Perception
Our conscious attention is selective.
Getting Started
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Reading Handout
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True false
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PsychSim 5
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Visual Illusions; complete for next class
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Which Line Is Longer?
From Perplexing Puzzles and Tantalizing Teasers, (p.75), by Martin Gardner, 1988, New York Dover.
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Perception
The process in which people take raw
sensation from the environment and
interpret them
 Uses knowledge, experience and
understanding
 Sensations thus become meaningful
experiences with perception
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Three Approaches to Perception
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Computational
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Constructivist
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Focuses on how the nervous system manipulates
any incoming signals
Emphasizes the inferences people make about
the environment
Ecological
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Emphasizes the information provided by the
environment
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From Sensation…
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Absolute Thresholds
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Subliminal Stimuli
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Stimuli too weak to be noticed
Supraliminal Stimuli
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Stimuli that are strong enough to be
consistently perceived
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The airplanes that fly overhead in this classroom!
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Why Does the “Absolute”
Threshold Vary?
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Sensitivity:
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Intensity of the signal.
Capacity of sensory systems.
Amount of background stimulation, or “noise.”
Response criterion reflects one’s
willingness to respond to a stimulus.
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Influenced by motivation and expectancies.
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Signal Detection Theory
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Signal-detection Theory
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Mathematical model used to determine
whether or not a near-threshold has occurred
Theorists seek to understand why people
respond differently to stimulus

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Newborn parents will miss a loud noise, but here a
baby’s whimper
In heightened security times you have more
false alarms
In peacetime, a stronger signal is required
before sensing danger
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Signal Detection Theory
Trade off: high criterion minimizes false alarms, but also increases misses
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Perceptual Illusion & Organization
Our mind structures information in several
ways:
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Form Perception
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Depth Perception
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Motion Perception
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Perceptual Constancy
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Form Percpetion: Reversible Images
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Form Perception: Grouping
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Summary: Form Perception
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Figure-Ground Organization
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Reversible images
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You can’t see both figure and ground at the same
time
Grouping
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Gestalt Principles
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Proximity
Continuity
Similarity
Connectedness
Closure
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Depth Percpetion: Visual Cliff
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Depth Perception
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How is it that we perceive a 3-dimensional world
when our eyes only project a 2-dimensional
image on our retinas??!

Our brain uses different cues to perceive depth

Binocular disparity

Since we use both our eyes to focus on an image, the
angles used by each eye to put the image on the fovea of
our retina is used by the brain to perceive distance
 Retinal disparity: a cure for perceiving depth; the greater the
disparity the closure the object

Monocular disparity

Our brain also uses information from the stimulus that
does not involve our use of both eyes (see next slide)
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Stimulus Cues for Depth Perception
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Monocular cues
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Interposition
Relative Size
Relative height
Textural Gradient
Linear Perspective
Relative Clarity
Light and Shadow
Motion
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Perception of Motion
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Many cues come from optical flow.
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Looming
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When an image looms, you tend to interpret it as an
approaching stimulus
Questions of Interest
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How do we know whether flow of images
across retina is due to movement of objects or
own movements?
How do we compensate for the time-lag in the
perception of movement?
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Perception of Motion
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Sometimes we perceive motion when
there is none.
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3D movies and feelings of nausea
Stroboscopic motion: Illusion of
movement when images are flashed in
rapid succession
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Flashing neon lights can appear to make the
sign move
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Perceptual Constancy
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Perceiving objects as unchanging; having
constant:
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Size
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Shape
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The tendency to view an object as constant in
size despite changes in the size of its image on
the retina (as we move)
The tendency to see an object as retaining its
form despite changes in orientation
Brightness
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Perceptual Constancy: Size Illusions
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Perceptual Constancy: Shape Illusion
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Brightness Contrast
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Perceptual Processing
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Bottom-Up Processing
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Analysis that begins with the sense receptors
and works up to the brain’s integration of
sensory information
4 legs, round surface; must be a table
Top-Down Processing
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Information processing guided by higher-level
mental processes
We construct perceptions based on our
experience and expectations
i.e. the jumbled sentence from lesson one
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Top-down processing!
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Top-Down Processing

Aided by higher level cognitive processes and
expectation we transform information into
meaningful experiences
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Our experiences create schemas.
Schemas can bias our perceptions by creating a
perceptual set (we perceive one thing not another).
Predisposition can also be shaped by the
immediate context of the stimulus.
Motivation can affect perception.
Can even influence elements of the brain’s bottomup processing (sensory receptors to brain).
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Attention
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We use attention to:
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Direct our sensory and perceptual systems
toward certain stimuli.
Select specific information for further
processing.
Allocate the mental energy required to do that
processing.
Regulate the flow of resources necessary
for performing a task or coordinating several
tasks at once.
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Characteristics of Attention
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Improves Mental Processing
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Takes Effort
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Usually we need to concentrate to do our best
Prolonged attention is tiring
Is Limited
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Focusing on this lecture and making notes
leaves less attention for hallway noises
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Directing Attention
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Voluntary, or goal-directed attention
control reflects top-down processing
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You purposely focus your attention in order to
perform a task (writing)
Involuntary attention control reflects
bottom-up processing
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Attention redirected because of a loud noise
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Divided Attention

Sometimes difficult to stop dividing
attention (multitask)

Drive car, listen to music, shift gears, watch
road signs
Attention is a limited resource.
 Easier to do two things at once if one task
is automatic.

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The Stroop Task
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Applications of Research on
Perception; the Human Factor
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Aviation Psychology
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Human-Computer Interaction
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Trash can for the delete button
Traffic Safety
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Landing the plane
Driving performance and cell phones
Architecture and Interior Design
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Window sizes to change the feel of the space
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Is there ESP?
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ESP is the controversial claim that
perception can occur apart from sensory
input
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Telepathy
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Clairvoyance
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Perceiving remote events
Precognition
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mind-to-mind communication
Perceiving future events (think Minority Report)
Parapsychologist
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Those who study paranormal
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Research on ESP
After thousands of experiments, a
reproducible ESP phenomenon has never
been discovered
 Remember scientific study blends curious
skepticism with open-minded humility
 We do assume some tings to be true
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God’s existence or non-existence
Someone else’s love
The finality of death or reality of life after it
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Review

Jeopardy: final two columns
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Some Questions
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Top-down processing and the influences of
expectancy, motivation and schemas.
Success of dual-attention tasks and why.
Describe how you would use at least five
monocular cues to give a scenery painting a
sense of depth.
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