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Sensation and Perception
Uta Wolfe
St. Olaf College
Outline
• Sensation, Perception, Behavior
• Process of sensation
• Perceived vs. “real” world
• Properties of perceptual processes
- Adaptation, pattern coding and aftereffects
- Receptive Fields
- Lateral Inhibition/ Contrast enhancements
- Perceptual constancies
• Top-Down Processing
• Attention and Perception
Sensation, Perception, Behavior
Sensation:
- basic processes by which sense organs and nervous system
respond to environmental stimuli
- basic experiences resulting from these processes (eg:
“yellowness”)
Perception:
- organizing and interpretation of sensory information
( eg: “The object is a banana”)
Purpose of both: guidance of behavior
most basic examples: phototaxis/ chemotaxis
External stimulus
Sensation (Perception)
Behavior
Process of sensation
reception
Absorption of physical energy
by specialized receptor
Photon hits
photoreceptor
transduction
transformation of one form of
energy into another
here: transformation of light,
sound etc. into
electrochemical change in
receptor
molecular change in
pigment leads to
channel closing and
change in potential
coding
representation of stimulus
aspects in temporal/ spatial
pattern of neural firing
Action potential
frequency signals
intensity
Important point: Once transduced, light, sound, touch etc.
are identical electrochemical signals.
Question: Why do these identical signals lead to
qualitatively different sensations? Why do we see light but
hear sound?
Answer: Law of specific nerve energies (J. Mueller, 1838):
quality of sensation depends on which set of nerve fibers is
stimulated.
Demo: poke eye, hit back of head!
Perceived vs. “real” world
Senses do not convey objective representation of environment:
1) Limitations of senses:
- sensitive to only certain types of energy
- sensitive to only small range of each energy spectrum
- threshold (absolute or difference threshold)
Senses do not convey objective representation of
environment:
2) Purposeful distortion of external stimulus:
- extracts and emphasizes important stimulus aspects
- leads to illusions
- eg: adaptation: Locke’s observation on temperature
lateral inhibition: Mach Bands
Properties of sensory systems:
- Adaptation, pattern coding and aftereffects
Adaptation: response of neuron decreases as a result of
prolonged stimulation
Neuronal
response
stimulus
time
Perceptual consequences: adaptation to pressure of clothes,
continuous sounds, ambient temperature, dark/ brightness
Pattern coding: (aka population or across-fiber coding)
Information about stimulus is conveyed by the pattern of firing
across a group of neurons, rather than by one individual neuron
Example: color
R
G
Perception depends on relative responses of R and G
R>G: Red
G>R: Green
R=G: White (Black)
Adaptation to green light:
G’s
response
stimulus
time
What is subsequent perception of white light?
R
G
Since G is adapted, R>G. Perception: Red
x
Other aftereffects:
Motion (Waterfall illusion)
Spatial Frequency
Properties of sensory systems:
- Receptive Field
Receptive Field: Region of body/ world within which the
activity of a neuron/ receptor can be influenced
Receptive field size and acuity/ two-point discrimination:
Neurons 1, 2
Neuron 3
Neurons 1 and 2 can resolve the points as separate, neuron 3 cannot.
Demos: two-point discrimination in touch and vision
Properties of sensory systems:
- Lateral inhibition
Lateral inhibition: mutual inhibition of adjacent neurons;
enhances contrast
Light
Neurons
Response
Number of inhibitions
0
1
1
2
1
1
0
Perceptual consequences of lateral inhibition
Mach Bands
Hermann Grid
New Florida flag?
Count and total black dots for Al Gore and white dots for
George W. Bush. Recount to confirm.
Simultaneous contrast
Properties of perceptual processes:
- Perceptual Constancies
The challenge: To form a logical, stable representation of objects despite
differences in viewing conditions, e.g.
- distance
- viewing angle
- illumination
size constancy
shape constancy
lightness constancy
“Excuse me for shouting- I thought you were farther away”
Size constancy scaling depends on the ability to correctly infer
relative distance from depth cues.
Misapplied size constancy:
The tendency to misjudge
the size
of 2D stimuli when depth
cues trigger
size constancy scaling
A conscious percept of depth is not necessary for misapplied
size constancy to occur.
The Mueller-Lyer Illusion
Lightness constancy scaling depends on the ability to correctly infer
illumination from contrast information.
Demo
Question: What color is the moon?
Illusions resulting from Lightness Constancy
A
B
Bottom-Up explanation:
Cell A receives less inhibition
than Cell B.
A
B
Less inhibition;
Signal: lighter
More inhibition;
Signal: darker
Top-Down explanation:
A is in the shadow, yet it reflects as much light as B. Hence A must be
lighter than B.
Interpretation: Shadow
Strong Illusion
Interpretation: Paint
Weaker Illusion
Interpretation: Single figure with equal illumination.
No illusion.
Interpretation: Two separate surfaces with unequal illuminations.
Illusion.
Properties of perceptual processes:
- Top-Down Processes
Top-Down Processes: Use knowledge and expectations to
form hypothesis about object/ scene
- Gestalt Principles
- Familiarity with objects
- Familiarity with environment
- Priming
Some Gestalt principles
Similarity
Figure-Ground
Proximity
Good continuation
Familiarity with objects: Hollow mask illusion
Familiarity with environment:
Assumption: Light source is above scene
Priming: Expectation influences interpretation of figure