Sensation and Perception Chapter 5 Vision: p. 135 - 156
Sensation and Perception
Chapter 5
Vision: p. 135 - 156
Sensation v. Perception
• Sensation vs. Perception
• Physical stimulus Physiological response
Sensory experience & interpretation
• Example vision research questions:
– How does the eye take light and transform it into a
message the brain can understand?
– How do we see a stable world even though our eyes
are constantly blinking and shifting?
– How do perceptual illusions trick the mind?
Visual system
Physical properties of light
• Wavelength
– Hue
• Amplitude
– Intensity/brightness
• Mix of wavelengths
– Saturation
The Eye
Transduction
Process to translate
light into an electro-
chemical message
for the brain
Blindspot demonstration
Demo in text p.141
http://faculty.washington.edu/chudler/chvision.html
Vision problems Accomodation = shape of lens changes to focus
Flexibility lost with increased age Presbyopia
http://faculty.washington.edu/chudler/sight.html
The three main layers of the retina
Photoreceptors • Rods
– Concentrated in
periphery of retina
– Low light ok
– No detail, no color
• Cones
– Concentrated in
fovea
– Needs full light
– High visual acuity
– Color receptors
Dark adaptation
Color theories
Trichromatic theory
Opponent-process theory
Negative afterimage
http://faculty.washington.edu/chudler/after.html
http://michaelbach.de/ot/col_rapidAfterimage/index.html
Text demo p155
Lilac Chaser
• http://michaelbach.de/ot/col_lilacChaser/index.html
Center-Surround Receptive Fields
• 126 million receptors in
retina
• Convergence allows many
receptors to use 1 neuron
for afferent signaling
• Center-Surround receptive
fields allow for more than 1
signal to be sent by 1
afferent neuron
• Either excitatory or
inhibitory for each region
Luminance and contrast
http://michaelbach.de/ot/lum_herGrid/index.html
Motion perception
• Phi phenomenon
– Max Wertheimer – 1912
– Motion via still images
– http://michaelbach.de/ot/mot_reverse-
phi/index.html
• Motion illusions
– Motion aftereffect
– http://michaelbach.de/ot/mot_adapt/index.html
Rotating Snake
• http://michaelbach.de/ot/mot_rotsnake/ind
ex.html
Thought paper • What perceptual work is required by a
baseball player to hit a baseball?
• Dynamic visual acuity: see moving object, see
rotation of object
• Depth perception: see how far away it is
• Tracking: keep eyes fixed on moving object
• Object recognition: separate object from field
• Contrast sensitivity: see object color against
background color
• Pick up on other cues specific to sport
Object (or Pattern)
Recognition
How do we interpret lines and patterns as objects?
Gestalt principles of organization
• Laws of “perceptual organization”: see whole
• Figure vs. ground
– Proximity
– Similarity
– Closure
– Good continuation
– Common fate
Which gestalt law?
Biological motion
• http://michaelbach.de/ot/mot_biomot/index
.html
• Which gestalt law??
What are the depth cues?
Depth perception
• Monocular cues
– Linear perspective
– Acuity
– Color and brightness
– Shadow or occlusion
– Relative height
– Relative motion
http://michaelbach.de/ot/mot_ske/index.html
Depth perception
• Binocular cues
– Retinal disparity
– Convergence
• Depth illusions
– Ames room
• Perceptual constancies
Pattern recognition
• Bottom-up processing
– Information from sensory receptors
• Top-down processing
– Information from knowledge and expectations
• Specialized receptors in
visual cortex
• Simple cells
– Orientation specific
• Complex cells
– Movement, faces, etc.
• How does brain pull
information together?
Feature detectors
Stimulus
Cell’s
responses
Visual disorders
• Agnosia: deficit in recognizing objects – Book: “The man who mistook his wife for a hat” by Oliver Sacks
– Prosopagnosia (faces) • http://www.youtube.com/watch?v=XLGXAiSpN00
• http://www.faceblind.org/
– Akinetopsia (objects in motion)
• “What” system
– Damage to occipital-temporal pathway
• “Where” system
– Damage to occipital-parietal pathway
Perceptual parsing
• Detect and identify primary 3d objects or geons
(Biederman, 1987)
Biederman’s Geons
• Intersections are important to recognition
Top-down processing
Top-down processing
Tox-Doxn Pxocxssxng
• To xllxstxatx, I cxn rxplxce xvexy txirx
lextex of x sextexce xitx an x, anx yox stxll
xan xanxge xo rxad xt – ix wixh sxme
xifxicxltx
• Why are you able to read the sentence
above?
Pattern Recognition
• Bottom-up AND top-down
• Bi-directional model
Perceptual problem solving:
Impossible figures
Thought paper
• Think of an example from your life where
you use top-down and bottom-up
processing.
• Explain the example.
• What parts of the example use top-down
processing and what parts use bottom-up
processing?