Chapter 4Sensation and Perception

©2013 W. W. Norton & Company, Inc.

Gazzaniga • Heatherton • Halpern

FOURTH EDITION

Psychological Science

4.1 How Do We Sense Our Worlds?

Learning Objectives• Distinguish between sensation and

perception.• Describe the process of transduction.• Distinguish between an absolute threshold

and a difference threshold.• Discuss sensory detection theory.• Define sensory adaptation.

How Do We Sense Our Worlds?

• Sensation: our sense organs’ detection and response to external stimulus energy and the transmission of those responses to the brain

• Perception: the brain’s processing of detected signals, resulting in internal representations of the stimuli that form a conscious experience of the world

• What we sense is the result of how we perceive

Stimuli Must Be Coded to Be Understood by the Brain

• Sensory coding: Sensory receptors translate the physical properties of stimuli into patterns of neural impulses

• Transduction: a process by which sensory receptors produce neural impulses when they receive physical or chemical stimulation

• The brain needs qualitative and quantitative information about a stimulus

• Sensation and perception result from a symphony of sensory receptors and the neurons those receptors communicate with

Psychophysics Measures the Relationship between

Stimuli and Perception • Psychologists try to understand the

relationship between the world’s physical properties and how we sense and perceive them

• Psychophysics is a subfield that examines our psychological experiences of physical stimuli

Sensory Thresholds• Absolute threshold: the minimum intensity of stimulation

that must occur before you experience a sensation– Example: The absolute threshold for hearing is the

faintest sound a person can detect 50 percent of the time

• Difference threshold: the minimum amount of change required for a person to detect a difference (i.e., the “just noticeable difference”)

• Weber’s law: states that the just noticeable difference between two stimuli is based on a proportion of the original stimulus rather than on a fixed amount of difference

Signal Detection Theory• Signal detection theory (SDT): states that detecting a

stimulus requires making a judgment about its presence or absence, based on a subjective interpretation of ambiguous information– Example: A radiologist is looking at a CAT scan for the kind of

faint shadow that signals an early-stage cancer. Her judgment will likely be affected by her knowledge of the patient, training, experience, motivation, attention and the knowledge of the consequences of being wrong

• Signal detection research involves a series of trials in which a stimulus is presented in only some trials. In each trial, the participant must state whether he or she sensed the stimulus

Sensory Adaptation

• Sensory adaptation: a decrease in sensitivity to a constant level of stimulation

• If a stimulus is presented continuously, the responses of the sensory systems that detect it tend to diminish over time; when a continuous stimulus stops, the sensory systems usually respond strongly as well

4.2 What Are the Basic Sensory Processes?

• For each of the five major senses — taste, smell, touch, hearing, and vision — identify the type of receptor and trace the neural pathway to the brain

• Distinguish between the neural processes associated with the experience of immediate pain and the experience of chronic pain

• Discuss color perception

In Taste, Taste Buds Detect Chemicals• Gustation: the sense of taste• Taste buds: sensory organs, mostly on the tongue; come in

the form of tiny, mushroom-shaped structures (papillae)– Stimulated taste buds send signals to the brain, which then

produces the experience of taste– Different regions of the tongue are not more sensitive to certain

tastes (Lindemann, 2001)

• Every taste experience is composed of a mixture of five basic qualities: sweet, sour, salty, bitter, and the relatively new taste sensation umami (Krulwich, 2007)

• Mothers can pass their eating preferences on to their offspring

In Smell, the Nasal Cavity Gathers Odorants

• Olfaction: the sense of smell• Basic process: – Odorants pass into the nose and nasal cavity– Contact a thin layer of tissue embedded with smell

receptors called the olfactory epithelium – Smell receptors transmit information to the olfactory

bulb, the brain center for smell– Has the most direct route to the brain

• Smell’s intensity is processed in brain areas also involved in emotion and memory (Anderson, Christoff et al., 2003)

In Touch, Sensors In the Skin Detect Pressure, Temperature, and Pain

• Haptic sense: the sense of touch• Sense conveys sensations of temperature, pressure, pain, and

where our limbs are in space• The integration of various signals and higher-level mental

processes produces haptic experiences• Examples:

– Stroking multiple pressure points can produce a tickling sensation, which can be pleasant or unpleasant, depending on the mental state of the person being tickled

– Brain areas involved in touch sensation respond less to self-produced tactile stimulation than to external tactile stimulation (Blakemore, Wolpert, & Frith, 1998)

Two Types of Pain

• Pain is part of a warning system that stops you from continuing activities that may harm you

• Two kinds of nerve fibers have been identified for pain: – Fast fibers for sharp, immediate pain; activated by

strong physical pressure and temperature extremes

– Slow fibers for chronic, dull, steady pain; activated by chemical changes in tissue when skin is damaged

In Hearing, the Ear Detects Sound Waves

• Audition: the sense of sound• Movements and vibrations of objects cause

the displacement of air molecules, which produce a sound wave (change in air pressure that travels through the air)

• A sound wave’s amplitude determines loudness; its frequency determines pitch

• The ears convert sound waves to brain activity, which produces the sensation of sound

The Cochlear Implant • Cochlear implantation has helped people with

severe hearing problems due to the loss of hair cells in the inner ear

• Works by directly stimulating the auditory nerve; does not not amplify sound

• When devices are implanted in children born deaf, the child’s hearing will be quite functional and he/she will learn to speak reasonably normally

• The problem of audism

In Vision, the Eye Detects Light Waves

• Most of the scientific study of sensation and of perception is concerned with vision

• Very little of what we call seeing takes place in the eyes, but rather as a result of constructive processes that occur throughout much of the brain

• Basic structures: cornea, lens, pupil, iris, retina

Rods and Cones• The retina has two types of receptor cells:– Rods: respond at extremely low levels of illumination;

responsible primarily for night vision; found on outer edges of the retina

– Cones: less sensitive to low levels of light; responsible primarily for vision under high illumination and for seeing both color and detail; found throughout the retina but concentrated at the fovea

• Contain photopigments that initiate the transduction of light waves into electrical neural impulses

Transmission From the Eye to the Brain• A variety of retinal cells perform a series of sophisticated

computations that help the visual system process the incoming information

• Cells include: bipolar, amacrine, and horizontal cells; converge on about a million retinal ganglion cells

• Ganglion cells are the first neurons in the visual pathway with axons, which are gathered into a bundle called the optic nerve

• At the optic chiasm, axons in the optic nerves cross to the left and right hemispheres, travel to visual areas of the thalamus and then to the primary visual cortex in the occipital lobe

The Color of Light is Determined by Its Wavelength

• An object appears to be a particular color because of the wavelengths of light it reflects

• Trichromatic theory: activity in three different types of cones that are sensitive to different wavelengths

• Opponent-process theory: Different types of ganglion cells, working in opposing pairs, create the perception that R/G, B/Y are opposites

• We categorize color along three dimensions: hue, saturation, and brightness

Subtractive Color Mixing• Color can be produced through either the subtractive or

the additive mixture of wavelengths• Subtractive color mixing: a process of color mixing that

occurs within the stimulus itself; a physical, not psychological, process– Mixing paints is one form of subtractive color mixing

because the colors are determined by pigments.– Wavelengths that a pigment does not absorb are

reflected and enter the eye• Red, yellow, and blue are the subtractive primary colors

because together these pigments absorb nearly all the colors of the visible spectrum and when mixed, produce black

Additive Color Mixing

• Additive color mixing: a process of color mixing that occurs when different wavelengths of light interact within the eye’s receptors; a psychological process

• Additive primary colors are red, green, and blue because mixing them yields white light

We Have Other Sensory Systems

• Humans, like other animals, have several internal sensory systems in addition to the five primary senses

• Kinesthetic sense: perception of the body’s position in space and movements of our bodies and our limbs (some include this with the sense of touch)

• Vestibular sense: perception of balance; uses information from receptors in the semicircular canals of the inner ear

4.3 How Does Perception Emerge from Sensation?

• Identify the primary sensory areas for touch, hearing, and vision.

• Discuss the gate control theory of pain.• Explain how the brain localizes sound.• Distinguish between the “what” and “where”

pathways of the visual system.• Describe blindsight.

How Does Perception Emerge from Sensation?

• With the exception of olfaction, all sensory information is relayed from the thalamus to cortical and other areas of the brain

• Information is projected separately from the thalamus to primary sensory areas of the cerebral cortex

• In these areas the perceptual process begins in earnest

In Touch, the Brain Integrates Sensory Information from

Different Regions of the Body

• Touch information from the thalamus is projected to the primary somatosensory cortex

• In the 1940s, Wilder Penfield discovered that electrical stimulation of the primary somatosensory cortex could evoke the sensation of touch in different regions of the body (Penfield & Jasper, 1954)

• The most sensitive regions of the body, such as lips and fingers, have a greater amount of cortex devoted to them

Gate Control Theory

• Pain is a complex experience that depends on biological, psychological, and cultural factors

• Melzack’s gate control theory of pain: Pain receptors must be activated and a neural “gate” in the spinal cord must allow the signals through to the brain

• Pain signals transmitted by small-diameter nerve fibers can be blocked at the level of the spinal cord by the firing of larger sensory nerve fibers

Controlling Pain

• Drug treatments (ibuprofen, acetaminophen, Novocain, anesthetics)

• Cognitive states (distraction, positive mood, relaxation) can close the pain gate

• Some mental processes, such as worrying about or focusing on the painful stimulus, seem to open pain gates– Well-rested research participants rated the same level of a

painful stimulus as less painful than did participants who were fearful, anxious, or depressed (Loggia, Mogil, & Bushnell, 2008; Sullivan et al., 2001)

In Hearing, the Brain Integrates Sensory Information from the Ears

• Auditory neurons in the thalamus extend their axons to the primary auditory cortex

• Neurons in the primary auditory cortex code the frequency (or pitch) of auditory stimuli

• To locate the origin of a sound (auditory localization), the brain integrates the different sensory information coming from each of our two ears

In Vision, the Brain Processes Sensory Information from the Eyes

• The study of perception has focused to a large extent on the primary visual cortex and the multiple areas in which the retinal image is processed

• According to some estimates, up to half of the cerebral cortex may participate in visual perception

What Versus Where• Visual areas beyond the primary visual cortex

form two parallel processing streams, or pathways

• Ventral stream appears to be specialized for the perception and recognition of objects

• Dorsal stream seems to be specialized for spatial perception (determining where an object is)

• These two processing streams are therefore known as the “what” stream and the “where” stream

Blindsight

• Blindsight: a condition in which people who are blind have some visual capacities in the absence of any visual awareness

• Example: A person might not be able to see anything on his or her left. However, when a stimulus is presented in this blind field, the patient can respond unconsciously to that stimulus

4.4 What Factors Influence Visual Perception?

• Describe the Gestalt principles of perceptual organization.

• Identify the brain regions associated with facial perception.

• Identify cues for depth perception.• Explain how the visual system perceives

motion.• Discuss how perceptual constancy is achieved.

Object Perception Requires Construction

• Perceptual psychologists believe that illusions reveal the mechanisms that help our visual systems determine the sizes and distances of objects in the visual environment

• Researchers rely on these tricks to reveal automatic perceptual systems that, in most circumstances, result in accurate perception

Gestalt Principles of Perceptual Organization

• The German word Gestalt means “shape” or “form.” As used in psychology, Gestalt means “organized whole.”

• Gestalt psychology postulated a series of laws to explain how our brains group the perceived features of a visual scene into organized wholes

Figure and Ground

• Among the most basic organizing principles is distinguishing between figure and ground

• A classic illustration of this is the reversible figure illusion, in which figure and ground switch back and forth (ambiguous)

• In identifying what is “figure,” the brain assigns the rest of the scene to the background

Proximity and Similarity

• Two of the most important Gestalt principles concern proximity and similarity

• Principle of proximity: The closer two figures are to each other, the more likely we are to group them and see them as part of the same object

• Principle of similarity: We tend to group figures according to how closely they resemble each other

The “Best” Forms

• Good continuation: the tendency to interpret intersecting lines as continuous rather than as changing direction radically

• Closure: the tendency to complete figures that have gaps

• Illusory contours: We sometimes perceive contours and cues to depth even though they do not exist

Bottom-Up and Top-Down Information Processing

• How do we assemble the information about parts into a perception of a whole object?– Bottom-up processing: Data are relayed in the brain

from lower to higher levels of processing– Top-down processing: Information at higher levels

of mental processing can influence lower, “earlier” levels in the processing hierarchy

• The flight crew of New Zealand Flight 901 failed to notice the 12,000-foot volcano looming in front of them because the pilots saw what they expected to see

Face Perception• The visual system is sensitive to faces:– We can more readily discern information about a person’s

mood, attentiveness, sex, race, and age by looking at a person’s face than by listening to them talk, watching them walk, or studying their clothing (Bruce & Young, 1986)

– Whites are much better at recognizing white faces than at recognizing black faces (Brigham & Malpass, 1985)

• Prosopagnosia: deficits in the ability to recognize faces • Cortical regions, even specific neurons, seem to be

specialized to perceive faces and are sensitive to facial expression and gaze direction

Depth Perception is Important for Locating Objects

• How are we able to construct a three-dimensional mental representation of the visual world from two-dimensional retinal input?

• Binocular depth cues: available from both eyes together and contribute to bottom-up processing

• Monocular depth cues: available from each eye alone and provide organizational information for top-down processing

Binocular Depth Perception• Binocular disparity (or retinal disparity): This cue is

caused by the distance between humans’ two eyes– The brain uses the disparity between these two

retinal images to compute distances to nearby objects

– Stereoscopic vision: the ability to determine an object’s depth based on that object’s projections to each eye

• Convergence: When eye muscles turn the eyes inward, the brain knows how much the eyes are converging and uses this information to perceive distance

Monocular Depth Perception

• We can perceive depth with one eye because of monocular depth cues

• Pictorial depth cues:– Occlusion– Relative size– Familiar size– Linear perspective– Texture gradient– Position relative to horizon

Motion Cues for Depth Perception• Motion parallax: The brain uses cues from the

relative movements of objects that are at various distances from the observer– When you watch the scenery from a moving car,

near objects such as mailboxes seem to pass quickly, far objects more slowly, whereas objects farther away appear to match your speed

– Objects at an intermediate distance (a house) move opposite the direction of closer ones (a mailbox), whereas distant objects (a mountain) move in the same direction relative to the intermediate-distance object

Size Perception Depends on Distance Perception

• The size of an object’s retinal image depends on that object’s distance from the observer

• To determine an object’s size the visual system needs to know how far away it is

• Depth cues can fool us into seeing depth when it is not there; a lack of depth cues can fool us into not seeing depth when it is there

Ames Boxes

• Ames boxes: first crafted in the 1940s by Adelbert Ames, a painter turned scientist

• Ames boxes’ rooms play with linear perspective and other distance cues to create size illusions

The Ponzo Illusion

• Classic example of a size/distance illusion• Explained: Monocular depth cues make the

two-dimensional figure seem three-dimensional (Rock, 1984)

• This illusion shows how much we rely on depth perception to gauge size; the brain uses depth cues even when depth is absent

Motion Perception Has Internal and External Cues

• How does the brain know what is moving?• After receiving damage to secondary visual

areas of her brain — areas critical for motion perception—M.P., a German woman, saw the world as a series of snapshots rather than as a moving image (Zihl, von Cramon, & Mai, 1983)

• Neurons specialized for detecting movement fire when movement occurs

Motion Aftereffects • Waterfall effect: If you stare at a waterfall and then

turn away, the scenery you are now looking at will seem to move upward

• Explained:– The visual cortex has neurons that respond to

movement in a given direction– When you stare at a moving stimulus long enough,

these direction-specific neurons adapt to the motion and become fatigued

– When the stimulus is removed, other motion detectors that respond to all other directions are more active than the fatigued motion detectors

Compensation for Head and Eye Movement

• When you see what appears to be a moving object, how do you know whether the object is moving, you are moving, or your eyes are moving?

• Explained: – The brain calculates an object’s perceived movements by

monitoring the movement of the eyes, and perhaps also of the head, as they track a moving object

– Motion detectors track an image’s motion across the retina

Stroboscopic Motion Perception• Stroboscopic movement: a perceptual illusion that

occurs when two or more slightly different images are presented in rapid succession

• Max Wertheimer conducted experiments in 1912 by flashing, at different intervals, two vertical lines placed close together– When the interval was about 60 milliseconds, the line

appeared to jump from one place to another– At slightly longer intervals, the line appeared to move

continuously — a phenomenon called phi movement

Perceptual Constancies Are Based on Ratio Relationships

• How does the brain know that a person is 6 feet tall when the retinal image of that person changes size?

• Perceptual constancy: The brain correctly perceives objects as constant despite sensory data that could lead it to think otherwise

• The brain computes a ratio based on relative magnitude rather than on sensations’ absolute magnitude; perceptual systems are tuned to detect changes from baseline conditions, not just to respond to sensory inputs

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