Biomedical Engineering Reference
In-Depth Information
amplitude of several hundreds of microvolts. As a steady-state response, oscillatory
EEG is usually time locked to the onset of an external stimulus or internal event,
without strict phase locking [25].
Some transient evoked potential-based BCIs, such as the P300 speller [18, 19],
show promising performance for real application with locked-in patients [26].
However, from the perspective of signal acquisition and processing, the oscillatory
EEG-based BCI has several advantages over the ERP-based BCI: (1) The oscillatory
EEG has a larger amplitude and needs no dc amplification, which greatly reduce the
requirement of the EEG amplifier; (2) the oscillatory EEG is much less sensitive to
low-frequency noise caused by eye movement and electrode impedance change,
comparing with ERP; (3) the oscillatory EEG is a sustained response and requires
merely coarse timing, which allows for the flexibility of asynchronous control,
whereas for ERP-based BCI, stimulus synchrony is crucial for EEG recording and
analysis; and (4) with amplitude and phase information easily obtained by robust
signal processing methods, such as the FFT and Hilbert transform, there are more
flexible ways of analyzing oscillatory EEGs than ERPs in a single trial fashion.
For these reasons, the oscillatory EEG-based BCI will be the focus of the follow-
ing two sections of this chapter. Two major oscillatory EEG-based BCIs, SSVEP and
SMR-based BCI, are introduced, along with details of their physiological mecha-
nism, system configuration, alternative approaches, and related issues.
8.2
SSVEP-Based BCI
8.2.1 Physiological Background and BCI Paradigm
Visual evoked potentials (VEPs) reflect the visual information processing along the
visual pathway and primary visual cortex. VEPs corresponding to low stimulus
rates or rapidly repetitive stimulations are categorized as transient VEPs (TVEPs)
and steady-state VEPs (SSVEPs), respectively [27]. Ideally, a TVEP is a true tran-
sient response to a visual stimulus that does not depend on any previous trial. If the
visual stimulation is repeated with intervals shorter than the duration of a TVEP,
the response evoked by each stimulus will overlap each other, and thus an SSVEP is
generated. The SSVEP is a response to a visual stimulus modulated at a frequency
higher than 6 Hz [25]. SSVEPs can be recorded from the scalp over the visual cortex,
with maximum amplitude at the occipital region (around EEG electrode Oz).
Among brain signals recorded from the scalp, VEPs may be the first kind used as
a BCI control. After Vidal's pilot VEP-based BCI system in the 1970s [28] and
Sutter's VEP-based word processing program with a speed of 10 to 12 words/min-
ute in 1992 [29], Middendorf et al. [15] and Gao et al. [30] independently reported
the method for using SSVEPs to determine gaze direction.
Two physiological mechanisms underlie SSVEP-based BCI. The first one is the
photic driving response [25], which is characterized by an increase in amplitude at
the stimulus frequency, resulting in significant fundamental and second harmonics.
Therefore, it is possible to detect the stimulus frequency based on measurement of
SSVEPw. The second one is the central magnification effect [25]. Large areas of the
visual cortex are allocated to processing the center of our field of vision, and thus
the amplitude of the SSVEP increases enormously as the stimulus is moved closer to
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