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25 Sensation vs. Perception

Learning Objectives

By the end of this section, you will be able to:

  • Distinguish between sensation and perception
  • Describe the concepts of absolute threshold and difference threshold
  • Discuss the roles attention, motivation, and sensory adaptation play in perception


What does it mean to sense something? Sensory receptors are specialized neurones that respond to specific types of stimuli. When sensory information is detected by a sensory receptor, sensation has occurred. For example, light that enters the eye causes chemical changes in cells that line the back of the eye. These cells relay messages, in the form of action potentials (as you learned when studying biopsychology), to the central nervous system. The conversion from sensory stimulus energy to action potential is known as transduction.

You have probably known since elementary school that we have five senses: vision, hearing (audition), smell (olfaction), taste (gustation), and touch (somatosensation). It turns out that this notion of five senses is oversimplified. We also have sensory systems that provide information about balance (the vestibular sense), body position and movement (proprioception and kinesthesia), pain (nociception), and temperature (thermoception).

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 and his colleagues 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 just barely detect it. 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 none 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). As you can see in Figure SAP.2, “Outcomes of a Signal Detection Analysis,” each judgment trial creates four possible outcomes: 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).

A 2x2 table presenting the outcomes of a signal detection analysis. The title of the top x-axis says "Perceiver's response" and the left y-axis says "Stimulus". The words "Yes" and "No" correspond to the top columns, and the words "Present' and "Absent" for the rows. Stimulus Present is a Hit if it is under the perceiver response "Yes" and a Miss if the perceiver's response is "No". The Stimulus Absent corresponds to a false alarm if the perceiver's response is "Yes" and a correct rejection if the perceiver's response is "No"

Figure SAP.2 Outcomes of a Signal Detection Analysis. Our ability to accurately detect stimuli is measured using a signal detection analysis. Two of the possible decisions (hits and correct rejections) are accurate; the other two (misses and false alarms) are errors.

The analysis of the data from a psychophysics experiment creates two measures. One measure, known as sensitivity, refers to the true 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. The other measure, response bias, refers to a behavioural tendency to respond “yes” to the trials, which is independent of sensitivity.

Imagine, for instance, that rather than taking a hearing test, you are a soldier on guard duty, and your job is to detect the very faint sound of the breaking of a branch that 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 (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) and yet 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. Again, a miss (in which the technician incorrectly determines that there is no tumour) can be very costly, but false alarms (referring patients who do not have tumours to further testing) also have costs. The ultimate decisions that the technicians make are based on the quality of the signal (clarity of the image), their experience and training (the ability to recognize certain shapes and textures of tumours), and their best guesses about the relative costs of misses versus false alarms.

Although we have focused to this point on the absolute threshold, a second 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. The 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 (say one teaspoon), adding another teaspoon of sugar will make a big difference in taste. But 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. But now imagine that you were comparing between two music systems, one that cost $397 and one that cost $399. Probably you would think that the cost of the two systems was “about the same,” even though buying the cheaper one would still save you $2.

Research Focus: Influence without Awareness

If you study Figure SAP.3, “Absolute Threshold,” you will see that the absolute threshold is the point where we become aware of a faint stimulus. 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. But can subliminal stimuli (events that occur below the absolute threshold and of which we are not conscious) have an influence on our behaviour?

A graph showing a mockup of the absolute threshold. As the intensity of stimulus increases on the X-axis , the percentage of correct detections increases on the Y-axis on an S-shaped curve. Data below the Absolute Threshold point on the X-axis corresponds to data below the 50% point of correct detections, and is labeled subliminal stimuli

Figure SAP.3 Absolute Threshold. As the intensity of a stimulus increases, we are more likely to perceive it. Stimuli below the absolute threshold can still have at least some influence on us, even though we cannot consciously detect them.

A variety of research programs have found that subliminal stimuli can influence our judgments and behaviour, at least in the short term (Dijksterhuis, 2010). But 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, Karremans, Stroebe, and 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 (Lipton Ice) or a control string containing the same letters as Lipton Ice (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 would sit 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 (we can call the technique “subliminal advertising” because it advertises a product outside awareness) 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. People cannot argue 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. Saegert (1987, p. 107) concluded that “marketing should quit giving subliminal advertising the benefit of the doubt,” 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 then, 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 don’t have to worry too much about being subliminally persuaded in your everyday life, even if subliminal ads are allowed in your country. But even if subliminal advertising is not all that effective itself, there are plenty of other indirect advertising techniques that are used and that do work. For instance, many ads for automobiles and alcoholic beverages are subtly sexualized, which encourages the consumer to indirectly (even if not subliminally) associate these products with sexuality. And there is the ever more frequent “product placement” technique, where images of brands (cars, sodas, electronics, and so forth) are placed on websites and in popular television shows and movies. Harris, Bargh, & 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, a condition in which people are unable to consciously report on visual stimuli but nevertheless are 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.


While our sensory receptors are constantly collecting information from the environment, it is ultimately how we interpret that information that affects how we interact with the world. Perception refers to the way sensory information is organized, interpreted, and consciously experienced. Perception involves both bottom-up and top-down processing. Bottom-up processing refers to sensory information from a stimulus in the environment driving a process, and top-down processing refers to knowledge and expectancy driving a process, as shown in Figure SAP.4 (Egeth & Yantis, 1997; Fine & Minnery, 2009; Yantis & Egeth, 1999).

The figure includes two vertical arrows. The first arrow comes from the word “Top” and points downward to the word “Down.” The explanation reads, “Top-down processing occurs when previous experience and expectations are first used to recognize stimuli.” The second arrow comes from the word “bottom” and points upward to the word “up.” The explanation reads, “Bottom-up processing occurs when we sense basic features of stimuli and then integrate them.”

Figure SAP.4 Top-down and bottom-up are ways we process our perceptions.

Imagine that you and some friends are sitting in a crowded restaurant eating lunch and talking. It is very noisy, and you are concentrating on your friend’s face to hear what she is saying, then the sound of breaking glass and clang of metal pans hitting the floor rings out. The server dropped a large tray of food. Although you were attending to your meal and conversation, that crashing sound would likely get through your attentional filters and capture your attention. You would have no choice but to notice it. That attentional capture would be caused by the sound from the environment: it would be bottom-up.

Alternatively, top-down processes are generally goal directed, slow, deliberate, effortful, and under your control (Fine & Minnery, 2009; Miller & Cohen, 2001; Miller & D’Esposito, 2005). For instance, if you misplaced your keys, how would you look for them? If you had a yellow key fob, you would probably look for yellowness of a certain size in specific locations, such as on the counter, coffee table, and other similar places. You would not look for yellowness on your ceiling fan, because you know keys are not normally lying on top of a ceiling fan. That act of searching for a certain size of yellowness in some locations and not others would be top-down—under your control and based on your experience.

One way to think of this concept is that sensation is a physical process, whereas perception is psychological. For example, upon walking into a kitchen and smelling the scent of baking cinnamon rolls, the sensation is the scent receptors detecting the odour of cinnamon, but the perception may be “Mmm, this smells like the bread Grandma used to bake when the family gathered for holidays.”

Although our perceptions are built from sensations, not all sensations result in perception. In fact, we often don’t perceive stimuli that remain relatively constant over prolonged periods of time. This is known as sensory adaptation. Imagine going to a city that you have never visited. You check in to the hotel, but when you get to your room, there is a road construction sign with a bright flashing light outside your window. Unfortunately, there are no other rooms available, so you are stuck with a flashing light. You decide to watch television to unwind. The flashing light was extremely annoying when you first entered your room. It was as if someone was continually turning a bright yellow spotlight on and off in your room, but after watching television for a short while, you no longer notice the light flashing. The light is still flashing and filling your room with yellow light every few seconds, and the photoreceptors in your eyes still sense the light, but you no longer perceive the rapid changes in lighting conditions. That you no longer perceive the flashing light demonstrates sensory adaptation and shows that while closely associated, sensation and perception are different.

There is another factor that affects sensation and perception: attention. Attention plays a significant role in determining what is sensed versus what is perceived. Imagine you are at a party full of music, chatter, and laughter. You get involved in an interesting conversation with a friend, and you tune out all the background noise. If someone interrupted you to ask what song had just finished playing, you would probably be unable to answer that question.

One of the most interesting demonstrations of how important attention is in determining our perception of the environment occurred in a famous study conducted by Daniel Simons and Christopher Chabris (1999). In this study, participants watched a video of people dressed in black and white passing basketballs. Participants were asked to count the number of times the team dressed in white passed the ball. During the video, a person dressed in a black gorilla costume walks among the two teams. You would think that someone would notice the gorilla, right? Nearly half of the people who watched the video didn’t notice the gorilla at all, despite the fact that he was clearly visible for nine seconds. Because participants were so focused on the number of times the team dressed in white was passing the ball, they completely tuned out other visual information. Inattentional blindness is the failure to notice something that is completely visible because the person was actively attending to something else and did not pay attention to other things (Mack & Rock, 1998; Simons & Chabris, 1999).

In a similar experiment, researchers tested inattentional blindness by asking participants to observe images moving across a computer screen. They were instructed to focus on either white or black objects, disregarding the other colour. When a red cross passed across the screen, about one third of subjects did not notice it (Figure SAP.5) (Most, Simons, Scholl, & Chabris, 2000).

A photograph shows a person staring at a screen that displays one red cross toward the left side and numerous black and white shapes all over.

SAP.5 Nearly one third of participants in a study did not notice that a red cross passed on the screen because their attention was focused on the black or white figures. (credit: Cory Zanker)

Motivation can also affect perception. Have you ever been expecting a really important phone call and, while taking a shower, you think you hear the phone ringing, only to discover that it is not? If so, then you have experienced how motivation to detect a meaningful stimulus can shift our ability to discriminate between a true sensory stimulus and background noise.

Our perceptions can also be affected by our beliefs, values, prejudices, expectations, and life experiences. As you will see later in this chapter, individuals who are deprived of the experience of binocular vision during critical periods of development have trouble perceiving depth (Fawcett, Wang, & Birch, 2005). The shared experiences of people within a given cultural context can have pronounced effects on perception. For example, Marshall Segall, Donald Campbell, and Melville Herskovits (1963) published the results of a multinational study in which they demonstrated that individuals from Western cultures were more prone to experience certain types of visual illusions than individuals from non-Western cultures, and vice versa. One such illusion that Westerners were more likely to experience was the Müller-Lyer illusion (Figure SAP.6): The lines appear to be different lengths, but they are actually the same length.

Two vertical lines are shown on the left in (a). They each have V–shaped brackets on their ends, but one line has the brackets angled toward its center, and the other has the brackets angled away from its center. The lines are the same length, but the second line appears longer due to the orientation of the brackets on its endpoints. To the right of these lines is a two-dimensional drawing of walls meeting at 90-degree angles. Within this drawing are 2 lines which are the same length, but appear different lengths. Because one line is bordering a window on a wall that has the appearance of being farther away from the perspective of the viewer, it appears shorter than the other line which marks the 90 degree angle where the facing wall appears closer to the viewer’s perspective point.

Figure SAP.6 In the Müller-Lyer illusion, lines appear to be different lengths although they are identical. (a) Arrows at the ends of lines may make the line on the right appear longer, although the lines are the same length. (b) When applied to a three-dimensional image, the line on the right again may appear longer although both black lines are the same length.

These perceptual differences were consistent with differences in the types of environmental features experienced on a regular basis by people in a given cultural context. People in Western cultures, for example, have a perceptual context of buildings with straight lines, what Segall’s study called a carpentered world (Segall et al., 1966). In contrast, people from certain non-Western cultures with an uncarpentered view, such as the Zulu of South Africa, whose villages are made up of round huts arranged in circles, are less susceptible to this illusion (Segall et al., 1999). It is not just vision that is affected by cultural factors. Indeed, research has demonstrated that the ability to identify an odour, and rate its pleasantness and its intensity, varies cross-culturally (Ayabe-Kanamura, Saito, Distel, Martínez-Gómez, & Hudson, 1998).

Children described as thrill seekers are more likely to show taste preferences for intense sour flavours (Liem, Westerbeek, Wolterink, Kok, & de Graaf, 2004), which suggests that basic aspects of personality might affect perception. Furthermore, individuals who hold positive attitudes toward reduced-fat foods are more likely to rate foods labeled as reduced fat as tasting better than people who have less positive attitudes about these products (Aaron, Mela, & Evans, 1994).


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