**1.2. Steady-state visual evoked potential**

Interestingly, EP components can be subdivided into steady-state evoked potential (SSEP) and event-related potential (ERP), and ERD/ERS from motor imagination. Eventually, there are three main approaches employed by researchers to study electric signals generated from the brain activities. Following sections will elaborate discussion about these approaches.

This event-related potential is a function of uncertainty of the external stimuli, and major changes in the positive amplitude of the EEG waveform appears at about 300 ms after the stimulus which is called P300 component of ERP, first used by Sutton et al. [3]. The P300 component of ERP was tested in human by Farwell and Donchin, and their experiment revealed that the rare event elicits P300 which can be used to develop mental prosthesis [4]. Farwell and Donchin proposed alphanumeric BCI speller consisting of 26 alphabets and 10 numbers (0–9) arranged in 6×6 matrix of rows and columns as shown in **Figure 1a** [4]. In this row-column (RC) paradigm, rows and columns are flashed randomly and the subject is asked to count the number of flashings of rows and columns corresponding to the target character. Flashed row/ column containing target stimulus elicits P300 from parietal, occipital, and temporal regions (majorly in parietal) of the brain based on Oddball Paradigm, i.e., occurrence of rare (odd ball) event. The higher amplitude P300 is evoked from stimulus with higher strength and low probability (rare event). However, this paradigm suffers from low information transfer rate

Various changes in visual aspects of RC paradigm in terms of background color, character distance, and character size is done [5] to test the system performance. In this experiment, various visual protocols such as black background, white background, large symbol size, small symbol size, larger inter-symbol distance, and smaller inter-symbol distance are tested to observe the performance in RC BCI speller. Visual protocol with white background outper-

A region-based (RB) BCI paradigm was designed by [6] to reduce human perpetual errors such as attentional blink, repetition blindness, habituation, and other spatial errors such as crowding effect and adjacency problems. Human perpetual errors in P300 speller was demonstrated by [7] to show the effect of adjacency problems. RB paradigm shown in **Figure 1b** and **c** uses seven regions flash at two levels instead of rows and columns. The region containing

**Figure 1.** (a) RC paradigm with second row flashing [4]. (b) and (c) RB paradigm where seven sets of characters in level

formed all the other protocols, while small symbol size resulted in poor performance.

**1.1. P300 event-related potential**

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(ITR) due to multiple trials.

1 (b) is expanded in level 2 (c) to spell a character "B" [6].

The concept of visual evoked potential (VEP) was given by [13] using flash light and calculated evoked EEG signal by averaging to measure visual evoked responses from four parietal and occipital regions of scalp with bipolar electrodes. A clear high amplitude plot after 80 and 145 ms of the stimulus was found. VEPs, due to low stimulus rates, are classified as transient VEPs (TVEPs) and the repetitive high stimulations are under steady-state VEPs (SSVEPs). TVEP responses are during brain resting stage and if visual stimuli duration is shorter, evoked responses by each stimulus overlap each other and SSVEP is generated at steady state of brain excitation [14, 15].

SSVEP based on gaze detection falls into dependent BCI and is not suitable for ALS patients who cannot move their eyes. Gaze-independent SSVEP using LED interlaced square pattern for stimulation has been designed by [16]. People can shift attention among visual stimuli without shifting gaze, called as covert attention and overlapping stimuli can evoke changes in SSVEP which is sufficient to control BCI without gaze shifting like two overlapped images. Training for selective attention like playing certain types of computer games can improve SSVEP performance, and SSVEP systems are suitable to operate in challenging environments with distractions and noises like in homes and hospitals [17].

SSVEP visual stimuli are three main types as categorized below among which LED stimulation results in highest bit rate. All visual stimuli have properties like frequency, color, and contrast which affect SSVEP. Stimuli frequency can be divided into low (1–12 Hz), medium (12–30 Hz), and high (30–60 Hz) bands. Visual fatigues and false positives can occur at low frequency bands, whereas flash and pattern reversal stimuli can provoke epileptic seizures above low frequency bands. Red light has strong SSVEP response at 11 Hz but decreases at other frequency levels. However, the response decreases further for blue and yellow light. The three major types of visual stimuli for SSVEP are categorized as follows [18].

SSVEP response not only has the same fundamental frequency as stimulus but also includes higher harmonics and use of three SSVEP harmonics has resulted higher classification accuracy than for one or two harmonics [22]. SSVEP-based BCI has many advantages over

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Asynchronous SSVEP-based BCI using flickering lights was used to control neuro-prosthetic devices for restoration of grasp function in spinal cord injured people [23] and as a functional electrical stimulation for abdominal stimulation to augment respiration in tetraplegia [24]. An emergency call system using SSVEP-based brain switch was developed for ALS patients and they successfully called their guardians by simply starring at stimulus in about 6.56 s, starting the experiment by themselves. This system had fairly good performance when experimented up to 4 weeks. A chromatic visual stimulus with isoluminant red color is used to reduce intensity of the stimulus [25]. SSVEP-based BCI using single flicker stimulus is coded spatially to control four channels for navigation of 2-D computer games. Control channels are coded by their spatial position rather than flickering frequency or phase which may provide alternative route toward a practical SSVEP BCI with reduced visual strain [26]. To reduce visual fatigue from traditional SSVEP using flickering lights, an equal-luminance, ring-shaped, red-green colored checkerboard paradigm is used which produces high SSVEP power around 15 Hz [27]. Most people, despite no prior BCI experience, can use SSVEP BCI system even in a very noisy environment and better performances is observed in young and female subjects [28].

Sensorimotor rhythms (SMRs) are synchronized brain waves over sensorimotor cortex in three different frequency bands: μ (8–12 Hz), β (18–30 Hz), and γ (30–200 Hz). EEG recording is mostly limited to μ and β bands. SMR amplitude is higher during idle stage called as event-related synchronization (ERS) and the amplitude decreases when the sensorimotor areas are active due to a certain motor task or even during motor imagery (MI). This decrease in SMR amplitude is called event-related desynchronization (ERD). The ERD signal is used for MI-related BCI. ERS immediately occurs after ERD [29]. For MI tasks, the subjects are instructed to imagine themselves performing a specific motor action without actual motor

A novel typewriter "Hex-O-Spell" was presented in [31] using imagined right-hand and right foot movements shown in **Figure 3**. Five letters or symbols are inside six hexagons surrounding a circle having center arrow. Imagination of right-hand movement turns arrow clockwise and imagination of right foot movement stops the rotation and arrow extends to select a character if the imagination persists longer. A synchronous MI-based "Oct-O-Spell" paradigm is designed by [32] using 2-D cursor control with simultaneous MI tasks and claimed to be

output and there exists contralateral lateralization of left-hand/right-hand/foot [30].

other EEG-based BCI systems due to the following reasons [16].

• less susceptibility to eye movements and blink artifacts

• high signal-to-noise ratio

**1.3. Motor imagery**

feasible with higher efficiency.

• high information transfer rate

• require very little or no training


The effect of visual distractions in SSVEP is dependent on the level of attention requirement during the task and the nature of distractions. SSVEP amplitude and identification accuracy decreases in dynamic screen condition compared to static condition [19]. Visual stimuli with a frequency resolution of 0.1 Hz were classified with high accuracy sufficient for practical BCI and the factors affecting the SSVEP speller are distance between adjacent stimuli, light source arrangements, stimulating frequencies, electrode arrangements, and visual angles [20].

The frequency response of SSVEP is experimented in [21] using visual stimulation at 14 different frequencies within the range of 5–60 Hz and found that the primary visual cortex follows an activation pattern similar to SSVEP and the SSVEP amplitude peaks at 15 Hz stimulation shown in **Figure 2**.

**Figure 2.** Variation of SSVEP amplitude with respect to stimulus frequency [21].

SSVEP response not only has the same fundamental frequency as stimulus but also includes higher harmonics and use of three SSVEP harmonics has resulted higher classification accuracy than for one or two harmonics [22]. SSVEP-based BCI has many advantages over other EEG-based BCI systems due to the following reasons [16].

• high signal-to-noise ratio

medium (12–30 Hz), and high (30–60 Hz) bands. Visual fatigues and false positives can occur at low frequency bands, whereas flash and pattern reversal stimuli can provoke epileptic seizures above low frequency bands. Red light has strong SSVEP response at 11 Hz but decreases at other frequency levels. However, the response decreases further for blue and yellow light. The three major types of visual stimuli for SSVEP are categorized

• Light stimuli: light sources are LEDs, fluorescent lamps, Xe lights, etc., and their intensity

• Single graphics stimuli: rectangle, square, or arrow on computer screen that appear and disappear at specific rate and stimulation rate are the number of full cycles per second

• Pattern reversal stimuli: periodic alternation of graphical patterns are usually black and

The effect of visual distractions in SSVEP is dependent on the level of attention requirement during the task and the nature of distractions. SSVEP amplitude and identification accuracy decreases in dynamic screen condition compared to static condition [19]. Visual stimuli with a frequency resolution of 0.1 Hz were classified with high accuracy sufficient for practical BCI and the factors affecting the SSVEP speller are distance between adjacent stimuli, light source arrangements, stimulating frequencies, electrode arrangements, and

The frequency response of SSVEP is experimented in [21] using visual stimulation at 14 different frequencies within the range of 5–60 Hz and found that the primary visual cortex follows an activation pattern similar to SSVEP and the SSVEP amplitude peaks at 15 Hz stimulation

white such as line boxes, checkerboards, etc., on computer screen.

**Figure 2.** Variation of SSVEP amplitude with respect to stimulus frequency [21].

as follows [18].

visual angles [20].

shown in **Figure 2**.

is measured in candela per sq. meter.

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called frequency of stimulus.


Asynchronous SSVEP-based BCI using flickering lights was used to control neuro-prosthetic devices for restoration of grasp function in spinal cord injured people [23] and as a functional electrical stimulation for abdominal stimulation to augment respiration in tetraplegia [24]. An emergency call system using SSVEP-based brain switch was developed for ALS patients and they successfully called their guardians by simply starring at stimulus in about 6.56 s, starting the experiment by themselves. This system had fairly good performance when experimented up to 4 weeks. A chromatic visual stimulus with isoluminant red color is used to reduce intensity of the stimulus [25]. SSVEP-based BCI using single flicker stimulus is coded spatially to control four channels for navigation of 2-D computer games. Control channels are coded by their spatial position rather than flickering frequency or phase which may provide alternative route toward a practical SSVEP BCI with reduced visual strain [26]. To reduce visual fatigue from traditional SSVEP using flickering lights, an equal-luminance, ring-shaped, red-green colored checkerboard paradigm is used which produces high SSVEP power around 15 Hz [27]. Most people, despite no prior BCI experience, can use SSVEP BCI system even in a very noisy environment and better performances is observed in young and female subjects [28].
