The functioning of an organism in a complex and unpredictable environment is supported by two complimentary processing systems, cognition and emotion. While the cognitive system is responsible for interpreting and making sense of the world, the emotion system is responsible for evaluating and judging events to assess their overall value with respect to the organism (Robot Emotion: A Functional Perspective; p.2). Paul Ekman in Basic Emotions chapter (Handbook of Cognition and Emotion; ch.3) offers a slightly different perspective: "...the primary function of emotion is to mobilize the organism to deal quickly with important interpersonal encounters, prepared to do so by what types of activity have been adaptive in the past."
More specifically, functions of emotions in animals (including humans) can be summarized as follows (based on Neural Networks and Brain Function; pp.138-40):
- Elicitation of automatic responses. Emotion may elicit an autonomic (for example, a change in a heart rate) or endocrine (for example, the release of adrenaline) response that prepares the body for action.
- Flexibility of reinforcement. Emotional states allow a simple interface between sensory inputs and motor outputs, because only the valence of of the stimulus to which attention is being paid needs to be passed to the motor system, rather than a full representation of the sensory world. In addition to that, when a stimulus elicits an emotional state, we can flexibly choose any appropriate response, which is more flexible than simply learning a fixed behavioural response to a stimulus.
- Motivation. For example, fear learned by stimulus-reinforcement association formation provides the motivation for actions performed to avoid noxious stimuli.
- Communication. Animals can communicate their emotional state to others. What is the role of mirror neurons?
- Storage of memories. Emotion may facilitate the storage of memories. This may be advantageous in that storing many details of the prevailing situation when a strong reinforcer is delivered may be useful in generating apropriate behavior in similar situations in the future. This may be facilitated in several ways: a) by bringing more attention to important situations the animal may be storing more information, b) by guiding attention toward what's important and away from distractions one may be storing more relevant information and c) by involving emotions stronger associations can be built.
- Evaluation of memories. The current mood state can affect the cognitive evaluation of events or memories.
- Memory recall: Emotion may trigger the recall of memories stored in neoroctical representations. Amygdala backprojections to the cortex could perform this for emotion in a way analogous to that in which the hippocampus could implement the retrieval of recent memories in the neocortex.
- Better performance. The slowing of cognitive processes caused by negative emotions may enable more careful and deliberate scrutiny of self and circumstances, allowing the individual to gain a new perspective to help improve performance in the future. Negative effect allows us to think in a highly focused way while positive effect allows us to think more creatively and to make broader associations.
Paul Ekman in Handbook of Cognition and Emotion identifies 15 basic emotions: amusement, anger, contempt, contentment, disgust, embarrassment, excitement, fear, guilt, pride in achievement, relief, sadness/distress, satisfaction, sensory pleasure, and shame. He also discusses enjoyment and surprise, but doesn't include those as basic emotions as they don't "subserve patterns of motor behavior which were adaptive for each of these emotions, preparing the organism for quite different actions." Also, interest "may be better regarded as a cognitive state rather than an emotion..." This list of emotions also doesn't include love, hate, grief, and jealousy as these are "emotional plots, more specific, more enduring than the basic emotions, specific contexts in which a number but not all of the basic emotions can be expected to occur." Ekman also defines moods -- which have different causes and last much longer, and are highly saturated with emotions -- and personaly traits, such as hostility.
Cynthia Breazel and Rodney Brooks in Robot Emotion: A Functional Perspective provide descriptions for a number of basic emotions that were implemented in the robot Kismet and gives details of that implementation (pp.17-20)
- Anger. Anger serves to mobilize and sustain energy and vigorous activity at high levels. If it often elicited when progress toward a goal is hindered or blocked. This mobilizes an organism to try alternative strategies.
- Disgust. Disgust is manifested as a distancing from some object, event, or situation, and can be characterized as a rejection of an unwanted stimulus.
- Fear. The function of fear is to motivate avoidance of escape from a dangerous situation.
- Joy. The emotion of joy is believed to heighten openness to experience. It often arises upon the success of achieving a goal or in the pleasure of mastery, exhibited even by very young children. The expression of joy operates as a universally recognizable signal of readiness for friendly interaction.
- Sorrow. Experiencing of sadness is causing to slow the cognitive and motor systems, which enables one to reflect upon a disappointing performace to gain a new perspective that will help improve future performance. The expression of sorrow communicates to others that one is in trouble and increases likelyhood that the others will feel sympathy and lend assistance.
- Interest. Interest motivates exploration, learning, and creativity. It mobilizes the creature for engagement and interaction. It serves as a mechanism of selective attention that keeps the creature focused on a particular object, person, or situation, and away from other distractions that impinge upon its senses.
- Boredom. Boredom is an emotion that arises when the organism is not stimulated for a while. Hofstadter has an interesting note about boredom in Godel, Escher, Bach (p.621): "you get bored with something not when you have exhausted its repertoire of behavior, but when you have mapped out the limits of the space that contains its behavior."
update 2006/03/21: This article by Edmund Rolls has an interesting diagram that put emotions on a scale from rage to relief and from terror to ecstasy.