- Review and summarize the capacities and limitations of human sensation.
- Explain the difference between sensation and perception, and describe how psychologists measure sensory and difference thresholds.
- Explain selective attention and sensory adaptation.
Sensory thresholds: What can we experience?
Humans possess powerful sensory capacities that allow us to sense the kaleidoscope of sights, sounds, smells, and tastes that surround us. Our eyes detect light energy, and our ears pick up sound waves. Our skin senses touch, pressure, hot, and cold. Our tongues react to the molecules of the foods we eat, and our noses detect scents in the air. The human perceptual system is wired for accuracy, and people are exceedingly good at making use of the wide variety of information available to them (Stoffregen & Bardy, 2001).
In many ways, our senses are quite remarkable. The human eye can detect the equivalent of a single candle flame burning 30 miles (48.3 km) away and can distinguish among more than 300,000 different colours. The human ear can detect sound — measured in hertz (Hz), which is a unit of frequency that determines vibrations per second — as low as 20 Hz and as high as 20000 Hz, and it can hear the tick of a clock about 20 feet (6.1 m) away in a quiet room. We can taste a teaspoon of sugar dissolved in two gallons (7.6 L) of water, and we are able to smell one drop of perfume diffused in a three-room apartment. We can feel the wing of a bee on our cheek dropped from one centimetre above (Galanter, 1962).
Although there is much that we do sense, there is even more that we do not. Dogs, bats, whales, and some rodents all have much better hearing than we do, and many animals (see Figure 5.1) have a far richer sense of smell. Birds are able to see the ultraviolet light that we cannot (see Figure 5.2) and can also sense the pull of the earth’s magnetic field. Cats have an extremely sensitive and sophisticated sense of touch, and they are able to navigate in complete darkness using their whiskers. The fact that different organisms have different sensations is part of their evolutionary adaptation. Each species is adapted to sensing the things that are most important to them, while being blissfully unaware of the things that do not matter.
Psychophysics is the branch of psychology that studies the effects of physical stimuli on sensory perceptions and mental states. The field of psychophysics was founded by the German psychologist Gustav Fechner (1801–1887), who was the first to study the relationship between the strength of a stimulus and a person’s ability to detect the stimulus.
The measurement techniques developed by Fechner are designed, in part, to help determine the limits of human sensation. One important criterion is the ability to detect very faint stimuli. The absolute threshold of a sensation is defined as the intensity of a stimulus that allows an organism to detect it 50% of the time. If the stimulus is detected less than half of the time, it is said to be subliminal, meaning it is below the threshold for reliable detection. The absolute threshold is different for different senses as well as for different people. It also varies between species.
In a typical psychophysics experiment, an individual is presented with a series of trials in which a signal is sometimes presented and sometimes not, or in which two stimuli are presented that are either the same or different. Imagine, for instance, that you were asked to take a hearing test. On each of the trials, your task is to indicate either “yes” if you heard a sound or “no” if you did not. The signals are purposefully made to be very faint, making accurate judgments difficult.
The problem for you is that the very faint signals create uncertainty. Because our ears are constantly sending background information to the brain, you will sometimes think that you heard a sound when nothing was there, and you will sometimes fail to detect a sound that is there. Your task is to determine whether the neural activity that you are experiencing is due to the background noise alone or is the result of a signal within the noise.
The responses that you give on the hearing test can be analyzed using signal detection analysis. Signal detection analysis is a technique used to determine the ability of the perceiver to separate true signals from background noise (Macmillan & Creelman, 2005; Wickens, 2002). Each judgment trial creates four possible outcomes (see Figure 5.3). A hit occurs when you, as the listener, correctly say “yes” when there was a sound. A false alarm occurs when you respond “yes” to no signal. In the other two cases, you respond “no” — either a miss, saying “no” when there was a signal, or a correct rejection, saying “no” when there was in fact no signal.
Signal detection is the culmination of two processes: the detection of stimuli and the judgment regarding its presence or absence. Detection of a stimulus depends on sensitivity, which is the ability of the individual to detect the presence or absence of signals. People who have better hearing will have higher sensitivity than will those with poorer hearing. However, many other factors can determine whether or not someone notices a stimulus, such as expectations (e.g., thinking you heard the phone ring when you were expecting a call) and level of physiological arousal. The second process, judgment (or response), refers to the decision about the presence or absence of a stimulus which is independent of sensitivity. Our judgments are decisions that can have consequences for our behaviour.
Imagine, for instance, that you are a soldier on guard duty, and your job is to detect the very faint sound of the breaking of a branch, which indicates that an enemy is nearby. You can see that in this case making a false alarm by alerting the other soldiers to the sound might not be as costly as a miss — that is, a failure to report the sound — which could be deadly. Therefore, you might well adopt a very lenient response bias in which whenever you are at all unsure, you send a warning signal. In this case, your responses may not be very accurate; your sensitivity may be low because you are making a lot of false alarms. However, the extreme response bias can save lives.
Another application of signal detection occurs when medical technicians study body images for the presence of cancerous tumours. A miss, in which the technician incorrectly determines that there is no tumour, can be very costly, but false alarms, in which patients who do not have tumours are referred to further testing, also have costs. The ultimate decisions that the technicians make are based on the clarity of the image (i.e., quality of the signal), their ability to recognize the certain shapes and textures of tumours (i.e., experience and training), and their best guesses about the relative costs of misses versus false alarms.
Another important criterion concerns the ability to assess differences between stimuli. The difference threshold, or just noticeable difference (JND), refers to the change in a stimulus that can just barely be detected by the organism. German physiologist Ernst Weber (1795–1878) made an important discovery about the JND — namely, that the ability to detect differences depends not so much on the size of the difference but on the size of the difference in relation to the absolute size of the stimulus. Weber’s law maintains that the just noticeable difference of a stimulus is a constant proportion of the original intensity of the stimulus. As an example, if you have a cup of coffee that has only a very little bit of sugar in it, just one teaspoon, adding another teaspoon of sugar will make a big difference in taste. However, if you added that same teaspoon to a cup of coffee that already had five teaspoons of sugar in it, then you probably wouldn’t taste the difference as much. In fact, according to Weber’s law, you would have to add five more teaspoons to make the same difference in taste.
One interesting application of Weber’s law is in our everyday shopping behaviour. Our tendency to perceive cost differences between products is dependent not only on the amount of money we will spend or save, but also on the amount of money saved relative to the price of the purchase. For example, if you were about to buy a soda or candy bar in a convenience store and the price of the items ranged from $1 to $3, you would likely think that the $3 item cost “a lot more” than the $1 item. Now, imagine that you were comparing between two music systems, one that cost $397 and one that cost $399. It seems likely you would consider the cost of the two systems was “about the same,” even though buying the cheaper one would still save you $2.
Influence without awareness
Absolute threshold is the point where we become aware of a faint stimulus (see Figure 5.4). After that point, we say that the stimulus is conscious because we can accurately report on its existence, or its nonexistence, more than 50% of the time. Yet, can subliminal stimuli, which are events that occur below the absolute threshold of our conscious awareness, have an influence on our behaviour?
A variety of research programs have found that subliminal stimuli can influence our judgments and behaviour, at least in the short term (Dijksterhuis, 2010). Whether the presentation of subliminal stimuli can influence the products that we buy has been a more controversial topic in psychology.
In one relevant experiment, Johan Karremans, Wolfgang Stroebe, and Jasper Claus (2006) had Dutch college students view a series of computer trials in which a string of letters such as BBBBBBBBB or BBBbBBBBB were presented on the screen. To be sure they paid attention to the display, the students were asked to note whether the strings contained a small b. However, immediately before each of the letter strings, the researchers presented either the name of a drink that is popular in Holland (e.g., Lipton Ice) or a control string containing the same letters as Lipton Ice (e.g., NpeicTol). These words were presented so quickly, for only about one-fiftieth of a second, that the participants could not see them.
Then, the students were asked to indicate their intention to drink Lipton Ice by answering questions — such as, “If you were sitting on a terrace now, how likely is it that you would order Lipton Ice?” — and also to indicate how thirsty they were at the time. The researchers found that the students who had been exposed to the “Lipton Ice” words, and particularly those who indicated that they were already thirsty, were significantly more likely to say that they would drink Lipton Ice than were those who had been exposed to the control words.
If they were effective, procedures such as this would have some major advantages for advertisers because it would allow them to promote their products without directly interrupting the consumers’ activity and without the consumers’ knowing they are being persuaded. As such, the technique is referred to as subliminal advertising because it advertises a product outside of our awareness. People cannot counterargue with, or attempt to avoid being influenced by, messages received outside awareness. Due to fears that people may be influenced without their knowing, subliminal advertising has been banned in many countries, including Australia, Canada, Great Britain, the United States, and Russia.
Although it has been proven to work in some research, subliminal advertising’s effectiveness is still uncertain. Charles Trappey (1996) conducted a meta-analysis in which he combined 23 leading research studies that had tested the influence of subliminal advertising on consumer choice. The results showed that subliminal advertising had a negligible effect on consumer choice. Joel Saegert (1987) concluded that “marketing should quit giving subliminal advertising the benefit of the doubt” (p. 107), arguing that the influences of subliminal stimuli are usually so weak that they are normally overshadowed by the person’s own decision making about the behaviour.
Taken together, the evidence for the effectiveness of subliminal advertising is weak, and its effects may be limited to only some people and in only some conditions. You probably do not have to worry too much about being subliminally persuaded in your everyday life, even if subliminal ads are allowed in your country. However, even if subliminal advertising is not all that effective itself, there are plenty of other indirect advertising techniques that do work. For instance, many ads for automobiles and alcoholic beverages are subtly sexualized, which encourages the consumer to indirectly, perhaps even subliminally, associate these products with sexuality. Additionally, there is the ever-more-frequent product placement technique where cars, sodas, electronics, and so forth are placed on websites, in movies, and in television shows. Jennifer Harris, John Bargh, and Kelly Brownell (2009) found that being exposed to food advertising on television significantly increased child and adult snacking behaviours, again suggesting that the effects of perceived images, even if presented above the absolute threshold, may nevertheless be very subtle.
Another example of processing that occurs outside our awareness is seen when certain areas of the visual cortex are damaged, causing blindsight. Blindsight is a condition in which people are unable to consciously report on visual stimuli, but they are still able to accurately answer questions about what they are seeing. When people with blindsight are asked directly what stimuli look like or to determine whether these stimuli are present at all, they cannot do so at better than chance levels. They report that they cannot see anything. However, when they are asked more indirect questions, they are able to give correct answers. For example, people with blindsight are able to correctly determine an object’s location and direction of movement, as well as identify simple geometrical forms and patterns (Weiskrantz, 1997). It seems that although conscious reports of the visual experiences are not possible, there is still a parallel and implicit process at work, enabling people to perceive certain aspects of the stimuli.
Another important perceptual process is selective attention, which is the ability to focus on some sensory inputs while tuning out others. Refer to the video below, and count the number of times the people in white shirts playing with the ball pass it to each other. You may find that, like many other people who view it for the first time, you miss something important because you selectively attend to only one aspect of the video (Simons & Chabris, 1999).
The following YouTube link provides an awareness test:
- Video: Selective Attention Test (Simons, 2010)
Selective attention also allows us to focus on a single talker at a party while ignoring other conversations that are occurring around us (Broadbent, 1958; Cherry, 1953). Without this automatic selective attention, we would be unable to focus on the single conversation we want to hear. Yet, selective attention is not complete; we also, at the same time, monitor what is happening in the channels we are not focusing on. Perhaps you have had the experience of being at a party and talking to someone in one part of the room, when suddenly you hear your name being mentioned by someone in another part of the room. You didn’t know you were attending to the background sounds of the party, but evidently you were. This cocktail party phenomenon shows us that although selective attention is limiting what we process, we are nevertheless simultaneously doing a lot of unconscious monitoring of the world around us.
A second fundamental process of perception is sensory adaptation, which is a decreased sensitivity to a stimulus after prolonged and constant exposure. When you step into a swimming pool, the water initially feels cold, but, after a while, you stop noticing it. After prolonged exposure to the same stimulus, our sensitivity toward it diminishes, and we no longer perceive it. The ability to adapt to the things that do not change around us is essential to our survival, as it leaves our sensory receptors free to detect the important and informative changes in our environment and to respond accordingly. We ignore the sounds that our car makes every day, which leaves us free to pay attention to the sounds that are different from normal and, thus, likely to need our attention. Our sensory receptors are alert to novelty and are fatigued after constant exposure to the same stimulus.
If sensory adaptation occurs with all senses, why does an image not fade away after we stare at it for a period of time? The answer is that, although we are not aware of it, our eyes are constantly flitting from one angle to the next, making thousands of tiny movements — called saccades — every minute. This constant eye movement guarantees that the image we are viewing always falls on fresh receptor cells. What would happen if we could stop the movement of our eyes? Psychologists have devised a way of testing the sensory adaptation of the eye by attaching an instrument that ensures a constant image is maintained on the eye’s inner surface. Participants are fitted with a contact lens that has a miniature slide projector attached to it. Because the projector follows the exact movements of the eye, stimulating the same spot, the same image is always on the retina. Within a few seconds, interesting things begin to happen. The image will begin to vanish, then reappear, only to disappear again, either in pieces or as a whole. Even the eye experiences sensory adaptation (Yarbus, 1967).
One of the major problems in perception is to ensure that we always perceive the same object in the same way, even when the sensations it creates on our receptors change dramatically. The ability to perceive a stimulus as constant despite changes in sensation is known as perceptual constancy. Consider our image of a door as it swings. When it is closed, we see it as rectangular, but when it is open, we see only its edge, and it appears as a line. We never perceive the door as changing shape as it swings because perceptual mechanisms take care of the problem for us, allowing us to see a constant shape.
The visual system also corrects for colour constancy. Imagine that you are wearing blue jeans and a bright white t-shirt. When you are outdoors, both colours will be at their brightest, but you will still perceive the white t-shirt as bright and the blue jeans as darker. When you go indoors, the light shining on the clothes will be significantly dimmer, but you will still perceive the t-shirt as bright. This is because we put colours in context and see that, compared with its surroundings, the white t-shirt reflects the most light (McCann, 1992). In the same way, a green leaf on a cloudy day may reflect the same wavelength of light as a brown tree branch does on a sunny day. Nevertheless, we still perceive the leaf as green and the branch as brown.
- Sensation is the process of receiving information from the environment through our sensory organs. Perception is the process of interpreting and organizing incoming information so we can understand it and react accordingly.
- Transduction is the conversion of stimuli detected by receptor cells to electrical impulses that are transported to the brain.
- Although our experiences of the world are rich and complex, humans have sensory strengths and sensory limitations, like all species do.
- Sensation and perception work together in a fluid, continuous process.
- Our judgments in detection tasks are influenced by both the absolute threshold of the signal as well as our current motivations and experiences. Signal detection analysis shows how both sensation and judgment are important in perception.
- The difference threshold, or just noticeable difference, is the ability to detect the smallest change in a stimulus about 50% of the time. According to Weber’s law, the just noticeable difference increases in proportion to the total intensity of the stimulus.
- Research has found that stimuli can influence behaviour even when they are presented below the absolute threshold (i.e., subliminally). The effectiveness of subliminal advertising, however, has not been shown to be of large magnitude.
- Selective attention is our ability to focus on some sensations while ignoring others.
- Sensory adaption occurs when repeated exposure to a stimulus results in lower perceived intensity.
- Perceptual constancy occurs when our perception of a stimulus is unchanged, even when the activation of sensory receptors by that stimulus has changed.
- Read a magazine or watch several advertisements on television and pay attention to the persuasive techniques being used. What impact are these ads having on your senses? Based on what you know about psychophysics, sensation, and perception, what are some of the reasons why subliminal advertising might be banned in some countries?
- If we pick up two letters, one that weighs one ounce and one that weighs two ounces, we can notice the difference. However, if we pick up two packages, one that weighs three pounds one ounce and one that weighs three pounds two ounces, we can’t tell the difference. Why?
- Take a moment to lie down quietly in a comfortable place. Notice the variety and levels of what you can see, hear, and feel. Does this experience help you understand the idea of the absolute threshold?
Figure 5.2. Used under a CC BY-NC-SA 4.0 license.
Figure 5.3. Used under a CC BY-NC-SA 4.0 license.
Figure 5.4. Used under a CC BY-NC-SA 4.0 license.
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