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Chapter 5
Sensation &
Perception
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Sensation and Perception
Sensation –
immediate, basic experience generated by a
stimulus
Perception –
interpretation, organization of internal or external
events (involves awareness and activity)
Sensation & Perception
Processes
Transduction
Transduction –
Transformation of one form of energy into
another
Receptors –
Specialized neurons that are activated by
stimulation and transduce (convert) it into
a nerve impulse
Thresholds
Absolute threshold –
Amount of stimulation necessary for a stimulus to
be detected
Difference threshold –
Smallest amount by which a stimulus can be
changed and the difference be detected (also
called just noticeable difference – JND)
Weber’s law –
Size of a JND proportional to intensity of stimulus
Signal Detection Theory
Signal detection theory –
Perceptual judgment as combination of
sensation and decision-making processes
Stimulus event
Neural activity
Comparison with
personal standard
Action (or no action)
Signal-Detection Theory
• A psychophysical theory that divides the
detection of a sensory signal into a sensory
process and a decision process.
Stimulus is
Present
Stimulus is
Absent
Response:
“Present”
Hit
False Alarm
Response:
“Absent”
Miss
Correct
Rejection
How Are the Senses Alike?
And How Are They Different?
The senses all operate in
much the same way, but
each extracts different
information and sends it to its
own specialized processing
region in the brain
Sensory Adaptation
Sensory adaptation –
Loss of responsiveness in receptor
cells after stimulation has remained
unchanged for a while
An Eye on the World
• Cornea
– Protects eye and
bends light toward
lens.
• Lens
– Focuses on objects by
changing shape.
• Iris
– Controls amount of
light that gets into
eye.
• Pupil
The Structures of the Retina
An Eye on the World
• Retina
– Neural tissue lining the back of the
eyeball’s interior
• Two Photoreceptors:
Rods: respond to dim light/movement (125 million)
Cones: involved in color vision/detail (7 million)
Why the Visual System is not a
Camera
• Much visual processing is done in the
brain.
– Some cortical cells respond to lines
orientations (e.g. horizontal).
– Other cells in the cortex respond to other
shapes (e.g., bulls-eyes, spirals, faces).
• Feature-detectors
Hubel & Wiesel’s Experiment
Constructing the Visual World
• Form perception.
• Gestault Principles
– “whole is more than the sum of the parts”
Form Perception
• Gestalt principles describe the brain’s
organization of sensory building blocks into
meaningful units and patterns.
Figure –
Part of a pattern that commands attention
Ground –
Part of a pattern that does not command
attention; the background
Examples “Gestault”
More “Gestault”
• Proximity
– Seeing 3 pair of lines
in A.
• Similarity
– Seeing columns of
orange and red dots in
B.
• Continuity
– Seeing lines that
connect 1 to 2 and 3
to 4 in C.
• Closure
– Seeing a horse in D.
Law of Prägnanz
Depth and Distance Perception
How can you tell what is closer?
Depth and Distance
Perception
• Binocular Cues:
– Convergence
– Retinal Disparity
• Monocular Cues:
– Relative size, overlap, texture gradient,
linear perspective, atmospheric perspective,
shadow/shading, motion parralax
Visual Constancies
• The accurate perception of objects as
stable or unchanged despite changes
in the sensory patterns they produce.
– Shape constancy
– Size constancy
– Color constancy
Shape Constancy
• Even though these images cast shadows of
different shapes, we still see the quarter as
round
Bottom-Up and Top-Down
Processing
Bottom-up processing –
Analysis that emphasizes characteristics
of the stimulus, rather than internal
concepts
Top-down processing –
Emphasizes perceiver's expectations,
memories, and other cognitive factors
Context Effects
• The same physical
stimulus can be
interpreted differently
• We use other cues in
the situation to
resolve ambiguities
• Is this the letter B or
the number 13?
Visual Illusions
Visual
Illusions
• Illusions are valuable in understanding
perception because they are systematic
errors.
• In the Muller-Lyer illusion (above)
The Ponzo Illusion
• Linear perspective
provides context
• Side lines seem to
converge
• Top line seems
farther away
– But the retinal
images of the red
lines are equal!
How Sound Waves Become
Auditory Sensations
Cochlea –
Where sound
waves are
transduced
Taste Buds
• Photograph of
tongue surface
(top), magnified 75
times.
• 10,000 taste buds
line the tongue and
mouth.
• Children have more
taste buds than
adults.
Four Tastes
• Four basic tastes
– Salty, sour, bitter and sweet.
• Different people have different tastes
based on:
–
–
–
–
Genetics
Culture
Learning
Food attractiveness
Smell: The Sense of Scents
• Airborne chemical molecules enter the nose and
circulate through the nasal cavity..
• Receptors on the roof of the nasal cavity detect
these molecules.
The Skin Senses
• Touch
• Temperature
• Pain
Perceptual Powers: Origins and
Influences
• James J. Gibson : Direct Perception
– Invariants and affordances
• Inborn abilities and perceptual lessons
Psychological and Cultural
Influences on Perception
•
“needs”, beliefs and emotions can influence perceptions
of sensory information.
•
Expectations based on our previous experiences
influence how we perceive the world.
– Perceptual Set
• Readiness to detect a particular stimulus in a given context
• We all are influenced by our culture/experience.
Perceptual Set
• What you see in the center figures depends
on the order in which you look at the figures:
– If you scan from the left, see an man’s face
– If you scan from the right, see a woman’s figure
Extrasensory Perception
• Extrasensory Perception (ESP):
– The ability to perceive something without
ordinary sensory information
– This has not been scientifically demonstrated
(issues with reliability/validity)
• Three types of ESP:
– Telepathy – Mind-to-mind communication
– Clairvoyance – Perception of remote events
– Precognition – Ability to see future events