Sensation and Perception

Chapter 5

Sensation & Perception

How do we construct our representations of the external world?

To represent the world, we must detect physical energy (a stimulus) from the

environment and convert it into neural signals. This is a process called sensation.

When we select, organize, and interpret our sensations, the process is called perception.

Sensation

• Sensation: the process by which our sensory receptors and nervous system receive and represent stimulus energies from our environment.

Perception

• The process of organizing and interpreting sensory information, enabling us to recognize meaningful objects and events.

Stimuli

• Different type of stimuli activate different sense organs – Example: light stimulates our sense of sight &

sound stimulates our sense of hearing

Sensing the World

Senses are nature’s gift that suit an organism’s needs.

A frog feeds on flying insects; a male silkworm moth is sensitive to female sex-attractant odor; and we as human beings are sensitive to sound frequencies that

represent the range of the human voice.

Exploring the Senses

What stimuli cross our threshold for conscious awareness?

Psychophysics

A study of the relationship between physical characteristics of stimuli and

our psychological experience with them.

Physical WorldPsychological

World

Light Brightness

Sound Volume

Pressure Weight

Sugar Sweet

Transduction

In sensation, the transformation of stimulus energy (sights, sounds, smells) into neural

impulses.

Thresholds

Absolute Threshold: Minimum stimulation needed to detect a particular stimulus 50% of the time.

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0 5 10 15 20 25 Stimulus Intensity (lumens)

Detecting Stimuli

• When does a stimulus become strong enough to be detected by our sense organs?– Absolute threshold: the smallest intensity of a

stimulus that must be present for it to be detected

– Our senses are very fine tuned• We can feel the wing of a bee when it is dropped

on our cheek from the distance of 1 cm

Subliminal Threshold

Subliminal Threshold: When stimuli are below one’s absolute threshold for conscious awareness.

Kurt Scholz/ Superstock

Subliminal Perception

• Refers to the perception of messages about which we have no awareness

• We can unconsciously sense subliminal stimuli and without our awareness, these stimuli have extraordinary suggestive powers.

Subliminal Perception

• There is a lack of evidence regarding how much subliminal messages can affect us. It appears that there is a subtle fleeting effect.– However, under certain conditions an invisible

image or word can prime our later answer.– Example: If you are told “We stood by the

bank” and the word “river” is whispered in your ear, this may influence how you interpret that sentence.

Subliminal Perception

• http://www.youtube.com/watch?v=2xPvYgTvr8I&NR=1

• http://www.youtube.com/watch?v=zwFiBWbegFM

Weber’s Law

Two stimuli must differ by a constant minimum percentage (rather than a constant amount), to be perceived as

different. Weber fraction: k = I/I.

StimulusConstant

(k)

Light 8%

Weight 2%

Tone 3%

Sensory Adaptation

Diminished sensitivity as a consequence of constant stimulation.

Put a band aid on your arm and after awhileyou don’t sense it.

VisibleSpectrum

The Stimulus Input: Light Energy

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Physical Characteristics of Light

1. Wavelength (hue/color)

2. Intensity (brightness)

Wavelength (Hue)

Hue (color) is the dimension

of color determined by the wavelength

of the light.

Wavelength is the distance

from the peak of one wave to the peak of the

next.

Wavelength (Hue)

Different wavelengths of light resultin different colors.

400 nm 700 nmLong wavelengthsShort wavelengths

Violet Indigo Blue Green Yellow Orange Red

Intensity (Brightness)

Intensity: Amount of energy in a

wave determined

by the amplitude. It is related to perceived

brightness.

Intensity (Brightness)

Blue color with varying levels of intensity.As intensity increases or decreases, blue color

looks more “washed out” or “darkened.”

The Eye

Parts of the eye

1. Cornea: Transparent tissue where light enters the eye.

2. Iris: Muscle that expands and contracts to change the size of the opening (pupil) for light.

3. Lens: Focuses the light rays on the retina.

4. Retina: Contains sensory receptors that process visual information and sends it to the brain.

The LensLens: Transparent

structure behind the pupil that changes

shape to focus images on the retina.

Accommodation: The process by which the eye’s lens changes shape to help focus

near or far objects on the retina.

Retina

Retina: The light-sensitive inner

surface of the eye, containing

receptor rods and cones in addition to layers of other neurons (bipolar,

ganglion cells) that process

visual information.

Optic Nerve, Blind Spot & Fovea

http://www.bergen.org

Optic nerve: Carries neural impulses from the eye to the brain. Blind Spot: Point where the optic nerve leaves the eye because there are no receptor cells located there. Fovea: Central point in the retina around which the eye’s cones cluster.

Photoreceptors

E.R. Lewis, Y.Y. Zeevi, F.S Werblin, 1969

Bipolar & Ganglion Cells

Bipolar cells receive messages from photoreceptors and transmit them to

ganglion cells, which converge to form the optic nerve.

Visual Information Processing

Optic nerves connect to the thalamus in the middle of the brain, and the thalamus

connects to the visual cortex.

Feature Detection

Nerve cells in the visual cortex respond to specific features, such as edges, angles,

and movement.

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Shape Detection

Specific combinations of temporal lobe activity occur as people look at shoes,

faces, chairs and houses.

Isha

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Visual Information Processing

Processing of several aspects of the stimulus simultaneously is called parallel processing. The

brain divides a visual scene into subdivisions such as color, depth, form, movement, etc.

From Sensation to RecognitionT

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Color VisionTrichromatic theory: Young and von

Helmholtz suggested that the eye must contain three receptors that are sensitive to

red, blue and green colors.

Blue Green Red

Medium LowMax

Standard stimulus

Comparison stimulus

Color Blindness

Genetic disorder in which people are blind to green or red colors. This supports the

Trichromatic theory.

Color Blind Test

Color Blind Test

Color Blind Test

Opponent Colors

Gaze at the middle of the flag for about 30Seconds. When it disappears, stare at the dot and report

whether or not you see Britain's flag.

After Image

The Other Senses Hearing

Touch

Pain

Taste

Smell

Body Position and Movement

Hearing

The Stimulus Input: Sound Waves

Sound waves are compressing and expanding air molecules.

Sound Characteristics

1. Frequency (pitch)2. Intensity

(loudness)

Frequency (Pitch)

Frequency (pitch): The dimension of

frequency determined by the wavelength

of sound.

Wavelength: The distance from

the peak of one wave to the peak

of the next.

Intensity (Loudness)

Intensity (Loudness):

Amount of energy in a wave,

determined by the amplitude, relates to the perceived

loudness.

Loudness of Sound

70dB

120dB

Richard K

aylin/ Stone/ Getty Im

ages

The Ear

Dr. Fred H

ossler/ Visuals U

nlimited

The Ear

Outer Ear: Collects and sends sounds to the eardrum.

Middle Ear: Chamber between eardrum and cochlea containing three tiny bones (hammer, anvil, stirrup) that concentrate the vibrations of the eardrum on the cochlea’s oval window.

Inner Ear: Innermost part of the ear, containing the cochlea, semicircular canals, and vestibular sacs.

Cochlea

Cochlea: Coiled, bony, fluid-filled tube in the inner ear that transforms sound

vibrations to auditory signals.

Localization of Sounds

Because we have two ears, sounds that reach one ear faster than the other ear

cause us to localize the sound.

Localization of Sound

1. Intensity differences2. Time differences

Time differences as small as 1/100,000 of a second can cause us to localize sound. The head acts as a “shadow” or partial sound

barrier.

Touch

The sense of touch is a mix of four distinct skin senses—pressure, warmth, cold, and pain.

Skin Senses

Only pressure has identifiable receptors. All other skin sensations are variations of

pressure, warmth, cold and pain.

Burning hot

Pressure Vibration Vibration

Cold, warmth and pain

Pain

Pain tells the body that something has gone wrong. Usually pain results from damage to the skin and other tissues. A rare disease exists in

which the afflicted person feels no pain.

Ashley Blocker (right) feels neither painnor extreme hot or cold.

AP Photo/ Stephen M

orton

Biopsychosocial Influences

Gate-Control Theory

Melzack and Wall (1965, 1983) proposed that our spinal cord contains neurological “gates” that either block pain or allow it to

be sensed.

Gary C

omer/ PhototakeU

SA.com

Pain Control

Pain can be controlled by a number of therapies including, drugs, surgery,

acupuncture, exercise, hypnosis, and even thought distraction.

Todd R

ichards and Aric V

ills, U.W

. ©

Hunter H

offman, w

ww

.vrpain.com

Taste

Traditionally, taste sensations consisted of sweet, salty, sour, and bitter tastes.

Recently, receptors for a fifth taste have been discovered called “Umami”.

Sweet Sour Salty Bitter Umami(Fresh

Chicken)

Sensory Interaction

When one sense affects another sense, sensory interaction takes place. So, the

taste of strawberry interacts with its smell and its texture on the tongue to produce

flavor.

Smell

Like taste, smell is a chemical sense. Odorants enter the nasal cavity to stimulate

5 million receptors to sense smell. Unlike taste, there are many different forms of

smell.

Smell and Memories

The brain region for smell (in red) is

closely connected with the brain

regions involved with memory (limbic system). That is why strong memories are

made through the sense of smell.

Body Position and Movement

The sense of our body parts’ position and movement is called kinesthesis. The

vestibular sense monitors the head (and body’s) position.

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Whirling Dervishes Wire Walk

Bob D

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Perceptual Organization

How do we form meaningful perceptions from sensory information?

We organize it. Gestalt psychologists showed that a figure formed a “whole”

different from its surroundings.

Perceptual Illusion

Perceptual Illusion

Organization of the visual field into objects (figures) that stand out from their

surroundings (ground).

Form Perception

Tim

e Savings S

uggestion, © 2003 R

oger Sheperd.

Grouping

After distinguishing the figure from the ground, our perception needs to organize the figure into a meaningful form using

grouping rules.

Grouping & Reality

Although grouping principles usually help us construct reality, they may occasionally lead us

astray.

Both photos by W

alter Wick. R

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ES

Magazine. .©

1983 PCS G

ames L

imited Partnership

Depth Perception

Visual Cliff

Depth perception enables us to judge distances. Gibson and Walk (1960) suggested that human infants (crawling age) have depth perception. Even newborn animals show depth perception.

Inne

rvis

ions

Binocular Cues

Retinal disparity: Images from the two eyes differ. Try looking at your two index fingers when pointing them

towards each other half an inch apart and about 5 inches directly in front of your eyes. You will see a

“finger sausage” as shown in the inset.

Monocular Cues

Relative Size: If two objects are similar in size, we perceive the one that casts a

smaller retinal image to be farther away.

Monocular Cues

Interposition: Objects that occlude (block) other objects tend to be perceived as

closer.R

ene Magritte, The B

lank Signature, oil on canvas, N

ational Gallery of A

rt, Washington. C

ollection of M

r. and Mrs. Paul M

ellon. Photo by Richard C

arafelli.

Monocular Cues

Relative Height: We perceive objects that are higher in our field of vision to be farther away than those that are

lower.

Image courtesy of S

haun P. V

ecera, Ph. D

., adapted from

stimuli that appered in V

ecrera et al., 2002

Monocular Cues

Relative motion: Objects closer to a fixation point move faster and in opposing direction to those objects that are farther away from a fixation

point, moving slower and in the same direction.

Monocular Cues

Linear Perspective: Parallel lines, such as railroad tracks, appear to converge in the distance. The more the lines converge, the

greater their perceived distance.

© T

he New

Yorker C

ollection, 2002, Jack Ziegler

from cartoonbank.com

. All rights reserved.

Monocular Cues

Light and Shadow: Nearby objects reflect more light into our eyes than more distant objects. Given two identical objects, the dimmer one appears to

be farther away.

From

“Perceiving S

hape From

Shading” by V

ilayaur S

. Ram

achandran. © 1988 by S

cientific Am

erican, Inc. A

ll rights reserved.

Perceptual Constancy

Perceiving objects as unchanging even as illumination and retinal

images change.

Perceiving familiar objects as having consistent color even when changing illumination filters

the light reflected by the object.

Color Constancy

Color Constancy

Size-Distance Relationship

The distant monster (below, left) and the top red bar (below, right) appear bigger

because of distance cues.

From Shepard, 1990

Alan C

hoisnet/ The Im

age Bank

Size-Distance RelationshipBoth girls in the room are of similar

height. However, we perceive them to be of different heights as they stand in the

two corners of the room.

Both photos from S. Schwartzenberg/ The Exploratorium

Ames Room

The Ames room is designed to demonstrate the size-distance illusion.

Perceptual Interpretation

Immanuel Kant (1724-1804) maintained that knowledge comes from our inborn ways of

organizing sensory experiences.

John Locke (1632-1704) argued that we learn to perceive the world through our

experiences.

How important is experience in shaping ourperceptual interpretation?

Sensory Deprivation & Restored Vision

After cataract surgery, blind adults were able to regain sight. These

individuals could differentiate figure and ground relationships, yet they had difficulty distinguishing a circle

and a triangle (Von Senden, 1932).

Facial Recognition

After blind adults regained sight, they

were able to recognize distinct features, but

were unable to recognize faces.

Normal observers also show difficulty in facial recognition when the

lower half of the pictures are changed.

Courtesy of R

ichard LeG

rand

Kittens raised without exposure to horizontal lines later had difficulty

perceiving horizontal bars.

Blakemore & Cooper (1970)

Sensory Deprivation

Perceptual Adaptation

Visual ability to adjust to an

artificially displaced visual field, e.g., prism glasses.

Courtesy of H

ubert Dolezal

Perceptual Set

A mental predisposition to perceive one thing and not another. What you see in the center picture is influenced by flanking pictures.

From

Shepard, 1990.

(a) Loch ness monster or a tree trunk; (b) Flying

saucers or clouds?

Perceptual Set

Other examples of perceptual set.

Frank Searle, photo Adam

s/ Corbis-Sygm

a

Dick R

uhl

To an East African, the woman sitting is balancing a metal box on her head, while the family is sitting

under a tree.

Cultural Context

Context instilled by culture also alters perception.

Is There Extrasensory Perception?

Perception without sensory input is called extrasensory perception (ESP). A large

percentage of scientists do not believe in ESP.

Claims of ESP

1. Telepathy: Mind-to-mind communication. One person sending thoughts and the other receiving them.

2. Clairvoyance: Perception of remote events, such as sensing a friend’s house on fire.

3. Precognition: Perceiving future events, such as a political leader’s death.