2-Minute Neuroscience: Amygdala

In this video, I discuss the amygdala. The amygdala is a collection of nuclei found in the temporal lobe; it is best known for its role in fear and threat detection, but its full range of functions is much more diverse. I discuss some of the major nuclei of the amygdala, a common scheme for the anatomical organization of the amygdalar nuclei, and some of the functions that have been associated with the amygdala ranging from threat detection to the processing of positive stimuli.

The amygdala: Beyond fear

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.

Prejudice in the brain

Despite the great strides that have been made toward a more egalitarian society in the United States over the past 50 years, events like what occurred in Ferguson last month are a bleak reminder of the racial tensions that still exist here. Of course, the United States is not alone in this respect; throughout the world we can see abundant examples of strain between different races, as well as between any groups with dissimilar characteristics. In fact, it seems that the quickness with which we form a negative opinion about those who are not members of the same group as us may be characteristic of human nature in general, as its effects have been pervasive throughout history, and it persists even when we attempt fastidiously to stamp it out.

Indeed, it may be that our inclination towards prejudicial thinking has its roots in what was once an adaptive behavior. Some argue that our ancient hominid ancestors may have benefited from living in small groups, as this allowed for joint efforts in gathering and protecting resources. A logical offshoot of the development of group living would have been the emergence of skill in being able to tell members of your group apart from those who were not. It might have paid off to be wary of those who were not part of your group, as they would have been more likely to pose a threat. According to this evolutionary hypothesis, prejudice--which can be defined as an opinion of someone that is formed based on their group membership--may be the result of this strategy being so effective in the past. In essence, we may be saddled with the mindset of our evolutionary ancestors, which makes us more skeptical at first of anyone whom we see as "different" than us.

Prejudice and the amygdala

If prejudice is a deep-seated human behavior, it would not be surprising to find networks in the brain that are selectively activated when someone has xenophobic thoughts. One area of the brain that has been investigated in this context is the amygdala.

The amygdala is often associated with emotion, and is perhaps best known for its role in fear and the recognition of threats. If you were walking in the woods and saw a bear, your amygdalae would immediately become activated, helping to bring about a fear response that would encourage you to run away (or maybe cause you to freeze in place).

Several neuroimaging studies have looked at what happens in the brains of people when they see images of others outside of their racial group (e.g. white people looking at images of black faces). Some findings from these studies include: the amygdala is activated upon seeing such images, amygdala activation is correlated with xenophobic attitudes of the viewer, and amygdala activity in white people is higher when viewing black faces with darker skin tone.

Thus, the amygdala may serve as a threat-detection mechanism that is reflexively activated when we see an outsider. Perhaps because this has been adaptive in the past, it may act to put our brain on alert when someone outside of our racial group is near. In many societies today, however, where we are attempting to make racial divisions less distinct, this knee-jerk reaction seems to be counterproductive.

Prejudice and the insula

Another area of the brain that has been associated with prejudice in neuroimaging studies is the insula. The insula is also involved in processing emotional states, and has been linked to mediating feelings of social disapproval. For example, one study found that the insula and amygdala were activated in individuals while they viewed pictures of of people deemed to be social outcasts, such as homeless people or drug addicts. Because the insula is also activated when viewing pictures of people outside one's racial group, it has been hypothesized that the insula is involved in feelings of distaste that may arise when experiencing prejudicial thoughts.

Prejudice and the striatum

The striatum, a subcortical area thought to play an important role in reward processing, also has been implicated in prejudice--albeit in a very different way than the amygdala and insula. Activity in the striatum correlates with rewarding experiences, and neuroimaging studies have found that the striatum is also activated when looking at pictures of individuals from one's own racial group. When white participants were tested for implicit preferences (i.e. preferences they may not state or even be aware of, but that they still seem to possess) for people of their own race, activity in the striatum was stronger in response to white faces in those who scored higher on the test for implicit preferences.

Thus, there may be activity in the brain that reinforces our tendency toward prejudice in at least two ways: 1) we may be more likely to feel fear and aversion when seeing someone of another race, and 2) we may be more likely to experience positive emotions in response to seeing someone of our own race.

So, if there are structures in our brains that promote prejudice, does it mean attempts to reduce our prejudices--both individually and societally--are a lost cause? Of course not. Just as there are brain structures that may make us more likely to recognize differences, there are also structures (e.g. areas of the frontal cortex) that allow us to exert control over those potentially reflexive reactions.

It's possible that the recognition of deep-seated mechanisms for prejudice could help us to understand racism a little better. It could, for example, provide insight into why people in high-stress situations may be more likely to see things as divided down racial lines. For, if their brains are already inclined to see people of another race as more threatening and they are in a stressful situation, they may be quicker to identify someone of a different race as the threat.

However, the extent to which such innate responses to outsiders affects our behavior is still somewhat unclear, and the hypothesis that such responses are remnants of once-adaptive behavior is just that: a hypothesis. For practical purposes, it may not matter exactly what the basis of prejudicial thinking is, as we are certain it's a thought pattern that doesn't have much remaining value in today's world. However, being open to the idea that we have some inclinations toward prejudicial thinking may help us to be able to train people to more mindfully deal with high-stress interactions with people of another race. For, instead of pretending these prejudicial thoughts don't (or shouldn't) happen, it would allow us to focus more on ways to mitigate the damage that might occur when they do.

Amodio DM (2014). The neuroscience of prejudice and stereotyping. Nature reviews. Neuroscience PMID: 25186236