Flavor Sociocultural Experience and Food Selection
304 After surgical damage to its ventromedial nucleus, this mouse ate enough to triple its body weight. Results such as this initially led many psychologists to conclude that food intake is regulated by a combination of “start-eating” signals from the lateral hypothalamus and “stop-eating” signals from the ventromedial nucleus. We now know that the regulation process is far more complex and involves more than just these two brain regions. ONE FAT MOUSE Chapter 8 Motivation and Emotion application of drive reduction theory, normal animals eat until their set point is reached, then stop eating until desirable intake falls below the set point (Cabanac & Morrissette, 1992). This theory turned out to be too simplistic. More recent research shows that the brain’s control of eating involves more than just the interaction of a pair of “stop-eating” and “start-eating” areas (Winn, 1995). For example, the paraventricular nucleus in the hypothalamus also plays an important role. As with the ventromedial nucleus, stimulating the paraventricular nucleus reduces food intake. Damaging it causes animals to become obese (Leibowitz, 1992). In addition, hunger—and the eating of particular types of food—is related to the effects of various neurotransmitters on certain neurons in the brain. One of these neurotransmitters, called neuropeptide Y, stimulates increased eating of carbohydrates (Kishi & Elmquist, 2005; Schwartz et al., 2000). Another one, serotonin, suppresses carbohydrate intake. Galanin motivates eating of high-fat food (Krykouli et al., 1990), and enterostatin reduces it (Lin et al., 1998). Endocannabinoids stimulate eating in general. They affect the same hypothalamic receptors as does the active ingredient in marijuana, which may account for “the munchies,” a sudden hunger that marijuana use often creates (Cota et al., 2003; Di Marzo et al., 2001). Peptide YY3–36 causes a feeling of fullness and reduced food intake (Batterham et al., 2002, 2003). In other words, several brain regions and many brain chemicals regulate hunger and food selection. These internal regulatory processes are themselves affected by the physical environment (e.g., what foods are available), by learning experiences with particular foods, and, for humans, by social and cultural traditions about eating. Flavor, Sociocultural Experience, and Food Selection For example, eating is powerfully affected by the ﬂavor of food—the combination of its taste and smell (Carlson, 2001). In general, people eat more when differently ﬂavored foods are served, as in a multicourse meal, than when only one food is served (Raynor & Epstein, 2001). Apparently the ﬂavor of a food becomes less enjoyable as more of it is eaten (Swithers & Hall, 1994). In one study, people rated how much they liked four kinds of food; then they ate one of the foods and rated all four again. The food they had just eaten now got a lower rating, whereas liking increased for all the rest (Johnson & Vickers, 1993). Eating is also affected by the appearance and smell of certain foods. These signals come to elicit conditioned physiological responses—including the secretion of saliva, digestive juices, and insulin—in anticipation of eating those foods (see the learning chapter for more on conditioned responses). So merely seeing a pizza on television may prompt you to order one. And if you see a delicious-looking cookie, you don’t have to be hungry to start eating it. In fact, many people who have just pronounced themselves “full” after a huge holiday meal still manage to ﬁnd room for an appetizing dessert. In other words, humans eat not just to satisfy nutritional needs but also to experience enjoyment. Eating is stimulated by other kinds of signals, too. Do you usually eat while reading or watching television? If so, you may ﬁnd that merely settling down with a book or your favorite show can trigger the desire to have a snack, even if you just ﬁnished dinner! This happens partly because situations associated with eating in the past can become signals that stimulate eating in the future (Birch et al., 1989; Weingarten, 1983). People also learn social rules and cultural traditions that inﬂuence eating. In North American culture, having lunch at noon, munching popcorn at movies, and eating hot dogs at ball games are common examples of how certain social situations can stimulate eating particular items at particular times. How much you eat may also depend on what others do. Politeness or custom might prompt you to try foods you would otherwise have avoided. Generally, the mere presence of others, even strangers, tends to increase food consumption. Most people consume 60 to 75 percent more food when they are with others than when eating alone (Clendenen, Herman, & Polivy, 1995), and the same effect has been observed in other species, from monkeys to chickens (Galloway et al., 2005; Keeling & Hurink, 1996).