What types of memory does the hippocampal formation contribute to?

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What types of memory does the hippocampal formation contribute to?

Storing memories has the stages encoding, storage and retrieval. A failure at any one of these stages could cause memory failure. Which types of memory the hippocampal formation contributes to can be studied by lesions, neuroimaging and electrophysiological recordings. The hippocampal formation is situated in the temporal lobe of the brain in humans. As it has a similar structure to hippocampal formations in animals, it is scientifically reasonable, to an extent, to study animals in order to understand humans.

By studying animals or humans who have had their hippocampus removed, we can study how it contributed to memory functions. Scoville and Miller (1957) studied an epileptic man called “HM” whose hippocampus was removed. This stopped his epileptic fits but he suffered severe memory loss and could not form new memories from the date of his operation. This suggests that the hippocampus plays a main part in the formation of new explicit memories; recollections of previously occurring events. HM’s implicit memory, such as his understanding and skills, was not so damaged. However, as this is a case study, it cannot be assumed that these findings can be generalised. What may affect one individual may not have the same effects on another. It also cannot be assumed that the hippocampal formation is the direct site of memory consolidation. It may be that the removal of the structure from the brain leads to another part of the brain not able to function properly and therefore affecting HM’s memory; in other words, cause and effect cannot be determined. For example, brain scans have captured the brain forming new memories and activity can be seen in the front lobes as well as the hippocampus (Wagner et al, 1998). This suggests that the hippocampus is not the sole site of memory formation. It may just be where memories are laid down before being stored elsewhere. There is evidence supporting this idea that the hippocampus is used as a temporary storage of information before being processed. Zola-Morgan and Squire (1990) gave primates pairs of objects to remember at different time periods before removal of the hippocampus. Those who did not receive lesions remembered recently learned objects better than objects learned longer ago. Primates who received lesions were much worse at remembering recently learned objects but remembered objects learned long ago the same as those without lesions. This suggests that memories are only stored in the hippocampal formation for a limited time before being processed and stored elsewhere. Therefore, it suggests that the hippocampus is needed to process memories and without it, memories cannot be stored in the brain.

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Morris et al’s (1982) study of rats in a water maze found that the hippocampus may play an important role in spatial memories. When rats were placed in milky water, they had to swim around until they found an underwater platform through trial and error. The rats who received lesions to the hippocampus had significantly poorer navigation than those who had lesions to their cerebral cortex or no lesion at all. The rats with hippocampal lesions also did not learn from previous experience as those without lesions spent longer in the quadrant where they had learnt the platform was before. ...

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