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)

With nerve cells, a potential is generally a difference in electric charge within the cell as compared to outside of it.

It is actively produced and maintained by transporting charged molecules, especially sodium and potassium, to either side of the cell's membrane. As long as a nerve cell is "silent", the charge is negative on the outside.

As soon as it is activated, though, the charging swops to positive on the outside, a state called "action potential" or AP. An AP can be conducted along the membrane of the nerve cell. If strong enough, it finally reaches the synapse, where it is "translated" into chemical molecules, the transmitters, that in turn activate other nerve cells or even a muscle.

It is this repeated change in electrical charge that can be measured by implanted electrodes.

What are action potentials?

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