woman in her early twenties damaged her knee in a fall. Follow- ing surgery, she experienced sharp, burning pain so excruciating that she could not eat or sleep. The pain ran from her ankle to the middle of her thigh, and the slightest touch—even a light brush with a piece of cotton—provoked a feeling of intense burning. Surgical attempts to relieve her pain gave her no relief or only temporary relief followed by even more severe pain (Gracely et al., 1992). Another case had a happier ending. A 50-year-old man whose chronic back pain failed to respond to exercise and medication finally underwent surgery. Like roughly 1 percent of patients who undergo this procedure (Sachs et al., 1990), he, too, developed severe burning pain and extraordinary sensitivity to any kind of stimulation of the skin. Fortunately, however, the pain disappeared after three months of treatment. These patients suffered from a disorder called painful neuropathy, which literally means a painful illness of the neurons. Painful neuropathy—caused by either an accident or surgery—results when the brain interprets as excruciating pain signals from receptors in the skin or joints that normally indicate only light touch, pressure, or movement.
Painful neuropathy raises some intriguing questions about the way the nervous system translates information about the world into psychological experience. Does the intensity of sensory experience normally mirror the intensity of physical stimulation? In other words, when pain increases or the light in a theater seems extremely bright following a movie, how much does this reflect changes in reality ver- sus changes in our perception of reality? And if neurons can become accidentally rewired so that touch is misinterpreted as burning pain, could attaching neurons from the ear to the primary cortex of the oc- cipital lobes produce visual images of sound?
Questions such as these are central to the study of sensation and perception. Sensation refers to the process by which the sense
organs gather information about the environment and transmit this information to the brain for initial processing. Perception is the
process by which the brain organizes and interprets these sensations. Sensations are immediate experiences of qualities—red, hot, bright, and so forth—whereas perceptions are experiences of objects or events that appear to have form, order, or meaning (Figure 4.1). The distinction between sensation and percep- tion is useful, though somewhat artificial, since sensory and perceptual processes form an integrated whole, translating physical reality into psychological reality.
Why do sensation and perception matter? They matter in part because of individual differences in sensation and perception. If I am color blind, my sensory world is differ- ent from yours. If I am depressed or schizophrenic, my perceptual world is different from yours. To understand the behavior of individuals, we need to have an appreciation of the varieties of sensory and perceptual experiences. To understand psychological disturbances, we need to have an understanding of the complexity and limitations of the
sensation the process by which the sense
organs gather information about the environment
perception the process by which the brain
selects, organizes, and interprets sensations
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BASIC PRINCIPLES 109
sensory systems and the role of perception in correcting or distorting our sensations.
Memory involves the mental reconstruction of past experience—but what would we remember if we could not sense, perceive, and store images or sounds to re-create in our minds? Or consider love. What would love be if we could not feel another person’s skin against ours? Without our senses, we are literally sense- less—without the capacity to know or feel. And without knowledge or feeling, there is little left to being human.
We begin the chapter with sensation, exploring basic processes that apply to all the senses (or sensory
modalities—the different senses that provide ways of
knowing about stimuli). We then discuss each sense
individually, focusing on the two that allow sensation at a distance, vision and hearing (or audition), and more briefly exploring smell (olfaction), taste (gustation), touch, and
proprioception (the sense of the body’s position and motion). Next we turn to percep-
tion, beginning with the way the brain organizes and interprets sensations and concluding with the influence of experience, expectations, and needs on the way people make sense of sensations.
I N T E R I M S U M M A R Y
Sensation is the process by which sense organs gather information about the environ-
ment and transmit it to the brain for initial processing. Perception is the related process by which the brain selects, organizes, and interprets sensations.
B A S I C P R I N C I P L E S
Throughout this discussion on sensation and perception, three general principles re- peatedly emerge. First, there is no one-to-one correspondence between physical and psy-
chological reality. What is “out there” is not directly reproduced “in here.” Of course,
the relation between physical stimuli and our psychological experience of them is not random; as we will see, it is actually so orderly that it can be expressed as an equation.
Yet the inner world is not simply a photograph of the outer. The degree of pressure or pain experienced when a pin presses against the skin—even in those of us without painful neuropathy—does not precisely match the actual pressure that is exerted. Up to a certain point, light pressure is not experienced at all, and pressure feels like pain only when it crosses a certain threshold. The inexact correspondence between physi- cal and psychological reality is one of the fundamental findings of psychophysics.
Second, sensation and perception are active processes. Sensation may seem passive— images are cast on the retina at the back of the eye; pressure is imposed on the skin. Yet sensation is first and foremost an act of translation, converting external energy into an internal representation of it. People also actively orient themselves to stimuli to capture sights, sounds, and smells that are relevant to them: We turn our ears to- ward potentially threatening sounds to magnify their impact on our senses, just as we turn our noses toward the smell of baking bread. We also selectively focus our
psychophysics branch of psychology that
studies the relationship between attributes of the physical world and the psychological experiences of them
FIGURE 4.1 From sensation to perception. Take a careful look at this picture before reading further, and try to figure out what it depicts. When people first look at this photo, their eyes transmit information to the brain about which parts of the picture are white and which are black; this is sensation. Sorting out the pockets of white and black into a meaningful picture is perception. The photograph makes little sense until you recognize a Dalmatian, nose to the ground.
consciousness on parts of the environment that are particularly relevant to our needs and goals (Chapter 9).
Like sensation, perception is an active process: It organizes and interprets sensa- tions. The world as subjectively experienced by an individual—the phenomenological
world—is a joint product of external reality and the person’s creative efforts to un-
derstand and depict it mentally. People often assume that perception is as simple as opening their eyes and ears to capture what is “really” there. In fact, perception involves constructing the phenomenological world from sensory experience, just as a quilt maker creates something whole from thread and patches.
The third general principle is that sensation and perception are adaptive. From an evolutionary perspective, the ability to see, hear, or touch is the product of millions of adaptations that left our senses exquisitely crafted to serve functions that facilitate survival and reproduction (Tooby & Cosmides, 1992). Frogs have “bug detectors” in their visual systems that automatically fire in the presence of a potential meal. Simi- larly, humans have neural regions specialized for the perception of faces and facial expressions (Adolphs et al., 1996; Phillips et al., 1997). Human infants have an innate tendency to pay attention to forms that resemble the human face, and, over the course of their first year, they become remarkably expert at reading emotions from other people’s faces (Chapter 12).
I N T E R I M S U M M A R Y
Three basic principles apply across all the senses: There is no one-to-one correspondence between physical and psychological reality; sensation and perception are active, not pas- sive; and sensory and perceptual processes reflect the impact of adaptive pressures over the course of evolution.
S E N S I N G T H E E N V I R O N M E N T
Although each sensory system is attuned to particular forms of energy, all the senses share certain common features. First, they must translate physical stimulation into sensory signals. Second, they all have thresholds below which a person does not sense anything despite external stimulation. Children know about this limitation threshold intuitively when they tiptoe through a room to “sneak up” on someone—who may suddenly hear them and turn around. The tiptoeing sounds increase gradually in in- tensity as the child approaches, but the person senses nothing until the sound crosses a threshold.
Third, sensation requires constant decision making, as the individual tries to dis- tinguish meaningful from irrelevant stimulation. We are unaware of most of these sensory “decisions” because they occur rapidly and unconsciously. Alone at night, people often wonder, “Did I hear something?” Their answers depend not only on the intensity of the sound but also on their tendency to attach meaning to small variations in sound.
Fourth, sensing the world requires the ability to detect changes in stimulation, like noticing when a bag of groceries has gotten heavier or a light has dimmed. Fifth and finally, efficient sensory processing means “turning down the volume” on in- formation that is redundant; the nervous system tunes out messages that continue without change. We examine each of these processes in turn.
1. All senses must translate physical stimulation into sensory signals.
2. All senses have thresholds below which a person does not sense anything despite external stimulation.
Sensation is an active process in which humans, like other animals, focus their senses on potentially important information.
SENSING ThE ENVIRONMENT 111
3. Sensation requires constant decision making to distinguish between meaningful and unimportant stimulation.
4. Sensation requires the ability to detect changes.
5. Efficient sensory processing requires the ability to tune out redundant information.
Transduction
Sensation requires converting energy in the world into internal signals that are psy- chologically meaningful. The more the brain processes these signals—from sensa- tion to perception to cognition—the more meaningful they become. Sensation typi- cally begins with an environmental stimulus, a form of energy capable of exciting the nervous system.
CREATING A NEURAL CODE Specialized cells in the nervous system, called sensory
receptors, transform energy in the environment into neural impulses that can be in-
terpreted by the brain (Loewenstein, 1960; Miller et al., 1961). Receptors respond to different forms of energy and generate action potentials in sensory neurons adjacent to them (Chapter 3). In the eye, receptors respond to wavelengths of light; in the ear, to the movement of molecules of air.
The process of converting physical energy or stimulus information into neural im- pulses is called transduction. The brain then interprets the impulses generated by sensory receptors as light, sound, smell, taste, touch, or motion. It then reads a neural code—a pattern of neural firing—and translates it into a psychologically meaningful “language.” CODING FOR INTENSITY AND QUALITY OF ThE STIMULUS For each sense, the brain codes sensory stimulation for intensity and quality. The neural code for intensity, or strength, of a sensation varies by sensory modality but usually involves the number of sensory neurons that fire, the frequency with which they fire, or some combination of the two. The neural code for quality of the sensation (such as color, pitch, taste, or temperature) is often more complicated, relying on both the specific type of receptors involved and the pattern of neural impulses generated. For example, some receptors respond to warmth and others to cold, but a combination of both leads to the sensa- tion of extreme heat.
I N T E R I M S U M M A R Y
Sensation begins with an environmental stimulus; all sensory systems have specialized cells called sensory receptors that respond to environmental stimuli and typically generate action potentials in adjacent sensory neurons. The process of converting stimulus informa- tion into neural impulses is called transduction. Within each sensory modality, the brain codes sensory stimulation for intensity and quality.