In Alzheimer's disease, the hippocampus is one of the first regions of the brain to be attacked; memory problems and disorientation are amongst the first symptoms. Damage to the hippocampus can also result from oxygen starvation (anoxia) and encephalitis.

In anatomy of animals, the hippocampus is among the phylogenically oldest parts of the brain. The hippocampal emergence from the archipallium is most pronounced in primates and Cetacean sea mammals. Nonetheless, in primates, the hippocampus occupies less of the telecephalon in proportion to cerebral cortex among the youngest species, especially humans. The significant development of hippocampal capacity in primates correlates more with overall increase of brain mass than with neocortical development.

Role in General Memory

There is some controversy in psychology and the neurosciences about the precise role of the hippocampus, but it is generally agreed that it is essential for the formation of new memories about personally experienced events ( episodic or autobiographical memory). Some researchers prefer to think of the hippocampus as part of a larger medial temporal lobe memory system responsible for general declarative memory (memories which can be explicitly verbalized - these would include e.g., memory for facts in addition to episiodic memory).

There is some evidence that, although these forms of memory often last a lifetime, the hippocampus ceases to be crucial for the retention of the memory after a period of consolidation. 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). Although the retrograde effect normally extends some years prior to the brain damage, in some cases older memories are spared - it is this sparing of older memories which leads to the idea that consolidation over time involves the transfer of memories out of the hippocampus to other parts of the brain. However, the sparing of older memories is difficult to test, and in some cases of retrograde amnesia is believed to have affected memories formed decades before the damage to the hippocampus occurred, so its role in maintaining these older memories remains controversial.

Some aspects of memory are not affected by damage to the hippocampus (e.g., the ability to learn new skills (such as playing a musical instrument), suggesting that such abilities depend on a different type of memory (procedural memory) and different brain regions.

Role in Spatial Memory and Navigation

There is also evidence, that the hippocampus is involved in storing and processing spatial information.

Studies in rats have shown that neurons in the hippocampus have spatial firing fields. These cells are called place cells. Each cell fires when the animal is in a particular location, regardless of which direction it is heading. Different cells fire at different locations so that by looking at the firing of the cells alone, its possible to tell where the animal is. Place cells have now been seen in humans involved finding their way around in a virtual reality town -- the individuals concerned had to have electrodes implanted in their brains as a diagnostic part of surgical treatment for serious epilepsy.

The discovery of place cells lead to the idea that the hippocampus might act as a cognitive map -- a neural representation of the layout of the environment which would permit one to figure out new routes and short cuts between places. Studies with animals have shown that an intact hippocampus is required for simple spatial memory tasks (for instance finding the way back to a hidden goal).

Without a fully functional hippocampus a person may not be able remember the places he/she has been to and how to get where they are going. The hippocampus is believed to be particularly important for finding shortcuts and new routes between familiar places. Some people are better at this than others, and brain imaging shows that these individuals have more active hippocampi when navigating.

London's taxi drivers are required to learn a large number of places -- and know the most direct routes between them (they have to pass a strict test, the Knowledge, before being licensed to drive the famous black cabs). One study showed that part of the hippocampus is larger in taxi drivers than in the general public, and that more experienced drivers have bigger hippocampi. It may be that having a bigger hippocampus helps you to become a cab driver. It also seems that finding shortcuts for a living may make your hippocampus grow.

History

Later, the importance of hippocampus in memory has been found in a patient HM that had most of the hippocampal formation in both hemispheres removed in a neurosurgery operation.

Related topics

• LTP
• LTD
• subiculum
• dentate gyrus
• List of regions in the human brain

In Greek mythology, the hippocampus (horse-like sea monster") was a mythical monster, half-horse, half-sea-monster. One of them pulled Poseidon's chariot. It looked like a horse with the rear part resembling a fish or dolphin.