emotion, term commonly and loosely used to denote individual, subjective feelings which dictate moods. In psychology, emotion is considered a response to stimuli that involves characteristic physiological changes—such as increase in pulse rate, rise in body temperature, greater or less activity of certain glands, change in rate of breathing—and tends in itself to motivate the individual toward further activity. Early psychological studies of emotion tried to determine whether a certain emotion arose before the action, simultaneously with it, or as a response to automatic physiological processes. In the 1960s, the Schachter-Singer theory pointed out that cognitive processes, not just physiological reactions, played a significant role in determining emotions. Robert Plutchik developed (1980) a theory showing eight primary human emotions: joy, acceptance, fear, submission, sadness, disgust, anger, and anticipation, and argued that all human emotions can be derived from these. Psychologists Sylvan Tomkins (1963) and Paul Ekman (1982) have contended that "basic" emotions can be quantified because all humans employ the same facial muscles when expressing a particular emotion. Studies done by Ekman suggest that muscular feedback from a facial expression characteristic of a certain emotion results in the experience of that emotion. Since emotions are abstract and subjective, however, they remain difficult to quantify: some theories point out that non-Western cultural groups experience emotions quite distinct from those generally seen as "basic" in the West.

Affective aspect of consciousness. The emotions are generally understood as representing a synthesis of subjective experience, expressive behaviour, and neurochemical activity. Most researchers hold that they are part of the human evolutionary legacy and serve adaptive ends by adding to general awareness and the facilitation of social communication. Some nonhuman animals are also considered to possess emotions, as first described by Charles Darwin in 1872. An influential early theory of emotion was that proposed independently by William James and Carl Georg Lange (1834–1900), who held that emotion was a perception of internal physiological reactions to external stimuli. Walter B. Cannon questioned this view and directed attention to the thalamus as a possible source of emotional content. Later researchers have focused on the brain-stem structure known as the reticular formation, which serves to integrate brain activity and may infuse perceptions or actions with emotional valence. Cognitive psychologists have emphasized the role of comparison, matching, appraisal, memory, and attribution in the forming of emotions. All modern theorists agree that emotions influence what people perceive, learn, and remember, and that they play an important part in personality development. Cross-cultural studies have shown that, whereas many emotions are universal, their specific content and manner of expression vary considerably.

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