Hippocampal prosthesis

A hippocampus prosthesis is a type of cognitive prosthesis (a prosthesis implanted into the nervous system in order to improve or replace the function of damaged brain tissue). Prosthetic devices replace normal function of a damaged body part; this can be simply a structural replacement (e.g. reconstructive surgery or glass eye) or a rudimentary, functional replacement (e.g. a pegleg or hook).

However, prosthetics involving the brain have some special categories and requirements. "Input" prosthetics, such as retinal or cochlear implant, supply signals to the brain that the patient eventually learns to interpret as sight or sound. "Output" prosthetics use brain signals to drive a bionic arm, hand or computer device, and require considerable training during which the patient learns to generate the desired action via their thoughts. Both of these types of prosthetics rely on the plasticity of the brain to adapt to the requirement of the prosthesis, thus allowing the user to "learn" the use of his new body part.

A cognitive or "brain-to-brain" prosthesis involves neither learned input nor output signals, but the native signals used normally by the area of the brain to be replaced (or supported). Thus, such a device must be able to fully replace the function of a small section of the nervous system—using that section's normal mode of operation. In order to achieve this, developers require a deep understanding of the functioning of the nervous system. The scope of design must include a reliable mathematical model as well as the technology in order to properly manufacture and install a cognitive prosthesis. The primary goal of an artificial hippocampus is to provide a cure for Alzheimer's disease and other hippocampus—related problems. To do so, the prosthesis has to be able to receive information directly from the brain, analyze the information and give an appropriate output to the cerebral cortex; in other words, it must behave just like a natural hippocampus. At the same time, the artificial organ must be completely autonomous, since any exterior power source will greatly increase the risk of infection.

The hippocampus is part of the human limbic system, which interacts with the neocortex and other parts of the brain to produce emotions. As a part of the limbic system, the hippocampus plays its part in the formation of emotion in addition to its other roles, such as consolidation of new memories, navigation, and spatial orientation. The hippocampus is responsible for the formation of long term recognition memories. In other words, this is the part of the brain that allows us to associate a face with a name. Because of its close relationship with memory formation, damage to the hippocampus is closely related to Alzheimer's disease.

The hippocampus is a bilateral structure, situated under the neocortex. Each hippocampus is "composed of several different subsystem[s] that form a closed feedback loop, with input from the neocortex entering via the entorhinal cortex, propagating through the intrinsic subregions of the hippocampus and returning to the neocortex." In an electronic sense, the hippocampus is composed of a slice of parallel circuits.

Since the prosthesis will be permanently implanted inside the brain, long term biocompatibility is required. We must also take into account the tendency for supporting braincells like astrocytes to encapsulate the implant. (This is a natural response for braincells, in order to protect neurons), thus impairing its function.

Being biomimetic means that the implant must be able to fulfill the properties a real biological neuron. To do so we must have an in–depth understanding of brain behavior to build a solid mathematical model to be based upon. The field of computational neuroscience has made headway in this endeavor.

First, we must take into account that, like most of biological processes, the behaviors of neurons are highly nonlinear and depend on many factors: input frequency patterns, etc. Also, a good model must take into account the fact that the expression of a single nerve cell is negligible, since the processes are carried by groups of neurons interacting in network. Once installed, the device must assume all (or at least most) of the function of the damaged hippocampus for a prolonged period of time. First, the artificial neurons must be able to work together in network just like real neurons. Then, they must be able, working and effective synaptics connections with the existing neurons of the brain; therefore a model for silicon/neurons interface will be required.