The limbic system in the brain helps mediate attention, memory, learning, and emotion. It lies beneath the cerebral cortex and is considered a more “primitive” system than the cortex, which is the seat of “higher” level brain processes such as reasoning, personality, and perception. Two important, paired limbic structures for those interested in memory are the hippocampus and the amygdala. Both structures impact memory with the hippocampus involved in memory for detached facts (e.g., the capitol of Italy, your address) and the amygdala involved in the emotional side of memory (e.g., fear, pain, disgust, pleasure).
The hippocampus has been much more the focus of attention in the study and treatment of memory as it underlies the ability to learn new things (e.g., learn a new name, learn a new route). Much of the stuff learned and remembered by the hippocampus is acquired slowly with effort (e.g., school learning, learning your way around a new city). We also have a brain system that operates quickly to identify the importance and possible threat of situations even before we are consciously aware. It requires no effort and is automatic and may circumvent the hippocampus. Indeed, it would do no good to become fearful after the lion attacks you. This is the role of the amygdala.
One way to determine the function of brain structures is to study individuals who have selective damage to the area of interest. HM is the most famous example of a case study of the effects of removal of the hippocampus, dense anterograde amnesia (i.e., difficulty learning new things) similar to that found in diseases like Alzheimer’s (Patient H.M., Luke Dittrich, 2016). S.M. has been a more recent object of research as she does not have a functioning amygdala. She appears to have a full range of emotions related to happiness and sadness but does not experience fear.
She was exposed to snakes, spiders, haunted houses, horror movies as well as queried into her fear of death and public speaking. None of these situations elected fear. For example, when asked she clearly states that she hates spiders and snakes. However, she can’t stay away from them in a pet store to the point that she touches them. She is perplexed by her own behavior. She lacks the skills necessary to detect and avoid danger. With damage to the amygdala, previously threatening stimuli are treated as benign.
Alternatively, persons suffering from posttraumatic stress disorder (PTSD) display an overactive amygdala. They perceive threat in benign situations (e.g., startle easily, are constantly scanning for danger). In short, dampening of the amygdala decreases fear learning and activating the amygdala promotes fear learning.
In short, the amygdala is particularly sensitive to “unsettling” circumstances. It directly receives sensory stimuli before it can reach the cortex and consciousness. It is involved in aggression, fear, anxiety, panic, disgust, and learning/memory. Most of the research on the amygdala has focused on threatening circumstances. The amygdala also activates in intense/important positive situations. It allows you to find the love of your life across a crowded room.
We know this from brain mapping studies that demonstrate that the amygdala responds to joyful as well as threatening stimulations. This is suggested by lateralization studies that differentiate the left from the right amygdala. As a brief aside, this raises the question of the importance of lateralization of brain function. The concept of right, left brain has been exaggerated by many (creativity does not reside in the right brain). The two sides of the brain display complementary functions. In real life behaviors such as emotion, learning, memory, and perception involve multiple integrated brain systems or networks and lateralization is subtle. (Indeed, Robert Sopolsky chose not to address laterality in his impressive Behavior, the biology of human behavior at our best and worst, 2017.)
Research into the actions of the amygdala add to the growing knowledge that memory has multiple brain systems that complexly interact in creating experience such as memory. Behavior is like symphony orchestra. When all of the diverse components (e.g., strings, brass, percussion, wind) are integrated correctly beautiful music is the result rather than noise. The amygdala is an instrument of the brain that allows us to quickly judge the importance of new experiences and the routes for their retention.
Dr. Beckwith is a neuropsychologist, speaker, and author of Managing Your Memory: Practical Solutions for Forgetting. He can be reached at
memoryseminars@aol.com. Visit Dr. Beckwith on Facebook at the Life and Memory Center or at www.lifeandmemorycenter.com.
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