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.
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. However, Olton et al (1979) argue that this does not test spatial memory but rather the working and short term memory, as the task involves the rats remembering information and using it to do another task.Therefore, the hippocampus may not be responsible for spatial memory at all but may merely be affecting the short term memory.
Furthermore, as this study was carried out on rats, it is questionable as to what extent we can generalise to humans. Although humans and rats have a similar hippocampus structure, there may be some functions which are specific to certain species and therefore we cannot assume that what the hippocampus does in a rat is the same as what it does in a human, however similar the structures may be.However, it has also been found that primates show a deficit in spatial memory (Redish and Touretzsky, 1997). As humans are most similar to primates in terms of species and structure, we can presume that perhaps there would be the same effect in humans.
As it is not ethically moral to conduct these experiments on humans, we may only estimate the effect in humans. There is evidence suggesting that different regions of the hippocampus contribute to different memories.It has been found that damage to the left hippocampus leads to trouble remembering verbal information whereas damage to the right hippocampus region leads to trouble remembering visual designs and locations (Schacter, 1996). When experienced taxi drivers were asked to describe the shortest route between two locations, a PET scan found brain activity more prominently in the right hippocampus than when asked to describe a landmark or film (Maguire et al, 1997). Spatial memory is remembering a path or route but can differ from knowledge of layout of a specific location.
This suggests that the hippocampus is involved in more than one type of explicit memory. It was found in a meta-analysis of hippocampal dysfunction that there was the largest impairment in positional memory tasks such as locating an X in a pile of letters, whereas there was less of an effect in tasks of navigation and on spatial working memory. In patients with mild to severe impairments, there was most damage to the right region of the hippocampus, supporting evidence that the right hand side is important for spatial memories (Kessels et al, 2001). Research on rats has shown that hippocampal neurons act as place cells.Place cells tell us about the spatial context and place that information was first retrieved in.
By using the spatial context of an event, humans can retrieve the details of the event. Therefore, the hippocampal neurons may also be the source of episodic memories. This can be studied by using electrophysiological recordings. This involves placing two sets of LEDs on a rat’s head so that head position and orientation is known. When the rat moves around its environment, certain neurons will fire in specific locations which can be recorded on a graph or diagram.
Different place cells may fire in different environments or fire in different positions in the same environment. The temporal structure of these experiences is replayed in slow wave sleep. Research has found that the more hippocampus activity there is during sleep, the better the next day’s memory is (Peigneux et al, 2004). This method of studying hippocampal activity provides a more extensive insight into the functions of the brain than neuroimaging.
In conclusion, it seems that the hippocampal formation contributes to mainly episodic and spatial memories.However, we cannot conclude that the hippocampus is responsible for these memories alone. The structure of the brain is designed so that all parts are interconnected. Hippocampal lesions or dysfunctions may merely mean that another part of the brain cannot function properly and therefore this may impair memory.
The mammalian and human brain is so complex that it may never be possible to conclusively say what the function of each part is. However, with increasingly more advanced technology, our understanding of what types of memory the hippocampus contributes to may become clearer in the future.
