CHAPTER 6

SENSATION AND PERCEPTION

Basic Concepts of Sensation and Perception

6-1 What are sensation and perception? What do we mean by bottom-up processing and top-down processing?

Sensation is the process by which our sensory receptors and nervous system receive and represent stimulus energies from our environment. Perception is the process of organizing and interpreting this information, enabling recognition of meaningful events. Sensation and perception are actually parts of one continuous process. Bottom-up processing is sensory analysis that begins at the entry level, with information flowing from the sensory receptors to the brain. Top-down processing is information processing guided by high-level mental processes, as when we construct perceptions by filtering information through our experience and expectations.

6-2 What three steps are basic to all our sensory systems?

Our senses (1) receive sensory stimulation (often using specialized receptor cells); (2) transform that stimulation into neural impulses; and (3) deliver the neural information to the brain. Transduction is the process of converting one form of energy into another. Researchers in psychophysics study the relationships between stimuli’s physical characteristics and our psychological experience of them.

6-3 How do absolute thresholds and difference thresholds differ, and what effect, if any, do stimuli below the absolute threshold have on us?

Our absolute threshold for any stimulus is the minimum stimulation necessary for us to be consciously aware of it 50 percent of the time. Signal detection theory predicts how and when we will detect a faint stimulus amid background noise. Individual absolute thresholds vary, depending on the strength of the signal and also on our experience, expectations, motivation, and alertness. Our difference threshold (also called just noticeable difference, or jnd) is the difference we can discern between two stimuli 50 percent of the time. Weber’s law states that two stimuli must differ by a constant minimum percentage (not a constant amount) to be perceived as different.

Priming (the often unconscious activation of certain associations that may predispose one’s perception, memory, or response)shows that we process some information from stimuli below our absolute threshold for conscious awareness.

6-4 Does subliminal sensation enable subliminal persuasion?

Subliminal stimuli are those that are too weak to detect 50 percent of the time. While subliminal sensation is a fact, such sensations are too fleeting to enable exploitation with subliminal messages: There is no powerful, enduring effect.

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6-5 What is the function of sensory adaptation?

Sensory adaptation (our diminished sensitivity to constant or routine odors, sounds, and touches) focuses our attention on informative changes in our environment.

6-6 How do our expectations, contexts, motivation, and emotions influence our perceptions?

Perceptual set is a mental predisposition that functions as a lens through which we perceive the world. Our learned concepts (schemas) prime us to organize and interpret ambiguous stimuli in certain ways. Our physical and emotional context, as well as our motivation, can create expectations and color our interpretation of events and behaviors.

Vision: Sensory and Perceptual Processing

6-7 What are the characteristics of the energy that we see as visible light? What structures in the eye help focus that energy?

What we see as light is only a thin slice of the broad spectrum of electromagnetic energy. The portion visible to humans extends from the blue-violet to the red light wavelengths. After entering the eye and being focused by a lens, light energy particles strike the eye’s inner surface, the retina. The hue we perceive in a light depends on its wavelength, and its brightness depends on its intensity.

6-8 How do the rods and cones process information, and what is the path information travels from the eye to the brain?

Light entering the eye triggers chemical reaction in the light-sensitive rods and color-sensitive cones at the back of the retina, which converts light energy into neural impulses. After processing by bipolar and ganglion cells, neural impulses travel from the retina through the optic nerve to the thalamus, and on to the visual cortex.

6-9 How do we perceive color in the world around us?

According to the Young-Helmholtz trichromatic (three-color) theory, the retina contains three types of color receptors. Contemporary research has found three types of cones, each most sensitive to the wavelengths of one of the three primary colors of light (red, green, or blue).

Hering’s opponent-process theory proposed three additional color processes (red-versus-green, blue-versus-yellow, black-versus-white). Research has confirmed that, en route to the brain, neurons in the retina and the thalamus code the color-related information from the cones into pairs of opponent colors.

These two theories, and the research supporting them, show that color processing occurs in two stages.

6-10 Where are feature detectors located, and what do they do?

Feature detectors, located in the visual cortex, respond to specific features of the visual stimulus, such as shape, angle, or movement. Supercell clusters in other critical areas respond to more complex patterns.

6-11 How does the brain use parallel processing to construct visual perceptions?

Through parallel processing, the brain handles many aspects of vision (color, movement, form, and depth) simultaneously. Other neural teams integrate the results, comparing them with stored information and enabling perceptions.

6-12 How did the Gestalt psychologists understand perceptual organization, and how do figure-ground and grouping principles contribute to our perceptions?

Gestalt psychologists searched for rules by which the brain organizes fragments of sensory data into gestalts (from the German word for “whole”), or meaningful forms. In pointing out that the whole may exceed the sum of its parts, they noted that we filter sensory information and construct our perceptions.

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To recognize an object, we must first perceive it (see it as a figure) as distinct from its surroundings (the ground). We bring order and form to stimuli by organizing them into meaningful groups, following such rules as proximity, continuity, and closure.

6-13 How do we use binocular and monocular cues to perceive the world in three dimensions, and how do we perceive motion?

Depth perception is our ability to see objects in three dimensions and judge distance. The visual cliff and other research demonstrate that many species perceive the world in three dimensions at, or very soon after, birth. Binocular cues, such as retinal disparity, are depth cues that rely on information from both eyes. Monocular cues (such as relative size, interposition, relative height, relative motion, linear perspective, and light and shadow) let us judge depth using information transmitted by only one eye.

As objects move, we assume that shrinking objects are retreating and enlarging objects are approaching. A quick succession of images on the retina can create an illusion of movement, as in stroboscopic movement or the phi phenomenon.

6-14 How do perceptual constancies help us construct meaningful perceptions?

Perceptual constancy enables us to perceive objects as stable despite the changing image they cast on our retinas. Color constancy is our ability to perceive consistent color in objects, even though the lighting and wavelengths shift. Brightness (or lightness) constancy is our ability to perceive an object as having a constant lightness even when its illumination—the light cast upon it—changes. Our brain constructs our experience of an object’s color or brightness through comparisons with other surrounding objects.

Shape constancy is our ability to perceive familiar objects (such as an opening door) as unchanging in shape. Size constancy is perceiving objects as unchanging in size despite their changing retinal images. Knowing an object’s size gives us clues to its distance; knowing its distance gives clues about its size, but we sometimes misread monocular distance cues and reach the wrong conclusions, as in the Moon illusion.

6-15 What does research on restored vision, sensory restriction, and perceptual adaptation reveal about the effects of experience on perception?

Experience guides our perceptual interpretations. People blind from birth who gained sight after surgery lack the experience to visually recognize shapes, forms, and complete faces.

Sensory restriction research indicates that there is a critical period for some aspects of sensory and perceptual development. Without early stimulation, the brain’s neural organization does not develop normally.

People given glasses that shift the world slightly to the left or right, or even upside down, experience perceptual adaptation. They are initially disoriented, but they manage to adapt to their new context.

The Nonvisual Senses

6-16 What are the characteristics of air pressure waves that we hear as sound?

Sound waves are bands of compressed and expanded air. Our ears detect these changes in air pressure and transform them into neural impulses, which the brain decodes as sound. Sound waves vary in amplitude, which we perceive as differing loudness, and in frequency, which we experience as differing pitch.

6-17 How does the ear transform sound energy into neural messages?

The outer ear is the visible portion of the ear. The middle ear is the chamber between the eardrum and cochlea. The inner ear consists of the cochlea, semicircular canals, and vestibular sacs. Through a mechanical chain of events, sound waves traveling through the auditory canal cause tiny vibrations in the eardrum. The bones of the middle ear amplify the vibrations and relay them to the fluid-filled cochlea. Rippling of the basilar membrane, caused by pressure changes in the cochlear fluid, causes movement of the tiny hair cells, triggering neural messages to be sent (via the thalamus) to the auditory cortex in the brain.

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Sensorineural hearing loss (or nerve deafness) results from damage to the cochlea’s hair cells or their associated nerves. Conduction hearing loss results from damage to the mechanical system that transmits sound waves to the cochlea. Cochlear implants can restore hearing for some people.

6-18 How do we detect loudness, discriminate pitch, and locate sounds?

Loudness is not related to the intensity of a hair cell’s response. The brain interprets loudness from the number of activated hair cells.

Place theory explains how we hear high-pitched sounds, and frequency theory explains how we hear low-pitched sounds. (A combination of the two theories explains how we hear pitches in the middle range.) Place theory proposes that our brain interprets a particular pitch by decoding the place where a sound wave stimulates the cochlea’s basilar membrane. Frequency theory proposes that the brain deciphers the frequency of the neural impulses traveling up the auditory nerve to the brain.

Sound waves strike one ear sooner and more intensely than the other. To locate sounds, the brain analyzes the minute differences in the sounds received by the two ears and computes the sound’s source.

6-19 How do we sense touch?

Our sense of touch is actually several senses—pressure, warmth, cold, and pain—that combine to produce other sensations, such as “hot.”

6-20 What biological, psychological, and social-cultural influences affect our experience of pain? How do placebos, distraction, and hypnosis help control pain?

Pain reflects bottom-up sensations (such as input from nociceptors, the sensory receptors that detect hurtful temperatures, pressure, or chemicals) and top-down processes (such as experience, attention, and culture). One theory of pain is that a “gate” in the spinal cord either opens to permit pain signals traveling up small nerve fibers to reach the brain, or closes to prevent their passage. The biopsychosocial perspective views our perception of pain as the sum of biological, psychological, and social-cultural influences. For example, our experience of pain is influenced by activity in the spinal cord’s large and small fibers (a biological influence), attention to pain (a psychological influence), and cultural expectations (a social-cultural influence).

Pain treatments often combine physical and psychological elements. Placebos can help by dampening the central nervous system’s attention and response to painful experiences. Distractions draw people’s attention away from painful stimulation. Hypnosis, which increases our response to suggestions, can also help relieve pain. Posthypnotic suggestion is used by some clinicians to control undesired symptoms.

6-21 In what ways are our senses of taste and smell similar, and how do they differ?

Taste and smell are both chemical senses. Taste is a composite of five basic sensations—sweet, sour, salty, bitter, and umami—and of the aromas that interact with information from the taste receptor cells of the taste buds.

There are no basic sensations for smell. We smell something when molecules of a substance carried in the air reach a tiny cluster of 20 million receptor cells at the top of each nasal cavity. Odor molecules trigger combinations of receptors, in patterns that the olfactory cortex interprets. The receptor cells send messages to the brain’s olfactory bulb, then to the temporal lobe, and to parts of the limbic system.

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6-22 How do we sense our body’s position and movement?

Through kinesthesia, we sense the position and movement of our body parts. We monitor our head’s (and thus our body’s) position and movement, and maintain our balance, with our vestibular sense.

6-23 How does sensory interaction influence our perceptions, and what is embodied cognition?

Our senses can influence one another. This sensory interaction occurs, for example, when the smell of a favorite food amplifies its taste. Embodied cognition is the influence of bodily sensations, gestures, and other states on cognitive preferences and judgments.

6-24 What are the claims of ESP, and what have most research psychologists concluded after putting these claims to the test?

Parapsychology is the study of paranormal phenomena, including extrasensory perception (ESP) and psychokinesis. The three most testable forms of ESP are telepathy (mind-to-mind communication), clairvoyance (perceiving remote events), and precognition (perceiving future events).

Skeptics argue that (1) to believe in ESP, you must believe the brain is capable of perceiving without sensory input, and (2) researchers have been unable to replicate ESP phenomena under controlled conditions.