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The brain is a collection of neurons and synapses arranged in a network that is incredibly complex. Replacing random parts of the brain with a completely fresh piece of brain tissue is not good enough, because every person’s network is encoded slightly differently in order to record memories and functions.
The safest part of a brain to replace, then, is a part that doesn’t store memories, but does something different, such as decode signals from ears or eyes and pass the decoded signals onwards.
The hippocampus is a part of the brain that does exactly that. It receives inputs in one end, and sends signals out the other end.
The first neural prosthetic was revealed in 2003 by Theodore Berger from the University of Southern California. It took ten years for his team to develop.
They chose to work on the hippocampus. The hippocampus is a well-ordered part of the brain that encodes experiences so that they can be recorded elsewhere in the brain. A damaged hippocampus doesn’t cause you to lose memories or your identity. You simply lose the ability to create new memories.
The brain is too complex for us to create simple replacements, like how we can replace the heart with a simple pump. Even the smallest brains are very complex, so you cannot simply replace with brain with something else that mimics its function. It must be exact, right down to the smallest neuron.
The prosthetic that Berger built was based on a rat hippocampus. The team decided that the best way to build the replacement was to completely replicate the hippocampus in a computer. To do that, they sliced the brain into slices and probed it millions of times with electrical signals until they were certain what effect each signal would produce in the slice.
After studying each slice individually, they were able to replicate the function of the entire hippocampus by sending signals in one end of a slice, and passing the resulting signals through to the next slice in the correct locations, etc.
When the entire hippocampus was entirely encoded in a computer, they then developed a computer chip which contained the entire network, connected| to two electrical arrays. The idea being that if you remove the hippocampus, you simply slice it cleanly on both ends, then connect the electrical arrays to the sliced areas left behind, and remove the damaged tissue. The chip then takes over the function of the removed hippocampus.
The prosthetic was tested against living tissue one year later, but it wasn’t until 2011 that it was tested in living rats and shown to work. And then a year later shown to also work in macaque monkeys.
In 2013, it was shown that the prosthetic also works in non-human primates.
Human trials are now underway and reportedly going well.
The hippocampus is similar in most mammals, and does not record personal information, so it does not need to be tailored individually to each person. You could literally have them on a shelf ready to be installed. And once it is installed, it simply works. You could walk into hospital one day unable to create new memories, and walk out a few days later with that problem fixed.