The Visual System
Light
Enters the eye as
electromagnetic
radiation
 Travels in a wave
that vary in
amplitude (height)
and wavelength
(between peaks)
 Amplitude=
brightness
 Wavelength= color
(hue)

Humans see a
mixture of several
wavelengths
 Varies purity
 Small portion of the
spectrum
 Animals- what do
they see?
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Parts of the eye
Parts of the Eye
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Cornea (1) transparent
covering/protection
Pupil and Iris (2) bright
light/contracts, dim
light/relax
Lens (3) focuses light on the
retina, accommodation
Retina (4) neural tissue that
absorbs light
Optic disk & Blind Spot (5)
Optic Nerve (6)
Fovea (7)

Blind Spot
Activity!!
The Retina
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Millions of
receptor cells
10 % of light
Rods- 100 to 125
million
Cones- 5-6 million
Expt- What Color
is it??
Light and Dark Adaptation
Light and Dark Adaptation
From the Eye to the Brain
Processing in the Visual Cortex
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Feature Detectors
Simple Cells
Complex cells
Ventral (what)
pathway
Dorsal (where)
pathway
Color Theory
 Psychological
interpretation- Blends of
three aspects or properties of light
 Three theories regarding color
Trichromatic Theory- Young and Von
Helmholtz
 The
eye does the mixing by varying the
ratio of neural activity among these 3
types
 Color blindness
– Dichromats
– Monochromats
Opponent Process Theory- Hering
Color perception depends on receptors
that make antagonistic responses to 3
pairs (red v. green) (yellow v. blue) (white
v. black)
 When excited they respond to one color
and when inhibited they respond to the
other

Opponent Process TheoryHering
 Complementary
colors (p.101)
Afterimage
Reconciling Theory
 Both
theories correct
 Perception of color is processed in stages
 Beginning stages- trichromatic
 Later stages- opponent processing
Form Perception
 Perceptual
Set - The influence of prior
assumptions and expectations on
perceptual interpretations
Perceptual Set

What do you see in the center picture: a male
saxophonist or a woman’s face? Glancing first
at one of the two unambiguous versions of the
picture is likely to influence your interpretation.
Feature Analysis

Bottom Up Processing & Top Down Processing
I cdnuolt blveiee taht I cluod aulaclty
uesdnatnrd waht I was rdanieg. It
dseno’t mataetr in what oerdr the ltteres
in a word are, the olny iproamtnt tihng is
taht the frsit and lsat ltter be in the rghit
pclae.
Gestalt Principles (p105)
 Figure
Ground
 Proximity
 Closure
 Similarity
 Simplicity
 Continuity
Phi Phenomenon
 The
illusion of motion when fixed lights are
turned on and off in a sequence
•
Stare at the X in the middle and notice what
happens. Is the Green Dot Moving?
•
http://www.weeville.com/eyetest.htm
Relationship Between Perceived
Size and Perceived Depth
To perceive the size
of objects accurately
we must also
perceive their
distance accurately
 Many visual illusions
occur because of a
lack of sufficient
depth cues

Retina
Image
Image
Pupil
A
B
A
This figure shows that image size depends
upon both object size and distance
Depth Perception
 Binocular
depth cues
– Retinal disparity- objects 25ft project
images to different locations on the right &
left retina
Monocular Depth Cues
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Linear Perspective
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Motion Parallax
Monocular Depth Cues
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Texture gradient
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Interposition
Monocular Depth Cues
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Relative Size

Height in a plane
Size
Constancy

People are the
same size even
though their
image sizes differ

The depth cues
such as linear
perspective and
relative size help
judge the size
accurately
Size Distance Relationship
Size Distance Relationship
Size Distance Relationship
Shape Constancy
 The
understanding that an object’s shape
remains the same even though the angle
of view makes the shape appear
changed
Shape Constancy

It is hard to tell if
the figure on the
upper right is a
trapezoid or a
square slanted
backward.

If we add texture,
the texture gradient
helps us see that it
is actually a square
Brightness Contrast
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Perceived lightness stays
roughly constant as long
as the context or
surroundings stay the
same.
When the context changes
you may perceive the
color as changing.
•Want to see more of these perceptions? Goto
http://www.skidmore.edu/~hfoley/Perc4.htm#lightcon
Sensory Deprivation & Restored Vision

Early visual experience can have a profound effect on
perception. Blakemore & Cooper’s restricted
environment with kittens.
Do the kittens ever fully regain normal sensitivity to
horizontal or vertical lines? NO.
 The “Use it or lose it” phenomenon.

From the time their eyes first opened, and until the
age of 5 months, these kittens were removed from
darkness each day to spend 5 hours alone in a
black-and-white striped cylinder with a clear glass
floor. A stiff collar prevented the kittens from seeing
anything else, even their own bodies. Afterward,
these kittens had difficulty perceiving horizontal
forms, compared with other kittens exposed only to
horizontal forms.
Sensory Deprivation & Restored Vision
These experiments show that lacking stimulation, the
cortical cells had not developed normal connections
making them functionally blind to shape.
 A sensory restriction does no permanent damage if it
occurs later in life. This suggests that visual
experiences during infancy are a critical period for
normal sensory and perceptual development.
Experience guides the organization of the brain’s
neural connections.
 If deafness or blindness is corrected as an infant, it
awakens the pertinent brain area. Nurture sculpts
what nature has endowed.
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Context
The setting or environment in which we interpret
sensory stimuli
 Culture can also influence how we perceive
information.

– People actively construct their perceptions by drawing on
their prior learning and cultural experiences.
– People living in urban and industrialized environments
where there are more right angles and straight lines will be
more susceptible to the Muller-Lyer Illusion than people in
non-carpentered natural environments.
Context & Culture

What is above the woman’s head? In one study, nearly all the East
Africans who were questioned said the woman was balancing a
metal box or can on her head and that the family was sitting under
a tree. Westerners, for whom corners and boxlike architecture are
more common, were more likely to perceive the family as being
indoors, with the woman sitting under a window.
Illusions
 When
we misperceive the true
characteristics of an object or image.
 Help researchers understand how
sensation and perception normally
works
Müller-Lyer Illusion
Perceptual psychologists
have hypothesized that
the top horizontal line
looks longer because it
also looks farther away
 Specifically, the inward
pointing arrows signify
that the horizontal line is
closest to you, and the
outward pointing arrows
signify the opposite case

Müller-Lyer Illusion
Müller-Lyer Illusion
Most people think segment AB equals BC.
In reality AB is much longer than BC.
Müller-Lyer Illusion
Müller-Lyer Illusion
Müller-Lyer Illusion
Ponzo Illusion
 Converging
lines indicate that top line is
farther away than bottom line
•
The interplay between perceived size and distance (a) The
monocular cues for distance make the pursuing monster look
larger than the pursued. It isn’t. (b) This visual trick, called the
Ponzo illusion, is based on the same principle as the fleeing
monsters. The two red bars cast identical-sized images on our
retinas. But experience tells us that a more distant object can
create the same-sized image as a nearer one only if it is actually
larger. As a result, we perceive the bar that seems farther away as
larger.
Moon Illusion
Moon appears larger when it is on the horizon than
when it is directly overhead.
 Objects on the horizon are perceived as farther away
than those above us
 The moon appears to be behind those objects on the
horizon. Since it is bigger than those object it is
perceived as huge! (click on box below for
explanation)

Click Below to View an
Explanation:
Moon Illusion
Ames Room Illusion: Secret
Revealed
We perceive the room to be as
we are used to, a perfect square
or rectangular.
When in fact it is a trapezoid!
Poggendorff Illusion
Impossible Figures
 These
grouping principles help us construct
reality but perceptual contradictions can lead us
astray
See how this and
others like it are
done
More Impossible Figures
More Impossible Figures
Escher’s Impossible
Scenes
Chrysanthemum
Is this 3-D?
Water or Monks?
Heads or Houses?
Rocks or Horses?
In or out of the picture?
http://www.rense.com/general67/str
eet.htm