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Chapter Six
Vision
CHAPTER 6
VISION
Characteristics of Light
• The Advantages of Light as a Stimulus
– Electromagnetic energy is abundant and travels quickly in
fairly straight lines
• The Electromagnetic Spectrum
– Range of energy visible to humans falls between 400 and
700 nanometers
• Absorption, Reflection, and Refraction
– Absorption and reflection determine colors we see
– Air and water refract, or change the direction, of traveling
waves of light
Figure 6.1 The Visual World of Dog and Human
Figure 6.2 Dimensions of Electromagnetic
Radiation
Figure 6.3 The Electromagnetic Spectrum
Figure 6.4 Light Interacts with the Environment
The Structures & Functions of the Visual System
• Protecting the Eye
– Located in bony orbit of the skull, cushioned by fat
– Eyelids and blinking
– Tears
• The Anatomy of the Eye
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Sclera
Cornea
Anterior chamber
Pupil
Lens
Vitreous chamber
Retina
Figure 6.6 The Structure of the Eye
Figure 6.7 Landmarks of the Retina
Figure 6.8 Demonstrating the Blind Spot of the
Eye
The Structures & Functions of the Visual System
• The Layered Organization of the Retina
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Ganglion cell layer
Inner plexiform layer
Inner nuclear layer
Outer plexiform layer
Outer nuclear area
• Photoreceptors
– Rods (scotopic vision) and cones (photopic vision)
– Transduction by photoreceptors – process of transmitting
physical stimulus into electrical signals
– The Dark Current – movement of positive ions into the
resting photoreceptor
Figure 6.9 The Structure of the Retina
Figure 6.10 Rods and Cones
Table 6.1 Scotopic and Photopic Vision
Figure 6.11 Transduction in the Rod
The Structures & Functions of the Visual System
• Differences between Rods and Cones
– Photopigments each have different peak sensitivitivities
– Three types of cones
• Blue/short wavelength
• Green/middle wavelength
• Red/long wavelength
– Rhodopsin in rods most sensitive to bluish-green
wavelengths
– Rods and cones need different amounts of light to respond
Figure 6.12 The Responses of Rods and Cones to
Different Wavelengths
The Structures & Functions of the Visual System
• Processing by Retinal Interneurons
– Horizontal Cells – form connections with photoreceptors
and bipolar cells
– Bipolar Cells
• Receptive fields
• Antagonistic center-surround organization
• Lateral inhibition
– Amacrine Cells – form connections between bipolar,
ganglion, and other amacrine cells
– Ganglion Cells – receive input from bipolar, amacrine cells
– Ganglion Receptive Fields – replicate the information
passed to them by the bipolar cells
– Three types of Ganglion Cells – M, P, and K cells
Figure 6.13 A Retinal Bipolar Receptive Field
Figure 6.14 Receptive Fields of Bipolar and
Ganglion Cells
Figure 6.15 Lateral Inhibition Account of the
Hermann Grid
Table 6.2 The Three Types of Ganglion Cells
The Structures & Functions of the Visual System
• Optic Nerve Connections
– Ganglion cell axons exit the eye through the optic disk,
forming an optic nerve leaving each eye
– The superior colliculus
• In humans used to guide movements of the eyes and head toward
newly detected objects
– The lateral geniculate nucleus (LGN) of the thalamus
• Six distinct stacked layers
• 80% of input comes from primary visual cortex
Figure 6.16 The Pathways from the Eye to Cortex
Figure 6.17 The Lateral Geniculate Nucleus
(LGN)
The Structures and Functions of the Visual
System
• The Striate Cortex – located in the occipital
lobe
– Cortical Receptive Fields
• Simple cortical cells
• Complex cortical cells
– Cortical Columns
– Cytochrome Oxidase Blobs
– Cortical Modules
Figure 6.18 The Striate Cortex of the Occipital
Lobe
Figure 6.19 Cortical Receptive Fields
Figure 6.20 Cortical Modules
The Structures & Functions of the Visual System
• Visual Analysis Beyond the Striate Cortex
– At least a dozen other areas of human cerebral cortex
participate in visual processing
– The Dorsal Stream
• Important role in processing motion
– The Ventral Stream
• Important for object recognition
• Fusiform face area
Figure 6.21 The Ventral “What” Stream and the
Dorsal “Where” Stream
Figure 6.22 The Fusiform Face Area
Visual Perception
• Hierarchies
– Simple cells input to increasingly complex cells
– Feature detectors
• Spatial Frequencies
– Gratings: simplest patterns of lines
– Contrast sensitivity function
Figure 6.23 Problems for the Hierarchical Model
of Vision
Figure 6.24 Spatial Frequencies
Figure 6.25 A Cat’s View of the World
Visual Perception
• Perception of Depth
– Monocular cues
• Perspective
• Texture and shading
• Comparison of size of familiar objects
– Binocular cues
• Retinal disparity
• Coding Color
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Trichromacy theory
Opponent Processes
Colorblindness
Color Contrast and Color Constancy
Figure 6.26 Stereograms Demonstrate Retinal
Disparity
Figure 6.27 Mixing Lights
Figure 6.28 Color Afterimages Illustrate
Opponency
Figure 6.29 Opponency
Figure 6.30 Looking Through the Eyes of a
Dichromat
Figure 6.31 Color Contrast
The Development of the Visual System
• Contrast Sensitivity Functions of Children
• Presbyopia – “old sight”
Figure 6.32 The Development of Contrast
Sensitivity
Disorders of the Visual System
• Ambylopia
– Lazy eye
• Cataracts
– Clouding of lens of the eye
• Visual Acuity Problems
– Myopia, hyperopia, astigmatism
• Blindness
• Visual Agnosias
– Difficulty recognizing what is seen
Figure 6.34 Eyeball Shape Influences the Quality
of Vision
Figure 6.35 Astigmatism