5.5 The Olfactory System

If you walk into a department store, among the first items you see—placed right up front because they’re so profitable—are perfumes. Consumers shell out big bucks for little bottles of scent. The prices and purchases attest to the power of olfaction, the perception of airborne chemical substances—or, more simply, the sense of smell.

You’d be joyful, too, if you could afford this Jean Patou product. It costs about $400 an ounce.

Perceiving Odors

Preview Question

Question

What are some types of stimuli we can smell?

Olfaction provides a range of information. It not only tells you that something smells. It also provides information about (1) intensity, or how strong the smell is, and (2) pleasantness, whether the odorant—the thing that smells—smells good or bad (Moskowitz, Dravnieks, & Klarman, 1976). These two dimensions are distinct; different parts of the brain process information about pleasantness and intensity (Anderson et al., 2003).

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Passing the smell test Dogs, including those trained to sniff out explosives, rely on olfaction more than vision.

Olfaction also detects a range of different odorants, including (1) foods, (2) pheromones, and (3) chemical signals of cell damage and disease, as you’ll see immediately ahead.

In our discussion of olfaction, we’ll focus primarily on olfaction by humans. Yet smell is even more important to other species. Consider the dog. In a study of dogs that had been trained to detect bombs, their detection ability was found to be equally good whether containers of explosive material were brightly or very dimly lit. The dogs relied so heavily on olfaction that impaired vision made no difference to them. Furthermore, if an explosive was moved around a room and then placed in plain sight, the dogs did not walk directly to it, as would an organism relying primarily on vision. Instead, they traced a path to the explosive that followed its past movements, which left a scent (Gazit & Terkel, 2003). Among dogs, then, olfaction dominates vision in the guidance of behavior. They trust their sense of smell more than their sense of sight. Humans, by comparison, “don’t trust their nose” (Sela & Sobel, 2010, p. 13) as much as their vision. This psychological difference is accompanied by a biological one: About 12.5% of dogs’ brain mass is devoted to olfaction, as compared with less than 1% for humans (Williams, 2011).

Does this information explain any seemingly bizarre canine behavior you’ve ever witnessed?

FOOD ODORS. One class of stimuli that you perceive olfactorily is food. The ability to detect food odors was critical over the course of evolution, for two reasons. One, of course, is that it enabled organisms to find food. The other is that it enabled them to avoid spoiled food, which can be dangerous to eat. Mammals have a keen ability to smell the spoilage and thus avoid the danger (Takahashi, Nagayama, & Mori, 2004).

How accurate are you at recognizing odor? The last time you walked into a home where dinner was cooking, were you able to identify it olfactorily?

When presented with good (not spoiled) foods, how accurately can people identify them by their odors? Intuitively, it seems that sometimes we can identify odors accurately, yet other times we can’t tell what a smell might be. Research is consistent with this intuition; accuracy varies considerably. In one study (Cain, 1979), participants presented with common odorants (e.g., chocolate, cinnamon, peanut butter) correctly identified them only 45% of the time. However, days later, participants returned to the lab and were asked to identify the odors again, using the labels they generated on the first day. After some practice, participants applied their original label consistently 77% of the time (Cain, 1979). Even if a label was inaccurate (e.g., they smelled nutmeg and labeled it “cloves”), participants used the label (“cloves”) consistently when smelling the substance, which means that their ability to recognize the smell was very good.

If you’re young, enjoy your food aromas now. Your ability to make subtle distinctions among foods by your sense of smell is likely to decline with age. One study presented food odors to two groups of people: young adults in their early twenties and older adults in their seventies. The younger adults were better able to classify different types of foods based on their smell (Schiffman & Pasternak, 1979).

PHEROMONES. Another class of stimuli detected by the olfactory system is pheromones. Pheromones are chemical signals that are produced within the body of one organism, secreted by that organism, and then detected by another organism of the same species. When detected, the pheromone triggers a distinctive reaction in the organism that receives it. Female fish, for example, secrete sex pheromones when they are ready to spawn; the pheromones, once detected by male fish, trigger reproductive behavior in the male (Wyatt, 2009).

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Pheromones are detected primarily by the olfactory system. The olfactory system, then, has evolved to be highly sensitive to the odors of pheromones. In fact, many mammals (but not humans) have a second olfactory system designed specifically to aid in the detection of pheromones (Firestein, 2001).

The precise chemical substances that function as pheromones have been identified in other species, but not in humans (Wyatt, 2009). This has led some to question whether communication by pheromones is important to our species. Yet some evidence supports that it is important. Researchers find that women living in close quarters, such as a college dorm, may develop menstrual cycles that are synchronous (i.e., they coincide). The cause appears to be pheromones detected by the olfactory system. Women naturally secrete different biological compounds (e.g., through sweat glands) at different stages in the menstrual cycle. Some of the compounds affect the length of other women’s menstrual cycles. As a result, over time, menstrual cycles tend to synchronize (Stern & McClintock, 1998).

CELL DAMAGE AND DISEASE. Researchers recently have discovered a third class of stimuli perceived through olfaction: chemicals associated with cell damage and disease (Munger, 2009).

Cellular damage and inflammation caused by disease create distinctive molecular activity in the body. Researchers have identified the exact molecular changes, and then have looked within the olfactory system to see whether organisms detect them. Indeed, they do (Rivière et al., 2009). Thanks to receptors in their olfactory systems, mammals are able to detect molecular markers of illness. This ability has practical implications for human health. Cancer can be detected in early stages of its development by dogs that recognize signs of cancer in patients’ breath (Sonoda et al., 2011).

The ability to detect cell damage and disease must have been an advantage throughout the course of evolution. To remain healthy themselves, organisms would benefit from avoiding others who are ill. When seeking mates for reproduction, organisms benefit from finding a mate healthy enough to reproduce. The struggle for survival, then, relies in part on the olfactory system.

WHAT DO YOU KNOW?…

Question 10

True or False?

  • vmooRgYBJyvXyj+duZEfwaHjAXyysBfXMC6k/PKsedCPTqiptIAj/pvZ6Zui3RnoznJXIKr5HXipi2GMBOdKysXuePvY0aeqH9x9PnKrRDVq39cEWSLwltQg0ALK4NFeQe3PBmAFO6kSL5oTVVw9MY777iKwB5rdc8CY+NAaGsH60Jd1Jn5uLAOvXlWc+0VXg4M83V01IvfkuMCmMBesmPrJ6i9Gp54k
  • kOlFDGQx18qRfs7Dh5ktrm+k7IiKYeCQfELynfsDerDdMKmfDFjdhAmT+tejy2RrTdT92AuK9UZQ91hTtAQ3uL95KApCZS/PZV5FDUvYFg/NDpT+dQkdKL/K3G/FPiTtHkEyiDe1dMnKjcBuOGuTJvwCoFx8Mn+9xMYeLqtGc3v4S4GucZoUZdRQT2QOWImIh370P06KZvzN15PBK1dQ7njlglWRFR5o0bO8JkfKT94o9QUJNgqTsQ==
  • 2HVX/bLTVggVGUVBfo8LmGOZ0TI52ob64gQrH+NIm/T8XFKaHwQx8CgRhR9BinWY5MxAGzdCAOoFbTA3HTlewahUvDLuDvPF3o0J4dTlGwWbOoUkYfDvNyS7QO+FUKGmfzkyl65xXRTm8pCFX0oLzSTk4hzi0hoCk5IOmrHcmTdldH1sAwPtnPi737+jjeevKheBL9OVlteu6H0+Vmdc1R6yjfGrPJkyDjuEso34tO2/5utJhmuND9MgI9RiuAHERdkMlQKZ/6H8N/+gJ8pv4i6uQgM=

From Nose to Brain: Biological Bases of Olfaction

Preview Questions

Question

Through what biological process are we able to convert airborne chemical signals into information that is sent to the brain?

Question

Why are some people more sensitive to odors than others?

You don’t need to take a psychology class to know that olfaction begins at the nose. When you sniff, airborne substances enter your nasal cavity. Let’s look at what happens next: the biological processes that produce the sense of smell.

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FROM NOSE TO BRAIN. The biological bases of olfaction involve a three-step process (Sela & Sobel, 2010; Figure 5.40):

figure 5.40 From nose to brain A series of processing steps leads from nose to brain, that is, from odorants entering the nose to the experience of smell that results from neural processing in the brain. Sensory receptors that are activated by odorants (1) send signals to the olfactory bulbs (2), which in turn send signals to the olfactory cortex (3). The olfactory cortex is interconnected with the thamalus (4), a “relay station” for information processing near the center of the brain. Both through the thalamus, and directly, the olfactory cortex is connected with regions near the front of the brain for further processing (5).
  1. Receptor cells. Both of your nostrils contain more than 10 million receptor cells that are activated by odorants, pheromones, and disease-related molecules in the air. Different types of receptor cells are sensitive to different molecular signals. The receptor cells convert chemical signals into electrical signals that eventually reach the brain.

  2. Olfactory bulbs. Signals from receptor cells are sent to the olfactory bulbs, collections of cells near the front of the brain. After the signals are received, cells in the olfactory bulbs begin the process of identifying odors; different patterns of activity in the olfactory bulbs’ cells are associated with different odors (Sela & Sobel, 2010).

  3. Olfactory cortex. Signals from the olfactory bulbs are sent to the olfactory cortex, a neural system in a lower region of the brain known as the limbic system (see Chapter 3). Neural processing in the olfactory cortex completes the processes through which smells are recognized. Exactly how this processing is conducted is not fully understood; however, evidence suggests that the duration of neural activity in the olfactory cortex is significant. Rather than there being specific sets of cells that correspond to particular smells, different smells—which variously linger over time—are associated with different durations of nerve cell activity over time (Poo & Isaacson, 2009).

INDIVIDUAL DIFFERENCES AND GENETICS. People differ considerably in the sensitivity of their olfactory system. Consider a study in which researchers (Cain & Gent, 1991) presented low levels of four odorants—chemicals that smelled like wood, gas, flowers, and pears—and determined the minimum chemical concentration of each odor that participants could detect. In a study with this design, there are two possible results:

  1. People’s ability to detect odors may vary from one odor to another; that is, one person might be good at detecting wood smells and not flower smells, and another might be good at detecting flowers but not wood.

  2. People’s ability to detect odors may be consistent from one odor to the next, but some people may have consistently better odor detection than others.

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The researchers obtained result #2: Some people had better odor detection than others. But the sensitivity of odor detection for any individual was consistent across various odors.

Genetic variations explain much of the variation in olfactory ability. Genes that encode instructions for the olfactory system’s receptor cells are key (Hasin-Brumshtein, Lancet, & Olender, 2009). Genetic variations create differences in the number and diversity of olfactory receptors that people possess. Differences in receptors, in turn, produce person-to-person differences in the sense of smell.

WHAT DO YOU KNOW?…

Question 11

Molecular signals activate 1u1ENamheMHCMUBnfbORvA== cells in the nostrils, which then relay those signals to the vERc46T+VelQQd5VuNzVht3ZO80=. These signals are sent to the HcClDnKcTbITV+Y25h3XIw== cortex in the H4YQ78LTa0DrY6II system. The t630gWAHX4efXCu1AgaktQ== of nerve cell activity in the brain seems to be associated with recognition of odors. Individual differences in the ability to detect odors are largely explained by rvI1AB83Re95WdSl variations.