Conceptually, BCIs fall into two categories (more): dependent and independent. A dependent BCI needs an at least partially intact peripheral system. A dependent BCI could flash letters on a screen. A user who is paralysed could then choose the letters by gazing at them, while an EEG is recorded above the particular part of the brain that is active when gazing. The brain's way of communication in this example is the EEG, but the EEG is dependent on the activity of the (moving) eyes.
An independent BCI, in contrary, works with signals that are totally independent of muscle activity wheresoever. In our example the flashing letters would be chosen by the user of the BCI not by gazing but by "thinking" which, if properly trained, produces certain electrical patterns that trigger action of the BCI. While dependent BCIs are easier to make, very severely handicapped people might only profit from independent ones. This web site will focus on the theoretically more challenging independent BCIs, although the exact line is not always easy to draw in practice. (more)