Biomedical Engineering Reference
In-Depth Information
successfully in their demonstration. Inspired by this work, we tried to further the
work by designing a BCI system with stimulating signals of six different phases
under the same frequency. Initial testing indicates the feasibility of this method.
For a phase-encoded SSVEP BCI, flickering targets on a computer screen at the
same frequency with strictly constant phase difference are required. We use the rela-
tively stable computer screen refreshing signal (60 Hz) as a basic clock, and six sta-
ble 10-Hz signals are obtained by frequency division as shown in Figure 8.5. They
are used for the stimulating signal of the flickering spots on the screen to control the
flashing moment of the spots. The flashing moments [shadow areas along the time
axis in Figure 8.5(a)] of the spots are interlaced by one refreshing period of the
screen (1/60 second). In other words, because the process repeats itself every six
times, the phase difference of the flashing is strictly kept at 60 degrees (taking the
flashing cycle of all the targets as 360 degrees). Six targets flickering at the same fre-
quency with different phases are thus obtained.
During the experiment, the subject was asked to gaze at the six targets respec-
tively. The spectrum value at the characteristic frequency (
f
0
=
10 Hz) was calculated
simply by the following formula:
1
N
[
]
()
∑
()
(
)
yf
=
xn
exp
−
j
2
π
f f n
s
(8.4)
0
0
N
n
1
where
f
s
is the sampling frequency (1,000 Hz) and data length
N
is determined by
the length of the time window. The complex spectrum value at 10 Hz can be dis-
played on a plane of complex value as shown in Figure 8.5(b). With a data length of
1 second, six phase clusters are clearly shown. The SSVEP and visual stimulus signal
are stably phase locked, sharing the same phase difference of 60 degrees between
targets. This makes it possible to set up several visual targets flickering under the
same frequency but with different phases so as to increase the number of targets
for choice. As an example, we used the system described to implement an
Imag
60 Hz
40
30
0°
20
°
10
*
60°
0
Real
°
120°
*
180°
−
10
−
20
−
30
°
240°
*
300°
−
40
−
40
−
30
−
20
−
10
0
10
20
30
(a)
(b)
Figure 8.5
An SSVEP phase interlacing design for discrimination of multiple screen targets. (a) Tim-
ing scheme for phase interlacing of six screen targets, with shadow areas indicating the ON time of
each screen target, with a reference to the cycles of CRT screen refreshing; and (b) phase clustering
pattern on the complex plane indicates a discriminability among the six targets.
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