Brain–Computer Interfaces - Index of

Brain Signals for Brain–Computer Interfaces 35
basic protocol to a 9 × 8 matrix of items and combined it with specific applications
such as word-processing with a predictive speller and e-mail [29].
P300-based BCI systems have several important advantages: the P300 is
detectable in most potential users; its relatively short latency (as opposed to that
of SCPs (see below)) supports faster communication; with an appropriate protocol
it does not attenuate significantly [30]; initial system calibration to each user
usually requires less than 1 h; and very little user training is needed. In people
with visual impairments, auditory or tactile stimuli could potentially be used instead
[31, 32].
Slow Cortical Potentials
The slowest features of the scalp-recorded EEG yet used in BCI systems are slow
voltage changes generated in cortex. These potential shifts occur over 0.5–10.0 s and
are called slow cortical potentials (SCPs). In normal brain function, negative SCPs
accompany mental preparation, while positive SCPs probably accompany mental
inhibition [33, 34]. Negative and positive SCPs probably reflect an increase and
decrease, respectively, in excitation of cortical neurons [35]. This change in activation
may be further modulated by dopaminergic and cholinergic systems. For
example, the contingent negative variation is a slowly developing negative SCP
that occurs between a warning stimulus and a stimulus requiring a response [22].
It reflects increased activation and decreased firing thresholds in cortical networks
associated with the preparatory process preceding the response.
In studies spanning more than 30 years, Birbaumer and his colleagues have
demonstrated that people can learn to control SCP amplitude through an operant
conditioning protocol, and can use this control to operate a BCI system [36–39]. In
the standard format, SCP amplitude is displayed as the vertical position of a cursor,
and after a 2-s baseline period, the user increases or decreases negative SCP amplitude
to select the top or bottom target, respectively. The system can also operate in
a mode that translates SCP amplitude into an auditory or tactile output. This SCPbased
BCI can support basic word-processing and other simple control tasks. People
who are severely disabled by ALS, and have little or no ability to use conventional
assistive communication technology, may be able to master SCP control and use it
for basic communication [e.g., 40].
2.1.2 Cortical Oscillations
Brain activity is reflected in a variety of oscillations, or rhythms, in scalp-recorded
EEG. Each rhythm is distinguished by its frequency range, scalp location, and
correlations with ongoing brain activity and behavioral state [41]. While these
rhythms are traditionally believed to simply reflect brain activity, it is possible
that they play functional roles (e.g., in synchronizing cortical regions) [42–44].
Rhythms that can be modulated independently of motor outputs are potentially
useful for BCI applications. Up to the present, mainly sensorimotor rhythms have
been applied to this purpose.