In 1977, Craig Thomas published his novel, "Firefox". It also became a movie, staring Clint Eastwood. In it the pilot can control the weapon systems of a MiG-31, code-named Firefox, by thought.
What about these visions? At present, even the most sophisticated EEG-based BCIs are not able to decipher more than 25 bits of information per minute (more). This can enable a completely paralysed person to use for example a mental typewriter (more) at a speed of two words per minute. It can also be used to perform routine tasks in the household. But it is many thousands of times too slow to control even simple movement (more). Thus: To bring powerful BCIs into existence, one has to either improve EEG-technology or use different modes of communication.
Apart from the speed of communication, the second big problem of BCI-research is the signal itself, as neither brainwaves nor individual firing rates of nerve cells were made to drive a prosthesis (more).

definitions / components / history of BCIs
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Firefox
definitions > problems

What to do about that nuisance?

A better temporal resolution and thus more functionality can be reached by using implantable electrodes in the brain. One won't be enough, though. It is estimated that electrical firing information of at least 1000 neurons, probably more, is necessary to control movement. This can be done, but the devices that result are currently too big for implantation, so that wireless communication would be needed through the bones of the skull.

Learn about definitions of BCI, how to build your own one, and what the history of BCIs was like.