Chapter 5. Sensation and Perception

5.1 Sensation versus Perception


What does it mean to sense something? Our sense organs contain sensory receptors, which are specialized neurons that respond to specific types of information in our physical world. They convert the external information into neural energy and then send this to the brain. These biological processes collectively are known as sensation. For example, light causes chemical changes in cells that line the back of the eye. These cells relay messages (For review see Chapter 3 on Biopsychology), to the central nervous system. Because information from the world around us stimulates our senses, we refer to various types of sensory information as stimuli (singular: stimulus).

You probably know about our five major senses: vision, hearing (audition), smell (olfaction), taste (gustation), and touch (somatosensation). However, we also have sensory systems that provide information about balance (vestibular sense), body position (proprioception), body movement (kinesthesia), pain (nociception), and temperature (thermoception).

The sensitivity of a given sensory system to a particular stimulus can be expressed as an absolute threshold. Absolute threshold refers to the minimum amount of stimulus energy that must be present for the stimulus to be reliably detected. Another way to think about this is by asking how dim can a light be, or how soft can a sound be for us to reliably notice it. The sensitivity of our sensory receptors can be quite amazing. For example, on a clear night, the receptors in the back of the eye can detect a candle flame 1.6 miles away (Silver, 2015). Also, under quiet conditions, the receptor cells of the ear can detect the tick of a clock 20 feet away (Galanter, 1962).

It is also possible for us to get messages that are presented below the threshold for conscious awareness—these are called subliminal messages. A stimulus reaches the absolute threshold when it is strong enough to excite sensory receptors and send nerve impulses to the brain. However, in some cases our brains may receive a message, but we are not consciously aware of it. Over the years there has been a great deal of speculation about the use of subliminal messages in advertising, rock music, and self-help audio programs. Research shows that in laboratory settings, people can process and respond to information outside of awareness. But this does not mean that we obey these messages like zombies; in fact, subliminal messages have little effect on behavior outside the laboratory (Kunst-Wilson & Zajonc, 1980; Rensink, 2004; Nelson, 2008; Radel et al., 2009; Loersch et al., 2013).

Sometimes, we are interested in how much we need to change a stimulus to detect a difference. This is known as the just noticeable difference (jnd) or difference threshold. The difference threshold changes depending on the stimulus intensity. For example, imagine you are in a very dark movie theater. If you receive a text message, it is likely that many people would notice your cell phone light up. However, if you were in a brightly lit arena during a basketball game, very few people would notice. The cell phone brightness does not change, but its noticeability changes dramatically between the two contexts. Ernst Weber described the importance of context on difference threshold in the 1830s, and it has become known as Weber’s law: The difference threshold is a constant fraction of the original stimulus. Thresholds are generally measured under incredibly controlled conditions in situations that are optimal for sensitivity.


Our sensory receptors are constantly collecting information from the environment. However, our interactions with the world are affected by how we interpret that information. Perception refers to the way sensory information is interpreted and consciously experienced. As shown in Figure 5.2, perception involves both bottom-up and top-down processing. Bottom-up processing refers to the information we receive from our sensory systems (sensation), and top-down processing refers to the ways we use knowledge and expectancy to process this information (Egeth & Yantis, 1997; Fine & Minnery, 2009; Yantis & Egeth, 1999). Bottom-up processing is automatic, but top-down processing is often under our conscious control.


Figure 5.2. Top-down and bottom-up processes affect 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 they are saying. Suddenly, you are startled by the sound of breaking glass and clang of metal pans hitting the floor. One of the servers dropped a large tray of food. Although you were attending to your meal and conversation, that crashing sound would likely grab your attention. You would have no choice but to notice it. Because this attentional capture was caused by the sound from the environment: it would be considered a bottom-up process.

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 chain, you would probably look for a yellow object of a certain size in specific locations where you might leave your keys. You would not look for yellow on your ceiling fan, because you know keys are unlikely to be there. 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. Even very simple perceptions, such as recognizing an object, are influenced by your expectations, prior experiences, and culture.

One way to think of the difference between sensation and perception, is to remember that sensation is a physical process, whereas perception is psychological. For example, imagine walking into a kitchen and smelling the scent of baking cinnamon rolls, the sensation is the smell receptors reacting to the odor of cinnamon and sending information to the brain, but the perception may be “Mmm, this smells like the holiday bread that Grandma used to bake when I was a kid”.

Our sensations (and perceptions) can change over time. In fact, when sensory stimuli remain relatively constant over prolonged periods of time, our perception of them often changes. This is known as sensory adaptation. Imagine that you have been baking a cake but forget to take it out of the oven and it burns. It fills the kitchen with a strong smell, but after a while you don’t notice it anymore and feel pretty convinced that no one will ever know about your cooking disaster. Then your roommate comes back and says – Wow! It really smells in here– what did you burn? If you go outside of your home for a short period of time and then return, you too will notice the smell. The chemicals were still present in the air, but your olfactory receptors had stopped responding to them.


Attention is another factor that affects sensation and perception. Attention plays a significant role in determining whether we perceive what is sensed. 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. This is a top-down process. Your ears are still being stimulated by the music, but, if someone interrupted you to ask what song had just finished playing, you would probably be unable to answer that question. We need to pay attention to stimuli (either voluntarily or automatically) in order for us to be aware of them.

The importance of attention for perception of the environment is demonstrated in a famous study conducted by Daniel Simons and Christopher Chabris (1999). In this study, participants watched a video of two teams of people (dressed in black or white) passing two basketballs among them. 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 everyone 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 it was in plain view for nine seconds. Because participants were so focused on counting the passes, 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).

Link to Learning

See for yourself how inattentional blindness works by checking out this selective attention test from Simons and Chabris (1999).

A similar attentional phenomenon is change blindness. Change blindness is when something changes in the environment (or a picture) and you fail to notice. Like inattentional blindness, change blindness is also very common. It explains why we often fail to notice when someone we see every day gets new glasses or a haircut, or shaves off their facial hair. Psychology experiments often investigate change blindness by alternating two slightly different pictures within a video and measuring how quickly participants spot the difference. Can you see the differences between the two pictures in Figure 5.3? – look at the end of this chapter (above  the References) to find the answer. Even when there is a large difference between two pictures, about 40% of people fail to find the difference. Simons and Levi (1988) showed that change blindness affects a similar number of people in real world settings. Fifty percent of participants failed to notice that the person to whom they were giving directions changed to a different person! It sounds like a magic trick but check out the Door Study video to see how the researchers managed to make the switch without people noticing.


Figure 5.3. Can you spot the differences between the two pictures? In change blindness studies, experimenters measure how long participants take to find the difference(s) between the pictures. We can see four differences – but let us know if you see more.


Cultural psychologists, like Shinoba Kitayama and Richard Nisbett and their colleagues, have conducted multiple studies on the effects of culture on visual perception (Masuda, 2017). In general, White people from the USA tend to be more individualistic in their outlook, whereas people from East Asia are more collectivistic and place a greater emphasis on the importance of community. Multiple studies have shown that US participants tend to focus on central elements in a picture or scene—like the globe in Figure 5.3, whereas East Asian participants attend more to context. Therefore, East Asian participants are more likely to notice the background objects as well as the larger objects in the foreground. So, in a change blindness study, participants from the US and East Asia take a similar amount of time to see changes in the large, central objects but US participants are slower to notice changes in the background (Masuda & Nisbett, 2006).

Culture can also affect our susceptibility to visual illusions. Marshall and colleagues conducted a multi-national study in which they found that individuals from industrialized cultures were more prone to experience certain types of visual illusions than individuals from non-industrialized cultures (Marshall et al., 1963). Westerners are more likely to experience the Müller-Lyer illusion than people living in non-industrialized countries (Figure 5.4). In the illusion, the two black lines in Figure 5.4a appear to be different lengths, but they are actually the same. This illusion also applies to the lines in Figure 5.4b.


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 5.4. 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.

Cultural differences in line length perception are consistent with differences in the types of environment that people experience. People who are more susceptible to the illusion are used to seeing buildings with straight lines, often referred to as a carpentered world (Segall et al., 1966). In contrast, the Zulu people of South Africa, whose villages are made up of round huts arranged in circles, are less susceptible to the illusion (Segall et al., 1999).


The cross-race effect is another example of how visual perception is affected by culture and experience. In general, we are better at recognizing faces of people of our own race, better than those from other races (Young et al., 2012). Most studies looking at the cross-race effect have focused on White participants, however, in one study Lee and Penrod found that participant race determines the extent of the cross-race effect. White participants showed larger cross-race effects than Asian participants. However, Asian, White, and Latinx participants all showed larger cross-race effects for Black faces compared to faces of other races (Lee & Penrod, 2022). In other words, people who are not Black, have more difficulty recognizing Black faces than White, Asian or Latinx faces. Poorer recognition of Black faces has widespread societal implications, ranging from hurtful classroom interactions where Black students are mistaken for other students (Griffith et al., 2019), to the increased rates of misidentification of Black people by people witnessing crimes and misdemeanors (Lee & Penrod, 2022). However, research by McKone et al. (2019) offers hope for addressing these issues. They found that exposure to faces of different races before the age of 12 years can help mitigate the cross-race effect. This suggests that by promoting diversity and inclusivity from a young age, we can work towards a future where BIPOC individuals experience less discrimination.


It is not just visual perception that is affected by cultural factors. Culture affects our perception across all sensory modalities. For example, Japanese participants rated Japanese foods (e.g., soy sauce, fermented soybeans etc.) as more pleasant smelling than German participants who were unfamiliar with them (Ayabe-Kanamura et al., 1998).


As we have already see, motivation can also affect perception. Have you ever been expecting a really important phone call and you keep thinking that 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. The ability to identify a stimulus when it is embedded in a distracting background is explained by signal detection theory—the idea that our ability to detect a stimulus depends not only on its physical properties, but also on the psychological state of the observer. This might also explain why a mother is awakened by a quiet murmur from her baby but not by other sounds that occur while she is asleep. Signal detection theory has practical applications, such as increasing air traffic controller accuracy. Controllers need to be able to detect planes among many signals that appear on the radar screen and follow those planes as they move through the sky (Swets, 1964).


Many other factors can influence perception. Basic aspects of personality are associated with differences in taste perception. Children described as thrill seekers are more likely to show taste preferences for intense sour flavors than children who are not (Liem et al., 2004). Our values and expectations also affect our taste ratings of food. Individuals with positive attitudes toward reduced-fat foods are more likely to rate foods labeled as reduced fat as better tasting than people with less negative attitudes about these products (Aaron et al., 1994). Our expectations about taste are also strongly influenced by brands and packaging. Some studies have used deceptive packaging to demonstrate its power on consumer preferences. For example, in a taste test, participants who identified as Coke drinkers preferred Pepsi when it was served from a Coke bottle more than Coke served from a Pepsi bottle, and vice versa for Pepsi drinkers (Woolcott et al., 1983). An fMRI study showed that participants rated the same wine as tasting better if it had an expensive price tag compared to a cheap one (Plassman et al., 2008). Also, popcorn tastes sweeter if served in a red bowl and saltier if served in a blue one (Wang & Chang, 2022).


Sensory experiences during early development are critical for perception. All sensory systems need to be stimulated early in life in order for them to develop normally (Cisneros-Franco et al., 2020). All of our senses are stimulated to some extent when we are still in the womb, with the exception of vision. Hence, when we are born, our visual system is less developed than our other sensory systems. The exact timing of critical periods varies across our senses. However, permanent deficits can arise if normal stimulation does not occur during a critical period. For example, if a newborn has visual problems that prevent them from seeing normally in the first year of their life they have permanent difficulties with face perception for the rest of their lives (Pascalis et al., 2020). Similarly, young children with eye problems often develop a condition called amblyopia (lazy eye), where they are unable to see fine details, such as small letters. Amblyopia is permanent if the eye problem is not corrected before the age of 8 years old. Therefore, it is important to detect and resolve any sensory issues, e.g., eye and ear problems as early as possible (Cisneros-Franco et al., 2020; Pascalis et al., 2020).


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Introduction to Psychology (A critical approach) Copyright © 2021 by Jill Grose-Fifer; Rose M. Spielman; Kathryn Dumper; William Jenkins; Arlene Lacombe; Marilyn Lovett; and Marion Perlmutter is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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