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Located in the limbic system of the brain, the amygdala is a brain structure involved in processing emotional stimuli, as well as committing them to memory. Amygdala damage may have several different effects, including an inability to create or execute fear responses to specific stimuli. Effects may also include lowered expressions of maternal behavior. Risky behaviors like gambling with poor odds may also increase following damage to this area of the brain.
Normally, the amygdala helps the brain to encode and express fear responses. After exposure to threatening stimuli, the brain memorizes these conditions, and a reappearance of the stimulus will elicit a fear reaction. Some damage will extinguish this response entirely. An individual with this damage will not associate fear responses with stimuli. Previously acquired fear responses will not be expressed in the presence of stimuli, either.
Emotions other than fear are mediated in this area. In studies involving monkeys, amygdala damage caused mothers to stop displaying maternal behavior. Mothers instead attacked or abused their babies, in some cases. Lesions to the entire amygdala on both sides of the brain led to Kluver-Bucy syndrome in monkeys. This disorder causes a lack of aggression and fear, as well as inappropriate sexual behavior.
Helping individuals to recognize emotions is another function that can be affected by damage. People that had amygdala damage on both sides of the brain were unable to recognize emotions related to facial expressions. At times, they would even confuse angry expressions with smiles. When these individuals were shown emotional expressions without faces, they were able to recognize the emotion shown. This result shows that the brain has other centers that help individuals to recognize emotions.
Loss aversion is a term that describes the avoidance of taking risks, especially when these risks would not result in significant gain. Amygdala damage in humans created an inability to practice loss aversion. These people tended to take large risks in gambling, even when they would not earn much money from winning. Risk-taking that did not have the potential for the individual to lose something was not affected.
Detecting other people in one's personal space is another function of the amygdala that may be affected. When the amygdala is damaged on both sides of the brain, people do not react to others standing in their personal space. This effect may be due to individuals not differentiating between standing right next to someone and standing some distance away from them.