N200-speller using motion-onset visual response Bo Hong *, Fei Guo, Tao Liu, Xiaorong Gao, Shangkai Gao Department of Biomedical Engineering, School of Medicine, Tsinghua University, Beijing 100084, China a r t i c l e i n f o Article history: Accepted 23 June 2009 Available online 28 July 2009 Keywords: Brain���computer interface Motion-onset visual evoked potential N200 P300 Support vector machine a b s t r a c t Objective: This study presents a brain���computer interface (BCI) named N200-speller. A matrix of motion stimuli are displayed for inducing the motion-onset visual response that allows the subject to spell out a message by scalp EEG. Methods: The brief motion of chromatic visual objects embedded in a 36 virtual button onscreen inter- face is employed to evoke a motion-onset specific N200 component. The user focuses attention on the button labeled with the letter to be communicated and performs color recognition task. The computer determines the target letter by identifying the attended row and column respectively. A support vector machine (SVM) is used in the target detection procedures of the BCI system. Results: Ten subjects participated in this study. The neurophysiological characteristics of the N200-spel- ler were compared with the classical P300-speller. The two paradigms elicit components with distinct spatio-temporal patterns. Classification of the data registered from all subjects reveals that the N200- speller achieves a comparable target detection accuracy with that of the P300-speller, given the same number of trials considered. Conclusions: With the advantages of low contrast and luminance tolerance, the proposed motion-onset stimulus presentation paradigm can be applied to brain���computer interface. Significance: A novel motion-onset paradigm N200-speller is proposed and assessed for BCI spelling application. �� 2009 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. 1. Introduction Brain���computer interface (BCI) used as a communication aid enables a subject to select a choice among a set of items (Wolpaw et al., 2002 Lebedev and Nicolelis, 2006 Schwartz et al., 2006). For the purpose of communication, an important application is the BCI speller, an example of which is the P300-speller, which allows the subject to spell out a message when successively choosing from the alphabet via his or her neural signal. Previous studies have demonstrated that the P300-based BCI, can serve as an effective speller device (Farwell and Donchin, 1988 Donchin et al., 2000 Krusienski et al., 2007 Sellers et al., 2006). Farwell and Donchin described the P300 BCI as a system that exploited an oddball par- adigm to select among a sequence of characters. The oddball para- digm was designed with randomly intensifying either a row or column of a six by six matrix that was displayed continually. The user should focus attention on one of the 36 cells of the matrix. The row and column intensifications that intersect at the attended cell represent the target stimuli, which occur with a probability of 1/6 and will elicit a P300 response. Farwell and Donchin demon- strated the feasibility of this version of BCI named ���P300-speller���. Up to the present, P300 related BCI has focused on feature extrac- tion and classification procedures to enhance the information transfer rate and applicability of the system (Xu et al., 2004 Serby et al., 2005 Thulasidas et al., 2006 Piccione et al., 2006 Zhang et al., 2008 Lenhardt et al., 2008 Nijboer et al., 2008). However, one problem associated with the P300-speller is that this paradigm uses visual stimuli of intensification, which may cause discomfort for some BCI users, especially in the case of long time use, and thus limits the user applicability for clinical or home use. This problem also exists in the flash or pattern reversal VEP based BCI. Conse- quently, seeking for new BCI modality has become a crucial need in the development of user-friendly BCI systems. Based on our pre- vious work on a visual motion-onset BCI paradigm (Guo et al., 2008), here we propose a new type of BCI speller, which employs visual motion response from the dorsal pathway of the visual sys- tem and therefore allows for the use of non-flash visual stimuli. Motion-onset paradigm has the advantage of low luminance and low contrast required by the stimuli to elicit prominent mo- tion visual evoked potentials (mVEP). Some mVEP studies indi- cate that a very low luminance of 0.003 cd/m2 and a contrast in the range of 2% are sufficient to evoke mVEP (Kuba and Kubova, 1992 Dodt and Kuba, 1995 Kubova et al., 2004). Motion-onset 1388-2457/$36.00 �� 2009 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2009.06.026 * Corresponding author. Tel.: +86 10 62795985 fax: +86 10 62792416. E-mail address: hongbo@tsinghua.edu.cn (B. Hong). Clinical Neurophysiology 120 (2009) 1658���1666 Contents lists available at ScienceDirect Clinical Neurophysiology journal homepage: www.elsevier.com/locate/clinph

VEP is composed of three main peaks, P100, N200 and P200, of which the N200 is the most distinct response to motion-onset. The motion-specific N200, with a latency of around 200 ms, seems to be generated from the extrastriate temporo-occipital and associated parietal cortical areas (Kuba and Kubova, 1992 Probst et al., 1993 Skrandies et al., 1998). Employing the mo- tion-onset VEP, Guo et al. reported a design of a novel BCI using the motion-onset paradigm, which provided a reliable way of determining the subject���s choice of visual target. In their five- class configuration, robust mVEP was evoked by using a small visual field (1.24�� 0.76��) which allows for a higher number of targets to be displayed in the BCI application. The accuracy of offline target detection confirmed that the motion-onset para- digm is feasible for a BCI system. In the current study, we design a visual BCI named ���N200-speller���, which shares spatial and tem- poral configurations that are similar to those of the P300-speller but is based on N200 visual motion response. The primary differ- ence between these two paradigms is that the N200-speller em- ploys the motion-specific N200 which is an exogenous component, while the P300-speller uses the P300 which is an endogenous component. The purpose of the present study is to evaluate the motion-on- set based ���N200-speller��� as a novel BCI paradigm. Motion stimuli are embedded into the six rows by six columns onscreen alphabet buttons for user selection. When the subject gazes at the desired button and performs recognition tasks, the motion stimuli corre- sponding to the target button will elicit motion-onset visual evoked potentials, which are used to determine the subject���s intention of choice. The spatiotemporal patterns of the motion- onset response were analyzed by the recorded EEG data from ten subjects. The motion-specific N200 component from occip- ito-temporal and parietal electrodes is selected as salient feature of the brain responses to the target in the N200-speller. Finally, the target detection performance of the N200-speller is assessed. Given the same number of trials considered, classification of the data registered from 10 subjects suggests that the proposed N200-speller can achieve a comparable performance with that of the P300-speller. 2. Methods 2.1. Experimental setup The visual stimuli are displayed on a 17-inch LCD monitor with 60 Hz refresh rate and 1280 1024 resolution, being viewed from a distance of 50 cm. Fig. 1 shows the spatial configuration of the N200-speller paradigm while Fig. 2 displays the temporal protocol of visual motion in the N200-speller. Fig. 1a gives a demonstration of the N200-speller interface, which is composed of 36 virtual buttons in the organization of six rows by six columns. In each button, a vertical bar with a height of 0.66�� visual angle appears (motion-onset) at the right border of a vacant rectangle and moves leftward at the velocity of 3.10 deg/s before it disappears (motion offset), forming a brief motion stimu- lus. The entire process of onset, motion and offset takes 140 ms. The color of the vertical bar, which might be red, green, blue, pur- ple, yellow or brown, is randomly designated by the stimulus soft- ware in such a manner that the colors of the six bars in the same row or the same column are different from each other. The random color is employed in order that the subject can perform color rec- ognition task to enhance attention on the desired target. The chro- matic moving bar and the rectangle inside which the bar appears compose a virtual button. A total of 36 such cells, consisting of 26 letters and 10 digits, form a virtual keyboard on a white background. Fig. 2 displays the timing protocol of the stimuli presentation in the N200-speller paradigm. The motion stimuli in each of the 36 cell buttons occur in a random order by row, or by column. A trial is defined as a complete series of motion stimuli in all the rows and columns of the keyboard. Each trial consists of 12 stimuli in a ran- dom sequence dedicated to the six rows and six columns respec- tively. The stimulus onset asynchrony (SOA) between two motion stimuli is 200 ms. Thus, the duration of a trial is 2400 ms, and the interval between two trials is 400 ms. During one acquisition period of 15 trials (one block), the par- ticipant is instructed to attend to a specific desired button (called ���target���) and hold immovable sightline. While watching the target, N2-speller C D E F B A I J K L H G O P Q R N M U V W X T S 0 1 2 3 Z Y 6 7 8 9 5 4 P300-speller C D E F I J K L O P Q R U V W X 0 1 2 3 Z Y 6 7 8 9 5 4 15 15 0.93 0.73 1.90 1.70 2.63 2.63 0.66 (a) (b) Fig. 1. Spatial configuration of the N200-speller and the P300-speller: 26 letters and 10 digits are arranged in the six rows and six columns of the virtual keyboard. (a) The N200-speller: A row or column of vertical bars appear and move leftward for 140 ms every 200 ms. The color of the vertical bar, which might be red, green, blue, purple, yellow or brown, is randomly designated in such a protocol that the colors of the six bars in the same row or the same column are different from each other. (b) The P300- speller: A row or column (filled with alphabet letters) intensifies for 140 ms every 200 ms. The intensification sequence is randomly designated in such a manner that all six stimuli (six rows or six columns) appear before any is repeated. (For interpretation of color mentioned in this figure legend, the reader is referred to the web version of the article.) B. Hong et al. / Clinical Neurophysiology 120 (2009) 1658���1666 1659