Tuesday, February 21, 2012

Limbic system Dysfunction

In 1953, a young man known as H.M. received bilateral removal of the amygdala and large parts of the hippocampus as a treatment for persistent, untreatable epilepsy. Although his epileptic condition was improved after this surgery, he was now afflicted with a specific type of amnesia in which new memories could not be formed due to limbic system dysfunction, which was the result of the surgery. He still had a normal IQ and a normal working memory. He could retain information for minutes as long as he was not distracted. However, he could not form long-term memories. If he was introduced to someone on one day, on the next day he did not recall having previously met that person. Nor could he remember any events that occurred after his surgery, although his memory for events prior to the surgery was intact. The case of H.M. illustrates that formation of declarative memories requires limbic structures of the temporal lobe.

Interestingly, H.M. had normal procedural memory, and could learn new puzzles and motor tasks as readily as normal individuals. This case was the first to draw attention to the critical importance of the temporal lobe in the formation of long-term declarative memories and to suggest that structures in this region are necessary for the conversion of short-term into long-term memories. Additional cases now demonstrate that the hippocampus is the primary structure involved in this process. Since H.M. retained memories from before the surgery, the hippocampus is not involved in the storage of declarative memories.

In another case, the case of S.M., illustrates that the amygdala processes fearful emotions. The patient in this study suffered from a rare disease (Urbach-Wiethe disease) in which the anterior and medial portions of the temporal lobe became atrophied, essentially completely destroying the amygdala bilaterally. Intelligence and memory formation remained intact. However, this individual now lacked the ability to express fear in appropriate situations and to recognize fearful expressions in other people. Therefore, in humans the amygdala is important for at least one emotion, fear.