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ELATION, COMPASSION, SADNESS, FEAR, and anger are commonly considered examples of emotions. These states have an enormous impact on our behavior and well-being. But what exactly is an emotion? Distinguishing different emotion states is difficult and requires an account of the environmentally or internally generated challenge an organism faces as well as its physiological responses. For example, before we can conclude that a rat is in a state of fear, we need to know that the rat is evaluating a specific threatening stimulus (a predator in its environment) and is mounting an adaptive response, such as high arousal and freezing.

Emotions are often represented along two dimensions: valence (ie, pleasantness to unpleasantness) and intensity (ie, low to high arousal), called “core affect” in many psychological theories. However, emotions can also be grouped into categories, such as categories of basic emotions (happiness, fear, anger, disgust, sadness) and categories of more complex emotions that help regulate social or moral behaviors (eg, shame, guilt, embarrassment, pride, jealousy). There is considerable debate about whether all the categories that are in common usage (like the ones just mentioned) will correspond to scientifically useful categories in a future neuroscience of emotion.

Within experimental contexts, the term emotion is used in several different ways, often related to the ways in which emotion is measured (Box 42–1). In everyday conversation, most people use the term “emotion” synonymously with “conscious experience of emotion” or “feeling,” and most psychological studies in humans have focused on this sense of “emotion” as well. Most research in animals has focused instead on specific behavioral or physiological responses, in good part because it is impossible to obtain verbal reports in animal studies. Yet emotions have been conserved throughout the evolution of species, as Charles Darwin first observed in his seminal book, The Expression of the Emotions in Man and Animals. The empirical approach we describe in this chapter thus considers emotions as central brain states that can be studied in humans as well as many other animals, provided that we distinguish between emotions and feelings.

Box 42–1 Ways of Measuring Emotion Measures Commonly Used in Humans

  • Psychophysiology. Psychophysiology uses several measures to assay the physiological parameters associated with emotional states. These measures include autonomic responses (Chapter 41) as well as some somatic responses. The most commonly used measure is the galvanic skin response (also known as the skin conductance response), a measure of sympathetic autonomic arousal derived from the sweatiness of the palms of the hands. Other measures include heart rate, heart rate variability, blood pressure, respiration, pupil dilation, facial electromyography (EMG), and the startle response (see below). Some of these measures mostly correlate with basic dimensions of emotion, such as valence (eg, the magnitude of the startle response) or arousal (eg, the galvanic skin response), whereas others (eg, facial EMG) can provide more fine-grained information about emotions. Facial expression has been ...

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