The amygdala---or, more appropriately, amygdalae, as there is one in each cerebral hemisphere---was not recognized as a distinct brain region until the 1800s, and it wasn't until the middle of the twentieth century that it began to be considered an especially significant area in mediating emotional responses. Specifics about the role of the amygdala in emotion remained somewhat unclear, however, until the 1970s and 1980s when it was studied in fear conditioning experiments in rodents. A typical fear conditioning experiment in rodents involves pairing an aversive stimulus (e.g. an electrical shock to the feet) with a previously neutral stimulus like an audible tone until the rodent begins to display signs of fear at simply hearing the tone. Using this experimental approach, researchers were able to demonstrate that functioning amygdalae are very important for rodents to learn the fear responses typically seen as a result of fear conditioning.
From this time on, research began to accumulate that identified the amygdala as having an integral role in fear in general. And thus was born the conception of the amygdala as a "threat-detector." According to this view, the amygdala helps us to identify threats in our environment and---if threats are present---to initiate a fight-or-flight response. This basic understanding of the function of the amygdala is repeated in many textbooks and classrooms---and has even found its way into popular culture. The problem is, however, that this is an oversimplified view of the amygdala. Yes, the amygdala seems to play a significant role in fear. But it is also likely involved in a slew of other behaviors and emotional responses.
An intricate structure with manifold connections
The name amygdala comes from the Greek word for almond, and the amygdala earned this designation because it is partially composed of an almond-shaped structure found deep within the temporal lobes. The almond-shaped structure, however, is just one nucleus of the amygdala (the basal nucleus)---for although it is often referred to as one entity, the amygdala is actually made up of a collection of nuclei along with some other distinct cell groups. The nuclei of the amygdala include the basal nucleus, accessory basal nucleus, central nucleus, lateral nucleus, medial nucleus, and cortical nucleus. Each of these nuclei can also be partitioned into a collection of subnuclei (e.g. the lateral nucleus can be divided into the dorsal lateral, ventrolateral, and medial lateral nuclei).
Exactly how the amygdala should be divided anatomically has been the subject of some debate, and no clear consensus has been reached. Many researchers group the lateral, basal, and accessory basal nuclei together into a structure referred to as the basolateral complex, and sometimes the cortical and medial nuclei are aggregated as the cortico-medial region. However, there is even a lack of consistency in the application of these terms. For example, some investigators use the basolateral designation to refer to the complex mentioned above, while others use it to refer to just the basal nucleus or basolateral nucleus specifically. Thus, the anatomy of the amygdala is much more complex than is often implied in simple descriptions of the structure. Indeed, the complexity is significant enough that neuroanatomists still have a hard time agreeing on how the different components of the amygdala should be categorized.
In addition to its anatomical diversity, the amygdala has abundant connections throughout the brain---connections that are widespread and divergent enough to suggest many functions beyond just threat detection. For example, many areas of the prefrontal cortex as well as sensory areas throughout the brain have bidirectional connections with the amygdala. The amygdala also has projections that extend to the hippocampi, basal ganglia, basal forebrain, hypothalamus, and a variety of other structures.
Evidence for diversity of function
It is true there is ample evidence that suggests the amygdala is important in the processing of fearful emotions and the identification of threatening stimuli. However, there is also a significant amount of evidence pointing to functions for the amygdala beyond simple threat detection. For example, studies have found the amygdala to be active not just during fear conditioning, but also when learning to link a previously neutral stimulus with a positive experience. Indeed, these studies suggest the amygdala may be involved in learning to assign a positive or negative value to a neutral stimulus, suggesting it has a role in assigning value in general and in the formation of positive and negative memories.
Due to its role in assigning value to stimuli and then creating memories about such valuations, it may not be surprising that some have implicated the amygdala in addictive behaviors. The amygdala has been shown to interact with reward areas of the brain like the ventral striatum, and it seems to play an important role in forming memories associated with drug use. Studies have found, for example, that disrupting amygdala function can inhibit the ability of rodents to learn positive associations with drugs like cocaine. Thus, disrupting activity in the amygdala can also disrupt the acquisition of drug-taking behavior in rodents.
Therefore, instead of being involved only with aversive memories and the learning of conditioned responses to fearful stimuli, the amygdala has come to be considered an important region for the consolidation of memories that have any strong emotional component---whether positive or negative. And this is still really only scratching the surface of the function of this complicated structure. Some studies have suggested, for example, that the amygdala plays a key role in social interaction, others have linked it to aggressive tendencies, and still others have indicated that amygdala connectivity may help to predict sexual orientation.
It may be involved with all of these things. Because the amygdala is a complex structure made up of multiple nuclei, it is unlikely it would serve only one function like "fear detection." Indeed, it is probably unlikely it would even be involved with only one large category of function like emotions. Simplifying the functions of a structure like the amygdala does help to make the brain easier to understand on a superficial level, but it's important to keep in mind that when we do so we are avoiding a more complicated reality in order to make the details of the organ more comprehensible. Although this can be a useful tactic, if we forget we are using it we can hinder the attainment of a more complete understanding of a structure by focusing too much on the simplified model.
LeDoux J. The amygdala. Curr Biol. 2007 Oct 23;17(20):R868-74.
Watch this 2-Minute Neuroscience video to learn more about the amygdala.