Somatosensation is all about physical changes in or on the body: Vision and audition sense energetic states of the world—light and sound waves—and touch is activated by physical changes in or on the body surface. The last set of senses we will consider share a chemical basis to combine aspects of distance and proximity. The chemical senses of olfaction (smell) and gustation (taste) respond to the molecular structure of substances floating into the nasal cavity as you inhale or dissolving in saliva. Smell and taste combine to produce the perceptual experience we call flavour.

Olfaction is the least understood sense and the only one directly connected to the forebrain, with pathways into the frontal lobe, amygdala, and other forebrain structures (recall from the Neuroscience and Behaviour chapter that the other senses connect first to the thalamus). This mapping indicates that smell has a close relationship with areas involved in emotional and social behaviour. Smell seems to have evolved in animals as a signaling sense for the familiar: a friendly creature, an edible food, or a sexually receptive mate.

Countless substances release odours into the air, and some of their odorant molecules make their way into our noses, drifting in on the air we breathe. Situated along the top of the nasal cavity shown in FIGURE 4.28 is a mucous membrane called the olfactory epithelium, which contains about 10 million olfactory receptor neurons (ORNs), receptor cells that initiate the sense of smell. Odorant molecules bind to sites on these specialized receptors, and if enough bindings occur, the ORNs send action potentials into the olfactory nerve (Dalton, 2003).

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Each olfactory neuron has receptors that bind to some odorants but not to others, as if the receptor is a lock and the odorant is the key (see Figure 4.28). Groups of ORNs send their axons from the olfactory epithelium into the olfactory bulb, a brain structure located above the nasal cavity beneath the frontal lobes. Humans possess about 350 different ORN types that permit us to discriminate among some 10 000 different odorants through the unique patterns of neural activity each odorant evokes. This setup is similar to our ability to see a vast range of colours based on only a small number of retinal receptor cell types or to feel a range of skin sensations based on only a handful of touch receptor cell types.

Some dogs have as many as 100 times more ORNs than humans do, producing a correspondingly sharpened ability to detect and discriminate among millions of odours. Nevertheless, humans are sensitive to the smells of some substances in extremely small concentrations. For example, a chemical compound that is added to natural gas to help detect gas leaks can be sensed at a concentration of just 0.0003 part per million. By contrast, acetone (nail polish remover), something most people regard as pungent, can be detected only if its concentration is 15 parts per million or greater.

The olfactory bulb sends outputs to various centres in the brain, including the parts that are responsible for controlling basic drives, emotions, and memories. Odour perception includes both information about the identity of an odour, which involves relating olfactory inputs to information stored in memory (Stevenson & Boakes, 2003), as well as our emotional response to whether it is pleasant or unpleasant (Khan et al., 2007). Which of these processes occurs first? When you walk into a house and encounter the wonderful smell of freshly baked chocolate chip cookies, does your positive emotional response come before you identify the smell or vice versa? According to the object-centred approach, information about the identity of the “odour object” is quickly accessed from memory and then triggers an emotional response (Stevenson & Wilson, 2007). According to the valence-centred approach, the emotional response comes first and provides a basis for determining the identity of the odour (Yeshrun & Sobel, 2010). Recent attempts to distinguish between these two views support that odour perception is guided first by memory and then by emotion (Olofsson et al., 2012).

The relationship between smell and emotion explains why smells can have immediate, strongly positive or negative effects on us. If the slightest whiff of an apple pie baking brings back fond memories of childhood or the unexpected sniff of vomit mentally returns you to a particularly bad party you once attended, you have the idea. Thankfully, sensory adaptation is at work when it comes to smell, just as it is with the other senses. Whether the associations are good or bad, after just a few minutes the smell fades. Smell adaptation makes sense: It allows us to detect new odours that may require us to act, but after that initial evaluation has occurred, it may be best to reduce our sensitivity to allow us to detect other smells.

Our experience of smell is determined not only by bottom-up influences, such as odorant molecules binding to sites on ORNs, but also by top-down influences, such as our previous experiences with an odour (Gottfried, 2008). Consistent with this idea, people rate the identical odour as more pleasant when it is paired with an appealing verbal label such as cheddar cheese rather than an unappealing one such as body odour (de Araujo et al., 2005; Herz & von Clef, 2001). fMRI evidence indicates that brain regions involved in coding the pleasantness of an experience, such as the orbiotofrontal cortex, respond more strongly to the identical odour when people think it is cheddar cheese than when they think it is a body odour (de Araujo et al., 2005) (see the Hot Science box, Taste: From the Top Down, for a related finding concerning taste perception).

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Smell may also play a role in social behaviour. Humans and other animals can detect odours from pheromones, biochemical odorants emitted by other members of its species that can affect the animal’s behaviour or physiology. Parents can distinguish the smell of their own children from other people’s children. An infant can identify the smell of its mother’s breast from the smell of other mothers. Pheromones also play a role in reproductive behaviour in insects and in several mammalian species, including mice, dogs, and primates (Brennan & Zufall, 2006). Can the same thing be said of human reproductive behaviour?

Studies of people’s preference for the odours of individuals of the opposite sex have produced mixed results, with no consistent tendency for people to prefer them over other pleasant odours. Recent research, however, has provided a link between sexual orientation and responses to odours that may constitute human pheromones. Researchers used positron emission tomography (PET) scans to study the brain’s response to two odours, one related to testosterone, which is produced in men’s sweat, and the other related to estrogen, which is found in women’s urine. The testosterone-based odour activated the hypothalamus (a part of the brain that controls sexual behaviour; see the Neuroscience and Behaviour chapter) in heterosexual women but not in heterosexual men, whereas the estrogen-based odour activated the hypothalamus in heterosexual men but not in heterosexual women. Strikingly, homosexual men responded to the two chemicals in the same way as heterosexual women did: The hypothalamus was activated by the testosterone- but not estrogen-based odour (Savic, Berglund, & Lindstrom, 2005) (see FIGURE 4.29). Other common odours unrelated to sexual arousal were processed similarly by all three groups. A follow-up study with lesbians showed that their responses to the testosterone- and estrogen-based odours were largely similar to those of heterosexual men (Berglund, Lindstrom, & Savic, 2006). Taken together, the two studies suggest that some human pheromones are related to sexual orientation.

One of the primary responsibilities of the chemical sense of taste is identifying things that are bad for you—as in poisonous and lethal. Many poisons are bitter, and we avoid eating things that nauseate us for good reason, so taste aversions have a clear adaptive significance. Some aspects of taste perception are genetic, such as an aversion to extreme bitterness, and some are learned, such as an aversion to a particular food that once caused nausea. In either case, the direct contact between a tongue and possible foods allows us to anticipate whether something will be harmful or palatable.

The tongue is covered with thousands of small bumps, called papillae, which are easily visible to the naked eye. Within each papilla are hundreds of taste buds, the organ of taste transduction (see FIGURE 4.30). The mouth contains 5000 to 10 000 taste buds fairly evenly distributed over the tongue, roof of the mouth, and upper throat (Bartoshuk & Beauchamp, 1994; Halpern, 2002). Each taste bud contains 50 to 100 taste receptor cells. Taste perception fades with age (Methven et al., 2012): On average, people lose half their taste receptors by the time they turn 20. This may help to explain why young children seem to be “fussy eaters,” since their greater number of taste buds brings with it a greater range of taste sensations.

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The human eye contains millions of rods and cones, the human nose contains some 350 different types of olfactory receptors, but the taste system contains just five main types of taste receptors, corresponding to five primary taste sensations: salt, sour, bitter, sweet, and umami (savoury). The first four are quite familiar, but umami may not be. In fact, perception researchers are still debating its existence. The umami receptor was discovered by Japanese scientists who attributed it to the tastes evoked by foods containing a high concentration of protein, such as meats and cheeses (Yamaguchi, 1998). If you are a meat eater and you savour the feel of a steak topped with butter or a cheeseburger as it sits in your mouth, you have an idea of the umami sensation.

Each taste bud contains several types of taste receptor cells whose tips, called microvilli, react with tastant molecules in food. Salt taste receptors are most strongly activated by sodium chloride (table salt). Sour receptor cells respond to acids, such as vinegar or lime juice. Bitter and sweet taste receptors are more complex. Some 50 to 80 distinct binding sites in bitter receptors are activated by an equal number of different bitter-tasting chemicals. Sweet receptor cells likewise can be activated by a wide range of substances in addition to sugars.

Although umami receptor cells are the least well understood, researchers are homing in on their key features (Chandrashekar et al., 2006). They respond most strongly to glutamate, an amino acid in many protein-containing foods. Recall from the Neuroscience and Behaviour chapter, glutamate acts as a neurotransmitter; in fact, it is a major excitatory neurotransmitter. The food additive monosodium glutamate (MSG), which is often used to flavour Asian foods, particularly activates umami receptors. Some people develop headaches or allergic reactions after eating MSG.

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Of course, the variety of taste experiences greatly exceeds the five basic receptors discussed here. Any food molecules dissolved in saliva evoke specific, combined patterns of activity in the five taste receptor types. Although we often think of taste as the primary source for flavour, in fact, taste and smell collaborate to produce this complex perception. As any wine connoisseur will attest, the full experience of a wine’s flavour cannot be appreciated without a finely trained sense of smell. Odorants from substances outside your mouth enter the nasal cavity via the nostrils, and odorants in the mouth enter through the back of the throat. This is why wine aficionados are taught to pull air in over wine held in the mouth: It allows the wine’s odorant molecules to enter the nasal cavity through this “back door.” (The taste of wine can also be influenced by cognitive factors, as illustrated in the Hot Science box.)

You can easily demonstrate the contribution of smell to flavour by tasting a few different foods while holding your nose, preventing the olfactory system from detecting their odours. If you have a head cold, you probably already know how this turns out. Your favourite spicy burrito or zesty pasta probably tastes as bland as can be.

Taste experiences also vary widely across individuals. About 50 percent of people report a mildly bitter taste in caffeine, saccharine, certain green vegetables, and other substances, whereas roughly 25 percent report no bitter taste. Members of the first group are called tasters and members of the second group are called nontasters. The remaining 25 percent of people are supertasters, who report that such substances, especially dark green vegetables, are extremely bitter, to the point of being inedible (Bartoshuk, 2000). Children start out as tasters or supertasters, which could help explain their early tendency toward fussiness in food preference. However, some children grow up to become nontasters. Because supertasters tend to avoid fruits and vegetables that contain tastes they experience as extremely bitter, they may be at increased health risk for diseases such as colon cancer. On the other hand, because they also tend to avoid fatty, creamy foods, they tend to be thinner and may have decreased risk of cardiovascular disease (Bartoshuk, 2000). Genetic factors contribute to individual differences in taste perception (Kim et al., 2003), but much remains to be learned about the specific genes that are involved (Hayes et al., 2008; Reed, 2008).

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