Sensation and Perception

Chapter Three

Lecture Slides

By Glenn MeyerTrinity University

Introduction: What Are Sensation and Perception?

6 major senses

vision

hearing

touch

taste

pain

smell

Vision has been studied most

extensively and is the most important sense for humans,

followed by hearing.

The list can be extended

with balance, joint senses, and others.

Basic Principles of Sensation and Perception

Sensation: process of detecting a physical stimulus, such as light, sound, heat, or pressure.

Example: Your eye’s physical response to splotches of color

Two Overlapping Processes:

Perception: process of integrating, organizing, and interpreting sensations

Example: The splotches of color are recognized as a basket of fruit.

Basic Terminology

Sensory receptors Specialized cells unique to each sense organ that respond to a particular form of sensory stimulation.

TransductionProcess by which a form of physical energy is converted into a coded neural signal that can be processed by the nervous system.

Sensory adaptation Decline in sensitivity to a constant stimulus.

Sensory receptors

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Sensory adaptation

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Transduction

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Sensory ThresholdsOne of the major problems in studying the senses is to study thresholds.

There are two issues:

Absolute threshold: smallest strength of a stimulus that can be detected

Example: The softest sound you can hear, the small concentration of sugar that can be tasted in your cup of coffee

Difference threshold: (just noticeable difference) smallest difference that can be detected

Example: How much lighter in weight can a company make a chocolate bar before you notice it

The Just Noticeable Difference

Weber’s law: for each sense the size of a just noticeable difference is a constant proportion of the size of the initial stimulus

Example: For weight, you can detect a 2% change. So if you can bench press 100 lbs., you will notice that a 102 lb. barbell is heavier.

Subliminal Perception• Detection of stimuli that are below the threshold of

conscious perception or awareness.

• Doesn’t profoundly influence behavior

• Can influence behavior if the stimulus is relevant to your goals

Subliminal stimuli used in Hassin & others, 2007 study.

Vision—What We SeeThe Nature of Light

• Light is electromagnetic energy that can be described as waves and by its wavelength

• Various types of electromagnetic energy differ in wavelength, which is the distance from one wave peak to another.

Vision—What We SeeThe Nature of Light

• Electromagnetic spectrum varies from…

• Note that as wavelength varies, color perception in humans varies.

• Wavelength itself is not color – color is an interpretation of wavelength

• Humans see in the 350 to 700 nanometer range.

Very shortVery long

Vision—What We SeeThe Nature of Light

Other animals can see in the infrared (pit vipers) and ultraviolet ranges (bees and some birds)

How We SeeThe Human Visual System –

The Visual Pathway

Light is focused by the cornea and lens

to project an image on the retina.

Pupil and iris:

• Colored part of the eye

(iris) and the hole formed

by the iris (pupil)

• Controls the amount of

light entering the eye

• Aids in controlling the

clarity of the image

(smaller pupils, clearer

image)

Cornea: clear membrane that

covers the front of the eye.

Does most of the focusing of

the image

Lens:

• Transparent structure

behind the pupil.

• Focuses the image on the

retina.

• Changes shape to focus

on far to near targets

through the processes of

accommodation.

Important parts of the pathway in order

Cornea

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Pupil and iris

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Lens

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The Retina

Thin, light-sensitive membrane located at back of eye, contains sensory receptors for vision

Rods and cones: Sensory receptor cells that respond to light

Called photoreceptors

Exposed to light, rods and cones undergo chemical reactions that result in neural signals.

Rods and Cones Cones:• Most located in the

center of the retina• Fovea: center of retina

with all cones and best vision

• Responsible for best acuity

• Responsible for color vision

• Active at daylight or photopic light levels

Rods:• Located in the

Periphery of the eye• Responsible for night

or scotopic vision• Have relatively poor

acuity• Take approximately 30

minutes to adapt to lowest light levels

Cones

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Rods

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The Blind Spot• The optic disc where ganglion nerve cells leave the retina

• Contains no photoreceptors

• Brain fills in the blind spot with the surrounding patterns

Visual Processing in the Retina

The Route:

Receptors to Bipolars to Ganglion

Cells through Thalamus (LGN) to

Visual Cortex

Bipolar cells • Collect information from the rods

and cones• Bipolar cells then funnel the

collection of raw data to the ganglion cells.

Ganglion cells• Combines, analyzes, and

encodes the information from photoreceptors in its receptive field

• Receptive fields are a particular area of the retina that feeds to a ganglion cell.

• Transmits information to the brain as axons form optic nerve

• Receptive FieldsAre usually circular in nature.

• A single ganglion cell receives information from only one or two cones

• Can receive information from a hundred or more rods.

Receptive fields

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Ganglion cells

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Bipolar cells

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Optic Nerve—1 million ganglion cell fibers

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• Left and right optic nerves meet at the optic chiasm.

• Fibers of the left and right optic nerves split in two.

• Images in the left visual field go to the right hemisphere, images in the right visual field go to the left hemisphere.

• Most of the optic nerve axons project to the brain structure called the thalamus.

• Responsible for form, color, brightness, and depth

• Midbrain: a smaller number of axons deal with location of objects

Visual Cortex

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• Detects edges, angles, lines, movement, distance

• Specialized neurons know as feature detectors

• Features are assembled in later visual cortical areas and frontal lobes

From Eye to Brain

Color Vision—The Experience of Color

• Our visual system interprets differences in the wavelength of light as color

• Rods are color blind, but the cones allow us to see different colors

• This difference occurs because we have only one type of rod but three types of cones

• ROYGBIV• Wavelengths of about 400

nanometers are perceived as violet.• Wavelengths of about 700

nanometers are perceived as red.• In between are orange, yellow,

green, blue, and indigo.

Hueproperty of wavelengths of light known as color; different wavelengths correspond to our subjective experience of color (hue)

Saturationproperty of color that corresponds to the purity of the light wave

Brightnessperceived intensity of a color, corresponds to amplitude of the light wave

Hue

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Saturation

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Brightness

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vs.Trichromatic Theory

Opponent-Process Theory

Theories of Color VisionTwo theories were proposed in the 1800s

Modern research indicates both have a grain of truth at different neural levels.

• Researchers found that by mixing only three primary lights (usually red, green, and blue), they could create the perceptual experience of all possible colors

Used in TV and computer displays

• Young and Helmholtz to propose that we have three different types of photoreceptors, each most sensitive to a different range of wavelengths

• Three kinds of cones have been found in the retina – one sensitive to long wavelengths, one sensitive to medium wavelengths and one to short wavelengths.

• These were thought to be a receptor set specific to red, green and blue

• Theory explains the most common forms of color blindness

• Can’t explain well – afterimages and the unique color yellow

Trichromatic Theory

Normal Color Vision

Appearance for someone who is red/green color blind

Opponent-Process Theory

• Color vision is the product of opposing pairs of color receptors

• This generates three systems: red–green, blue–yellow, and black–white

• When one member of a color pair is stimulated, the other member is inhibited

• Explains afterimages and color blindness

Hearing – Audition (the technical term for hearing)

From Vibration to Sound • Sound waves are

produced by the rhythmic vibration of air molecules

• Auditory perception occurs when sound waves interact with the structures of the ear

The intensity (or amplitude) of a sound wave, measured in decibels.

The intensity or amount of energy of a wave —height of a wave; amplitude of a sound wave determines a sound’s loudness.

Unit of measurement for loudness.

The rate of vibration, or the number of sound waves per second – measured in Hertz (Hz)

Relative highness or lowness of a sound, determined by the frequency of a sound wave.

Distinctive quality of a sound, determined by the complexity of the sound wave.

Loudness

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Amplitude

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Decibel

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Frequency

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Pitch

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Timbre

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Characteristics of Sound

Intensity of Various Sounds

Decibels

Softest detectable sound

Soft whisper

Quiet neighborhood

Average conversation

Loud music from a radio

Heavy automobile traffic

Very loud thunder

Jet airplane taking off

Loudest rock band on record

Spacecraft launch from 150 ft.

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ExampleP (in sound-

pressure units) Log P

How We HearThe Path of Sound

Sound waves are:

• Collected in the outer ear

• Amplified in the middle ear

• Transduced in the inner ear

Outer Ear: Collects sound waves;

consists of pinna, ear canal, and

eardrum (tympanic membrane)

Middle Ear: Amplifies sound waves; consists of three small bones (ossicles): hammer, anvil, and stirrup

Inner Ear: Where sound is transduced into neural impulses; consists of cochlea and semicircular canals

Structures of the Inner Ear

• Cochlea: coiled, fluid-filled inner-ear structure that contains basilar membrane and hair cells.

• Basilar membrane:membrane within cochlea of ear that contains hair cells.

• Hair cells: hair-like sensory receptors for sound, which are embedded in basilar membrane of cochlea. They get brittle and damaged as you age, especially by loud noises (music, work, hair dryers, gunfire)

Transduction of Sounds

• The structures of the ear transform changes in air pressure (sound waves) into vibrations of the basilar membrane

• As the basilar membrane vibrates it causes the hairs in the hair cells to bend

• The bending of the hairs leads to a change in the electrical potential within the cell

Frequency of Sound Waves

• Highest frequency we can hear20,000 Hz

• Highest note on a piano4,186 Hz

• Highest pitch of human voice1,000 Hz

• Lowest pitch of human voice100 Hz

• Lowest note on a piano27 Hz

The frequency of a sound wave is measured as the number of cycles per second (Hertz).

Distinguishing Pitch

• Humans can hear 20 to 20,000 Hz

• Frequency theory—basilar membrane vibrates at the same frequency as the sound wave

• Place theory—different frequencies cause larger vibrations at different locations along the basilar membrane

Both frequency theory and place theory are involved in explaining our discrimination of pitch. • Frequency theory helps explain our discrimination

of frequencies lower than 500 hertz. • Place theory helps explain our discrimination of

higher-pitched sounds. For intermediate frequencies or midrange pitches, both place and frequency are involved.

The Chemical and Body Senses

• Olfaction -Technical name for the sense of smell

• Gustation - Technical name for the sense of taste

• Touch and temperature

• Pain

• Kinesthetic (location of body)

• Vestibular (balance)

How We Smell (Don’t answer that!)

Sensory stimuli for odor are molecules in the air.

Molecules encounter millions of olfactory receptor cells located high in the nasal cavity.

Odor receptor seems to be specialized to respond to molecules of a different chemical structure.

Olfactory receptor cells stimulation is converted into neural messages

Messages pass along their axons, bundles of which make up the olfactory nerve.

Hundreds of different odor receptors have been identified.

Brain identifies an odor by interpreting the pattern of olfactory receptors that are stimulated.

Olfactory bulb – part of olfactory cortex.Project from there to

temporal lobe for conscious recognition and limbic system for emotional responses.

Humans have 12,000,000 receptors as compared with hundreds of millions in other animals.

Taste• Tongue covered with little

bumps and grooves lined with taste buds

• Each taste bud contains 50 receptors for 5 basic tastes

• Five basic tastes – aid us in seeking nutrient-rich food

• Sweet

• Sour

• Salty

• Bitter

• Umami – monosodium

glutamate

The Skin and Body Senses

Touch – receptors in skin are sensitive to pressure, warmth, cold or a combination of these• Pacinian corpuscle is located beneath the skin. • When stimulated by pressure, Pacinian corpuscle

converts stimulation into a neural messagePain - The unpleasant sensation of physical discomfort or suffering that can occur in varying degrees of intensity.• Pain receptors are called nociceptors. Nociceptors are

actually small• Composed of sensory fibers, called free nerve endings, in

the skin, muscles, or internal organs.

Fast and Slow Pain SystemsA-delta fibers and C fibers

A-delta fibers• Myelinated A-delta fibers represent the fast pain

system. • Pathway – thalamus to somatosensory cortex• A-delta fibers transmit the sharp, intense, but

short-lived pain of immediate injury.C fibers• Smaller, unmyelinated C fibers represent the slow

pain system.• C fibers transmit longer-lasting throbbing, burning

pain of injury • Pathway – hypothalamus and thalamus and then

to the limbic system (amygdala)

Factors That Influence Pain Gates

Sensitization:

Pain pathways become more responsive

Example: Phantom limb pain—when a person continues to experience intense painful sensations in a limb that has been amputated

Gate-control theory of pain—pain is a product of both physiological and psychological factors that cause spinal gates to open and relay patterns of intense stimulation to the brain; the brain perceives them as pain

Movement, Position, and Balance

Vestibular Sense:

Sense of balance, or equilibrium, by responding to changes in gravity, motion, and body position.

• Sources of vestibular sensory information – semicircular canals and vestibular sacs, located in ear

Kinesthetic Sense: The technical name for the sense of location and position of body parts in relation to one another

• Proprioceptors: sensory receptors, located in the muscles and joints, that provide information about body position and movement

PerceptionTwo Major Processes in Perceptual Processing

• Bottom-up processing—emphasizes the importance of sensory receptors in detecting the basic features of a stimulus; moves from part to whole; also called data-driven processing

• Top-down processing—emphasizes importance of observer’s cognitive processes in arriving at meaningful perceptions; moves from whole to part; also called conceptually driven processing

• The use of either can be influenced by cultural differences or nuances, such as found in collectivist vs. individualistic societies

Process of integrating,

organizing, and interpreting

sensory information into

meaningful representations.

ESP: Can Perception Occur Without Sensation?

• ESP, or extrasensory perception: detection of information by some means other than through the normal processes of sensation.

• Parapsychology: scientific investigation of claims of various paranormal phenomena. Contrary to what many people think, very few psychologists conduct any kind of parapsychological research

Ongoing controversy: Majority of scientifically

oriented psychologist are very skeptical of reports of

ESP

The Perception of Shape - The Influence of Gestalt Psychology

Figure–Ground Relationship

• Gestalt psychologists also thought an important part of our perception was the organization of a scene into its figure(the object of interest) and its ground (the background)

• Gestalt psychologist Edgar Rubin (1921) observed, “In a certain sense, the ground has no shape.” We notice the shape of the figure but not the shape of the background, even when that ground is used as a well-defined frame

Gestalt psychology,

Founded by German psychologist Max Wertheimer in the early 1900s (Wertheimer, 1923).

Emphasized that we perceive whole objects or figures (gestalts) rather than isolated bits and pieces of sensory information.

Figure–Ground Relationship

• The separation of a scene into figure and ground is not a property of the actual elements of the scene at which you’re looking. Rather, your ability to separate a scene into figure and ground is a psychological accomplishment

• Demonstrated in Figure-Ground Reversal

Gestalt Grouping Principles

• General principle - law of Prägnanz, or the law of simplicity:

• When several perceptual organizations of an assortment of visual elements are possible, the perceptual interpretation that occurs will be the one that produces the “best, simplest, and most stable shape”

Gestalt Grouping Principles Proximity

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Similarity

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Closure

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Good continuation

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Specific Principles

Depth Perception and Cues

Types of Cues

Monocular—depth cues that appear in the image in either the left or right eye

Binocular—depth cues that involve comparing the left and right eye images

• One of our more important perceptual abilities involves seeing in three dimensions

• Depth perception is difficult because we only have access to two-dimensional images

• How do we see a 3D world using only the 2D retinal images?

• Cue—stimulus characteristics that influence our perceptions

• We are able to see in 3D because the visual system can use depth cues that appear in the retinal images

Monocular Cues

Overlap, Aerial perspective, Texture gradient

Relative size, Linear perspective, Aerial perspective

Motion parallax

Binocular Cues

Binocular disparity: • Because our eyes are set a few

inches apart, a slightly different image of an object is cast on the retina of each eye.

• These distances are interpreted as depth.

• 3D movies present you with slightly different images in each eye and when fused give you a powerful sense of depth.

• A stereogram is a picture that uses the principle of binocular disparity to create perception of a 3D image

• An early problem with vision can cause this ability not to develop correctly if the eyes don’t work together

Convergence: the degree to

which muscles rotate your eyes to focus on an

object

Motion Illusions

Induced motion • Was first studied by Gestalt

psychologist Karl Duncker in the 1920s

• Duncker (1929) had subjects sit in a darkened room and look at a luminous dot that was surrounded by a larger luminous rectangular frame. When the frame slowly moved to the right, the subjects perceived the dot as moving to the left.

Stroboscopic motion• First studied by Gestalt psychologist Max Wertheimer in the early

1900s, stroboscopic motion creates an illusion of movement with two carefully timed flashing lights

• If the time interval and distance between the two flashing lights are just right, a very compelling illusion of movement is created.

• Basis of movies, TV and computer displays

• When retinal image of an object enlarges, we perceive object as moving toward us.

• Perception of the speed of the object’s approach is based on our estimate of the object’s rate of enlargement

• Some neurons are specialized to detect motion in one direction but not in opposite direction.

• Other neurons are specialized to detect motion at one particular speed.

Perceptual Constancies Perceptual constancy: tendency to

perceive objects, especially familiar

objects, as constant and unchanging

despite changes in sensory input

Example: As a person walks away from you their retinal image decreases in size but you know they are still the same size.

Example: You turn a quarter over in your hand but it still looks round at any reasonable angle

Size constancy: perception of an

object as maintaining the

same size despite changing images on

retina

Shape constancy: perception of a

familiar object as maintaining the same shape,

regardless of image produced on the

retina

Perceptual Illusions• Misperception of true characteristics of an object or an

image.

• Perceptual illusions underscore the idea that we actively construct our perceptual representations of the world according to psychological principles.

The Müller-LyerIllusion

Based on interpreting the angles of lines as

depth cues. The inward arrows mean the line is close and thus smaller

for is retinal image

The Illusory Contour (Kanizsa Square)

Based on Gestalt principles of closure

and good continuation as well as the depth cue of interposition

The Moon Illusion

Based in part on the fact that people

perceive objects on the horizon as farther away

than objects that are directly overhead in the

sky, leading to misjudging size

Famous Illusions

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The ShephardTables Illusion

Impossible Figure: Escher’s Waterfall Illusory Contours

The Müller-Lyer Illusion The Moon Illusion Dispelled

Example: • People in industrialized societies are

far more susceptible to the Müller-Lyer illusion than are people in some nonindustrialized societies

• May be due to the carpentered-world hypothesis that people living in urban, industrialized environments have a great deal of perceptual experience in judging lines, corners, edges, and other rectangular, objects

The Effects of Experience on Perception

The tendency to perceive objects or situations from a

particular frame of reference

Controlling PainStrategies:

Biofeedback

Acupuncture

Self-Administered Strategies:

Distraction• By actively focusing your attention on some nonpainful task, you

can often reduce pain

Imagery• Creating a vivid mental image can help control pain

Relaxation and meditation• Deep relaxation can be a very effective strategy for deterring pain

sensations

Counterirritation• Decreases pain by creating a strong, competing sensation that’s

mildly stimulating or irritating

Positive self-talk and reappraisal• Make positive coping statements, either silently or out loud, during a

painful episode or procedure

Distraction… Click here

Counterirritation… Click here

Relaxation and meditation… Click here

Imagery… Click here

Positive self-talk and reappraisal… Click here