We are aware of the world only because we have a number of sense organs able to receive messages. These organs enable us to see, hear, taste, smell, touch, balance, and experience such feelings as body stiffness, soreness, fullness, warmth, pleasure, pain, and movement. Sense organs operate through sensory receptor cells, which receive outside forms of energy (light, vibrations, heat) and translate them into neural impulses that can be transmitted to the brain for interpretation. Sense organs do the job of the basketball announcer who translates what he or she sees into words that can be transmitted on the radio. The process of receiving information from the outside world, translating it, and transmitting it to the brain is called sensation . The process of interpreting that information and forming images of the world is called perception .
Stimuli: What Messages Can Be Received?
The term stimulus refers to any aspect of the world that infl uences our behavior or conscious experience. The term stimulus comes from the action of stimulating sensory receptor cells.
Virtually anything that can excite receptor cells can be a stimulus. When you take a seat at a dinner party, the chair is a stimulus through your senses of sight and touch.
When you eat, the food becomes a stimulus through your senses of taste, smell, and sight. The compliments you lavish on your hosts are also stimuli for them. Whenever a person is aware of, or in some other way responds to, a part of the outside world, she or he receives a stimulus.
When I say that any part of the outside world can be a stimulus, I am using the term outside broadly. Even parts of the internal world of the body can be stimuli. If you ate too much at the dinner party, the bloated stretching of your stomach would be a very noticeable stimulus.
Transduction: Translating Messages for the Brain
Energy from stimuli cannot go directly to the brain. Light, sound, and other kinds of energy from the outside world are not able to travel through the nerves. To reach the brain, sensory messages must be translated into neural impulses carried by neurons to the brain. The translation of energy in the environment into neural impulses is called transduction.
Energy is transduced into neural impulses in the sense organs by sensory receptor cells. These are specialized neurons that are excited by specifi c kinds of sensory energy and transmit neural impulses along their axons. Some sensory receptor cells respond to sound waves, some to light waves, some to chemicals, and so on. Note that we can be aware of a stimulus only if we have receptor cells that can transduce it (see fi gure 5.1 ).
For example, we cannot see radio waves or hear some high-frequency sound waves, and we fi nd some chemicals to be “tasteless” and “odorless,” because we do not have receptors that can transduce those stimuli. Although a radio wave is just as real as a light wave refl ected to our eyes from an apple, we cannot transduce the radio wave. We know that radio waves exist only because radios physically transduce them into sound waves, which are in turn transduced by our ears into neural messages to the brain.
Sensory Limits: How Strong Must Messages Be?
Even when we have receptor cells that can transduce a form of energy, not every signal is strong enough to be detected. The term threshold refers to the lower limits of sensory experience. The two primary kinds of thresholds are (a) the smallest mag- nitude of a stimulus that can be detected and (b) the smallest difference between two stimuli that can be detected.
The absolute threshold is the smallest magnitude of a stimulus that can be detected. Measuring such thresholds is no simple matter. People differ considerably in their sensitivity to weak stimuli, and each person’s sensitivity differs from time to time. For this reason, absolute thresholds are defi ned as the magnitude of a stimulus that subjects can detect half the time. The smallest difference between two stimuli that can be detected half the time is called the difference threshold . For example, the smallest change in intensity of your stereo that you can distinguish as “louder” 50% of the time is your difference threshold for that stimulus. Detailed knowledge of absolute and difference thresholds has, in fact, been used by the electronics industry to design better stereo systems.
Sensory Adaptation. Recall that an individual’s sensitivity to a stimulus differs from time to time. There are many reasons why this happens, such as inattention, but sensory adaptation is one of the major causes. When a stimulus is continuously pres- ent or repeated at short intervals, the sensation that the same amount of sensory energy causes becomes gradually weaker, in part because the receptor cells become fatigued.
transduction (trans-duk ´ shun) Translation of energy from one form to another.
absolute threshold The smallest magnitude of a stimulus that can be detected half the time.
difference threshold The smallest difference between two stimuli that can be detected half the time.
sensory adaptation Weakened magnitude of a sensation resulting from prolonged presentation of the stimulus.
What humans see
What bees “see”
Figure 5.1
The visual receptor cells of bees allow them to transduce ultraviolet light better than we can with our normal visual receptor cells. Therefore, bees
“see” more of this form of energy.
The top fl ower is as the human sees it; the bee, however, is able to see an ultraviolet “landing strip” on the fl ower that we do not see. If you are color blind, you may not be able to see the blue marks on the red fl ower—more on color blindness later.
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When I was a teenager, I frequently went skin diving in an extremely cold spring in central Florida. At fi rst the water was almost unbearably cold; when I jumped in from the dock, the intensity of the cold grabbed my attention so totally that for a moment I felt like only the cold skin of a person rather than a whole person. But after a few minutes the water felt comfortably cool. The water did not change in temperature, of course, but the sensation changed considerably because the temperature receptors in the skin adapted to the temperature of the water. This is sensory adaptation. It happens to some extent in all the senses; loud sounds and offensive odors, fortunately, also seem less intense as time goes by.
Psychophysics. The specialty area within the fi eld of psychology that studies sen- sory limits, sensory adaptation, and related topics is called psychophysics . The sub- ject matter of this fi eld is the relationship between the physical properties of stimuli and the psychological sensations they produce. Psychophysics is an important fi eld, because frequently there is not a direct or simple relationship between stimuli and sensations.
Because our knowledge of the outside world is limited to what we sense, we need to understand that under some conditions our sensations do not directly refl ect the physical nature of the stimulus. Sensory adaptation is one process that alters the relationship between stimuli and sensations, but numerous other circumstances pro- vide examples of this lack of a one-to-one relationship. The concept of the difference threshold provides another good example.
Psychophysicists have been fascinated since the 19th century with the fact that the difference threshold increases as the strength of the physical stimulus increases. When a stimulus is strong, changes in it must be bigger to be noticed than when the stimulus is weak. You can see this for yourself the next time you turn on a three-way light in a dark room. Many three-way bulbs provide light energy in three approximately equal steps (such as a 50-, 100-, and 150-watt bulb). The greatest difference in brightness in the room is noticeable after the fi rst click of the switch—the sofa that you just tripped over in the darkness is now plainly visible. Turning up the light to the 100-watt level adds a less noticeable increase in perceived brightness, and the third level adds even less in apparent brightness. At each level of increasing illumination, the difference threshold is greater, so the perceived increase in brightness is smaller. If you were to turn on another 50-watt bulb at this point—with the three-way bulb at its highest illumination—you might not see any increase in apparent brightness because your dif- ference threshold is so high.
The ability to detect small changes in the intensity of weak stimuli, but only large changes in the intensity of strong stimuli, was fi rst formally noted by German psycho- physicist Ernst Weber. Today this phenomenon is known as Weber’s law . Interest- ingly, the amount of the change needed to be detected half the time (the difference threshold) is almost always in direct proportion to the intensity of the original stimu- lus. Thus, if a waiter holding a tray containing four glasses were just able to detect the added weight of one glass, he would just be able to feel the added weight from two more glasses if the tray already held eight glasses. The amount of detectable added weight would always be in the same proportion, in this case 1/4.
What is the relevance of this bit of information? Weber’s law tells us that what we sense is not always the same as the energy that enters the sense organ. The same magnitude of physical change in intensity can be obvious one time, yet go undetected under different circumstances. This fact has important practical implications. Sup- pose, for example, that you are chosen to help design the instruments for a new air- plane. The pilot wants an easier way to monitor the plane’s altitude, so you put in a light that increases in intensity as the plane nears the earth—the lower the altitude, the more intense the light. That way, you assume, the pilot can easily monitor changes in altitude by seeing changes in brightness. Right? According to Weber’s law, this would be a dangerous way to monitor altitude. At high altitudes, the intensity of the light
psychophysics (sı¯ ´´ko¯ -fi z ´ iks) Specialty area of psychology that studies sensory limits, sensory adaptation, and related topics.
Weber’s law Law stating that the amount of change in a stimulus needed to detect a difference is in direct proportion to the intensity of the original stimulus.
At fi rst, cold water bombards us with sensations. After being in cold water for a while, receptors in the skin adapt to changes in temperature and lessen the sensation of coldness.
would be low, so small changes could be detected easily; at low altitudes, however, the intensity would be so great that large changes in altitude—even fatal ones—might not be noticed. That is why the people who design instruments for airplanes and cars need to know about psychophysics. We now know that Weber’s law is slightly incor- rect mathematically but is correct in concept.
The world is known to us only indirectly because our brains are not in direct contact with the outside world. Sensory receptor cells transduce physical energy into neural messages sent to the brain (sensation), where they are interpreted (perception). Not all forms of physical energy can become part of our perception of the world: we must have sensory receptor cells that can transduce that form of energy, and the stimulation must be strong enough to exceed the sensory threshold. Our perception of external reality is complicated, because there is no simple and direct relationship between the properties of physical stimuli and our conscious sensations. For example, a small change in the intensity of sound from a stereo is noticeable when the stereo is being played softly, but the same size change might go unnoticed if the stereo were at high volume. The complicated relationship between physical stimuli and conscious sensations is the subject matter of psychophysics.
There are no right or wrong answers to the following questions. They are presented to help you become an active reader and think critically about what you have just read.
1. How would life be different if human beings had a much higher absolute threshold for the chemicals involved in the sense of taste?
2. What is the difference between sensation and perception? Can you have a percep- tion without a sensation?
To be sure that you have learned the key points from the preceding section, cover the list of correct answers and try to answer each question. If you give an incorrect answer to any question, return to the page given next to the correct answer to see why your answer was not correct. Remember that these questions cover only some of the important information in this section; it is important that you make up your own questions to check your learning of other facts and concepts.
1. The is the smallest magnitude of a stimulus that can be detected half the time.
a) absolute threshold c) difference threshold b) visual threshold d) relative threshold
2. When a stimulus is continuously present or repeated at short intervals, the sensa- tion gradually becomes weaker. This is termed .
a) sensory adaptation c) desensitization b) psychophysics d) none of these
3. According to , the amount of the change in a stimulus needed to be detected half the time is in direct proportion to the intensity of the original stimulus.
a) psychophysical dualism c) Weber’s law
b) McGurty’s law d) threshold variation
4. The specialty area within the field of psychology that studies sensory limits, sensory adaptation, and related topics is called .
Review
Thinking Critically about Psychology Check Your Learning
Correct Answers: 1. a (p. 122),
2. a (p. 122), 3. c (p. 123),
4. psychophysics (p. 123)
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