Perception
The process of organizing and interpreting
information, enabling us to recognize meaningful
objects and events.
We see not how the world is but how it is useful to us
Selective Attention
• The focusing of conscious
awareness on a particular
stimuli
• You didn’t notice consciously
feel your shirt until you read
this 
• Inattentional Blindness: failing
to see visible objects when our
attention is directed elsewhere
• Change Blindness: failing to
notice changes in the
environment
Click on the mouse to
see an example of
change blindness.
Cocktail-party phenomenon
• The cocktail party effect
describes the ability to
focus one's listening
attention on a single talker
among a mixture of
conversations and
background noises, ignoring
other conversations
• Form of selective attention
Top-Down Processing
• We identify something
b/c our brain tells us what
the object is (prior
knowledge)
• We perceive by filling the
gaps in what we sense.
• I _ant ch_co_ate ic_
cr_am.
• Based on our experiences
and schemas.
• If you see many old men in
glasses, you are more apt
to process a picture of an
old man (even when you
may be in error).
• Also called feature
analysis
• We use the features on
the object itself to build
a perception
• No prior knowledge
• Takes longer than topdown but is more
accurate
Bottom-Up
Processing
Click to see an example
of bottom –up
processing.
Thresholds
• Absolute Threshold: the minimum
amount of stimulation needed to detect a
light, sound, pressure, taste, odor… 50%
of the time
– Psychophysics: the study of relationships
between physical characteristics of stimuli
(like intensity) and our psychological
experience of them (look for absolute
thresholds)
– Ex: mtn on dark night, see light miles away
– Ex: hearing tests
– Ex: mosquito ring tones
– Ex: shopkeeper
Signal Detection Theory
• Theory that predicts when we detect
weak signals amid background
stimulation
– Says absolute thresholds are not
really absolute
• So many factors affect what we can
sense
– time of day? did you eat
breakfast? do they exercise? Did
you get enough sleep? …
• 4 possibilities:
– Hit-signal present & sensed
– Miss-signal present but not
sensed
– False Alarm-signal not present but
sensed
– Correct Rejection-signal absent &
not sensed
Ex: wartime
(Just Noticeable Difference)
Difference Threshold
•
•
The smallest amount of change
needed to detect in a stimulus
before we detect a change
Why would you need this?
• Ex: hearing your friend’s
voice over other voices in
the hallway
• Ex: My mom’s whistle
• Ex: Musician detecting
differences when tuning
an instrument
Weber’s Law
• The greater the magnitude
of the stimulus, the larger
the differences must be to
be noticed
• Ex: if you are listening to the
tv at volume 40, the JND
occurs when you decrease the
volume by 4
• If you are listening to tv
volume at 20, the JND occurs
when you decrease the
volume by 2
Perceptual Ideas
Figure Ground Relationship
• We organize what we
see into objects (the
figures) that stand out
from their surroundings
(the background)
• We always organize
stimulus into a figure
seen against the ground
Gestalt Psychology
• Gestalt psychologists focused on how
we GROUP objects together
• We innately look at things in groups
or as a WHOLE, not as isolated
elements (we want to see the forest,
not the trees)
• Proximity (group objects that are
close together as being part of same
group- 3 sets of 2 lines, not 6)
• Similarity (objects similar in
appearance are perceived as being
part of same group- vertical columns
of similar shapes not horizontal of
different)
• Continuity (objects that form a
continuous form are perceived as
same group- 1 wavy, 1 straight line)
• Connectedness (b/c they are uniform
& linked we think of the 2 dots and
line as one unit)
• Closure (like top-down processing…we
fill gaps in if we can recognize it)
CLOSURE
Depth Cues
• Depth Perception: ability to see
3D, allows us to judge distance
– Our retinas pick up 2D but we
see 3D
• Eleanor Gibson and her Visual Cliff
Experiment.
• If you are old enough to crawl, you
are old enough to see depth
perception.
• We see depth by using two cues
that researchers have put in two
categories:
– Monocular Cues
– Binocular Cues
Binocular Cues
• Binocular Cue: depth cues
that depend on the use of
2 eyes
• Retinal Disparity: the
difference between the
images each eye perceives
– Each eye sees different
things
– important for depth
perception
• Let’s see this in action!!!
Monocular Cues
• Monocular Cue: You
really only need one eye
to use these (used in
art classes to show
depth)
• Linear Perspective:
parallel lines appear to
converge w/ distance
• Interposition: if one
object partially block
our view of another we
think it’s closer
More Monocular Depth Cues
• Relative Height: we
perceive objects higher in
our field of vision as
farther away
• Relative Size: the smaller
the object the farther away
we think it is
• Light and Shadow: given 2
identical objects the
dimmer one seems farther
away
Perceived Motion
• Stroboscopic effect
(flip book effect,
cartoons)
• Phi phenomenon:
movement illusion when
2 or more adjacent
lights are blink on and
off in quick succession
• Autokinetic Effect (if
people stare at a white
spotlight in a dark
room, it appears to
move)
Perceptual Constancy
• Perceptual Constancy:
perceiving objects as
unchanging even when
light and retinal images
change
• Shape Constancy
• door
• Size Constancy
• Car carrying people
• Color Constancy
Perceptual Constancy
•
•
•
•
Size Constancy
Lightness Constancy
Color Constancy
You saw all of these
in the “Brain
Games” video
•
•
•
•
Perceptual Set
What do these letters spell? FOLK
How about these? CROAK
And these? SOAK
What do we call the white of an egg?
– NO!!! IT’S AN EGG WHITE!!!
• Perceptual Set: a mental predisposition to perceive
one thing and not another