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C h a p t e r 4 S E N S A T I O N & P E R C E P T I O N

P S Y C H O L O G YT h i r d E d i t i o n

by

Drew Westen

John Wiley & Sons, Inc.

Lecture Outline

Sensation versus Perception Psychophysics Vision Skin Senses Perceptual Organization

© 2002 John Wiley & Sons, Inc.

Sensation and Perception Sensation is the process by which the

body gathers information about the environment

Perception is the process by which the brain organizes and interprets sensory information Sensation and perception are akin to two

sides of the same coin Perception is an active process

• Ambiguous figures give rise to different perceptions


Basic Principles

Three principles emerge with regard to sensation and perception: There is not a one-one correspondence

between physical and psychological reality Sensation and perception are active processes Sensation and perception are adaptive


Common Features of Sensory Systems

Sensory receptors Translate physical stimulation into neural signals

(“transduction” process) Detect physical energy (e.g. light, sound waves)

Each system has a minimum amount of energy required to activate the system (termed the threshold)

Sensation involves decision making Sensory systems are sensitive to changes in

stimulation level© 2002 John Wiley & Sons, Inc.

Thresholds Sensory receptors are tuned to a particular

form of energy Auditory receptors in the ear code for sound

pressure changes, not for light Sensory systems require a minimum

amount of energy for activation (absolute threshold) Just-noticeable difference (JND): The minimum

difference in stimulation that is just noticeable• Weber noted that the JND is a fraction of the

original stimulus intensity: the fraction is different for different senses


Weber’s Law


ROC Curves


Vision The eye detects electromagnetic radiation

(light) Vision is functional

• Allows for detection of movement (predator or prey?)

• Allows for detection of color (ripe or spoiled?)

Light detection is useful because:• Light travels rapidly (no delay, unlike hearing)• Light travels in straight lines (no distortion)• Light interacts with the surfaces of objects in the

environment (is reflected or absorbed)


Electromagnetic Spectrum


Details of the Human Eye


Details of the Retina

Photoreceptors

Light

Impulses produced bylight


Transduction of Light Light travels through the retina to impinge on

photoreceptors at the back of the eye Light bleaches a pigment contained within the

photoreceptors:• Bleaching leads to a graded receptor potential that

eventually produces an action potential in the ganglion cell

Two types of photoreceptors• Cones: found in center of retina (fovea) and are

sensitive to fine detail and color• Rods: found in periphery of retina and are sensitive to

movement but not fine detail


Receptive Fields Receptive field: That

aspect of the external world that produces a change in firing rate of a given sensory cell

Insert a microelectrode into a retinal ganglion cell: then record potentials as various stimuli are shown onto the cell

• Center-surround shape


Visual Pathways in Brain:“Where” versus “What”


Perception of Color Color is a psychological perception

Three dimensions of color:• Hue is the apparent color of an object (blue)• Brightness is the intensity of a color• Saturation is the purity of the color

Three different types of cones are found in the eye

• Each is sensitive to different wavelengths of light– S-cones: blue– M-cones: green– L-cones: red


Retinal Cones Absorb Different Wavelengths of Light


Color Vision Theories Young-Helmholtz:

Color is explained by differential activation of 3 color elements in eye

• Could not account for negative color afterimages

Hering: Opponent-Process Theory Colors are derived from activity of 3

antagonistic systems• Black-white• Red-green• Blue-yellow


Negative Color Afterimages

Stare at the globe for 10 seconds and then look away at a white surface


Skin Senses Receptors are localized within the skin

to detect physical stimuli in the outside world

Three classes of skin receptors: Touch: physical contact of objects with the

skin Temperature of objects Pain: destruction of skin tissue


Skin Receptors


Pain Pain is a source of information that relates to tissue

destruction Force Heat Chemical

Pain is a adaptive to a degree (motivates behaviors to terminate the source of the pain)

Pain is influenced by Cultural beliefs Emotional states Exogenous drugs (opiates) Internal transmitters (endorphins)


A Demonstration of Reversible Perception


(Figure adapted from Boring, 1930, p. 42)

Gestalt Principles of Form Perception

A: SimilarityB: ProximityC: Good ContinuationD: SimplicityE: Closure


Monocular Depth Perception Cues Interposition: One object blocks another Linear perspective: Parallel lines converge in the

distance Texture gradient: Texture of distant objects appears to

be finer rather than coarser Shading: 3D objects cast shadows Aerial Perspective: Far objects are fuzzy Familiar size: Familiar objects that appear small are

inferred to be distant Relative size: When looking at 2 objects of known

similar size, the smaller is seen as further away


Motion Perception Visual system is wired to detect motion

Rods in retina are sensitive to motion Neurons in visual cortex respond to motion

Two systems for processing movement: A: eye is stationary as object moves B: eye moves to maintain object at same place on the retina; muscle signals

are used to infer movement


Linear Perspective as a Depth Cue


Perceptual Constancy Color: Tendency to perceive object color

as stable even under conditions of changing illumination

Shape: We recognize an object as having the same shape although we may view it in a different angle, at a different distance...

Size: Objects do not differ in size when viewed from different distances


Copyright

Copyright 2002 by John Wiley & Sons, Inc., New York, NY. All rights reserved. No part of the material protected by this copyright may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without written permission of the copyright owner.


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