Amygdala

An amygdala is a small, almond-shaped group of neurons. One amygdala is located in each temporal lobe of the brain. The amygdalae are part of the limbic system, which is responsible for regulating numerous emotional responses including fear, anger, pleasure, hunger, and sexual reproduction. The amygdalae are closely associated with fear. They are the reason humans fear events that they cannot control.

In the earliest days of studying the amygdalae, scientists were unable to study live human brains. They conducted experiments on rats instead. After having their amygdalae removed, the rats showed no fear at all; they were not even afraid of predators such as cats.

Scientists conducted numerous, more complicated experiments on primates. When an amygdala was removed or damaged, researchers noted that the primate displayed a distinct lack of fear responses. As further tests were conducted, researchers came to theorize that the amygdalae were important in processing emotions associated with survival.

The invention of functional magnetic resonance imaging (fMRI) devices allowed researchers to view live images of the brain of a living human without harming the subject. Through fMRI research, scientists discovered that the amygdalae became increasingly active during times when the subject presented a fearful facial expression. This study suggested that in humans the amygdalae were associated with fear. Later research showed that the amygdalae are essential in the fear-response process. This process pairs a memory with a negative result, teaching the brain to fear the original event to avoid the negative result.

Studies show that humans with damaged amygdalae have difficulty processing or feeling several emotions. In many cases, they have difficulty recognizing and interpreting the others' facial expressions. They may also have trouble understanding or empathizing with others' emotions.

Malfunctioning or damaged amygdalae may also be responsible for numerous mental illnesses. Researchers suspect that a malformed, enlarged, or hyper-functioning amygdala may be responsible for numerous anxiety-related disorders. These include obsessive-compulsive disorder (OCD), borderline personality disorder (BPD), and social anxiety disorder (SAD).

Background

In the early twentieth century, scientists and medical researchers held a limited understanding of the brain and its functions. One way scientists were able to study the brain was through dissecting the brains of mammals. Because the brains of apes are so similar to those of humans, scientists were able to draw conclusions about the human brain from experiments on apes. Experimenting on living humans was considered immoral, and the brains of deceased humans provided extremely limited information.

In 1939, neuroscientists Henrich Kluver and Paul Bucy were experimenting with the behavioral effects of surgery on the brains of primates. Their experiments showed that surgically removing the primates' temporal lobes resulted in numerous distinct behavioral changes in the animals. The primates reacted in an entirely neutral manner toward humans who had elicited an extremely fearful response prior to the surgery. Additionally, the primates conducted themselves in a more tame, less aggressive manner overall.

Neuroscientists continued to study the temporal lobe, quickly realizing that modifying it profoundly changed the way emotion was experienced. In 1952, the scientist Lawrence Weiskrantz conducted additional surgical trials on primates. However, Weiskrantz did not remove the entire temporal lobe. Instead, he removed only the parts of the temporal lobe that would later be called the amygdalae. Weiskrantz quickly noted that upon recovery, the surgically altered primates exhibited the same lack of fear as those altered by Kluver and Bucy.

Based on the results of these experiments, many neuroscientists theorized that the amygdala was responsible for fear and other base emotions. However, because the surgical techniques used in those procedures caused substantial damage to other parts of the brain, the resulting changes could not conclusively be determined as the result of the removed amygdalae. Another less-understood part of the brain could have been damaged during the surgery, leading to the dramatic results exhibited after the procedure.

In 1999, neuroscientists led by Mortimer Mishkin used a new surgical technique that disabled the amygdala without badly damaging the connective fibers surrounding it. After recovering from surgery, the primates showed significantly reduced symptoms when compared to the results of previous experiments. This showed scientists that some of the symptoms recorded in earlier experiments resulted from damaged tissue surrounding the amygdala, not the amygdala itself. However, the primates did exhibit significantly reduced fear and other emotions, showing that the amygdala was essential for normal emotional processing.

Amygdala research in humans has begun relatively recently. In prior scientific eras, the inability to dissect a living human brain made research into emotional processing difficult. However, new digital imaging technologies have allowed neuroscientists to study the brains of living humans without surgery. When using functional magnetic resonance imaging (fMRI), scientists in numerous studies have observed an increase in amygdalae activity during experiences that evoked fearful facial expressions.

Additional studies showed that humans with damaged amygdala tend to show a number of other difficulties resulting to emotion. Scientists have observed that individuals with severely or nonfunctioning amygdalae often have trouble identifying others' emotions. When shown pictures of other people exhibiting a variety of facial expressions, individuals with nonfunctioning amygdalae are often unable to pair an emotion with a facial expression. They tend to be particularly confused by expressions of fear.

Overview

The amygdalae are part of the limbic system. The limbic system is a specific group of brain structures located on top of the brainstem and under the cortex. Most limbic structures are involved in processing emotions associated with survival. These include fear, anger, hunger, sexual reproduction. Many parts of the limbic system are also responsible for memory storage.

In addition to the amygdalae, the limbic system encompasses numerous important parts of the brain. These include the cingulate gyrus, which is associated with sensory input related to aggressive behavior, the hippocampus, which acts as a memory indexer, the hypothalamus, which helps regulate emotions and adrenaline, the olfactory cortex, which is involved in the identification of odors, and numerous areas of connective tissues between brain regions.

The amygdalae are two almond-shaped clusters of neurons located inside the medial temporal lobes. Each node is a cluster of nuclei, which can be divided into several complexes. The largest complex, the basolateral complex, is connected to several other parts of the limbic system. Other sections connect to several other parts of the brain, including the endocrine system, the hypothalamus, and the brainstem.

The amygdalae are involved in regulating or causing numerous biological functions in the human body such as fear, anger, various hormone secretions, arousal, and memory. Studies suggest that the amygdalae are essential to triggering various emotional states.

Recent studies have shown that the amygdalae are responsible for fear conditioning, the process by which a person learns to fear something over time. It causes humans to associate a stimulus with an unpleasant result, leading to a sudden, negative reaction whenever that stimulus is encountered. It helps humans learn to avoid situations that they believe have been harmful in the past.

The amygdalae are instrumental in activating the fight-or-flight response. Once a stimulus associated with fear has been encountered, it activates the sympathetic division of the peripheral nervous system. This activation results in the symptoms of the fight-or-flight response, which include increased blood flow to muscles, accelerated heart rate, and dilated pupils. It helps humans defeat a threat or flee from a perceived threatening situation.

A damaged amygdala can result in numerous psychological abnormalities. The amygdalae can become damaged or nonfunctional in several ways. A stroke or an aneurism can severely damage the amygdalae. In other cases, the amygdalae may become partially damaged by disease or trauma, such as a seizure, a lack of nutrition, and even excessive stress. In rare cases, genetic abnormalities can cause an individual to lose total function in his or her amygdalae. This causes extreme difficulty in recognizing facial expressions and in understanding others' emotional reactions. It also leads to an inability to feel fear, despite rationally understanding that a situation is potentially threatening. Abnormal amygdalae may also play a part in several mental illnesses. Experts believe that hyperactive, malformed, or unevenly sized amygdalae may play a role in numerous anxiety disorders, including post-traumatic stress disorder (PTSD), social anxiety disorder (SAD), obsessive compulsive disorder (OCD), and borderline personality disorder. In these situations, a hyperactive or malfunctioning amygdala activates the sympathetic division of the peripheral nervous system when no genuine external threat is present. This causes the afflicted individual to form a fear response to a nonthreatening event.

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