1. Chapter 4 Section 2 The Visual System
Goal Two: Explain how the visual system enables us to see, and by communicating with the brain, to perceive the world.

2. The Visual Stimulus and the Eye
Light is a form of electromagnetic energy. Light travels through space in waves.
The wavelength of light is the distance from the peak of one wave to the peak of the next.
Amplitude is the height of the wave and it is associated with the brightness of a visual stimulus.
Purity is the mixture of wavelengths in light.

4. The Structure of the Eye
The eye is set up like a camera in that it gets the picture of the world.
The sclera is the white outer part of the eye that gives the eye its shape and protects the eye from injury.
The iris is the colored part of the eye.
The pupil, which appears black, is the opening in the center of the iris. The iris contains muscles that allow the pupil to get larger or smaller depending on how much light is being let into the eye.

5. The Structure of the Eye
The cornea is the clear membrane on the outer part of the eye. The curved surface on the cornea bends light on the surface of the eye in order to focus it to the back of the eye.
The lens is transparent and somewhat flexible. When a person is looking at an object far away, the lens has a relatively flat shape. However, when a person is looking at an object that is closer, more bending of the light is needed.

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Structure of the Eye
IM: The Structure of the Eye Activity
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7. Structure of the Eye: Retina
Rods
sensitive to even dim light, but not color
function well in low illumination
humans have ≈ 120 million rods
Cones
respond to color
operate best under high illumination
humans have ≈ 6 million cones
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9. Structure of the Eye: Retina
Fovea
densely populated with cones
vital to many visual tasks
Bipolar and Ganglion Cells
Optic Nerve
Blind Spot
where optic nerve leaves the eyeball
perception involves top-down processing
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10. Structure of the Eye: Retina
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Visual Processing
Pathway of Visual Information
optic nerve  optic chiasm  …  visual cortex
Optic Chiasm: Optic Nerve Fibers Divide
left visual field  right hemisphere
right visual field  left hemisphere
Primary Visual Cortex
occipital lobe
initial visual processing
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Visual Processing
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Visual Processing
Feature Detectors
highly-specialized cells in the visual cortex
size, shape, color, movement, or combination
deprivation studies: brain “learns” perception
Parallel Processing
Binding
IM: Activity Handout 5.3: What All Do You See?
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14. Parallel Processing
What” and “where” are two questions that need to be answered in order for people to respond appropriately to a visual stimulus.
The “what” pathway is in the temporal lobe and processes information about what the object is. The “where” pathway is located in the parietal lobe and processes information about an object’s location.
Parallel processing is a simultaneous distribution of information across different neural pathways. It helps information move rapidly through the brain.

15. Binding
Binding is when the different pathways and cells bring together and integrate information.
Through binding a person can integrate information about various parts of an object. For example, if a person sees a chair, then through binding they not only see just the chair, but they also see the size, the color, the motion, etc. of the chair.

16. Color Vision: Theories
Trichromatic Theory
Three Types of Receptors
green, red, and blue cones
Color Blindness
one or more cone types is inoperative
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17. Color Vision: Theories
The term color blind refers to seeing some colors but not others. Color blindness depends on which of three kinds of cones (green, red, and blue) is not working.
The opponent-process theory states that cells in the visual system respond to red-green and blue-yellow colors. A cell excited by red and green could be inhibited, or a cell excited by blue and the yellow could be inhibited.

18. Color Vision: Afterimages
Note: Direct students to stare at this image for seconds, then switch quickly to next (blank) slide.
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19. Color Vision: Theories
Afterimage
sensation remains after a stimulus is removed
trichromatic theory cannot explain afterimages
Opponent Process Theory
complementary color pairs
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20. Perceiving Shape, Depth, Motion, and Constancy
To perceive a visual stimulus, the fragments of information that the eye sends to the visual cortex must be organized and interpreted.
The figure-ground relationship occurs when a person organizes the perceptual field into stimuli that stand out (figure) and those that are left over (ground).
Gestalt psychology explains how people naturally organize their perceptions according to certain patterns. Closure is a gestalt principle; when a person sees a disconnected or complete figure, they see a whole. Proximity is a second gestalt principle; when individuals see objects as close to each other, they tend to group them as together. The third gestalt principle is similarity; when objects are similar, individuals tend to group them together.

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Visual Perception
organizing and interpreting visual signals
dimensions
- shape
- depth
- motion
- constancy
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22. Visual Perception: Shape
Gestalt Psychology
perceptions are naturally organized according to certain patterns
whole is different from the sum of the parts
Gestalt Principles
figure-ground relationship
closure
proximity
similarity
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23. Figure-Ground Relationship
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24. Visual Perception: Shape
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25. Visual Perception: Depth
the brain constructs perception of 3D from 2D images processed by the retina
binocular cues
- disparity
- convergence
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26. Depth Perception
Depth perception is the ability to see objects in three dimensions.
Binocular cues are depth cues that depend on the combination of the images in the left and right eyes and on the way the two eyes work together. For example, if a person holds their hand over one eye and focuses on an object and then switches to cover their other eye, the switching back and forth between the eyes will cause the object to jump back and forth.
Monocular cues are depth cues that are available from the image in one eye, either the left or the right eye. Some examples of monocular cues are: familiar size, height in the field of view, linear perspective, overlap, shading, and texture gradient.

27. Visual Perception: Depth
Monocular Cues – Pictorial Cues
familiar size
height in the field of view
linear perspective
overlap
shading
texture gradients
IM: Monocular Cues Activity
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28. Monocular Cues: Shading, Texture Gradient
Note: Instructors should feel free to replace this figure with any picture or pictures illustrating the various monocular cues of depth perception
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29. Monocular Cues: Linear Perspective, Height in Field
Note: Instructors should feel free to replace this figure with any picture or pictures illustrating the various monocular cues of depth perception.
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30. Visual Perception: Motion
humans have specialized motion detectors
apparent movement
Note: As with most visual illusions, there are a number of excellent demonstrations of movement aftereffects available online, or use the next slide.
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31. Visual Perception: Motion
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32. Visual Perception: Constancy
Perceptual Constancies
recognition that objects do not physically change despite changes in vantage point and viewing conditions
sensory information (retinal image) changes, but perceptual interpretation does not
Size, Shape, and Color Constancies
Activity/Demonstration: You can easily demonstrate shape and size constancy by walking around the classroom and asking students if you are growing larger or smaller as you move closer to them, and hold up and rotate a book and ask students if the shape of the book changes. Of course, you should point out that the resulting retinal image does change in both cases.
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33. Motion Perception
Apparent motion occurs when an object is stationary but it is perceived as moving. An example of apparent motion is watching an IMAX movie. Two forms of apparent motion are stroboscopic motion and movement aftereffects.
Perceptual constancy refers to the recognition of objects as remaining stationary and unchanging even though sensory input about them is changing. There are three types of perceptual constancy: size constancy, shape constancy, and brightness constancy.