Biomedical Engineering Reference
In-Depth Information
EEG-operated TV controller, demonstrating the practicability of phase coding in
SSVEP- based BCI systems.
8.2.3.2 Coding Approach: Frequency Domain Versus Temporal Domain
According to the VEP signals used for information coding, VEP-based BCIs fall into
two categories: transient VEPs and SSVEPs. The first category uses TVEPs to detect
gaze direction. Spatial distributions of TVEPs elicited by a stimulus located in differ-
ent visual fields were used by Vidal in the 1970s to identify visual fixation [28].
According to the approach for information coding, the SSVEP-based BCIs can be
further divided into time-coded and frequency-coded subgroups. Hereafter, we refer
to them as
t
SSVEP and
f
SSVEP, respectively. The BCI system described in Section
8.2.2 employs the
f
SSVEP approach. Instead of using a periodic flashing with fixed
time interval between flashes, in Sutter's VEP-based BCI system, the occurrence time
of visual flashes was not periodic (although it has a short interval as required by
SSVEPs). The varying temporal patterns of these flashing sequences make it possible
to discriminate among targets, thereby falling into the category of
t
SSVEP.
So far, both frequency decoding and temporal decoding strategies have been
employed in VEP-based BCI research. Feature extraction of the TVEP is based on
waveform detection in the temporal domain [38, 39]. Similarly, a template matching
approach by cross-correlation analysis was used to detect the
t
SSVEP in the BRI sys-
tem [29]. For a frequency-coded design, the amplitude of the
f
SSVEP from multiple
flashing targets is modulated by gaze or spatial attention, and detected by using
power spectral density estimation. Note that analysis of the TVEP and
t
SSVEP
methods needs accurate time triggers from the stimulator, which can be omitted in
frequency amplitude-based detection of the SSVEP.
8.2.3.3 Muscular Dependence: Dependent Versus Independent BCI
According to the necessity of employing the brain's normal output pathways to gen-
erate brain activity, BCIs are divided into two classes: dependent and independent
[2, 40]. The VEP system based on gaze detection falls into the dependent class. The
generation of the desired VEP depends on gaze direction controlled by the motor
activity of extraocular muscles. Therefore, this BCI is inapplicable for people with
severe neuromuscular disabilities who may lack reliable extraocular muscle control.
Totally different from amplitude modulation by gaze control, recent studies on
visual attention also reveal that the VEP is modulated by spatial attention and fea-
ture-based attention independent of neuromuscular function [41, 42]. These find-
ings make it possible to implement an independent BCI based on attentional
modulation of VEP amplitude. Only a few independent SSVEP-based BCIs have
been reported, in which the amplitude of SSVEPs elicited by two flashing stimuli
were covertly modulated by the subject's visual attention, without shifting gaze [34,
40, 43]. Compared with the dependent type, this attention-based BCI needs more
subject training, attention, and concentration. The amplitude of SSVEP elicited by
attention shifting is much lower than that elicited by gaze shifting, which poses a
challenge when pursuing a high information transfer rate [34, 40].
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