What is the Hippocampus?

The hippocampus, so named because its shape vaguely resembles that of a seahorse, is responsible for encoding long-term memories and helping with spatial navigation. It's one of the phylogenetically oldest parts of the brain, and the first part chosen to be artificially replicated as a brain prosthesis. The hippocampus is known to be associated with the consolidation of episodic memories, which are memories of personally experienced events and their associated emotions. In contrast to semantic memories of abstract facts and their associations, episodic memories can be represented as stories. Damage to the hippocampus results in an inability to form new long-term episodic memories, though new procedural memories, such as motor sequences for everyday tasks, may still be learned.

In schizophrenia and certain types of severe depression, the hippocampus shrinks. The hippocampus is also known as one of the most highly structured and studied parts of the brain, which is why it was chosen for prosthetic emulation. Although the exact neural algorithms are not known, they have been modeled in their entirety. Because the hippocampus is so old, it has been optimized extensively by evolution and is basically the same across all mammal species. This is why it was possible to design a hippocampal prosthesis through the exhaustive study of rat hippocampi suspended in cerebrospinal fluid.

For navigation, the hippocampus contains "place cells" which activate depending on the perceived location of the animal. A strong case can be made that these cells exist in the hippocampus because memories must be employed to determine current location from more fundamental variables like orientation and speed. The activation of these place cells has been observed in humans navigating virtual reality towns. An intact hippocampus is required for many spatial navigation tasks. The hippocampus was originally associated, incorrectly, with the sense of smell, which is actually processed by the olfactory cortex.

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Post 5

@fify-- The hippocampus is important both for learning and memory, so there is no doubt that changes in its function would have consequences for these activities.

People with certain conditions that lead to lower function of the hippocampus will have a harder time retaining and remembering memories. It's a major link for Alzheimer's where both severe memory issues and changes in the hippocampus occur.

Post 4

@fify-- I'm not sure what the consequences of a shrunk hippocampus. I read a few studies on it because I wrote about it in a paper. Scientists believe that a small hippocampus may result in poor communication between nerves in the brain.

It was actually thought that depression shrunk the hippocampus by reducing the number of neurons. But it was later discovered that the reduction in size is due to loss of nerve fibers and substances that aid communication in this part of the brain.

There are still studies that need to be done on this but it's very possible that depression causes people to think irrationally because of this reduction in the size of the hippocampus.

Post 3

What does it mean for the hippocampus to shrink in severe depression? Does it mean that people have trouble remembering? Does it lead to some of the negative side effects of depression?

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