The amygdala is a relatively small structure composed of several nuclei located deep within the temporal lobe. The name amygdala is derived from the Greek word amygdale, which means “almond,” denoting its general size and shape. Although its Latin name is corpus amygdaloideum, it is also called the amygdala nuclei, amygdaloid region, and corpus amygdalae. The amygdala is considered a component of the limbic system; however, the criteria for determining what is part of the limbic system are not uniformly accepted among scientists. Nonetheless, the amygdala has been clearly associated with certain emotions, especially fear. This association with emotions has determined its classification as a limbic system component.
The first scientists to formally recognize the amygdala in the early 19th century named it for its size and shape since its function was still unknown. Until recently, researchers had only vague notions about the amygdala and were not interested in exploring its function. Early studies suggested that it was tied to vigilance and attention, much like an alarm system. More recent research has confirmed this as one of its many functions and has explored further connections that allow it to participate in a wide range of behaviors such as fear conditioning and decision making. The simplest summary of the amygdala’s function is its ability to recognize and respond to fear stimulus. Further research, however, is continuing to expand knowledge of its function and connections with other regions of the brain, and many researchers are finding that its function extends beyond simple emotion.
Anatomy and Physiology
The number of nuclear groups associated with the amygdala varies depending on how neurologists choose to split up the 13 interconnected nuclei. To date, the number of groups may range from about three to six. Without a standard set of terms that all scientists agree upon and refer to in their studies, literature on the amygdala can be tricky to decipher. Scientists have not yet agreed upon anything because neuroimaging technology is not precise enough for the complexity of the connections of the amygdala. This makes it difficult to separate nuclei into clearly bordered regions. Most authors subdivide it into three portions: (1) the basolateral nuclear group, which is the largest portion; (2) the centromedial group, the second largest portion; and (3) the cortical nucleus, also known as the olfactory amygdala for its connections with the olfactory bulb and olfactory cortex.
The stria terminalis and the ventral amygdalofugal pathway are two main fiber bundles connecting the amygdala to other parts of the brain. The stria terminalis enables the amygdala to activate the hypothalamus, which activates the autonomic nervous system, or the “fight-or-flight” response. The ventral amygdalofugal pathway connects the amygdala to the brainstem, affecting hormones and behavior such as eating, drinking, and sex. The basolateral group of the amygdala is directly connected to the insular cortex, orbital cortex, and frontal lobe. There are also projections to the hippocampus, a part of the brain responsible for storing memories. Some research has also suggested that it affects which memories are stored by categorizing the importance of a memory. Neuroimaging studies indicate that the amygdala helps to recognize both basic and complex facial emotions through access to emotional memory in the hippocampus.
Individual neurons within the amygdala have been found to have specific functions. Some neurons respond separately to sight, sound, smell, taste, and touch stimuli. Some also respond specifically to faces. The most abundant type is the visual responder neurons, which may explain why the strongest neural activity in the amygdala occurs in response to fearful faces. Studies in which only selective parts of a person’s face, such as the eye or mouth region, were revealed show that the amygdala responds more strongly to expression in the eye region than any other part of the face.
The amygdala, particularly the central nucleus, has been shown to play a role in detecting and resolving ambiguity, that is, when a situation can be interpreted in more than one way. For example, because fearful expressions recruit a stronger response from the amygdala than angry expressions, scientists hypothesize that fearful expressions signal ambiguity in a situation—they present a sign of imposing threat without giving very much information about the threat. Angry expressions cause a response in the amygdala to a lesser degree because anger in the eyes is less ambiguous than fear in the eyes.
Once the amygdala is activated by a frightening stimulus, it begins the process of attention. Amygdala experiments with cats have shown that after a loud noise, the cat ceases all activities, perks up its ears, and orients itself in the direction of the noise. This series of actions is termed “attention response” or “arrest response.” The researchers conducting this experiment hypothesized that this is the amygdala’s attempt to resolve the ambiguity of the noise, urging the cat to see if there is a threat by finding out where the noise came from.
Alyssa M. Wienecke
See also: Autonomic Nervous System; Brain Anatomy; Limbic System; Olfactory System
Cohen, Lisa J. (2011). The handy psychology answer book: Your smart reference. Detroit, MI: Visible Ink Press.
Díaz-Mataix, Lorenzo, Lucille Tallot, & Valerie Doyère. (2014). The amygdala: A potential player in timing CS-US intervals. Behavioral Processes, 101, 112—122. http://dx.doi.org/10.1016/j.beproc.2013.08.007