The discovery of place cells led to the idea that the hippocampus might act as a cognitive map - a neural representation of the layout of the environment. The findings resulted from research with individuals with electrodes implanted in their brains as a diagnostic part of surgical treatment for serious epilepsy.
Place cells have now been seen in humans involved in finding their way around in a virtual reality town. Different cells fire at different locations, so that, by looking at the firing of the cells alone, it becomes possible to tell where the animal is. In rats, some cells, termed context-dependent cells, may alter their firing depending on the animal's past ( retrospective) or expected future ( prospective). Some cells fire when the animal finds itself in a particular location, regardless of direction of travel, while most are at least partially sensitive to head direction and direction of travel. Studies in rats have shown that neurons in the hippocampus have spatial firing fields. Role in spatial memory and navigationĮvidence suggests the hippocampus is used in storing and processing spatial information. And there is evidence to suggest that patient HM (who had his medial temporal lobes removed bilaterally as a treatment for epilepsy ) can form new semantic memories. However, experimentation has difficulties in testing the sparing of older memories and, in some cases of retrograde amnesia, the sparing appears to affect memories formed decades before the damage to the hippocampus occurred, so its role in maintaining these older memories remains uncertain.ĭamage to the hippocampus does not affect some aspects of memory, such as the ability to learn new skills (playing a musical instrument, for example), suggesting that such abilities depend on a different type of memory ( procedural memory) and different brain regions. Although the retrograde effect normally extends some years prior to the brain damage, in some cases older memories remain - this sparing of older memories leads to the idea that consolidation over time involves the transfer of memories out of the hippocampus to other parts of the brain. Damage to the hippocampus usually results in profound difficulties in forming new memories ( anterograde amnesia), and normally also affects access to memories prior to the damage ( retrograde amnesia). Some evidence supports the idea that, although these forms of memory often last a lifetime, the hippocampus ceases to play a crucial role in the retention of the memory after a period of consolidation. Some researchers prefer to consider the hippocampus as part of a larger medial temporal lobe memory system responsible for general declarative memory (memories that can be explicitly verbalized - these would include, for example, memory for facts in addition to episodic memory). Psychologists and neuroscientists are not entirely sure of the precise role of the hippocampus, but, in general, agree that it has an essential role in the formation of new memories about experienced events ( episodic or autobiographical memory).
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