1

Sensation and Perception

Chapter 5

Vision: p. 135 - 156

Sensation vs. 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

Accommodation

Lens changes shape

to focus light on retina

Blindspot demonstration

Demo in text p.141

http://faculty.washington.edu/chudler/chvision.html

2

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

Photopigments

have to be

regenerated by

cells

Color theories

Trichromatic theory

H. von Helmholz

Opponent-process theory

E. Herig

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

3

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?

4

Gestalt principles of organization

• Laws of “perceptual organization”: see whole

• Figure vs. ground

– Proximity

– Similarity

– Closure

– Good continuation

– Common fate

Which gestalt law?

Gestalt principles

• Group objects together to find the 13

faces

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

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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

– Hubel & Weisel , 1962

• 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 ( deficit for faces) • http://www.youtube.com/watch?v=XLGXAiSpN00

• http://www.faceblind.org/

– Akinetopsia (deficit for objects in motion)

• Suggests regions of the cortex designed to

processes certain parts of visual input

• “What” system

– Damage to occipital-temporal pathway

• “Where” system

– Damage to occipital-parietal pathway

Ungerleider & Mishkin (1983)

• Method with monkeys:

– Object discrimination task

– Landmark discrimination problem

• IV: lesion site (temporal lobe vs. parietal lobe)

• Result:

– Object discrimination deficit with temporal lesion

– Landmark discrimination deficit with parietal lesion

Perceptual parsing

• Detect and identify primary 3d objects or geons

(Biederman, 1987)

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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?

Another example

7H15 M3554G3 53RV35 7O PR0V3 H0W

0UR M1ND5 C4N D0 4M4Z1NG 7H1NG5!

1MPR3551V3 7H1NG5! 1N 7H3

B3G1NN1NG 17 WA5 H4RD BU7 N0W, 0N

7H15 LIN3 Y0UR M1ND 1S R34D1NG 17

4U70M471C4LLY W17H 0U7 3V3N

7H1NK1NG 4B0U7 17, B3 PROUD! 0NLY

C3R741N P30PL3 C4N R3AD 7H15.

PL3453 F0RW4RD 1F U C4N R34D 7H15

Pattern Recognition

• Bottom-up AND top-down

• Bi-directional model

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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?