The drive reduction hypothesis The drive reduction hypothesis was the first attempt to explain motivation and reinforcement. According to this theory, biological needs, caused by deprivation of the necessities of life, are unpleasant. The physiological changes associated with going without food or drink for several hours produce unpleasant states called hunger and thirst. These unpleasant states serve as drives to behaviour (to energise us to find food or drink). The acts of eating and drinking reduce hunger and thirst and this drive reduction is reinforcing.

Food metabolism When the digestive system contains food, glucose nourishes the brain and muscles. Extra glucose is stored in the liver and converted to fat. When the digestive system is empty, glucose obtained from glycogen stored in the liver nourishes the brain until this short-term reservoir is used up. Fatty acids from fat tissue nourish the muscles, and glycerol is converted to glucose to nourish the brain.

Sensory-specific satiety The term ‘sensory specific satiety’ refers to the decrease in the pleasantness and consumption of a specific food after eating it to satiety. This issue is explored in this slide. (a) The amount eaten of a one-flavoured and a three-flavoured meal. (b) The effect of varying the type of pasta shape on food intake and pleasantness ratings of the taste of the shapes. In both examples, the sensory properties of the food influence intake and ratings of pleasantness. Source: Reprinted from Rolls, B.J., Rolls E.T. and Rowe E.A., How sensory properties of foods affect human feeding behaviour, Psychology and Behaviour, 1982, 29, 409–417. © 1982, with permission from Elsevier Science; and Rolls and Rolls (1997).

BMI and the problem of obesity Body mass index (BMI) is calculated as an individual’s weight (kg) divided by their height (m2). To be diagnosed as obese, a person’s BMI must be equal to or greater than 30 kg/m2. In the USA, it is estimated that around 61% of adults are either overweight or obese. Source: www.shapeup.org and Wadden, T.A., Brownell K.D. and Foster, G.D., Obesity: Responding to the global epidemic. Journal of Consulting and Clinical Psychology, 2002, 70(3), 510–525. Copyright © 2002 by the American Psychological Association, reprinted with permission.

The role of leptin in obesity The ob mouse is genetically obese; it has a low metabolism, overeats and grows monstrously fat. A particular gene, called OB, normally produces the protein leptin which is normally secreted by fat cells that have absorbed a large amount of triglyceride. Because of a genetic mutation, the fat cells of ob mice are unable to secrete leptin. If ob mice are given daily injections of leptin, their metabolic rate increases, their body temperature rises, they become more active and they eat less. As a result, their weight returns to normal. Source: Photo courtesy of Dr J. Sholtis, The Rockefeller University. © 1995 Amgen, Inc.

Ekman’s six basic emotions shown in facial expression, and a combination of two Psychologists have argued that a set of basic emotions exists. The number of basic emotions has ranged from six or seven, to eight or ten. According to Ekman, the basic emotions are (clockwise from top left of the figure) surprise, anger, sadness, joy (happiness), fear and disgust. These basic emotions correspond to universally recognised facial expressions. Source: Pinel, J., Biopsychology (3rd edition). Boston, MA: Allyn & Bacon. Copyright © 1997 by Pearson Education, reprinted by permission.