Physiological Role of Pleasure

Abstract

A given stimulus can induce a pleasant or unpleasant sensation depending on the subject's internal state. The word alliesthesia is proposed to describe this phenomenon. It is, in itself, an adequate motivation for behavior such as food intake or thermoregulation. Therefore, negative regulatory feedback systems, based upon oropharingeal or cutaneous thermal signals are peripheral only in appearance, since the motivational component of the sensation is of internal origin. The internal signals seem to be complex and related to the set points of some regulated variables of the "milieu interieur," like set internal temperature in the case of thermal sensation (15). Alliesthesia can therefore explain the adaptation of these behaviors to their goals. Only three sensations have been studied- thermal, gustatory, and olfactory, but it is probable that alliesthesia also exists in such simple ways as in bringing a signal, usually ignored, to the subject's attention. For example, gastric contractions, not normally perceived, are felt in the state of hunger (16). Since alliesthesia relies on an internal input, it is possible that alliesthesia exists only with sensations related to some constants of the "milieu interieur" and therefore would not exist in visual or auditory sensations. As a matter of fact, luminous or auditory stimuli can be pleasing or displeasing in themselves, but there seems to be little variation of pleasure in these sensations, that is, no alliesthesia. There may be some esthetic value linked to these stimuli but it is a striking coincidence that they are in themselves rather neutral and that it is difficult to imagine a constant of the "milieu interieur" which could be possibly modified by a visual or an auditive stimulus-such as light of a certain wavelength or sound of a given frequency. In the light of this theory, it is possible to reconsider the nature of the whole conscious experience. The existence of alliesthesia implies the presence of internal signals modifying the concious sensations aroused from peripheral receptors. It is therefore necessary to question the existence of sensations aroused by direct stimulation of central receptors, such as hypothalamic temperature detectors, osmoreceptors, and others. Does their excitation arouse sensations of their own, or does the sensation have to pass through peripheral senses? Only human experimentation could answer this question. In the same way, it is possible that selfstimulation of the brain is pleasant, not by giving a sensation in itself, but because the electrical stimulus (17), renders peripheral stimuli pleasant.

Sensory specific satiety in man☆

“…It is worthwhile to consider some of the possible mechanisms through which such specific satieties might act. Cabanac and his colleagues suggested that the change they observed in the pleasantness of sucrose solutions or food-related odors after a preload of glucose was due to changes in the internal state or need for particular nutrients [3,8]. The decrease in pleasantness was found only if the substance ingested was similar to that tasted, i.e.…”

“…This led to the present investigation of satiety and its specificity in man. In relation to satiety in man it has been shown that the pleasantness of the taste or smell of food-related test stimuli was decreased by a 50 g load of glucose which was either swallowed or delivered intragastrically [3,4,6,7]. Cabanac and his colleagues I6,8] showed that the glucose load decreased the pleasantness of sucrose solutions but not of salt solutions, and that eating a meal decreased the pleasantness of food-related odors but not of non-food-related odors.…”

“…Cabanac and his colleagues I6,8] showed that the glucose load decreased the pleasantness of sucrose solutions but not of salt solutions, and that eating a meal decreased the pleasantness of food-related odors but not of non-food-related odors. They have argued from these findings that there is modulation of the hedonic responses to food produced by the internal nutritional state which could be important in controlling feeding, and have called the phenomenon "alliesthesia" (literally, changing sensation) [3].…”

“…Cabanac and his colleagues I6,8] showed that the glucose load decreased the pleasantness of sucrose solutions but not of salt solutions, and that eating a meal decreased the pleasantness of food-related odors but not of non-food-related odors. They have argued from these findings that there is modulation of the hedonic responses to food produced by the internal nutritional state which could be important in controlling feeding, and have called the phenomenon "alliesthesia" (literally, changing sensation) [3].On the basis of these findings in the monkey and man, we performed the experiments described here, to investigate whether in man there is a satiety mechanism specific for foods eaten. In Experiment 1, the possibility that the pleasantness of the taste of food might decline more for a food eaten than for other foods not eaten was investigated.…”

Food-conditioned odour rejection in the late stages of the meal, mediating learnt control of meal volume by aftereffects of food consumption

“…The findings strongly support this interpretation of earlier, behaviourally less conclusive data. Thus, the volume of a meal is not determined just by the decrease in all palatability as deprivation arousal declines (Booth, 1972a;Cabanac, 1971) and the decrease in the palatability specifically of the food just eaten (e.g. Berridge, 1991;Booth, 1976;Clifton et al, 1987;Rolls et al, 1983).…”