Biomedical Engineering Reference
In-Depth Information
trodes. Generally, the electrode giving the strongest SSVEP, which is generally
located in the occipital region, is selected as the signal channel.
The location of the reference channel is searched under the following consider-
ations: The amplitude of this channel's SSVEP should be lower and its position
should lie in the vicinity of the signal channel such that the noise component is simi-
lar to that in the signal channel. A high SNR can then be gained when the potentials
of the two electrodes are subtracted. Figure 8.4 shows an example of a significant
enhancement of the SSVEP SNR derived from the lead selection method.
Most of the spontaneous background activities are eliminated after the subtrac-
tion; the SSVEP component, however, is retained. Details of this method can be
found in previous studies [31, 36]. According to our observations, although the
selection varies across subjects, it is relatively stable for each subject over time. This
finding makes the electrode selection method feasible for practical BCI application.
For a new subject, the multichannel mapping only needs to be done once to optimize
the lead position.
In tests of the system based on frequency features (dialing a telephone number),
with optimized system parameters for five participants, an average ITR of 46.68
bits/min was achieved.
8.2.3 Alternative Approaches and Related Issues
8.2.3.1 SSVEP Feature: Amplitude Versus Phase
In the SSVEP BCI system based on frequency coding, the flickering frequencies of
the targets are not the same. To ensure sufficiently high classification accuracy, a
sufficient interval should be kept between two different frequencies such that the
number of targets is restricted. If phase information embedded in SSVEPs is added,
the number of flickering targets may be increased and a higher ITR should be
expected. An SSVEP BCI based on phase coherent detection was proposed [37], in
which two stimuli with the same frequency but different phases were discriminated
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Figure 8.4
Monopolar and optimal bipolar EEGs and their normalized power spectral density of one
subject. The frequency of target stimulation is 13 Hz, shown as a clear peak in the bipolar electrode
derivation, along with its harmonics at 26 Hz.
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