© Cengage Learning 2016 Vision Chapter Six

© Cengage Learning 2016 Environmental stimuli –Transduced into action potentials for interpretation by the nervous system –Perceived by the process of sensation and interpreted by the process of perception Bottom-up processing vs. top-down processing From Sensation to Perception

© Cengage Learning 2016 Evolution of Sensory Systems Is Species- Specific

© Cengage Learning 2016 Electromagnetic radiation –Moving waves of photons Visible light –Wavelength determines the colors perceived –Amplitude determines the brightness perceived The advantages of light as a stimulus –Electromagnetic energy is abundant, travels quickly and in straight lines The Visual Stimulus: Light

© Cengage Learning 2016 Dimensions of Electromagnetic Radiation

© Cengage Learning 2016 The electromagnetic spectrum –Range of energy visible to humans falls between 400 and 700 nanometers Absorption, reflection, and refraction –Absorption and reflection of visible light by objects determine the colors we see –Air and water refract, or change the direction, of traveling waves of light The Electromagnetic Spectrum

© Cengage Learning 2016 The Electromagnetic Spectrum

© Cengage Learning 2016 Light Interacts with the Environment

© Cengage Learning 2016 Protecting the eye –Located in bony orbit of the skull –Cushioned by fat –Eyelids, blinking, tears The anatomy of the eye –Sclera, cornea, anterior chamber (aqueous humor), pupil, lens, vitreous chamber (vitreous humor), retina The Structures and Functions of the Visual System

© Cengage Learning 2016 The Structure of the Eye

© Cengage Learning 2016 The Color of the Iris

© Cengage Learning 2016 Demonstrating the Blind Spot

© Cengage Learning 2016 Contains visual interneurons and photoreceptors Optic disk Macula Fovea (central vision vs. peripheral vision) Features of the Retina

© Cengage Learning 2016 Ganglion cell layer Inner nuclear layer Outer plexiform layer Outer nuclear area Inner plexiform layer The Layered Organization of the Retina

© Cengage Learning 2016 Landmarks of the Retina

© Cengage Learning 2016 The Structure of the Retina

© Cengage Learning 2016 Transduction of visual stimuli is performed by photoreceptors Rods: scotopic vision –High density in the peripheral retina –Sensitivity to dim light Cones: photopic vision –High density in the fovea –Best visual acuity The Structures and Functions of the Visual System: Photoreceptors

© Cengage Learning 2016 Rods and Cones

© Cengage Learning 2016 Scotopic and Photopic Vision

© Cengage Learning 2016 Depolarized photoreceptors at rest –Dark current –Sodium channels are open due to cGMP –Stimulates glutamate release Light stimulus splits rhodopsin –Photoreceptor becomes hyperpolarized –Enzymes break down cGMP –Release less glutamate With return to darkness, rhodopsin rejoins Transduction in Photoreceptors (Rods)

© Cengage Learning 2016 Transduction in Rods

© Cengage Learning 2016 Rods –Contains rhodopsin photopigment –Sensitive to dim light in the blue to green range of the electromagnetic spectrum Cones –Requires more light than rods in order to respond –Three different cones Blue/short, green/middle, red/long wavelength Differences between Rods and Cones

© Cengage Learning 2016 The Responses of Rods and Cones to Different Wavelengths

© Cengage Learning 2016 Horizontal cells –Form connections with photoreceptors and bipolar cells –Graded potentials Bipolar cells: receptive fields –Antagonistic center-surround organization –Lateral inhibition –Two types: diffuse and midget –Graded potentials Processing by Retinal Interneurons

© Cengage Learning 2016 Amacrine cells –Form connections between bipolar, ganglion, and other amacrine cells Ganglion cells –Receive input from bipolar, amacrine cells –Receptive fields replicate the information passed to them by the bipolar cells –Antagonistic center-surround organization –Three types of ganglion cells: M, P, and K cells Processing by Retinal Interneurons (cont’d.)

© Cengage Learning 2016 Retinal Bipolar Cells Have Receptive Fields

© Cengage Learning 2016 Receptive Fields of Bipolar and Ganglion Cells

© Cengage Learning 2016 The Three Types of Ganglion Cells

© Cengage Learning 2016 Optic nerves –Ganglion cell axons exit each eye through the optic disk, forming an optic nerve leaving each eye –Optic nerves cross at the optic chiasm The lateral geniculate nucleus (LGN) –Receives 90% of optic tract axons –Six distinct stacked layers –Keeps input from each eye separate Optic Nerve Connections

© Cengage Learning 2016 The superior colliculus –Guides movements of the eyes and head toward a newly detected object Optic Nerve Connections (cont’d.)

© Cengage Learning 2016 The Pathways from Eye to Cortex

© Cengage Learning 2016 The Lateral Geniculate Nucleus (LGN)

© Cengage Learning 2016 Cortical mapping –Correlate location of neural activity with the position of an object in the visual field Cortical receptive fields –Simple and complex cortical cells –End-stopped cells Cortical columns and hypercolumns Cytochrome oxidase blobs Cortical modules The Striate Cortex (Primary Visual Cortex; V1)

© Cengage Learning 2016 Simple Cortical Cells

© Cengage Learning 2016 Complex Cortical Cells

© Cengage Learning 2016 Cortical Modules

© Cengage Learning 2016 At least a dozen other areas of human cerebral cortex participate in visual processing The dorsal stream (“where”) –Important role in processing motion –Areas MT and MST; akinetopsia The ventral stream (“what”) –Important for object recognition –Areas V4, IT, and the fusiform face area (FFA) Visual Analysis Beyond the Striate Cortex: V2

© Cengage Learning 2016 The Dorsal “Where” Pathway and the Ventral “What” Pathway

© Cengage Learning 2016 Hierarchies –Simple cells input to increasingly complex cells –Bottom-up processing Spatial frequencies –Spatial frequency analysis of contrasts and frequencies in the visual field Gratings –Contrast sensitivity function Visual Perception

© Cengage Learning 2016 Bottom-Up Processing?

© Cengage Learning 2016 Problems for the Hierarchical Model of Vision

© Cengage Learning 2016 Spatial Frequencies

© Cengage Learning 2016 A Cat’s View of the World

© Cengage Learning 2016 Monocular cues –Perspective –Texture and shading –Comparison of size of familiar objects Binocular cues –Retinal disparity –Disparity-selective cells Visual Perception: Perception of Depth

© Cengage Learning D Movies Take Advantage of Retinal Disparity

© Cengage Learning 2016 Trichromatic theory –Interpretation of the output of three types of cones Opponent process theory –Opposition of four colors: green/red, blue/yellow Visual Perception: Coding Color

© Cengage Learning 2016 Mixing Lights

© Cengage Learning 2016 Color Afterimages Illustrate Opponency

© Cengage Learning 2016 Opponent Process Theory

© Cengage Learning 2016 Dichromacy: two cone photopigments –Red-green is sex-linked and most common –Absence of blue photopigment is rare Monochromacy: one/no cone photopigments Anomalous trichromacy –The cone photopigments have peak responses at abnormal wavelengths Visual Perception: Color Deficiency

© Cengage Learning 2016 Looking Through the Eyes of a Dichromat

© Cengage Learning 2016 Color contrast –The same color appears different depending upon the context of surrounding colors Color constancy –Colors look the same regardless of the type of light illuminating the object Visual Perception: Color Contrast and Color Constancy

© Cengage Learning 2016 Color Contrast

© Cengage Learning 2016 Contrast sensitivity functions of children –Infants see less fine detail at a distance –Preferences for high-contrast, colorful objects Age-related vision changes –Presbyopia – “old sight” –Slower adaption to changes in light –Yellowing of the lens; smaller pupils –Less selectivity in cortical responses to visual stimuli The Life-Span Development of the Visual System

© Cengage Learning 2016 The Development of Contrast Sensitivity

© Cengage Learning 2016 Ambylopia: lazy eye Cataracts: clouding of lens of the eye Visual acuity problems –Myopia, hyperopia, astigmatism Blindness –Scotomas, blindsight Visual agnosias –Difficulty recognizing what is seen Disorders of the Visual System

© Cengage Learning 2016 Eyeball Shape Influences the Quality of Vision

© Cengage Learning 2016 Scotoma