| |
| |
| 8.
Direct Brain–Computer Communication |
| |
8.3.
Components of graz BCI
| |
8.3.1.
Parameter Estimation and Classification |
| |
|
8.3.1.3.
Common Spatial Patterns |
|
Frequently, the ERD/ERS focus during motor
imagery is not located beneath electrode positions C3
and C4.
One possibility to improve classification accuracy in these
circumstances is to use the method of common spatial
patterns (CSP), which constructs spatial filters that
are optimal for discrimination of two populations of EEG.
The method is based on the simultaneous diagonalization of two
matrices. The picture displays
examples of the most relevant common spatial patterns for three
subjects.
Left motor imagery causes relatively increased EEG over the
left hemisphere because on the contralateral hemisphere event-related
desynchronization of EEG takes place.
This behavior is reflected in large coefficients for electrodes
on the left hemisphere. The most important patterns show their
strongest modulation at electrodes above the motor cortex.
However, for none of the subjects was the focus exactly centered
at electrode positions C3 and C4,
which are used for EEG classification in most BCI experiments.
The CSP method can be effectively used in real-time on standard
PC software. The online classification accuracy for three healthy
subjects in a two-imagery task experiment ranged from 87%–98%.
| |
 |
| |
Picture:
Most relevant common spatial patterns of three subjects. The left
(right) column shows the pattern most suited for detection of
left (right) hand motor imagery where light colors represent relevant
regions. Electrode positions are marked with a dot except for
electrodes C3 and C4, which are marked with a “+.”
The contour plots are obtained with cubic interpolation between
the CSP values calculated for each electrode.
Source:
Motor Imagery and Direct Brain–Computer Communication, Gert Pfurtscheller and Christa Neuper |
|
|
