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| 8.
Direct Brain–Computer Communication |
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8.1.
A short overview of EEG-based BCI systems |
EEG-based brain–computer interfaces (BCI)
can be realized in an externally (stimulus)-paced mode
(synchronous BCI) or in an internally paced
mode (asynchronous BCI).
In externally (stimulus)-paced
mode, specific mental states have to be generated in response
to an external event that means changes of brain activity occur
in a predefined time window.
In internally paced mode, the subject
is free to intend a mental state or a specific thought.
As early
as 1996, the Graz group reported on an EEG based BCI able to
discriminate between three brain states, where the subject was
instructed to execute a specific movement (right hand, left
hand, right foot) in the first three sessions and to imagine
the same movement in the next 1–2 sessions.
The next important step in the Graz BCI was to select the most
reactive frequency bands for each subject by using the distinction
sensitive learning vector quantization.
Source:
Motor Imagery and Direct Brain–Computer Communication, Gert Pfurtscheller and Christa Neuper 
