BCI - the signals

Signals are the one important thing about BCIs. They are used to tell the prosthesis what to do. Signals represent nerve cell activity in certain parts of the brain under given circumstances. These include EEG-rhythms that reflect oscillations in neural circuits of certain areas of the brain's cortex (more).
SIgnals can also be so called evoked potentials which are visible in the EEG as response to stimulation, for example visual stimulation (more). Finally, action potentials (more) produced by particular neurons can act as signals. The latter ones are usually not recorded by an EEG but by implanted microelectrodes that are in direct contact with brain cells.
It is important to realise that although nerve cells naturally communicate with muscles and the rest of the body, the sampled activity that is recorded from the brain's cortex by either EEG or electrodes is not the way the brain usually talks. In other words: Signals reflect brain acticity rather than being brain activity themselves. This has some deep theoretical impact. (more)

The normal "communication channels" for the brain are the nerves that leave it or reach it. All over the body, these nerves come in contact with either muscles or sensory organs. The "language" the brain uses to communicate with its muscles or sensory organs is a mixture between electricity and chemistry. The nerves conduct information like a wire conducts electricity. At the small gap between two cells, though, molecules are used to transmit information chemically.

A BCI can help when these electrico-chemical channels do not work properly and thus need replacement. In the motor neuron disease ALS, for example, it is exactly the nerve cells that make contact to the muscles which are affected by the disease.

It is unclear how much learning processes rely on the normal communication infrastructure of the brain. Complex activities like movement, which involve quiet a substantial amount of subconscious learning, might simply not be realisable with a purely electrical substitute channel like an EEG or even cortical neural activity.

At the very least, the brain might need some form of feedback from its muscles, limbs or whatever. Depending on the disease again, these feedback channels can still be available. If not, artificial feedback can be considered.

Redirecting brain activity

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