The phylogenetically ancient Amygdala: The neuroanatomical correlate of Fear, Anxiety, and Aggression

 

The amygdala (also corpus amygdaloideum; Greek, ἀμυγδαλή, amygdalē, ‘Almond’, ‘tonsil’) is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain in complex vertebrates, including humans. Shown in research 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.

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 is associated with negative emotion. t 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.

www.brainfacts.org/3d-brain#intro=false&focus=Brain-limbic_system-amygdala

Structure

Subdivisions of the mouse amygdala

The regions described as amygdala nuclei encompass several structures with distinct connectional and functional characteristics in humans and other animals. 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.

MRI coronal view of the amygdala

MRI coronal view of the right amygdala

Anatomically, the amygdala, and more particularly its central and medial nuclei have sometimes been classified as a part of the basal ganglia.

Hemispheric specializations

There are functional differences between the right and left amygdala. 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. Other evidence suggests that the left amygdala plays a role in the brain’s reward system.


Further References

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