The amygdala is one of two clusters of nuclei in the brain’s temporal lobes. They have an almond-like shape and are responsible for many important functions that involve emotional responses, decision-making, and processing memory. One amygdala lies in either hemisphere of the brain, and research suggests they are heavily involved with stress, anxiety, and fear. The amygdala remains relatively unknown, though there are many prevailing theories and ideas about the roles that this area of the brain plays.
The amygdala is composed of at least 13 subnuclei. Of these subnuclei, experts understand three better than the others. The central nuclei regulate the fear response and control the release of certain hormones. The basal and lateral nuclei assist with learning and associative processes. Notably, the lateral nuclei receive audio and visual cues that may result in conditioned fear responses. Some experts classify the amygdala and its nuclei as part of the basal ganglia.
To perform its many duties, the amygdala communicates with other areas of the brain and the nuclei within itself. For example, the amygdala can receive inputs from sensory organs; the olfactory bulb and olfactory cortex send scent inputs here. The nuclei within the amygdala take these inputs and formulate a response. The amygdala's additional involvement in the formation of emotional memories may explain why we recall specific events after smelling a certain scent.
Medical professionals can easily determine the difference between male and female amygdalae. By the age of 11, the male amygdala has become larger than the female counterpart. Additionally, memory seems to operate more in the left amygdala in females but more in the right in males. One study found evidence that female amygdalae allow females to retain stronger memories for emotional events than males. The right amygdala also leads to taking action, which may explain why males tend to respond physically to emotional events.
Beyond the differences in male and female amygdalae, each performs different functions depending on the hemisphere in which it resides. For example, the left amygdala can induce happiness as well as negative emotions such as fear, anxiety, and sadness. It is possible that the left hemisphere amygdala helps the reward system of the brain. The right hemisphere amygdala can only induce negative emotions, with fear and sadness being the most prevalent, explaining its connection to fear conditioning and the fear response.
Fear conditioning is a complex form of learning that develops following punishment for an action. Quickly or eventually, depending on the severity of the event, the brain learns to predict the outcome of similar actions. If a person touches a hot stove and injures themselves, they can predict the outcome of touching hot objects in the future. This goes for any negative stimulus, including loud noises, unpleasant odors, and pain. The amygdala is responsible for fear conditioning. Experts believe the brain stores fear memories in the connections between the nuclei in the amygdala.
The amygdala also performs positive or appetitive conditioning. Researchers do not understand the amygdala’s role in appetitive conditioning as well as they understand its role in fear conditioning. However, they do believe different nuclei within the amygdala trigger different responses and have unique functions. Through testing rodents, researchers found that pheromones trigger the amygdala nuclei, resulting in attraction. Appetitive conditioning is how the brain’s reward system functions. When a person succeeds or enjoys an activity, the amygdala sends projections that allow the release of dopamine. This is why people find overcoming a challenge enjoyable.
One of the amygdala’s primary functions is modulating memory consolidation. After an event occurs, the brain stores the memory temporarily. Over time, it converts the memory information into long-term memory. Researchers do not fully understand this process. During the conversion period, the amygdala can affect or modulate the memory. If the memory was particularly emotional, the amygdala strengthens the memory and allows a person to recall it better. This may explain why post-traumatic stress disorder often involves vivid recollections of past trauma.
Beyond its roles in emotion and memory, the amygdala also appears to respond to social interactions. Amygdala volume correlates directly with the size and complexity of a person’s social network. If a person has many contacts and belongs to many groups, their amygdala is larger and more active. These individuals are can make more accurate social judgments about other people’s faces, such as their trustworthiness and attractiveness. The amygdala also handles reactions involving personal space violations. A recent test found that the amygdala activates when a person notices another individual standing close to them.
The amygdala may also have something to do with a person’s sexual orientation, though there is still a lack of research in this area. Notably, homosexual men and women may have different activations in their amygdala than their heterosexual counterparts. Homosexual males tend to have more activations in their left amygdala than heterosexual men, making their processes more similar to those of heterosexual females. The same is true of homosexual women. Whether this is the reason for a person’s sexual orientation or simply a byproduct of it is not yet known. The same study suggests that the amygdala’s response to pheromones may be part of determining sexual orientation.
In general, more women than men develop anxiety disorders. This may be due to how the amygdala handles stressful experiences. Feelings of anxiety begin with a catalyst -- a sight, smell, or feeling that then results in anxiety. The amygdala handles the catalyst by either preparing to fight or by preparing to run. As part of this process, males of various species produce more serotonin receptors in their amygdalae, whereas females lose them. This may result in males being less responsive to stressful stimuli and feelings of anxiety. The fight or flight response is responsible for the various physical effects that anxiety can have.
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