The amygdala is a collection of nuclei found deep within the temporal lobe of the Brain. The term amygdala comes from Latin and translates to “almond,” because one of the most prominent nuclei of the amygdala has an almond-like shape. Although we often refer to it in the singular, there are two amygdalae—one in each cerebral hemisphere.
What is the amygdala and what does it do?
The amygdala is recognized as a component of the limbic system, and is thought to play important roles in emotion and behavior. It is best known for its role in the processing of fear, although as we’ll see, this is an oversimplified perspective on amygdala function.
Interestingly, research suggests that information about potentially frightening things in the environment can reach the amygdala before we are even consciously aware that there’s anything to be afraid of. There is a pathway that runs from the thalamus to the amygdala, and sensory information about fearful stimuli may be sent along this pathway to the amygdala before it is consciously processed by the cerebral cortex. This allows for the initiation of a fear reaction before we even have time to think about what it is that’s so frightening.
Our modern understanding of amygdala function can be traced back to the 1930s, when Heinrich Kluver and Paul Bucy removed the amygdalae of rhesus monkeys and saw drastic effects on behavior. Among other things, the monkeys became more docile and seemed to display little fear. The constellation of behavior that resulted from amygdalae removal was called Kluver-Bucy syndrome, and it led to the amygdala being investigated for its role in fear.
Since, the amygdala has become best known for its role in fear processing. When we are exposed to a fearful stimulus, information about that stimulus is immediately sent to the amygdala, which can then send signals to areas of the brain like the hypothalamus to trigger a “fight-or-flight” response (e.g. increased heart rate and respiration to prepare for action).
This type of reflexive response can be useful if we really are in great danger. For example, if you are walking through the grass and a snake darts out at you, you don’t want to have to spend a lot of time cognitively assessing the danger the snake might pose. Instead, you want your body to experience immediate fear and jump backward without having to consciously initiate this action. The direct pathway from the thalamus to the amygdala may be one way to achieve this type of response.
In addition to its involvement in the initiation of a fear response, the amygdala also seems to be very important in forming memories that are associated with fear-inducing events. For example, if you take mice with intact amygdalae and play a tone right before you give them an uncomfortable foot shock, they will very quickly begin to associate the tone with the unpleasant shock. Thus, they will display a fear reaction (e.g. freezing in place) as soon as the tone is played, but before the shock is initiated. If you attempt this experiment in mice with lesions to the amygdalae, however, they display an impaired ability to “remember” that the tone preceded the foot shock. You can play the tone and they will continue about their business as if they have no bad memories associated with the noise.
It shouldn’t be too surprising (given its role in fear processing) that the amygdala might also play a role in anxiety. While fear is considered a response to a threat that is present, anxiety involves the dread that accompanies thinking about a potential threat—one that may or may not ever materialize. A number of studies suggest that the amygdala is involved in experiencing anxiety, and that it may be overactive in people with anxiety disorders. However, as is the case with most human behaviors, anxiety likely involves a network of brain areas, so activity in the amygdala doesn’t tell us all we need to know about the emotion.