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SCREEN TASK EXPERIMENTS FOR EEG SIGNALS BASED ON SSVEP BRAIN COMPUTER INTERFACE.

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Development BCI system based stay state visual evoked potential (SSVEP), require establish the characteristics of the stimuli presented the user for optimal development of the extraction of signal characteristics; for it is necessary to determine the stimulation system and the evidence perform for detecting events. There are many types of stimulators that can be used to evoke the SSVEP: monitors include cathode ray tube (CRT) and liquid crystal display (LCD) or an array of light emitting diode (LED). This paper aims to show the different tests and methodologies that have been presented in different studies for generating visual stimuli in a short period of time.
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  ISSN: 2320-5407 Int. J. Adv. Res. 6(2), 1718-1732 1718    Journal Homepage: -  www.journalijar.com   Article DOI:  10.21474/IJAR01/6568 DOI URL:  http://dx.doi.org/10.21474/IJAR01/6568 RESEARCH ARTICLE SCREEN TASK EXPERIMENTS FOR EEG SIGNALS BASED ON SSVEP BRAIN COMPUTER INTERFACE. S. M. Fernandez-Fraga 1 , * M. A. Aceves-Fernandez 2 ,   J. C. Pedraza-Ortega 2  and J. M. Ramos-Arreguín 2 . 1.   Instituto Tecnológico de Querétaro, Av. Tecnológico s/n esq. Mariano Escobedo, Centro, 76000 Santiago de Querétaro, México. 2.   Universidad Autónoma de Querétaro, Cerro de las Campanas S/N, Querétaro, México. ……………………………………………………………………………………………………....   Manuscript Info Abstract …………………….   ………………………………………………………………    Manuscript History Received: 07 December 2017 Final Accepted: 09 January 2018 Published: February 2018 Key words:-   Steady State Visual Evoked Potential, SSVEP, Brain Computer Interface, BCI, EEG Signal Analysis.  Development BCI system based stay state visual evoked potential (SSVEP), require establish the characteristics of the stimuli presented the user for optimal development of the extraction of signal characteristics; for it is necessary to determine the stimulation system and the evidence perform for detecting events. There are many types of stimulators that can be used to evoke the SSVEP: monitors include cathode ray tube (CRT) and liquid crystal display (LCD) or an array of light emitting diode (LED). This paper aims to show the different tests and methodologies that have been presented in different studies for generating visual stimuli in a short period of time. Copy Right, IJAR, 2018,. All rights reserved. ……………………………………………………………………………………………………....   Introduction:- Brain Computer Interfaces (BCI) are computer systems that translate the electrophysiological brain activity signals that can be measurable by an electro / electronic device. BCI aim to provide a channel of non-muscle communication for sending commands to the outside world using the electrical activity of the brain. Brain interfaces monitor the brain activity of the user and translate this signals into commands without activating any muscle or  peripheral nerve [16]. Brain states are the result of different patterns of neural interaction. These patterns result in waves, which are characterized by different amplitudes and frequencies. The human brain electrical activity present due to two causes. The first is internal, that is, due to inadvertent operation and control of respiration, digestion etc. and will of the individual, to move your body, speak or think, etc. The second cause of brain activity is the occurrence of external stimuli, through a bodily sense. BCI systems can be classified according to their implementation: ã   Invasive systems: Are implanted directly into the brain and its main application is in the area of prosthesis to restore limb movement. ã   Non-invasive systems: The signal acquisition is performed using electrodes placed on the scalp surface (Figure 1) [3]. Corresponding Author:- M. A. Aceves-Fernandez. Address:- Universidad Autónoma de Querétaro, Cerro de las Campanas S/N, Querétaro, México.  ISSN: 2320-5407 Int. J. Adv. Res. 6(2), 1718-1732 1719 A.   (b) Figure 1:-  (a) Invasive System, implemented electrodes directly into the brain. (b) Non-invasive system, EEG cap  based on international 10-20 system. The BCI systems can be classified according to the acquisition of signals: ã   Endogenous Systems:  based on brain rhythms, or systems depend on the user's ability to control their electrophysiological activity, such as EEG amplitude in a specific frequency band over a particular area of the cerebral cortex. We can classify the endogenous systems based on motor imagery (MI), sensorimotor rhythms or based on slow cortical potentials (SCP) BCI systems. Endogenous systems require a period of intensive training. A MI-based BCI is a paradigm of two or more classes of motor images (moving left or right hand, feet, tongue, etc.) or other brain tasks (rotation of a cube, performing arithmetic, etc.). MI-based BCI systems have variations for both the execution of a real movement to the imagination of a move or preparing to it. A BCI-based on SCP involves slow changes in voltage generated on the cerebral cortex, with a variable duration between 0.5 and 10 seconds. They are typically associated with movement and other functions involving cortical activation. It has shown that people can learn to control these potential. ã   Exogenous Systems: These are based on event-related potentials (ERP) depend on the electrophysiological activity evoked by external stimuli and do not require intensive training phase systems. We can classify the BCI systems in potential exogenous events P300 systems based on visual events potentials (VEP), based on potential for steady-state visual events (SSVEP) or based on auditory events potentials (AEP) systems. BCI systems based on P300. They refer to a peak amplitude on the EEG approximately 300 ms after a rare auditory or visual stimulus occurred. VEP based BCI systems and SSVEP detected in the EEG recorded on the visual area of the cerebral cortex after the user a visual stimulus has been applied. Based AEP BCI systems are detected in the EEG recorded on the auditory area of the cerebral cortex presenting the user with sound sources at different frequencies, the user to focus on any of them, generates a potential of the same frequency as the stimulus [3],[5]. ã   Steady State Visually Evoked Potentials (SSVEP):- The retina of human eye contains rod and cone cells. The rod cells detect the amount of light and cone cells distinguish the color. 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 (Figure 2). 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 to different colors; as a result, the SSVEP strength elicited with different colors of the stimuli will be different [14]. Figure 2:-  (a) Cross section through a human eye. (b) Schematic view of the retina including rod and cone light receptors (adapted from Encyclopedia Britannica), 1994-Shubert, 2006.    ISSN: 2320-5407 Int. J. Adv. Res. 6(2), 1718-1732 1720 Figure 3:-  Training time versus communication bitrate for the three main types of noninvasive BCIs [8].   In SSVEP based BCIs, visual stimulus modulated at different frequencies are simultaneously presented to the user. Each pattern is associated with an action in an output device. When the user focuses attention on a certain pattern, the corresponding stimulating frequency, or its harmonics, dominantly appears in the spectral representation of the EEG signals recorded at occipital sites. The action associated to the dominant frequency is performed [8]. The amplitude of the SSVEP is not the same for different stimulation frequencies or different subjects. In fact, the largest SSVEP amplitude occurs, in average, at a stimulation frequency of about 15 Hz, Figure 4 [8]. Figure 4:-  Average SSVEP amplitude in function of the stimulation frequency [8].   SSVEP-based BCI experiments need to reproduce a flickering stimulus at a constant frequency. We use the computer screen as the flickering device. Thus, a flickering object (or “target”) is represented by a shape which changes color at a constant frequency or/and object to appear suddenly. In order for the SSVEP to work, this change of color/object must occur at precise intervals. It is thus necessary to redraw the targets in a synchronized way with the screen. This means, that the frequencies, which can be achieved, depend on the refresh rate of the screen. In  practice, only frequencies that are entire divisions of the screen‟s base refresh rate will be possible to achieve [13].  On a screen with refresh rate of 60 Hz, displaying of frequencies of 30, 20, 15, 12, 10 Hz and lower will be possible. On a 50 Hz screen, you will be able to use 25, 16.66, 12.5, 10 and lower frequencies as shown on Figure 5. Figure 5:-  Stimulation frequencies on a 60 Hz screen [13].  ISSN: 2320-5407 Int. J. Adv. Res. 6(2), 1718-1732 1721 ã   SSVEP Task Experiments ã   Repetitive Visual Stimulus (RVS) In SSVEP research, three main categories of repetitive visual stimuli exist: ã   Light stimuli .- are rendered using light sources such as LEDs, fluorescent lights and Xe-lights, which are modulated at a specified frequency. These devices are generally driven by dedicated electronic circuitry, which enables them to accurately render any illumination sequence or waveform (Figure 6). The intensity (time integrated luminance) of the light stimulus is measured in candelas per square meter on the second (cd/sm 2  or nits/s) because the light luminance changes over time. An important parameter to quantify the stimulus strength is the modulation depth  which is defined as lmax − lmin/lmax+ lmin , where lmin, lmax are the minimum and maximum luminance, respectively [20]. Figure 6:-  Led Light dives for visual stimuli ã   Single graphics stimuli.-  Some geometric figures (e.g., rectangle, square, or arrow) are rendered on a computer screen and appear from and disappear into the background at a specified rate (Figure 7). The stimulation rate is reported as the number of full cycles per second, normally simply referred to as the frequency of the stimulus [20].  Figure 7:-  Single graphic stimuli, the graphical object alternately appears and disappears in the background [20]. ã   Pattern reversal stimuli.- Are rendered on a computer screen by oscillatory alternation of graphical patterns, for example, checkerboards. They consist of at least two patterns that are alternated at a specified number of alternations per second. Frequently used patterns include checkerboards and line boxes (Figure 8). Patterns are usually colored in black and white. A checkerboard stimulus is characterized by the subtended visual angle of each tile (spatial frequency), the number of reversals per second, the mean luminance, the field size, and the  pattern contrast. It is worth noting that single graphic stimuli could be viewed as a special case of pattern reversal stimuli where the graphic is the first pattern and the second pattern is the background. An important difference is that single graphic stimuli elicit an SSVEP response at the frequency of one full cycle (e.g., two alternations), whereas real pattern reversal stimuli elicit an SSVEP response at the frequency of one alternation. All repetitive visual stimuli have various properties such as frequency, color, and contrast. Both the type and  properties of stimuli affect the elicited SSVEP response [20]. Figure 8:-  Pattern reversal stimuli, at least two patterns are alternated at a specified frequency [20].

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Mar 17, 2018
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