The results were published this week in the early online edition of the Proceedings of the National Academy of Sciences.
In a study using rats, researchers Emily L. Malin and James L. McGaugh of UCI's Center for the Neurobiology of Learning and Memory demonstrate that while one part of the brain, the hippocampus, is involved in processing memory for context, the anterior cingulate cortex, a part of the cerebral cortex, is responsible for retaining memories involving unpleasant stimuli. A third area, the amygdala, located in the temporal lobe, consolidates memories more broadly and influences the storage of both contextual and unpleasant information.
"These results are highly intriguing," said McGaugh, a member of the National Academy of Sciences who pioneered the study of drug and stress-hormone influences on memory. "It is the first time we have found this fragmentation in the brain of what we would think of as a single experience. For example, different aspects of an experience, such as a car accident, would be processed by different parts of the brain. The experience is fragmented in our brain, even though we think of it as one event."
According to Thomas J. Carew, Donald Bren Professor and chair of UCI's Department of Neurobiology and Behavior, understanding which parts of the brain process which types of memories gives scientists a better grasp on why particular types of memory impairment can occur and why, for example, different types of strokes might affect different memory systems. "This study is a terrific demonstration of how different components of our neural real estate can be allocated to process different aspects of memory," said Carew. "The more we know about the specialization of memories, the more we can understand how and why the processing of memory can go awry, which in turn can critically inform clinical problems involving a wide range of cognitive deficits."
McGaugh's previous work has shown the key role emotional arousal and the accompanying release of stress hormones play in creating lasting memories. The amygdala has been shown to be activated by the release of these hormones.
ABOUT THE STUDY: In the study, the rats were placed inside a box to familiarize themselves with that context. On the second day, they were confined to a dark compartment of the same box for only a few seconds and given a mild foot shock. The drug oxotremorine, which mimics the neurotransmitter acetylcholine in the brain and enhances memory retention, was injected into the hippocampus, the anterior cingulate cortex or the amygdala immediately after either the contextual training on day one or after the foot-shock training on day two. All the rats were then tested two days later to see how quickly they would return to the chamber where they had received the foot shock, an indication of how well they remembered the previous training.
Rats given oxotremorine in the hippocampus after just the contextual training stayed out of the foot-shock chamber longer, meaning that they remembered the past event. But the injections into the hippocampus after the foot-shock training had no effect on memory retention. This is consistent with evidence that the hippocampus is involved in contextual memory consolidation but not with consolidation of unpleasant information. Likewise, those rats given injections into the anterior cingulate cortex had enhanced memory when the drug was administered after the foot-shock training but not after the contextual experience.
In contrast, the rats with injections in the amygdala showed better memory retention regardless of whether they had received the drug after the context training or the foot-shock training. The results support the hypothesis that the amygdala is involved in overall consolidation of memories of different kinds of experiences.
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