The amygdala (/əˈmɪɡdələ/; pl.: amygdalae /əˈmɪɡdəli, -laɪ/ or amygdalas; also corpus amygdaloideum; Latin from Greek, ἀμυγδαλή, amygdalē, 'almond', 'tonsil'[1]) is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex vertebrates, including humans.[2] Shown to perform a primary role in the processing of memory, decision making, and emotional responses (including fear, anxiety, and aggression), the amygdalae are considered part of the limbic system.[3] The term "amygdala" was first introduced by Karl Friedrich Burdach in 1822.[4]
The regions described as amygdala nuclei encompass several structures of the cerebrum with distinct connectional and functional characteristics in humans and other animals.[5] Among these nuclei are the basolateral complex, the cortical nucleus, the medial nucleus, the central nucleus, and the intercalated cell clusters. The basolateral complex can be further subdivided into the lateral, the basal, and the accessory basal nuclei.[3][6][7]
Anatomically, the amygdala [8] and more particularly its central and medial nuclei,[9] have sometimes been classified as a part of the basal ganglia.
In one study, electrical stimulations of the right amygdala induced negative emotions, especially fear and sadness. In contrast, stimulation of the left amygdala was able to induce either pleasant (happiness) or unpleasant (fear, anxiety, sadness) emotions.[10] Other evidence suggests that the left amygdala plays a role in the brain's reward system.[11]
Each side holds a specific function in how we perceive and process emotion. The right and left portions of the amygdala have independent memory systems, but work together to store, encode, and interpret emotion.
The right hemisphere of the amygdala is associated with negative emotion.[12][13] It plays a role in the expression of fear and in the processing of fear-inducing stimuli. Fear conditioning, which occurs when a neutral stimulus acquires aversive properties, occurs within the right hemisphere. When an individual is presented with a conditioned, aversive stimulus, it is processed within the right amygdala, producing an unpleasant or fearful response. This emotional response conditions the individual to avoid fear-inducing stimuli and more importantly, to assess threats in the environment.