Information Technology Reference
In-Depth Information
Fig. 2.6
Example of P300
VEP paradigm
—
Donchin
'
s
speller matrix
The principle is based on the oddball paradigm where the frequency of the target
stimulus is lower than the non-target stimulus. In this case, the target frequency is
one sixth since only either one row or one column
flashes at a time. A variation of
this paradigm is where each alphanumeric character
flashes thereby decreasing the
frequency to one thirty-six
the lower this frequency, the higher is the P300
amplitude response, which allows easier detection, however resulting in slower
response overall as it takes longer to complete the cycle.
—
2.6
EEG-based BCI Paradigm 4
—
Mental Task BCI
In this paradigm, users think of different mental tasks and since different tasks activate
different areas of the brain, a set of multichannel EEG recordings will have distinct
EEG patterns to differentiate the tasks, which could be used to design a BCI.
Examples of mental tasks used are (Keirn and Aunon
1990
; Palaniappan
2006
):
Baseline task where users are asked to relax and think of nothing in particular;
Computation task where users do nontrivial multiplication problems;
Mental letter task composing where users mentally compose a letter to someone;
Visual counting task where users visually imagine numbers written on a board
with the previous number being erased before the next number is written;
Geometric
figure rotation task where users imagine a
figure being rotated about
an axis.
These mental tasks exhibit inter-hemispheric differences, and hence, the EEG
pattern will be distinct (Keirn and Aunon
1990
). For example, computation task
involves the left hemisphere more while the visual task exhibits more activity in the
right hemisphere. The detection of the inter-hemispheric activity can be done using
asymmetry ratio where the powers of EEG channels in the left and right
