What Is the Hippocampal Formation?

The hippocampal formation is part of the limbic system and plays a key role in memory and learning. A hippocampal formation is made up of the hippocampus and the dentate gyrus, and is located on the medial temporal lobe in the brain. Shaped like a seahorse, the hippocampus has two halves, each located on one side of the brain.

Communication to and from the hippocampal formation is conducted through the arch-shaped fornix, which connects to a section of the brain called the mamillary bodies. Fibrous material compromises the fornix, and the mamillary bodies are considered part of the hypothalamus. These sections of the brain are also part of the limbic system.

In addition to helping control memory and learning, the limbic system is also responsible for body functions such as eating and drinking. This includes the entire process, from finding edible food to running the body processes to digest it. When considered in terms of evolution, the hippocampal formation itself is an older part of the brain. It seems possible that the brain partially evolved memory and learning through successful location of edible food sources.

Fear and emotional responses are also a responsibility of the limbic system. It controls the physiological effects of each, such as pulse, blood pressure, and facial expressions. The fight or flight response to fear is also controlled in this part of the brain, as is spatial navigation and awareness. Learning, memory, emotions, and key body functions are all bound together as part of the same processes of the brain.

The hippocampal formation is often one of the first areas affected by Alzheimer's Disease, and can also be damaged by seizures, herpes encephalitis, and severe oxygen loss to the brain. Oxygen loss can occur as a result of almost drowning, heart attacks, sleep problems, and carbon monoxide poisoning. Lesions found in the mamillary bodies and fornix also affect the hippocampal formation and can lead to memory changes.

Damage to the hippocampal formation affects both short-term and long-term memory. Short-term memory is still possible but it becomes shorter than it was before, lasting only a couple of minutes. Long-term memory can be permanently damaged. Memories obtained prior to the damage are unaffected, but any new memories acquired cannot be stored or recalled. Most of the research and information obtained about the function of the hippocampal formation is learned from studying patients who have suffered damage to the area.