**2.1. Stimulus types**

In SSVEP-based experiments, the user is asked to identify the target with eye-gaze. The attention of the user is supposed to be visually fixed on the target and the target is identified by feature extraction and its analysis [42]. In case of single graphic stimuli, stimulus appears and disappears at some particular rate just like displayed in **Figure 1**. In case of pattern reversal stimuli, at least two graphical patterns are displayed by alternative oscillations like shown in **Figure 2**. Such stimulus maybe of checkerboard or grating type.

With flashing stimulus, SSVEP appears as a sinusoidal-like waveform with fundamental frequency as that of blinking frequency of the stimulus. With graphic pattern stimulus, SSVEP appears at the reversal rate and their harmonics [8]. The SSVEP discrete frequency components stay intently constant in terms of amplitude and phase for long time [9].

SVM with One against All (OAA) strategy. The control signals were assigned for each of the five classes detected (7, 9, 11, 13 and rest signal). Corresponding to five classes, five movement positions such as forward (F), backward (B), left (L), right (R) and stop (S) were obtained.

SSVEP-Based BCIs

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http://dx.doi.org/10.5772/intechopen.75693

The SSVEP stimulus produces a response in the EEG signal, which is characterized by oscillations of the order of the stimulation frequency and sometimes at harmonics or sub harmonics

The ability of the human eye to distinguish colors is based upon the varying sensitivity of cone cells to the light of different wavelengths [46]. There are three kinds of cone cells and are conventionally labeled as short (S), medium (M), and long (L) cones according to the wavelengths of the peaks of their spectral sensitivities. S, M and L cone cells are therefore sensitive to blue (short-wavelength), green (medium-wavelength) and red (long-wavelength) light respectively. The brain combines the information from each cone cells to give different perceptions for different colors and as a result the SSVEP strength elicited with different colors of the stimuli will be different [46]. In this work blue, green, red and violet were selected as stimuli colors to explore how different colors influence the elicited SSVEPs and the performance of SSVEP based system. The research used, repetitive visual stimuli (RVS) with four different flickering frequencies was designed by using LabVIEW software (National Instrument Inc., USA). The front panel of RVS is shown in **Figure 3**. RVS with violet, red, green and blue flickering bars were designed as four different sets. The back ground color of the RVS was selected as black. The visual stimuli were square (4 × 4 cm) in shape and were placed on the four corners of the LCD screen. Four frequencies 7, 9, 11 and 13 Hz, i.e., in the low frequency range were selected by considering 60 Hz refreshing rate of LCD monitor [45]. In order to select any particular stimuli the four visual stimuli were separated in pair of two each, i.e., 7, 11 and 9, 13. Further in an interval of

(i) Low frequency subsystem (near 10 Hz). It gives the greatest SSVEP amplitudes.

(ii) Medium frequency subsystem (16–18 Hz). It gives medium amplitude. (iii) High frequency subsystem (40–50 Hz). It shows the smallest response.

of it. The visual system can be divided into three subsystems [45].

**Figure 2.** Pattern reversal stimuli.
