasm optic tract POST-‐CHIASMAL lateral geniculate nucleus (LGN) optic radiation (cortical ‘white matter’) primary visual cortex (V1) Area V1 has a reEnotopic map ‘scotoma’ * -‐ a circumscribed region of visual field loss, caused by punctate damage to a small region of area V1; -‐ e.g. as caused by a bullet wound. calcarine sulcus -‐ First accurate map of V1 produced by Gordon Holmes, studying casualEes of WW I. f o v e a -‐ Also Tatsuji Inouye for the Russo-‐Japanese war (1904-‐5) leN visual cortex 90 inferior vertical meridian 270 f o v e a 315 2.5 5 10 20 40 HM 45 90 1 cm 45 40 20 10 10 20 40 superior vertical meridian 270 right visual field 0 = HM 315 ‘magnificaEon factor’ = mm cortex per degree of visual field Area V1 has a reEnotopic map ‘scotoma’ -‐ a circumscribed region of visual field loss, caused by punctate damage to a small region of area V1; -‐ e.g. as caused by a bullet wound. -‐ First accurate map of V1 produced by Gordon Holmes, studying casualEes of WW I. f o v e a -‐ Also Tatsuji Inouye for the Russo-‐Japanese war (1904-‐5) leN visual cortex 90 = VM inferior vertical meridian 270 f o v e a 315 2.5 5 10 20 40 HM 45 90 1 cm 45 40 20 10 10 20 40 superior vertical meridian 270 = VM right visual field 0 = HM 315 ‘magnificaEon factor’ = mm cortex per degree of visual field 1 mm 1 2 3 4 Primary Visual Cortex Cells in all other layers are binocular, but dominated by one eye. Cells in layer 4C are monocular L eye relay Lateral Geniculate Nucleus R eye relay 5 6 white mafer FUNCTIONAL ARCHITECTURE OF PRIMARY VISUAL CORTEX David Hubel & Torsten Wiesel V1 (left) two independent modular subsystems: - ocular dominance columns - orientation columns LGN (left) 6 monocular layers; - each layer maps a right, or a left eye half-retina The possession of monocular neurons is a unique feature of V1, that helps to confirm its idenEty as a discrete area of cortex ... -‐ although, historically, this feature was never used as an operaEonal means of defining V1. leg eye right eye To recap: mulEple terminology reflects historical convergence of separate concepts: striate cortex (myeloarchitecture; stria of Gennari) = area 17 (cytoarchitecture; e.g. Brodmann) = primary visual cortex (connecEvity, i.e. area of distribuEon of opEc radiaEon) = area V1 (first map of visual field; e.g. Holmes) * DefiniEon of other, non-‐primary visual areas depends on similar combinaEons of separate criteria; -‐ experimental aim is to find congruent evidence for borders between neighbouring areas. 2. Discovery of areas in monkey visual cortex; functional specialisation Brodmann cytoarchitectonic areas in macaque monkey Area 17 = striate cortex Areas 18 & 19 = prestriate cortex MulEple outputs from V1 to sites in prestriate cortex of macaque monkey -‐ implies parallel pathways & mul;ple visual maps (Zeki, 1969) MulEple visual areas in prestriate cortex of macaque monkey (Zeki 1978) Using callosal connecEons to chart the borders of visual areas (Zeki 1978) V.M. H.M. The corpus callosum is the major inter-‐hemispheric commissure; – callosal fibres connect representaEons of the verEcal meridian monocular crescent Prestriate areas have varying specialised funcEons e.g. areas V5 & V5A (or MT, MST & FST) are mo;on areas The discovery of directional selectivity in area V5 (Zeki, 1974) DefiniFon of area V5: -‐ V5 is an isolated projecEon field of V1 (neighbouring cortex within STS does not receive input from V1). -‐ V5 has a very high proporEon of direcEon-‐selecEve cells V4 has lifle direcEon tuning; V5A also has many direcEon-‐selecEve cells, but they have larger recepEve fields than V5 cells. -‐ V5 has a disEnct myeloarchitecture BUT.. The visual map in V5 lacks a high degree of topographic order, and it is therefore difficult to use the map to define the border of V5. Felleman & Van Essen 1991 visual areas in flatmap of macaque cortex The ‘area hypothesis’: - that all * cortex is composed of discrete areas Can we use the same methods to identify human visual areas? -‐ Invasive methods for tract-‐tracing are impermissible; -‐ Single unit physiology is only obtainable under special circumstances; -‐ Post-‐mortem corEcal architecture cannot be correlated with other criteria; -‐ BUT... 3: Use of imaging to chart areas in human visual cortex -‐ Func;onal magne;c resonance imaging (fMRI) can: -‐ obtain reEnotopic maps; -‐ examine funcEonal specialisaEon; -‐ trace fibre bundles through white mafer = DTI (‘diffusion tensor imaging’). Functional Magnetic Resonance Imaging (fMRI) Detects BOLD signal (Blood Oxygenation Level Dependent): oxyhaemoglobin gives higher signal than de-oxyhaemoglobin. NB. BOLD signal increases in active regions of the brain, because increased blood supply overcompensates for increased tissue oxygen demand. CHARTING VISUAL AREAS WITH fMRI [ref 3] (a) (b) ‘Travelling wave’ technique Checkerboard stimuli, formed as an alternately expanding and contracting annulus (a), or a slowly revolving quadrant (or octant) (b). Phase mapping BOLD signal analysis to reconstruct map of eccentricity (a), and polar angle (b). CHARTING VISUAL AREAS WITH fMRI A B [ref 2] C Determine separate visual areas by charEng visual field maps and determining ‘local sign’. These can be rendered: (A) on brain surface image; (B) on brain ‘balloons’ (inflate volume to flafen out sulci, shown in dark grey; (C) on totally flafened 2D surface (NB this requires ‘tearing’ part of the surface to minimise distorEon). CHARTING VISUAL AREAS WITH fMRI 1 this chart shows: a representaEon of the inferior VM at the borders between V1 / V2d & V3d / V3A; a representaEon of the superior VM at the borders between V1 / V2v & V3v / V4v; a representaEon of the HM at the borders between V2v / V3v & V2d / V3d; Schema for the arrangement of maps in areas V1, V2 & V3 of primate visual cortex verZcal meridian horizontal meridian verZcal meridian Schematic view of occipital pole of ‘inflated’ right hemisphere foveal vision horizontal meridian verZcal meridian horizontal meridian verZcal meridian Horton & Hoyt 1991 [ref 4] Quadrantanopia from V2 lesion tumour Use of fMRI to determine areas in human visual cortex (i) By charEng reEnotopic maps; (ii) By idenEfying regions with specific funcEon (e.g. ‘face’ area). Functionally identified areas of human cortex using fMRI Area V4 greyscale v colour V3A V7 V4d V2 V1 V4 lesion gives rise to achromatopsia V5 LO V3 V4v FFA PPA area V4v (found on fusiform gyrus) HEMI-ACHROMATOPSIA plus SUPERIOR QUADRANTANOPIA Area V5 Functionally identified areas of human cortex using fMRI a.k.a. area MT sta;c v dynamic V5 lesion gives rise to akinetopsia The re;notopic organiza;on of the human middle temporal area MT/V5 and its cor;cal neighbors. Kolster et al. (2010) J. Neurosci. 30: 9801-‐9820. area V5/MT BILATERAL LESION OF V5 (akinetopsia, paEent LM) (case of thrombosis of superior sagittal sinus) Functionally identified areas of human cortex using fMRI Area V6 -‐ a relaEve emphasis on peripheral visual field; -‐ strong response to opEc flow; V6 -‐ iniEates a visual pathway to premotor cortex. V3A V7 V4d V2 V1 V5 LO V3 V4v FFA PPA area V6 (‘medial moFon area’) Functionally identified areas of human cortex using fMRI Area LO non-‐object v object V3A V7 V4d V2 V1 LO lesion gives rise to agnosia V5 LO V3 V4v FFA PPA area LO (Lateral Occipital) Functionally identified areas of human cortex using fMRI Areas FFA & PPA a`end face v a`end house V3A V7 V4d V2 V1 Parahippocampal Place Area, [ topographic disorientation ] V5 LO V3 V4v FFA PPA Fusiform Face Area [ prosopagnosia ] Functionally identified areas of human cortex using fMRI Area VWFA mirror text v normal V3A V7 LEFT hemisphere V4d V2 V1 V5 LO V3 V4v FFA PPA Visual Word Form Area [alexia or pure word blindness] Functionally identified areas of human cortex using fMRI Human V6: the medial moFon area. Pitzalis S. et al. (2010) Cereb. Cortex 20, 411-‐424 DirecFon-‐selecFve moFon blindness aWer unilateral posterior brain damage Blanke C. et al.. (2003) Eur. J. Neurosci. 18, 709-‐722 Two distinct regions where cortical lesions produce motion deficits ‘Area MT’ is an alternaEve term for V5 : composite reconstrucEon of lesioned regions across subjects FFA PPA V5 V6 Visual field maps in human cortex. Wandell BA et al (2007) Neuron 56:366-‐383 Brewer et al 2002 [ref 3] CHARTING VISUAL AREAS WITH fMRI Visual areas in macaque cortex measured using func;onal magne;c resonance imaging. Brewer et al (2002) J Neurosci, 22: 10416-‐10426. Visual map in macaque area V1: -‐ this diagram shows the map of polar angle 4. Why are there multiple areas? A ‘theory’ of vision [= area 18] [= area 17] Campbell 1905 X ‘homunculus’ theory of vision & brain funcEon visual processing requires acEve synthesis of ‘feature detectors’ -‐ colour -‐ form/edges -‐ moEon -‐ stereo depth + hierarchical analysis of feature combinaEons Lessons from AI: machine vision DAVID MARR ‘SEEING’: to know what is where by looking Marr’s 3 levels of analysis by which to understand any seeing system (natural or ar;ficial): 1. ComputaZonal goal 2. Algorithm 3. Physical implementaZon by computaZonal hardware (biological or electronic) Why are there so many visual areas... ? COLOUR FORM STEREOSCOPIC DEPTH MOTION All require very different processing strategies -‐ most efficient if performed separately The logic of funcEonal specialisaEon: mulEple areas enable more efficient visual computaEon
© Copyright 2024 ExpyDoc
