Download 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.
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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.
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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.
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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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Rods
sensitive to even dim light, but not color
 function well in low illumination
 humans have ≈ 120 million rods
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Cones
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respond to color
operate best under high illumination
humans have ≈ 6 million cones
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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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Pathway of Visual Information
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optic nerve  optic chiasm  …  visual cortex
Optic Chiasm: Optic Nerve Fibers Divide
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left visual field  right hemisphere
right visual field  left hemisphere
Primary Visual Cortex
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occipital lobe
initial visual processing
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© 2011 The McGraw-Hill Companies,
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Feature Detectors
highly-specialized cells in the visual cortex
 size, shape, color, movement, or combination
 deprivation studies: brain “learns”
perception
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Parallel Processing
Binding
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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.
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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.
Trichromatic Theory
Three Types of Receptors
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green, red, and blue cones
Color Blindness
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one or more cone types is
inoperative
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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.
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Afterimage
sensation remains after a
stimulus is removed
 trichromatic theory cannot
explain afterimages
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Opponent Process Theory
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complementary color pairs
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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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organizing and
interpreting visual signals
dimensions
- shape
- depth
- motion
- constancy
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Gestalt Psychology
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perceptions are naturally organized
according to certain patterns
whole is different from the sum of the
parts
Gestalt Principles
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figure-ground relationship
closure
proximity
similarity
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the brain constructs perception of
3D from 2D images processed by
the retina
binocular cues
- disparity
- convergence
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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.
Monocular Cues – Pictorial Cues
 familiar size
 height in the field of view
 linear perspective
 overlap
 shading
 texture gradients
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humans have specialized
motion detectors
apparent movement
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Perceptual Constancies
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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
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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.