Ordinarily, the physiological substrate of appetite is thought to respond to energy considerations as previously outlined. However, these reactions are also subject to conditioning, such that particular environmental stimuli may, through repeated pairing, come to elicit the physiological states associated with the onset or offset of eating. For instance, it has been demonstrated that previously neutral stimuli repeatedly paired with the presentation of food can induce sated animals to continue eating (4). Visual, olfactory, or social cues associated with hunger may induce feeding in people who are not otherwise hungry; comparable cues that have been paired with satiety may likewise terminate eating in individuals who are not otherwise sated. It has been suggested that such external, conditioned cues do not control behavior directly, but rather operate on the physiological substrate of hunger and satiety; for example, the mere sight of attractive food cues (of the sort that have reliably predicted eating in the past) may induce physiological reactions (eg, insulin release) associated with hunger (18). Even temporal cues may stimulate appetite; people become hungry as mealtime approaches, but if for some reason the meal is skipped, hunger may subside (C. P. Herman, J. Polivy, and I. Biernacka, unpublished data, 1998).
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