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Brain–Computer Interfaces - Index of

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314 Y. Li et al.<br />

P300-based, and ERD/ERS-based BCIs. For more details on these brain patterns,<br />

please refer to Chapter “Brain signals for Brain–Computer <strong>Interfaces</strong>” <strong>of</strong> this book<br />

as well as the related references listed below.<br />

3.1 SSVEP-based BCIs<br />

SSVEP is one <strong>of</strong> the neurophysiological signals suitable for BCIs. Figure 5 shows<br />

a stimulation paradigm for a SSVEP-based BCI in which each button flashes with a<br />

specific frequency labeled in the figure. The labeled frequencies do not appear in the<br />

real interface. The user can select a number by gazing at a corresponding button. The<br />

SSVEP elicited by the visual stimulus is composed <strong>of</strong> a series <strong>of</strong> components whose<br />

frequencies are exact integer multiples <strong>of</strong> the flickering frequency <strong>of</strong> the stimulus.<br />

Several SSVEP-based BCI systems have been developed [6, 25, 26]. The amplitude<br />

and phase <strong>of</strong> SSVEP are highly sensitive to stimulus parameters such as the flashing<br />

rate and contrast <strong>of</strong> stimulus modules [5]. Gaze shifting may improve performance<br />

with SSVEP-based BCIs, but is not required, at least for some users. The importance<br />

<strong>of</strong> gaze shifting depends heavily on the display and task. For example, if there are<br />

many targets, or if they are located outside the fovea, gaze shifting may be essential<br />

[25, 26]. One possible feature extraction method used in SSVEP-based BCIs is as<br />

follows [6]:<br />

First, two EEG channels are recorded from electrodes placed over the visual<br />

cortex. Next, the EEG signal is filtered using a band-pass filter for removing noise.<br />

The EEG signal is subsequently segmented using a Hamming window for estimating<br />

the dominant frequency that corresponds to the flashing rate <strong>of</strong> the button <strong>of</strong> interest.<br />

The spectra <strong>of</strong> each segment is estimated using a fast Fourier transformation (FFT).<br />

Finally, a feature vector is then constructed from the spectra for classification.<br />

3.2 The P300-based BCI<br />

P300 is a positive displacement <strong>of</strong> EEG amplitude occurring around 300 ms after a<br />

stimulus appears. The P300, like SSVEP, is based on selective attention. This mental<br />

Fig. 5 A stimulation<br />

paradigm for eliciting SSVEP

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