Interfacing with the central nervous system is an important part of replacing many structures in the body, whether with new tissues or artificial structures that accomplish at least some of the same functions. Much of the work in this direction is concerned with the development of more functional artificial limbs and powered exoskeletons, but there is a lot more than just that going on in the research community:
Neural control of a prosthetic device for medical applications is now becoming commonplace in labs around the world. In its simplest form, a neuroprosthetic is a device that supplants or supplements the input and/or output of the nervous system. For decades, researchers have eyed neuroprosthetics as ways to bypass neural deficits caused by disease, or even to augment existing function for improved performance. Today, several different types of surgical brain implants are being tested for their ability to restore some level of function in patients with severe sensory or motor disabilities. [Perhaps] the most visible recent demonstration of the power of neuroprosthetics was a spinal cord-injured patient using a brain-controlled exoskeleton to kick off the 2014 World Cup in Brazil. In short, tinkering with the brain has begun in earnest.
When connecting an external device to the human nervous system, researchers have traditionally used a setup that records brain signals from the user, computationally analyzes those signals to infer the user's intentions, and then relays the information to an external effector that acts on those intentions. Inputs can be the firing of individual neurons in the brain, the cumulative voltages across areas of cortex encompassing millions of neurons, or the action potentials conducted by peripheral nerves anywhere in the body. In terms of output effectors, researchers have demonstrated that brain or nerve signals can be used to control computer cursor movements and robotic arms, or enable the reanimation of paralyzed limbs.
But information transfer via neuroprostheses is not a one-way street; some systems are able to convert environmental stimuli into perceptions by capturing an external input and translating it into an appropriate stimulus delivered directly to the nervous system. In this light, researchers have developed cochlear implants and functional retinal prostheses. Such reversal of information transfer can also be beneficial for limb prostheses. Under normal circumstances, meaningful movements of the body can only be accomplished in conjunction with appropriate sensation of the limb or body part. While this area of research is still young, researchers are beginning to create "bidirectional" brain-computer interfaces.