Visuospatial Function Review of Visual Neuroanatomy Hierarchical organization and somatotopic, functional segregation of visual information from retina to cortex Magnocellular (dorsal, ‘how/where’ pathway) Parvocellular (ventral, ‘what’ pathway) Disorders of higher-order visual function and how they are assessed Variants of Blindness (Pre-striate and Striate Syndromes) Ventral (‘what’) Stream Syndromes Dorsal (‘where’) Stream Syndromes

Organizing Principles of Visual System Somatotopy Spatial relations in environment map to primary cortical areas that represent them Hierarchy information is summarized and passed on to higher levels in simplified form information modified/combined at next level final output reflects total information in system Functional segregation Lower level features are integrated into higher level representations of objects and location objects in space are further integrated into meaningful environmental ‘scenes’ Parallel processing across hierarchical and functionally segregated systems (‘cross-talk’) ensures smooth integration of information

P P Anatomy of the eye Cornea Pupil Lens Retina Fovea Optic nerve Outer layer of eye Pupil Aperture through which light enters eye Lens Focuses light on fovea Retina Layer of cells which process light Fovea Focal point of images Optic nerve Transmits visual information to brain P P

Rods Cones Fovea absent from fovea high concentration in periphery of retina low resolution (acuity) respond to movement, orientation primary input from ganglion cells to magnocellular pathways high concentration in fovea lower concentration in periphery of retina high resolution (acuity) respond to color, edges primary input from ganglion cells to parvocellular pathways

Somatotopic Representation of Visual Information The left LGN receives input from the right visual field via nasal half of retina of right eye temporal half of retina of left eye The right LGN receives input from the left visual field via nasal half of retina of left eye temporal half of retina of right eye Each LGN projects to the same-side (ipsilateral) primary visual cortex. Objects in right visual field are processed in left visual cortex Objects in left visual field are processed in right visual cortex Left Visual Field Right Visual Field Temporal Nasal Temporal Optic nerve Optic chiasm Optic tract Lateral geniculate nucleus Optic radiation Primary visual cortex

Hierarchical processing From Retina to Cortex Photoreceptors sensitive to four primary types of visual attributes Attributes individually processed in photoreceptors Color & shape (cones) Orientation & movement (rods) Photoreceptors connect to retinal ganglion cells which respond to only one of the attributes. The ganglion cells forward their signals to cells in different layers of the lateral geniculate nucleus (LGN). These respond uniquely to that one type of information LGN cells forward information to primary visual cortex and further cortical areas. A complete and dynamic visual scene is assembled at higher levels

Functional Segregation of the Visual System Multimodal Parietal Cortex Magnocellular Pathway (responsive to movement & orientation) MT V3 V2 Dorsal Pathway (‘where’) V1 M-ganglion cell (info from rods) Magno LGN P-ganglion cell (info from cones) Parvo LGN V1 Ventral Pathway (‘what’) V2 Parvocellular Pathway (responsive to color & shape) V4 IT Multimodal Temporal Cortex

Hierarchical Organization of Visual System Area Neurons Respond to V1 colors, angled lines V2 linear motion, changes in depth V3 shapes, specific velocities of motion V4 complex patterns & shapes MT optic flow (movement through environment)

Segregation of dorsal and ventral streams Propagation of visual information from primary sensory cortex to multimodal integrative cortex Dorsal stream continues superiorly ending in posterior parietal cortex Ventral stream continues inferiorly ending in inferotemporal cortex Not mutually exclusive- much cross talk between these processing streams.

Disorders of Visual Abilities Primary Visual (striate/peri-striate cortex) Primary Blindness/Visual field defects Blindsight Visuoperceptual (ventral ‘what’ stream) Impairment of visual object/feature recognition (agnosias) Defective visual feature analysis and synthesis Visuospatial (dorsal ‘where’ stream) Defective somatic and topographical orientation Defective judgment of direction and distance Visual attention deficits Visuoconstructive (Sensory-Motor Integration) Defective assembling performance Defective drawing performance

Primary Visual Syndromes (Pre-Striate and Striate) Blindness/Visual Field defect failure to respond to information presented to visual field damage between retina and V1 Monocular Blindness Tunnel Vision Homonymous Hemianopsia Superior Quadrantanopsia Inferior Quadrantanopsia Superior Quadrantanopsia

Primary Visual Syndromes (Pre-Striate and Striate) Blindness/Visual Field defect failure to respond to information presented to visual field damage between retina and V1 ‘Blind-sight’ ability to respond to visual stimuli in scotoma (‘blind spot’) without overt awareness describe characteristics of objects adjust grasping movement to location/size avoid objects in path semantic priming Proposed mechanism: damage to V1, but preserved LGN or collicular input to higher visual association areas

Ventral Stream (‘What’) Disorders (Posterior Temporal / Inferior-Parietal Damage) Associative visual Agnosia – inability to recognize an object or its distinguishing feature(s) inability is not explained by other deficits (e.g., sensory, language, attentional) Deficit in assigning meaning to object Apperceptive visual agnosia Can describe, reproduce or discriminate objects in normal context Changing orientation or context disrupts this ability Deficit in visual perception

Gollin Incomplete Figures

Ventral Stream (‘What’) Disorders (Posterior Temporal Inferior-Parietal Damage) Associative Visual Agnosia – inability to associate visual forms with meaning visual-perceptual and constructive functions (discrimination, matching, copying) are preserved deficit in connecting visual information with semantic system may affect verbal label only (anomia) or other semantic information (agnosia) can be specific to physical features Color Letter/word forms (alexia) Objects within a class (faces, cars, cows, bird species) (prosopagnosia)

Benton Facial Recognition Test Famous Faces Test David Letterman Barack Obama George Bush Tiger Woods or Talk-show Host Current president Former president Golfer

WAIS-III Picture Completion Ventral Stream (‘What’) Disorders (Posterior Temporal Inferior-Parietal Damage) Defective visual analysis and synthesis inability to discriminate/integrate abstract or meaningful visual features may reflect primary visual-perceptual or attention deficits subtle deficits may interfere with important functions (e.g. driving) WAIS-III Picture Completion

Hooper Visuospatial Organization Test

WAIS-IV Matrix Reasoning

Dorsal Stream (‘Where’) Disorders (Superior Parietal Damage) Deficits in Spatial Orientation Intrapersonal Right - left confusion (difficulty discriminating left and right on self or others) Finger agnosia (difficulty discriminating fingers) Anosagnosia (lack of awareness of motor deficit) Extrapersonal Topographic disorientation (inability to represent extrapersonal space) Deficits of Visual Attention (another lecture) Visual neglect Balint’s Syndrome

R/L Orientation

Topographical Disorientation Disruption of mental representation of environment within and beyond range of immediate perception (Aguirre & D’Esposito, Brain, 1999)

Double Dissociation of Topographic Disorders Egocentric Disorientation ‘Landmark’ Agnosia Visual Object Recognition Intact Impaired (subtle) Way-finding (Landmarks) Not assessed Impaired Spatial Span Visual Reaching Judging Relative Position Verbal Route Description Way-finding (Spatial Map) Lesion lateralization Bilat or Right Lesion localization Posterior parietal Lingual, fusiform, parahippocampal gyri

Visuo-Constructional Deficits (Frontal and/or Parietal Damage) Deficit in reproducing 2D/3D representations of complex objects Drawing tasks (Cross, Clock, Rey-Osterreith Figure) Construction Tasks (WAIS-III Block Design

WAIS Block Design

Broken Configuration Error

3D Constructional Tasks Patient’s Reproduction Model

Summary Visual information is represented in the brain in somatotopic, hierarchically organized, functionally segregated (‘what’ and ‘where’) systems Localized brain lesions produce dissociations between and within these systems, due to damage to specific functional subsystems Apperceptive vs. Associative Agnosias Category-specific agnosias