HUNGER
How the motivation to eat (and food intake) are controlled, we first need to consider the functions of peripheral factors (i.e. factors outside the brain), such as taste, smell and gastric distension, and control signals, such as the amount of glucose in the bloodstream. Then we can examine how the brain integrates these different signals, learns about which stimuli in the environment represent food, and initiates behaviour to obtain the correct variety and amount.
The functions of some peripheral factors in the control of eating can be demonstrated by the sham feeding preparation in this preparation, the animal tastes, smells and eats the food normally, but the food drains away from the stomach. This means that, although the animal consumes the food, the stomach does not become full, since the food does not enter the stomach or intestine. Experiments have shown that rats, monkeys and humans will work for food when they are sham feeding often continuing to eat for more than an hour. This demonstrates that it is the taste and smell of food that provide the immediate reward for food-motivated behaviour. Further evidence for this is that humans are more likely to rate the taste and smell of food as being pleasant when they are hungry. A second important aspect of sham feeding is that satiety (reduction of appetite) does not occur. From this we can conclude that taste, smell and even swallowing (i.e. oropharyngeal factors) do not of themselves make us feel satisfied, or satiated. Instead, satiety is produced by food accumulating in the stomach and entering the intestine. Gastric distension is an important satiety signal, and intestinal signals also have a part to play (Gibbs et al., 1981).
When an animal is allowed to eat to normal satiety and then has the food drained from its stomach, it starts eating again immediately. Moreover, small infusions of food into the duodenum (the first part of the intestine) decrease feeding, indicating satiety. Interestingly, however, animals have difficulty learning to perform a response that brings a reward of food if the food is delivered directly into the stomach, demonstrating that this form of feeding is not very rewarding in itself (see Rolls, 1999). We can draw important conclusions about the control systems for motivated behaviour from these findings: n Reward and satiety are different processes. n Reward is produced by factors such as the taste and smell of food.
Satiety is produced by gastric, intestinal and other signals after the food is absorbed from the intestine.
Hunger and satiety signals modulate the reward value of food (i.e. the taste and smell of food are rewarding when hunger signals are present and satiety signals are not). To put this in more general psychological terms, in most behavioural situations the motivational state modulates or controls the reward or reinforcement value of sensory stimuli. So, for example, in certain species the female may apparently find the male of the species ‘sexually attractive’ only during certain phases of the female’s reproductive cycle.
Since reward and satiety are produced by different bodily (i.e. peripheral) signals, one function of brain (i.e. central) processes in the control of feeding is to bring together the satiety and reward signals in such a way that satiety modulates the reward value of food.
How the motivation to eat (and food intake) are controlled, we first need to consider the functions of peripheral factors (i.e. factors outside the brain), such as taste, smell and gastric distension, and control signals, such as the amount of glucose in the bloodstream. Then we can examine how the brain integrates these different signals, learns about which stimuli in the environment represent food, and initiates behaviour to obtain the correct variety and amount.
The functions of some peripheral factors in the control of eating can be demonstrated by the sham feeding preparation in this preparation, the animal tastes, smells and eats the food normally, but the food drains away from the stomach. This means that, although the animal consumes the food, the stomach does not become full, since the food does not enter the stomach or intestine. Experiments have shown that rats, monkeys and humans will work for food when they are sham feeding often continuing to eat for more than an hour. This demonstrates that it is the taste and smell of food that provide the immediate reward for food-motivated behaviour. Further evidence for this is that humans are more likely to rate the taste and smell of food as being pleasant when they are hungry. A second important aspect of sham feeding is that satiety (reduction of appetite) does not occur. From this we can conclude that taste, smell and even swallowing (i.e. oropharyngeal factors) do not of themselves make us feel satisfied, or satiated. Instead, satiety is produced by food accumulating in the stomach and entering the intestine. Gastric distension is an important satiety signal, and intestinal signals also have a part to play (Gibbs et al., 1981).
When an animal is allowed to eat to normal satiety and then has the food drained from its stomach, it starts eating again immediately. Moreover, small infusions of food into the duodenum (the first part of the intestine) decrease feeding, indicating satiety. Interestingly, however, animals have difficulty learning to perform a response that brings a reward of food if the food is delivered directly into the stomach, demonstrating that this form of feeding is not very rewarding in itself (see Rolls, 1999). We can draw important conclusions about the control systems for motivated behaviour from these findings: n Reward and satiety are different processes. n Reward is produced by factors such as the taste and smell of food.
Satiety is produced by gastric, intestinal and other signals after the food is absorbed from the intestine.
Hunger and satiety signals modulate the reward value of food (i.e. the taste and smell of food are rewarding when hunger signals are present and satiety signals are not). To put this in more general psychological terms, in most behavioural situations the motivational state modulates or controls the reward or reinforcement value of sensory stimuli. So, for example, in certain species the female may apparently find the male of the species ‘sexually attractive’ only during certain phases of the female’s reproductive cycle.
Since reward and satiety are produced by different bodily (i.e. peripheral) signals, one function of brain (i.e. central) processes in the control of feeding is to bring together the satiety and reward signals in such a way that satiety modulates the reward value of food.
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