pleasure, biological basis Pleasure might be defined as a subjective state associated with a reward. Termination of punishment or the omission of expected punishment may also produce pleasure. A reward is anything that we will work to obtain; a punishment anything that we will work to avoid or escape from. Reward has an important function in evolution, for it produces responses that usually help to adapt the organism to its particular environment. Natural selection has presumably favoured those genes that influence the organization of the brain so as to produce adaptive behaviour.

One example of pleasure is the taste and smell of food, which we find particularly rewarding and pleasant when we are hungry. Water too produces more pleasure sensation when we are thirsty. These examples make the point that some types of pleasure are modulated by internal need states, so that behaviour is appropriately regulated and directed. They also show that pleasure can be selectively reduced. For example, the flavour of a food that we have eaten until satiated becomes less desirable, while other foods that we have not just eaten can remain pleasurable. The advantage of this sensory-specific decrease in the pleasantness of the taste of a much-eaten food is that it encourages us to consume a variety of different foods, and hence to obtain a wider range of nutrients. More generally, this tendency for initially pleasurable stimulation to become less pleasant if repeated is that it encourages exploration of different potential rewards. This modulation of reward represents a mechanism for enabling other behaviours to have their turn, and perhaps prompts us to explore a wide variety of different stimuli in the environment, which could enhance survival.

For many animals, pleasure is presumably solely associated with sensory stimulation. For human beings, and possibly other primates, mental reflection can excite pleasure. A good example is Schadenfreude — taking pleasure in someone else's misfortune. Why should we find this rewarding? It may be that we have developed mechanisms that can recognize when we are more likely to succeed than someone else. At a more abstract level, the pleasure associated with solving difficult mental, social, or intellectual problems may represent mechanisms by which our genes have built our brains so as to favour problem solving.

Touching and being touched can produce intense pleasure, presumably homologous with the evident pleasure that many animals experience as a result of being groomed by another individual. The pleasure associated with touch may relate, in part, to its adaptive value in reproductive behaviour, and in maintaining stable social communities (in that taking time to touch someone else implies an investment by one human in another).

It is more difficult to give a simple, scientific account of the aesthetic pleasure associated with many visual and auditory stimuli that are not obviously biologically adaptive, although sociobiologists have been ingenious in their efforts to provide such explanations. Flowers, for instance, give pleasure, because they are predictors of fruit later in the season. The waist-to-hip ratio that defines feminine beauty might serve as a marker of youth and reproductive potential. Features that make men attractive to women might be produced by wealth and social status, as markers of the ability to provide resources. The pleasure produced by music may be related to the fact that prosody (the rhythmic aspect of speech), as in lullaby, can reduce activation and stress (or, in the converse, can produce martial behaviour). The arts and cultural activities, so extensively developed by human societies as a source of pleasure, may tap into evolutionarily ancient links between emotion and adaptive behaviour.

The brain has specialized systems for evaluating the reward value of sensory stimuli, and this evaluation normally occurs after the stimulus has been identifed and recognized. For example, in the parts of the cerebral cortex devoted to taste and to vision, the firing of neurons seems to represent the identity of the taste or the seen object, independent of whether the taste is pleasant or the object seen is associated with reward. But once an object has been identified by this preliminary stage of analysis, information is sent to the limbic system and related areas of the brain, including the amygdala and orbitofrontal cortex. Here, the reward value of the taste is determined, in conjunction with, from other brain areas, information about the state of hunger; and any reward associated with a visual object is decoded by reference to evidence that this stimulus has previously been associated with a rewarding situation.

This separation in the brain of the representation of what we taste and see from whether it is pleasant enables us to perceive and learn about objects (e.g. where they are) independently of their current reward value. In that emotions can be defined as states elicited by rewards and punishments, those brain regions (such as the orbitofrontal cortex and amygdala) which analyse whether stimuli are rewarding or punishing are centrally important in emotions, including pleasure.

Edmund T. Rolls, and Gemma Calvert

Bibliography

Rolls, E. T. (1999). The brain and emotion. Oxford University Press.

See also brain; cerebral cortex; evolution; limbic system; sociobiology.