Primary Questions in the
Study of Perception
• The physiological question: How are the
properties of objects in the environment
represented by the activity in the nervous
system?
• The stimulus question: How do we use
information from the environment to create
perceptions?
• The cognitive question: How do experiences
and prior knowledge influence perceptions?
Distance Cues
• Monocular cues: Relative size of images
on retina, motion parallax, linear
perspective, aerial perspective, relative
size, occlusion, shadowing, texture
gradient, height in plane.
• Binocular cues: Retinal disparity.
Relative Size of Retinal Images
Which one is closer?
Why do you think so?
Monocular Depth Cues
Relative size
Occlusion
Height in plane or
position relative to
horizon
Aerial perspective
Shadowing
Linear
perspective
Texture gradient
Relative Size
Motion Parallax
Size Perception
• Our perception of size depends on our
perception of the distance of the object—
we judge the size based on how far away
we think the object is—the size of the
image on the retina.
• Because of this the eye can be fooled into
seeing things differently that they really
are.
These are
identical twins.
Why do they
appear so
different?
They are
standing in an
Ames room.
An Ames room is designed to produce this illusion. One views the objects or
people through a peep hole monocularly, so that the room appears normal
and we thus judge that the objects must be of different sizes.
The Ames Room
Ames was an
optometrist who
designed this
room to create
perceptual
distortion.
Another Size Illusion
Mueller- Lyer Illusion
Which line appears longer?
Another Size Illusion
Mueller- Lyer Illusion
They are the same length. The brain is fooled
by the arrows at the ends of the lines.
Motion Parallax
• Think of looking out the window of a
moving automobile.
• Are different parts of the scene moving in
different directions?
• Relative to the motion of the car, what
direction are the nearest objects moving?
• What direction are the distant objects
moving, again relative to the movement of
the car?
Binocular Depth Cues
• Retinal Disparity: Each eye has a slightly different image
of the world, although both are in the same plane. There
are disparate images on the two retinas.
• The process of putting these images together in the
brain is called stereopsis.
• Stereograms are flat two-dimensional images designed
in such a way that if we can feed each eye a slightly
different perspective of this image the result will be one
three-dimensional image.
• Optical devices can be used to produce this effect.
When the two-dimensional picture is designed to do this
when view correctly we call it an autostereogram.
Autostereograms
• The images that follow are autostereograms. There are
two possible ways that you might view these images to
create a three-dimensional effect.
– Parallel vision.
– Cross-eyed vision.
• Parallel vision requires that each eye lock on the image
separately. Each eye is staring straight ahead.
• Cross-eyed vision requires that each eye lock on the
opposite sight, but again each is viewing the image
separately.
• One procedure may work for some stereograms and not
for others. You need to try both.
• Often there are two little dots or crosses at the bottom.
Trying to make these converge can sometimes help.
Trying to look behind the picture can also help.
Viewing Autostereograms
Try Parallel Vision
What You Might Have
Seen Is a Cube
You might have
seen a tea cup
floating in this
picure.
Did you see
anything else?
Some people see a
field of mushrooms.
Motion Perception
• How do we know that the person walking toward
us is the one moving?
• How do we know that when we move our eyes
and the scene changes, that the scene has not
moved?
• How do we explain motion after-effects such as
this: http://dogfeathers.com/java/spirals.html.
Please be cautious in observing this—some
people have trouble with this kind of stimulation.
• How is the illusion of movement created on
movie marquees, or road signs?
Motion Perception
(Phi Phenomenon)
Stroboscopic Motion
• The foregoing illusion tells us that slightly
different images presented in rapid
succession can be perceived as a moving
object if the rate of presentation is rapid
enough.
• The object will appear to move smoothly at
faster rates, and will appear to hop at
slower rates.
• This is called stroboscopic movement.
Explaining Motion Perception
• There are neurons in the brain that respond only
when neurons on the retina fire in sequence as
an image moves across the retina.
• Different sets of neurons are specialized to
respond to only one direction of movement.
• The interpretation of that experience depends on
things like motion after-effects and the
monitoring of our head and eye movements.
• The combination of all these neural signals is
interpreted to provide us with the perceptual
experience that we have.
Object Perception
• How do we separate
an object from the
background? How do
we know which is the
object and which is
the background?
• It seems to depend on
recognition of form.
• How do you know
there is a dog in this
picture?
Object Perception
• Do we build our perception of objects and the
environment piecemeal, collecting together all the parts
to form a whole? This is called bottom-up or feature
driven perception.
• We have neural pathways that are specialized to
respond to certain features, colour, shapes, motion that
are interconnected in the association areas. Early
processing seems to be feature driven.
• Do we look at the world as wholes, and then break what
we perceive into its component parts? This is called topdown processing.
• Evidence says we do both.
Object and Form Perception
Pop-out Evidence
• A: A search for a green line is
easy—it pops out. Only one
feature is different.
• B: A search for a horizontal line is
easy—it pops out. Only one
feature is different.
• C: A search for a horizontal red
line is much more difficult—two
features are different. The search
becomes methodological, plodding
analyzing feature after feature,
feature driven, or bottom-up.
The pop-out evidence is often taken to indicate that the
simpler scenes are processed all at once—a gestalt.
Object and Form Perception
Gestalt Principles
• Gestalt psychology: A school of
pyschology that focused on the ‘whole’
perception, arguing that the whole is
greater (different) than the sum of its parts.
• Developed a set of principles suggesting
how we group items together when
perceiving them.
• This argues for top down processing in
perceiving form.
Gestalt Principles
• Law of Good Form:
We will perceive
objects in the simplest
way possible. Is the
following a multi-sided
irregular figure or an
oblique triangle on top
of a rectangle?
Gestalt Principles
• Law of Closure: We will fill in
missing information so as to
create fully-formed figures. We
perceive “illusory contours”—
they aren’t really there.
Law of Good Continuation:
The eye naturally follows a
line or curve, and will not
break the flow even if there is
an obvious interruption.
Images from:
http://graphicdesign.spokanefalls.edu/tutorials/process/gestaltprinciples/gestaltprinc.htm
Gestalt Principles
• Law of Proximity: Items
placed close together tend to
be perceived as a group.
Twelve happy faces, or three
rows of four, or four columns of
three?
Law of Similarity: Objects
that are similar tend to be
perceived as a group. Twelve
happy faces, or four columns
of happy faces, alternating
colours and direction?
Perceptual Constancies
The tendency is to see a
series of circles whose angles
are changing, rather than four
rows of different shapes. It
seems as if the three black
circles at the bottom are
repeated into depth, as if
printed on a cylinder.
Describe what
you see here.
This is referred to as shape
constancy—we tend to
perceive the shapes as
constant and moving away
from us.
Perceptual Constancies
As it opens, the image of the door changes shape on our retina but we do
not perceive that its shape changes—we know it is still a rectangle—
shape constancy.
http://www.aber.ac.uk/media/Modules/MC10220/visper02.html
Perceptual Constancies
• Size constancy
– As objects move away from us their image on the
retina becomes smaller, but we perceive that their
size remains constant. Because of this we can be
tricked, e.g., the Ames room.
• Colour and lightness constancy
– The wave lengths and amount of light that reach our
eye from a given object vary with the lighting. In spite
of this, the object is always perceived to be its original
colour and lightness.
– White snow actually reflects much less light at dusk,
so that the neural activity from the retina is similar to
that of a grey object. In spite of that, we perceive the
snow as white—lightness constancy.
– A red circle that is half in shadow is not perceived as
two shades of red. We know it is still all red—colour
constancy.
Perceptual Constancies
• Our perception of an object’s size, shape, colour,
or lightness does not vary with changing the
angle, distance, or illumination on that object.
• Perception is based on understanding the
relationship between objects and these factors.
The brain computes a ratio of one sensation to
another, e.g., the illumination from the object to
the background illumination, or the appearance of
the object to the angle of viewing.
• Gibson’s direct perception theory: The visual
system is designed to interpret the world.
– Visual processing performed by the brain is prewired.
– These innate processes occur at the same time (in
parallel) with learned processes.
Two Stage Model of Perception
• Stage 1: Preattentive or automatic
processing: Gestalt laws apply, as do
binocular and monocular cues to depth
and other automatic perceptual processes.
• Selector or Filter: Cocktail party
phenomenon—selective listening—some
things get through. The capacity is limited.
• Stage 2: Selective attention.
Physically orient to stimulus.
Cognitively orient to stimulus.