The Biology of Emotions

The scientists examining the biology of emotions have discovered some similarities between the brains of humans and animals that help to explain the basic primary emotions. Emotions seem to arise from the parts of the brain that are located below the cortex and are part of the limbic system. These regions of the brain have remained intact across many species throughout evolution. So far, the amygdala has been identified as the central site of emotion. This almond-shaped structure is at the center of the brain. Neuroscientists have found that rats will show a pattern of fear when a particular section of the amygdala is stimulated. If the amygdala is damaged, a rat will not show the normal behavioral responses to danger, such as freezing or running away. The rat with a damaged amygdala also will not demonstrate the accompanying physiological reactions to danger, such as increased heart rate or blood pressure. Research with humans has highlighted the amygdala's critical role in the learning of emotional associations and the recognition of emotional expressions in other individuals. Magnetic resonance imaging studies have shown that the amygdala shows activation to fearful stimuli. In humans, the brain is also involved in the control of emotional facial expressions. Smiles that occur spontaneously as a result of genuine happiness are involuntary. The extrapyramidal motor system, which depends on subcortical areas, governs involuntary smiles and fear reactions.

The chemistry of the brain also plays an important part in animal and human emotions. The neu-rotransmitter dopamine is released in copious amounts during periods of pleasure and excitement. Researchers have found that rats experience an increase of dopamine when engaging in activities that appear to suggest play. Research has also shown that if dopamine production is blocked in rats through the administration of a dopamine-blocking agent, the rat's play activity disappears. The effects of the hormone oxytocin have been studied in small mammals and appear to be related to sexual activity and bonding behaviors. In humans, oxytocin is released in mothers who are nursing their infants and is considered to aid in the mother-child bond. Researchers have investigated the role of oxytocin in bonding among voles. If a female vole is injected with oxytocin, the animal will quickly select a mate. When a female vole is given a drug to block oxytocin, however, mate selection never takes place.

Many scientists contend that it is illogical to believe that emotions appear suddenly in humans. If evolution takes place through the process of natural selection, the emotions found in humans would be present in early evolutionary ancestors. The similarities in brain anatomy and chemistry between animals and humans would then support the idea that some basic emotions exist in various species. Darwin believed that some facial expressions in humans are universal. These expressions are genetically determined and evolved as the most effective at telling others something about how a person is feeling. Research with infants shows the innate capacity to grimace in pain or to smile in pleasure. For the most basic emotions, people in all cultures show similar facial responses to similar emotional situations. For example, anger is linked with a facial expression recognized by almost all cultures. Perhaps it is this line of rea soning from the evolutionary context that provides the strongest support for the existence of a wide range of emotional reactions in animals.

See also: Apes to hominids; Brain; Communication; Evolution: Historical perspective; Hormones and behavior; Instincts; Learning; Offspring care; Pair-bonding; Nervous systems of vertebrates; Reproductive strategies; Rhythms and behavior; Territoriality and aggression.

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