Information Technology Reference
In-Depth Information
Fig. 2.8
SSVEP-based
cursor
—
each shaded edge
flickers with a certain distinct
frequency
colours. The detection of the colours/characters through this mechanism overcomes
problems like shoulder sur
ng.
3
2.10 Application 2
—
Cursor Control
Most of the SSVEP-based BCIs focus on discrete control, for example selecting a
menu on screen. In Wilson and Palaniappan (
2011
), an analog pointer was
developed where instead of discrete control, the cursor on screen moved analo-
gously based on the phase locking index (PLI) of the SSVEP frequency. The cursor
was designed as shown in Fig.
2.8
, where each edge
flickers with either 15, 12, 10
or 8.57 Hz (the frequencies were chosen based on the refresh rate of the LCD screen
of 60 Hz). The EEG was recorded from channel Oz in the visual cortex referenced
to channel PO3. Depending on which block the user was looking at, the SSVEP
will contain the respective frequency and its harmonics which can be detected using
discrete Fourier transform (DFT) and other spectral analysis methods. Using this
frequency measure,
the stronger the SSVEP
response (measured by the PLI measure from DFT), the further the cursor moved on
screen.
the cursor moved accordingly
—
2.11
Challenges in BCI
The most dif
cult challenge at the moment for general BCI use is on the require-
ment of using gel to improve the conductance though the advances in electrode
design (such as active electrodes) have reduced the set-up time considerably. Dry
capacitive electrodes have been invented but the quality of the EEG signals is still
poor. When it comes to patient usage, most of the advances are being tested on
healthy, abled bodied subjects and the required adaptation for disabled people and
in real noisy environments are not being studied extensively. Asynchronous (or
self-paced) BCIs are more suitable for the disabled as these give additional ON/OFF
3
Peeking over the shoulder to steal another person
'
s password.
