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Published on: April 13, 2012
by Life Extension
Amnesia occurs when the portion of the brain responsible for retrieving stored memories is somehow compromised. This region of the brain is known as the limbic system; it comprises the hippocampus, the amygdala, and portions of the cortex.
Besides retrieving memory, the limbic system is responsible for coordination of emotion and motivation and for some of the functions of the endocrine system.
People are amnesiac when the memory retrieval portion of the limbic system isn’t working properly but there is otherwise no change in language, attention span, visual/spatial functioning, or motivation.
Memories are not actually stored in the limbic system or the hippocampus. Rather, several areas of the brain are involved in memory; the type of information being assimilated determines where it is stored. For example, visual and auditory patterns are stored in the temporal lobe, whereas the parietal lobe stores language, speech, word usage, and comprehension.
Forms of Amnesia: Different Ways to Forget
There are two types of memory. Short-term or “working” memory stores information one needs to remember in the next few seconds, minutes, or hours (e.g., a telephone number or driving directions). Long-term memory includes relational and procedural memory. Relational memory is concerned with relationships among objects and depends on the hippocampus.
In amnesia, both relational memory and short-term memory may be impaired. Procedural memory represents memory for single objects or tasks (e.g., riding a bicycle) and depends on cortical processors that remain intact in amnesia. This helps explain why amnesiacs often remember basic skills and motor function.
There are several forms of amnesia:
There are many possible causes of amnesia. The most common include Alzheimer’s disease, traumatic brain injury (head trauma), brain infection (such as encephalitis or meningitis), dementia, seizures, and stroke. Less common causes include a brain tumor or psychiatric disorders (schizophrenia, depression, criminal behavior, or psychogenic amnesia).
Psychogenic amnesia usually happens in close association with a stressful event that involves serious threat to life or health. Criminals frequently present with amnesia: reports indicate that 23 percent to 65 percent of murderers claim amnesia for their crimes (Taylor PJ et al 1984).
Amnesia can occur because of brain damage that interferes with memory storage, retrieval, or consolidation. What ultimately causes the memory loss—a failure to store memories or a failure to retrieve them—remains unclear. However, a study using rats suggested that memory loss is probably due to an error in memory retrieval, which explains why amnesiacs can usually recover their memories (de Hoz L et al 2004).
Amnesia is also a symptom of Wernicke-Korsakoff syndrome. Wernicke-Korsakoff is caused by a severe thiamine (vitamin B1) deficiency due to chronic alcoholism or malnourishment. Thiamine is necessary for the body to process carbohydrates. Besides amnesia, symptoms of Wernicke-Korsakoff include confusion, loss of balance, drowsiness, and problems with vision, such as double vision or rapid movement of the eye.
In severe cases, the memory loss may be accompanied by agitation and dementia.
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