Neural Reader vs Neurostimulator
One of the most fundamental differences possible in neuroprosthetic devices, is that between a neural reader and a neurostimulator. Once you understand this fundamental difference, it is hard to see how you could possibly have gotten the two confused before.
Sadly, particularly in lay persons, the two types are often hopelessly mixed together. This document is intended to set things straight in the simplest manner possible, and explain the limitations of both.
At the most basic level with all systems you have three basic elements:
The human brain - and all brains for that matter - works on these basic principles, as do all computer systems, and physical processes. You need something to detect the output of one system, and use that as the input of another system. Typically this is what our senses and ability to speak/move are for. In a manner of speaking, neuroprosthetics are a sensory bridge between a brain and an external processing system.
A neural reader is a neuroprosthetic which is dedicated to detecting the electrical spiking of neurons, or even general brainwaves, and decoding that into meaningful data to be used by the computer process.
It reads output data from the brain and inputs it into the control computer system, whatever that may be.
A neural reader cannot influence or alter the electrical patterns or thoughts of the brain, it can only detect them, and pass them on to an external device.
Sometimes neural readers are placed on the outside of the cranium, sometimes they are placed under the skull. The most precise ones are embedded directly into the brain itself, deep within the tissue. Other than the initial damage of surgery, even the deep implants do not affect the functioning of the brain. They are there simply to detect signals, and pass them on to an external device. This is the single greatest point of confusion for such devices.
A neurostimulator is the opposite. It is not possible with such a device, to gather any information about the thought patterns or electrical signals currently in the brain. The neurostimulator's job is to output data to the brain, overwriting what is already there.
It reads output data from a computer system, decodes it, and outputs what is usually a blanket electrical pulse into a specific region of the brain, in order to reset or regulate the firing patterns there. One day it should be possible to do this on a neuron by neuron basis, but for now blanket coverage is all we have.
It is worth noting that in most cases, a neural reader and neurostimulator cannot coexist peacefully. If they are used together in one device, they must be phase offset, such that they operate alternatively. Otherwise when the neurostimulator writes data back into the brain, it obliterates any electrical patterns present in the local area - giving the neural reader purely artificial data to read. One of the long-term goals of neuroprosthetics is to have neurostimulators that are capable of writing data to individual neurons, leaving others unaffected. This is unfortunately still very much the realm of pure science fiction at this current time. All a stimulator can do is inundate an area with charge.
A neural reader reads output data from the brain and inputs it into the control computer system.
A neurostimulator takes data from the control computer system and writes it as input directly into the brain, overwriting what was already there.
Used together you have a true I/O system with the brain, however care has to be taken to ensure the reader and stimulator operate offset from one another. Otherwise, all the reader will ever read is the output wave from the stimulator - removing the brain from the loop.