The hippocampus is a horseshoe shaped region of the subcortical brain. As part of the limbic system, located in the temporal lobe, it has a role in emotion and sexuality. It also contains "place" cells that construct mental maps of position, and with the parahippocampal gyrus, is implicated in the learning and remembering of space (spatial orientation). Studies with monkeys show that damage to the hippocampus results in the inability to navigate through familiar areas.

From Michio Kaku's book "Visions" (How Science will Revolutionize the 21st Century): "The hippocampus helps construct a three dimensional "mental map" of our surroundings, and is crucial for our ability to move around in the real world.

"Memories are probably first processed and kept in the hippocampus for several weeks, before they are transferred to the cerebral cortex for permanent storage. This may explain why people with brain damage to their hippocampal region retain previous memories of faces and places, which are stored in the cortex, but have difficulty forming new short-term memories.

"The hippocampus of the mouse brain contains about a million large nerve cells, called "place cells," which enable mice to record where they are located in space. These place cells consolidate their memory via the protein kinase.

Scientists at MIT and Columbia University independently announced in 1996 that they could change memory in mice by altering a gene that codes for kinase...In both studies the mice appeared perfectly normal, but were severely impaired in their ability to find their way around..."

Damage to the hippocampal region results in a failure to remember spatial layouts or landmarks. This may be one reason why people with Alzheimer's disease become progressively unable to navigate in space despite normal vision. In a 4 year study of 405 elderly volunteers, de Leon and coworkers found that probable cases of Alzheimer's disease often emerge in people who start out with a small hippocampus and mild memory problems. Hippocampal volume declined from 20 to 50 percent in victims of Alzheimers compared to healthy elderly controls.

Stroke patients who experience navigation problems inevitably manifest brain damage in an area just below (and connected to) the hippocampus, a region called the parahippocampal gyri. This area is crucial to the storage and recall of spatial information. After stroke damage to the parahippocampus, patients develop topographical disorientation. They lose the ability to learn new routes or to travel familiar routes.

There may be a connection between children with navigational disabilities, stroke patients, and victims of Alzheimer's disease. It may be that the blood supply to the hippocampus and parahippocampus is vulnerable, ie. more easily damaged by infarction, more prone to damage faster when oxygen (and/or nutrients) is reduced. (speculation on my part).

A study in the July, 1997 issue of the magazine Science indicates that semantic memory (memory of context-free facts) and episodic memory (memory of context-rich events) take place in separate areas of the brain. The hippocampus regulates episodic memory, but plays a very small role in the remembering of factual information. (study done by a research team at University College London Medical School).

Another study at Dartmouth College by psychologists Golob and Taube (reported in the July 8, 1997 issue of the Proceedings of the National Academy of Sciences), suggests that "Despite the hippocampus' powerful influence on episodic memory, other brain areas may independently gather and retrieve information that animals use to orient themselves in a new setting."

The hippocampus seems to be a major hub through which all new experiences must pass on their way to forming a new memory. When the region is damaged, patients become prisoners of the past. Yet they can learn new skills like golf or bridge. They improve their skills each time they play golf or bridge for example, all the while thinking that this is the first time they have ever tried the game. This suggests that skill development is its own kind of memory, governed by another brain region.

Patients with damage to the basal ganglia display exactly the opposite: they can practice a skill like playing the piano hundreds of times, retaining clear memories of every previous lesson, yet make absolutely no progress with the skill.

Many patients with hippocampal damage remember things from before an injury, but not since. A patient studied at Rutgers University convulses in agony every time he hears that his father has died, yet he awakes each morning fresh thinking that his father is alive.

notes:

The hippocampus is a center for short term memory. It weighs the importance of episodic acts and decides which should be kept as memories. Therefore it has an important role in learning.

Brain scan evidence shows that healthy people experience a drop in hippocampal volume of about 15 percent as they age.

The hippocampus appears to shrink in people who have been exposed to severe trauma; like sexual abuse cases or combat victims.

Since the hippocampus is a center for event memory, does this suggest a connection with the visual movement center, area 5 (?) of the visual cortex? I am assuming that an event primarily involves movement recall.

It receives input from auditory as well as visual tracts

The hippocampus is sometimes removed in epilepsy surgery causing a loss of the ability to form new long term memories. Problems in the thalamus can result in the inability to retrieve memories.

Damage to the hippocampus disrupts recent memory, but leaves remote (already learned?) memory intact.

Researchers think there may be at least five memory systems operating in parallel.

The hippocampus allows for rapid learning of new items.

According to Jeff Hawkins in the book On Intelligence: The hippocampus is the "uppermost" level of the neocortex.

The hippocampus creates longterm memories (which are then stored in the neocortext).