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?