1/21/2019. to see : to know what is where by looking. -Aristotle. The Anatomy of Visual Pathways: Anatomy and Function are Linked

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1 The Laboratory for Visual Neuroplasticity Massachusetts Eye and Ear Infirmary Harvard Medical School to see : to know what is where by looking -Aristotle The Anatomy of Visual Pathways: Anatomy and Function are Linked Nasal retina images cross at the optic chiasm Left visual field is processed by the Right hemisphere Right visual field is processed by the Left hemisphere Right Hemisphere Left Hemisphere 1

2 The Anatomy of Visual Pathways: Anatomy and Function are Linked Retinotopic Organization: neurons of the visual system are organized in an orderly fashion. A two dimensional representation of the visual image is formed on the retina in such a way that neighboring points of an image remain neighbors along the entire visual pathway. The Anatomy of Visual Pathways: Retinotopic Organization Notes: Both eyes have slightly Different views - disparity Left is right/right is left Left is right/right is left Up is down/down is up Central vision (fovea) gets more cortical representation The Anatomy of Visual Pathways: Retinotopic Organization Notes: Greater representation of The central/foveal region (cortical magnification factor) from Bear, Connors, Paradiso. Neuroscience 2

3 The Anatomy of Visual Pathways: Clinical Manifestations Scotoma Left eye Right eye Monocular defect; in front of the chiasm Hemianopia Left eye Right eye Binocular defect; behind the chiasm The Anatomy of Visual Pathways: Clinical Manifestations of Damage (Pathology and dysfunction are also linked) The Anatomy of Visual Pathways: Ocular Dominance Columns Normal Binocular Input Cortex Impaired Right Eye Input Monocular Deprivation leads to Amblyopia Deprived eye After Patching Left (dominant) Eye Injected eye Intact eye Autoradiographs of the ocular dominance columns in two young monkeys. A radioactive dye was injected into one eye and taken up by neurons in the input layer of primary visual cortex (a) normal pattern (b) one year of monocular occlusion (Hubel, Weisel, Levay 1977) 3

4 The Human Visual System Optical Component: refractive elements bringing a point focus Hyperopia ("far sighted"): focus is behind the retina, corrected by a magnifying lens Myopia ("near sighted"): focus is in front of the retina, corrected by a minifying lens Neural Component: The light gathering elements of the Retina. Photoreceptors converting light energy into neural signals optic nerve: approximately 1 million nerve fibers 4

5 The Retina macular degeneration optic nerve hypolasia Peripheral Vision Loss Central Vision Loss General Vision Loss Retinal-subcortical-cortical Component (Afferent visual pathway): neural signals leaving the eye via the optic nerve are sent to the thalamus (lateral geniculate nucleus) and then sent along the optic radiations to the occipital primary visual cortex (V1) for early processing Note: Subcortical structures (i.e. Deep inside the brain) LGN Pul SC Primary Visual Cortex V1 Occipital (Visual) Cortex optic nerve optic radiations Subcortical Structures: LGN: Lateral Geniculate Nucleus (relay station - signal amplification*) Pul: Pulvinar (visual attention) SC: Superior Colliculus (eye movements) Cortico-cortical Component: higher order visual cortical areas are responsible for further analysis and eventual integration Primary Visual Cortex V1 MOTION COLOR FORM Occipital (Visual) Cortex 5

6 The Visual System Feature specialization V4 COLOR V3 FORM EBA BODY V2 STEREO V5 MOTION V1 FFA FACES Cortical Feedback Component (Efferent visual pathway): neural signals leaving the cortex and returning to the thalamus or earlier cortical areas to modulate incoming information. LGN Pul SC Primary Visual Cortex V1 Occipital (Visual) Cortex Eye movement control Integrated cortical processing Human Visual Development: - Early vision (i.e. first few months) is largely subcortical - Progressively integrated with and dominated by cortical process during the first year Thalamus Cortex White (Gray matter) Matter Connections Cortical Subcortical Tamietto and de Gelder Nature Rev Neurosci

7 Human Visual Development: Simulations of vision as seen by a newborn at various ages (Alex Wade) The Human Visual System: A Division of Labor Dorsal or Where Stream: Spatial Processing (location, movement, spatial relations) Ungerleider and Mishkin (1983) Parietal Temporal Ventral or What Stream: Object Processing (color, texture, shape, size, faces) What and Where Pathways Ungerleider and Mishkin (1983) Object discrimination task Monkey is shown an object Then presented with two choice task Reward given for detecting the target Landmark discrimination task Monkey is trained to pick the food well next to a cylinder A Double Dissociation 7

8 Spatial Processing Disorders Neglect Syndrome The Milan Square Experiment Bisiach and Luzzatti (1978) from Raichle and Posner, 1994; Images of Mind simultaneous agnosia - extinction Object Processing Disorders Visual Agnosia from Greek: a- (not or lack of) + - gnōsia (from gnôsis "knowledge") Many types: e.g. - prosopagnosia (faces) - object agnosia (may be very selective, e.g. animate objects) The Human Visual System: A Division of Labor - Functional Organization (version 2) Goodale and Milner (1992) Dorsal or Vision for Action Stream: Visually guided behavior dorsal action stream transforms incoming visual information to coordinate skilled motor planning Parietal Ventral or Vision for Identification Stream: Recognition/identification ventral perceptual stream computes a detailed map of the world from visual input, which can then be used for cognitive operations Temporal 8

9 The Anatomy of Visual Pathways: Multiple Visual Areas Approximately 30 different cortical areas that contribute to visual perception. Category-Specific Visual Areas objects Lateral Occipital (LO) faces Fusiform Face Area (FFA) places Parahippocampal Place Area (PPA) Source: Nancy Kanwisher Source: Jody Culham 9

10 The Visual System: "Simplified View" Felleman and Van Essen (1991) The Visual System: "Putting it All Together" - Serial (Sequential) Processing View: Suggests that increased processing stages are associated with combining and analyzing progressively more complex information "Grandmother cell Hypothesis" However: Visual perception is likely not the product of continuous serial convergence ( combinatorial explosion ) - Parallel Processing View: Visual perception combines individually identified properties of visual objects achieved by simultaneous, parallel processing of several visual pathways "Emergent Hypothesis" Thought Experiment Inferior visual field: dorsal bank optic radiations runs below Parietal Cortex Superior visual field: ventral bank optic radiations runs below Temporal Cortex ( PITS mnemonic) What happens if I suspect damage to the dorsal bank of the optic radiations near the ventricles and to the parietal cortex (e.g. periventricular leukomalacia, PVL)...what type of visual deficits would I expect? Ventricle 10

11 "Waves" Wilma Hurskainen 11

12 Vision is an Active Process We don t notice (or process) the majority of the visual information that enters the eyes. - There are limits to the processing capacity of the visual system/brain. The eye does not provide high resolution (i.e. fine detailed) information across the entire field of view. - Central vision (central 2 ) provides high resolution and vivid color (cone photoreceptors) - Peripheral vision provides low resolution, but is very sensitive to motion, black & white, and low light (rod photoreceptors) Visual Scene Simulation of a captured image Vision is an Active Process Solution: Move the eyes! - Rapid movements of the eyes sample the entire visual scene Over time, the brain stiches together a high resolution representation of the visual scene. Eye movement patterns reflect the content and saliency of the visual scene. In reality, the brain decides where to look and what we should pay attention to. Tracking Eye movements Free Examination Estimate the material wealth of the family Zilbovicius et al. Open Journal of Psychiatry

13 Vision is an Active Process We don t notice (or process) the majority of the visual information that enters the eyes. - There are limits to the processing capacity of the visual system/brain. NOTE: This is not the same as saying we only use 10% of our brain capacity! Find the Blue O Find the Blue O 13

14 Find the Blue O Unique Shape Shape/Form Processing Area Unique Color Color Processing Area Visual Perception Requires Coordination and Time "Perceptual Binding" Both shape and color cues are present Shape and Color Processing Areas have to talk to each other and this requires integration! 14

15 The Effect of Task Load Perceptual capacity in reserve No perceptual capacity in reserve No perceptual capacity in reserve Resources used by lowload primary task Resources used by highload primary task Resources used by lowload primary task but with less robust system Rationale that low load tasks that use few cognitive resources may leave resources available for processing unattended or task-irrelavant stimuli. High load tasks that use all the person's cognitive resources don't leave any resources to process unattended or task-irrelavant stimuli (From Goldstein) Vision is an Active Process: How Much Information Can We Handle? Visual Complexity: The Case of Crowding Crowding: the inability to recognize objects in clutter. It sets a fundamental limit on conscious visual perception and object recognition throughout most of the visual field. 15

16 Vision is an Active Process: How Much Information Can We Handle? "The Door Study" Copyright: Simons and Levin Your Task: Can you spot the toothbrush in the messy bathroom? Why We Miss Objects That Are Right in Front of Us By HEATHER MURPHY OCT. 6, 2017 The New York Times What Happened?!?! 16

17 Sensory Expectations/knowledge accessing prior memories (Top Down Influence) The Perceptual Process Sensory Input/incoming data Data-based processing, stimuli reaching the brain (Bottom Up Influence) The Brain Sees What it Wants Us to See Top Down Bottom Up 17