Laurent Bonnet

Two Brains, One Game: Design and Evaluation of a Multi-User BCI Video Game Based on Motor Imagery

Keywords: Brain Computer Interface, Game Design, Multiplayer games, Evaluation

How can we connect two brains to a video game by means of a BCI, and what will happen when we do so? How will the two users behave, and how will they perceive this novel common experience? In this paper we are concerned with the design and evaluation of multi-user BCI applications. We created a multi-user videogame called "BrainArena" in which two users can play a simple football game by means of two BCIs. They can score goals on the left or right side of the screen by simply imagining left or right hand movements. To add another interesting element, the gamers can play in a collaborative manner (their two mental activities are combined to score in the same goal), or in a competitive manner (the gamers must push the ball in opposite directions). Two experiments were conducted to evaluate the performance and subjective experience of users in the different conditions. In the first experiment we compared single-user situation with one multiuser situation: the collaborative task. Experiment 1 showed that multi-user conditions are significantly preferred in terms of fun and motivation compared to the single-user condition. The performance of some users was even significantly improved in the multi-user condition. A subset of well-performing subjects was involved in the second experiment, where we added the competitive task. Experiment 2 suggested that competitive and collaborative conditions may lead to similar performances and motivations. However the corresponding gaming experiences can be perceived differently among the participants. Taken together our results suggest that multi-user BCI applications can be operational, effective, and more engaging for participants.

I. INTRODUCTION

Brain-Computer Interface technology enables a user to send commands to a computer or other system using only his/her brain activity. The most common way to acquire such physiological signal is by using electroencephalography (EEG): several sensors are placed on the user's scalp to acquire the micro currents produced by the activity of neurons in the brain. The past decade has seen a widespread enthusiasm for this technology and its potential applications. Many researchers now drift from the original objective, helping disabled people to recover a means of interaction with their environment and surrounding [START_REF] Wolpaw | Brain-Computer Interfaces for communication and control[END_REF], to multimedia applications such as video games [START_REF] Lécuyer | Brain-Computer Interfaces, virtual reality and videogames[END_REF].

The video game context adds many new challenges for the BCI research community, as both the physical and virtual environments are usually highly complex [START_REF] Lotte | Brain-Computer Interfaces for 3D Games: Hype or Hope?[END_REF]. Indeed, the typical gamer is a healthy user that may produce a wide range of movements during the gaming experience, most of them disrupting the BCI itself. The virtual environment may also disturb the BCI usage as it produces many distractors: visual, tactile or auditory stimuli.

In spite of these challenges, video games hold a lot of potential for use in BCIs as they aim to entertain and motivate the users. As the motivation plays a major role in the success of BCI interaction [START_REF] Nijboer | The influence of psychological state and motivation on braincomputer interface performance in patients with amyotrophic lateral sclerosis a longitudinal study[END_REF], video games thus represent a highly relevant application field for training and mastering BCI systems. Previous studies by Leeb et al. [START_REF] Leeb | Brain-computer communication: Motivation, aim, and impact of exploring a virtual apartment[END_REF] or Ron-Angevin et al. [START_REF] Ron-Angevin | Brain-computer interface: Changes in performance using virtual reality techniques[END_REF] show how using Virtual Reality in BCI feedback improves the performances of the system, especially with naive or untrained users. Furthermore, the recent advances in the acquisition technologies, resulting in low cost EEG devices [7][8], make the use of BCI feasible in a gaming context outside the laboratories. Multiple studies have already tackled the use of EEG-based BCI in a video game context [START_REF] Nijholt | Turning shortcomings into challenges: Braincomputer interfaces for games[END_REF], regarding the interaction techniques and nature of feedback [START_REF] Lotte | Self-Paced Brain-Computer Interaction with Virtual Worlds: A Quantitative and Qualitative Study "Out of the Lab[END_REF], the performances [START_REF] Lotte | Self-Paced Brain-Computer Interaction with Virtual Worlds: A Quantitative and Qualitative Study "Out of the Lab[END_REF], or the subjective experience [START_REF] Plass-Oude Bos | A study in user-centered design and evaluation of mental tasks for bci[END_REF].

In this paper we focus on a particular interaction paradigm, which is already widely used in gaming in general but mostly neglected in BCI research so far: the multi-user interaction. The objective is to connect multiple users to the same video game application in real-time, through their brain activity.

We address several challenging questions. The first ones are related to the technical feasibility and design of the system itself. We seek to conceive and implement a multi-user BCI system for gaming purposes, which implies merging multiple BCI inputs. A tradeoff must be found between immersion and simplicity for the design of the feedback: while a complex and immersive feedback will be close to a commercial video game, a simpler one with reduced distractors could lower the mental workload, thus facilitating BCI use.

Secondly, how multi-user interaction differs from singleuser control is yet to be studied. Thus, we aim to qualify and quantify the influence of a multi-user paradigm on the BCI interaction, with regards to the performances and user impressions (e.g. motivation or enjoyment).

Therefore, we propose in this paper three contributions:

• BrainArena, a new multi-user BCI video game based on Motor Imagery. We present its concepts, architecture and implementation.

• The introduction of two paradigms of interaction for multi-user BCI applications : a collaborative mode (i.e. the users share a common goal and a common action), and a competitive mode (i.e. the two users have conflicting intentions, and their actions are opposed). • An evaluation of our system in these conditions, on 20 naive subjects. This paper is organised as follows. After reviewing the current state of the art regarding BCI video games, and multi-user interaction and feedback design, we present the BrainArena concepts, architecture and implementation. We then move on to the results and analysis of two evaluations we conducted on 20 naive subjects playing the BrainArena BCI game. The first experiment was comparing single-user condition with multiuser condition, whereas the second experiment was focused on collaborative versus competitive situations. We finally discuss these results and the potential applications of the multi-user BCI interaction in video games.

II. RELATED WORK

A variety of games using BCI interaction has already been developed. Some use the BCI explicitly as a control channel. In the MindBalance [START_REF] Lalor | Brain-Computer Interface based on the Steady-State VEP for immersive gaming control[END_REF] game the player was able to control the balance of his avatar by focusing on two flickering targets triggering different SSVEP responses. In the application of Lotte et al. Use the force! [START_REF] Lotte | Self-Paced Brain-Computer Interaction with Virtual Worlds: A Quantitative and Qualitative Study "Out of the Lab[END_REF], the player was able to lift a virtual spaceship by performing Motor Imagery (MI) of the feet (i.e. imagination of feet movements [START_REF] Pfurtscheller | Motor imagery and direct braincomputer communication[END_REF]). The control paradigm used Event-Related Synchronization (ERS) of the Beta rhythm. The Graz-BCI Game Controller developed by Scherer et al. [START_REF] Scherer | New input modalities for modern game design and virtual embodiment[END_REF] was able to connect any BCI or physiological sensor input to a game, using intelligent and context-aware tools. They used the Emotiv software suite to identify users' mental states (e.g. excitement or meditation) and facial expressions, to control an avatar in the famous online game World of Warcraft (WoW). The austrian company g.Tec presented at the CeBIT expo 2012 (Hanover, Germany) the IntendiX-SOCI (for Screen-Overlay Control Interface) [START_REF]Press release: intendixsoci: g.tec introduces mind-controlled computer gaming at cebit 2012[END_REF], that relied on SSVEP to select different items on a screen. Visitors could test it by controlling an avatar in WoW. Recent research has also been focused on implicit interaction with video games [START_REF] George | An overview of research on "passive" Brain-Computer Interfaces for implicit human-computer interaction[END_REF]. Nijholt et al. introduced in AlphaWow [9] the use of alpha activity to detect the stress level of the player, automatically adapting the avatar's form. Bacteria Hunt [START_REF] Muhl | Bacteria hunt: A multimodal, multiparadigm BCI game[END_REF] was a multimodal BCI game based on both explicit and implicit BCI interaction: while the avatar movements were modified by the user stress level (alpha), a target selection was done using SSVEP detection. For a more comprehensive overview of BCI use in video games, the interested reader can refer to [START_REF] Lécuyer | Brain-Computer Interfaces, virtual reality, and videogames[END_REF][9] [START_REF] Plass-Oude Bos | Brain-computer interfacing and games[END_REF].

While researchers were aiming at making BCI interaction possible in a gaming context, the question of multi-user interaction rose. Acknowledging the current trends in the video game usage (e.g. network connection, multimodal interaction) Nijholt in [START_REF] Nijholt | Towards Multimodal, Multi-Party, and Social Brain-Computer Interfacing[END_REF] described his objectives toward a multi-party and social application of BCI in the future. The author foresaw an integration of BCI interaction in our media-based social life (e.g. Video Games, Internet). A first step toward such multi-party social gaming was presented by Obbink in [START_REF] Obbink | Social interaction in a cooperative brain-computer interface game[END_REF], in which the author studied the influence on social interaction of using BCI control (Selection using SSVEP) in a 2-player game (Mind the sheep!) on the social interaction. As any cooperative task implies interactions between users, both physical and vocal, it may conflict with the BCI usage itself. EEG systems are prone to muscular artifacts and noise in such situations. The author qualified and quantified the social behaviors between the two players, such as utterances and empathic gestures. He addressed the influence of the BCI on the collaboration level, compared to mouse control condition. However this paper did not compare performances nor preferences of users in multiuser versus single-user scenarios. Blankertz et al. reported using the Berlin-BCI system in a 2-player environment [START_REF] Blankertz | A note on brain actuated spelling with the berlin brain-computer interface[END_REF], inspired by the famous video game Pong. This application was successfully used during the CeBIT expo 2006 (Hanover, Germany) on 2 subjects performing demonstrations all day. However to the authors' best knowledge there exists no formal description and evaluation of the system.

Another issue of BCI game design is the influence of the feedback presented to the user. The importance of feedback was raised early in the BCI community [START_REF] Wolpaw | Brain-computer interface technology: a review of the first international meeting[END_REF]. Neuper and Pfurtscheller reviewed the trends in BCI feedback in [START_REF] Neuper | Neurofeedback training for bci control[END_REF] and pointed out the effect of feedback during the training phase, and how its nature and computation mode influence future performances. Barbero et al. were interested in the feedback accuracy in [START_REF] Barbero | Biased feedback in brain-computer interfaces[END_REF]: during a motor-imagery task, participants were presented a biased feedback (strong or weak, positive or negative bias). This study concluded that subjects with low performances benefit from a strong positive bias in their feedback. Designing an appropriate feedback for BCI applications in general is still considered as a an interesting topic in the community. Introducing the BCI interaction in the video game world, where the feedback must be entertaining above everything else, is thus a challenge on its own. When working on BCI games, another key point is motivation, which plays a major role in any BCI interaction. Nijboer et al. studied this topic on severely paralyzed patients suffering from Amyotrophic Lateral Sclerosis (ALS) [START_REF] Nijboer | The influence of psychological state and motivation on braincomputer interface performance in patients with amyotrophic lateral sclerosis a longitudinal study[END_REF]. The importance of motivation also emerged from the work of Nijboer et al. [START_REF] Nijboer | An auditory brain-computer interface (bci)[END_REF], while the authors were designing an auditorybased BCI and evaluating it on disabled subjects. Influence of motivation has also been assessed by Kleih et al. on healthy subjects [START_REF] Kleih | Motivation modulates the p300 amplitude during bci use[END_REF]: by raising the motivation level of P300 speller users (with financial incentive), authors observed an increase in their performance. User acceptance and motivation is usually assessed through questionnaires and subjective appreciation. Plass-Oude Boss et al. emphasized the role of user experience evaluation [START_REF] Plass-Oude Bos | User experience evaluation in bci: Mind the gap![END_REF] in order to improve further designs of any BCI application, especially in an entertainment context, e.g., video games. This work was extended by Van de Laar et al. in [START_REF] Van De Laar | Perspectives on user experience evaluation of brain-computer interfaces[END_REF].

We can conclude from the literature that multi-user interaction using BCI control in a gaming application has been introduced only recently, and has not been studied in depth. There has at present not been any experiment that specifically compares solo and multi-player motor imagery-based BCI gaming,and how it may influence performances and user experience/preferences.

III. BRAINARENA: A MULTI-USER BCI GAME BASED ON MOTOR IMAGERY

We designed a multi-player video game concept called BrainArena, a football game controlled by hand motor imagery. The objective of two users is to imagine left or right hand movements to move a virtual ball towards a goal located on the left or right side of the screen, respectively. The design of this application covers a large panel of technical issues: synchronization of the EEG acquisition and processing, EEG signal processing and classification, video game visual rendering, multi-user feedback design, and real-time feedback rendering. The flexibility of our implementation enables easy switching between three different modes: single-user condition, collaborative and competitive multi-user conditions (see section III-D).

In this section we present first several definitions of multiuser interactions with video games to contextualise our work. We then move on to the hardware and software components of the BrainArena BCI game, and the different game modes developed.

A. Multi-user BCI interaction with video games

A single-user BCI interaction with a video game environment, presented in Figure 1, is a closed loop system. The EEG signals are first acquired from a user, then classified by a signal processing unit. From this classification the decision maker chooses in which mental state the user is considered, then the interaction technique (i.e. the system for transforming the interface input data to comprehensible information for the application [START_REF] Lécuyer | Using Eyes, Hands, and Brain for 3D Interaction with Virtual Environments: A Perception-Based Approach[END_REF]) uses this decision to produce a command. Finally the application gives a feedback to the user according to the effects of the command.

We consider a situation of multi-user BCI interaction when two or more people equipped with a BCI system share an interaction in a common environment. In a gaming context, multiuser interaction is also referred to as multi-player interaction. We can extend this principle to an environment where actors may be indistinctly people, objects or intelligent systems. This configuration is related to the multi-party interaction presented by Nijholt [START_REF] Nijholt | Towards Multimodal, Multi-Party, and Social Brain-Computer Interfacing[END_REF].

Multi-user BCI systems can connect users at four different levels in the loop, as illustrated in Figure 2. 2 presents a decision making from two motor imagery classifications, the mental state deduced from the most probable class. 3) At the level of the interaction technique, several dependent decisions can be combined to issue a multi-user command given to the application. Cumulating several actions on the same virtual or physical objects is a possible use. For example an object can be pushed according to two decisions, one for the direction and one for the distance. In Figure 2, the example #3 shows how two decisions (one hand motor imagery mental state for a direction and one SSVEP outcome for a selection) can be joined to produce a unique command. 4) At the level of the application, the feedback can be defined as the result of several independent interaction commands. A simple example would be several users controlling their independent avatars in a virtual world.

The example #4 in Figure 2 uses 2 separated BCI pipelines that issue different commands based on motor imagery, moving two different objects on screen. Single-user BCI interaction loop, illustrated by an example (in red/italic font) inspired by the use of motor imagery paradigm. Two bipolar channels are acquired on users scalps during a hand motor imagery trial. The signal processing outputs a class label with a probability (e.g. class left with 75% probability). The decision making concludes that the probability is high enough to decide the user mental state: a left hand motor imagery. In the interaction technique this decision is translated into a command for the application: move left. From this command the application produces a visual feedback: the ball on screen is moving left.

Any multi-user interaction in a video game can also be characterized regarding the objective of the interaction.

• Independent multi-user interaction: the users have distinct objectives while interacting with the environment. These objectives are independent and unrelated. • Collaborative multi-user interaction: two or more users share the same objective. They may choose different actions but their ultimate goal remains the same. • Competitive multi-user interaction: users objectives are in conflict. The goal may be different but ultimately the course of actions collides. Only a subset of users can win. As the objectives may be multiple in the same situation, collaborative and competitive interactions can both be present in the same context. For example in a sport video game, with two teams of users: the interaction within the teams are collaborative (e.g. move, pass), but competitive when facing the opposite team members (e.g. score a goal). Even in a fully collaborative game (e.g. two users resolving a puzzle game), a form of competition can always be found if the users define themselves a certain success metric (e.g. number of pieces of puzzle resolved). We predict that competitive behaviors are likely to happen with people experienced with gaming in general, even if the game principles and objectives are not designed as competitive. 2): Two hand motor imagery classifications are merged in the decision making, which here simply chooses the most probable mental state between two classes. (3): Two decisions, one hand motor imagery state and one focused target a BCI paradigm such as SSVEP, are merged to issue a unique command for the application: move the selected target to the left. ( 4): Two independant commands issued by 2 separated motor imagery BCI pipelines, are merged into the application to produce a complex feedback where two different objects (a ball and a goal keeper) are moved on a screen.

The BCI system we present in this paper makes the connection between users at the decision level. Each user has his/her own EEG acquisition and tuned BCI pipeline, that outputs a classification (a class label, left or right motor imagery, and a value depending on the classifier result). BrainArena uses the two classification results to decide which mental state is dominant and to what extent. In our case, mastering a BCI interface based on motor imagery is known to be difficult for untrained users. Thus we chose a simple environment with limited distractors.

B. Architecture

The architecture of the BrainArena BCI game is represented in Figure 3.

It is composed of several layers:

• EEG acquisition: caps and amplifiers acquire the EEG signals of both users in parallel. • EEG processing: EEG signals are processed and classified to identify the user's mental state. • Interaction paradigm: BrainArena takes the 2 BCI processing outputs to decide which is the dominant mental state, and to control the feedback. • Feedback: 3 possible feedbacks can be displayed according to the game mode (solo, collaborative, competitive). Our implementation of this architecture relied on 3 interconnected computers. We assigned the different functions described previously onto these regular desktop computers. The first one acquired the signals from 2 g.USBAmp amplifiers (g.Tec GMbH, Austria) and GAMMACaps with up to 16 active electrodes. This station also ran the multi-user video game mode when required. The two other stations were assigned to each subject, for the signal processing and the singleuser video game mode. The acquisition computer was used as a monitoring station for the experimenter. We based our implementation on the OpenViBE software platform [START_REF] Renard | Openvibe: An open-source software platform to design, test and use Brain-Computer Interfaces in real and virtual environments[END_REF] for the EEG acquisition, the signal processing and the output of commands for the external game application, and the interconnection between every software component. The video game programs were written using the Ogre3D rendering engine [START_REF]Ogre3d -open source 3d graphics engine[END_REF]. A picture of the apparatus is shown in Figure 4, and Figure 5 shows the system in use.

C. Video game design

The application was designed as a tradeoff between the complexity of the visual feedback and user's visual workload. We chose a very simple environment to avoid distractors that could disrupt BCI usage. We displayed a ball at the center of the screen on a black background. Goals were symbolized by two triangles on each side of the screen (see Figure 6 for an annotated screenshot). A green cross could be displayed in the center, with green or blue gauges going left or right during the game sessions.

The feedback gave two complementary pieces of information. First, the real-time feedback on the "intensity" of the commands given by the users was provided: a gauge which went left or right depending on whether left or right motor imagery was recognized, and whose length represented the actual intensity of the command. When two users were connected 3 gauges were displayed: the two single-user feedbacks plus a multi-user gauge in the middle for the resulting overall command, summing both users' commands. The lengths of the user gauges were directly proportional to the normalized output of the classifier(s) used in the BCI process (see section III-E). In multi-user modes, the sum of the output was divided by two, so that the resulting multi-user feedback was in the same range as the real-time feedback in the single-user mode. The sizes and directions of the gauges were updated in realtime, 16 times per second.

The second form of feedback was that the ball could roll horizontally when pushed by the mental commands (i.e. left or right hand motor imagery), acting like a cumulative feedback for the user. The real-time gauges could be viewed as a representation of these push forces applied to the ball, each push being added to the previous ones to move the ball. This ball had a physics-based behaviour, thus when pushed it acquired a velocity. This speed was attenuated by a viscosity parameter which slowed the ball gradually, or by an opposite push. Moving the ball to a given goal was the objective of the game, and would increase the users scores. Figures 7 and8 present this feedback during collaborative and competitive trials respectively.

D. Different paradigms: solo, collaborative and competitive

We designed three different paradigms, one single-user and two multi-user interactions. The Solo mode involves only one user, thus one BCI pipeline. The system asks the user to score a goal on the left or right side of the screen, by performing an imaginary movement of the corresponding hand. Figure 6 presents the application during a Solo trial. This paradigm was used to compare single and multi-user control in a BCI game. Fig. 6. Single-user mode: the user was instructed to imagine movements of the hands to move the ball into the designated goal. The background is presented here in white for better visibility, however the original configuration uses a black background.

In multi-player gaming, people are expected to work together to achieve a goal and/or work against each other to be the best. Thus we designed two versions of our multi-user BCI game: a collaborative mode, where players are supposed to join forces to achieve the goal and improve their score, and a competitive version where they must perform better than their opponent.

The Collaborative version (Figure 7) received inputs from two BCI systems. The two users shared the same objective: moving the ball to the left or right goal. The instruction was displayed for each user beside their name (USER 1 and USER 2, see Figure 7). The application displayed the gauge feedback of both users. Between users' individual feedbacks, the multiuser feedback was presented in blue as the sum of both feedbacks. The ball was pushed by the multi-user feedback. In this context, scoring a goal could become very challenging as one user performing poorly could easily prevent the ball from reaching the goal. This was confirmed by beta testers before the evaluation. To avoid the frustration that may be created by such situation, the ball was automatically animated at the end of each trial to reach the nearest goal.

The Competitive mode (Figure 8) was similar to the collaborative one except that the instructions given to the two users were opposite. For example when the first user had to score in the left goal, the second user had to score in the right one. Scoring a goal in this situation was even more challenging. To enhance the competition between users the ball was again animated at the end of each trial, reaching the nearest goal automatically. Thus one user would always score a goal against the other, at the end of each trial. Fig. 7.

A collaborative trial: users should score in the right goal. The background is presented here in white for better visibility. Fig. 8.

A competitive trial: users should score in opposite goals. The background is presented here in white for better visibility.

E. EEG signal processing

Our BCI was built around a classical pattern recognition scheme, which involves the following steps:

1) Acquisition of a training data set 2) Training of a subject-specific BCI model, i.e., of subjectspecific features and classifier. More precisely we used:

• subject-specific spatial filters obtained with the Common Spatial Pattern (CSP) algorithm [START_REF] Ramoser | Optimal spatial filtering of single trial eeg during imagined hand movement[END_REF]. • a Linear Discriminant Analysis (LDA) as classifier [START_REF] Lotte | A review of classification algorithms for EEG-based Brain-Computer Interfaces[END_REF], which employs the obtained CSP features as input. 3) Online use of the resulting subject-specific model. During the acquisition of the training set, the user was asked to perform left and right hand motor imagery according to visual instructions (see Section IV for details). A total of 40 training trials were collected for each subject (20 trials from each MI class). Each trial lasted 8 seconds, the instruction being displayed at t=3s. The subject had to perform the instructed imaginary hand movement continuously for 5 seconds.

Once the training data was acquired, it was used to optimize CSP spatial filters. To do so, EEG signals were first band-pass filtered in the mu and beta frequency bands (8-30Hz). Then, the CSP filters were optimized on these filtered EEG signals, using a 4 second time window extracted from each trial, starting 500ms after the visual instruction. As recommended in [START_REF] Blankertz | Optimizing spatial filters for robust eeg single-trial analysis[END_REF], we selected 6 CSP spatial filters, corresponding to the highest and lowest eigenvalues of the Generalized Eigen Value Decomposition used to optimized the filters.

Once the CSP spatial filters were obtained, they were used to compute input features for the LDA classifier. The features used were the logarithmic band power of the spatially filtered EEG signals in the 8-30Hz band, averaged over one second of signal. To train the LDA classifier, again a 4 second time window starting 500 ms after the visual instruction was extracted from each trial. Then, each of these 4 second windows were sub-divided into 48 1-second segments, with a step of 1/16th second (with overlap) between two consecutive segments. A CSP feature vector (as described above) was computed for each of these segments, and labeled according to the trial label (left or right). The resulting 40 × 48 feature vectors were used as training data for the LDA classifier.

During the online phase, CSP features and the LDA classifier were used to continuously classify the last 1-second segment of EEG signal (with a step of 1/16th second, with overlap, between two consecutive segments) into one of the two classes (left or right). Since LDA classification is based on the distance to a separating hyperplane, this distance was normalized and used as the basis of the feedback provided to the users. This signal processing scheme was applied to each user, each one ending with a specific set of CSP filters and an LDA classifier. The CSP and LDA training was performed before each session. It should be stressed that this EEG signal processing pipeline is independent from the paradigm used for the feedback (single-user, collaborative or competitive). In other words, when more than one user was involved the same processing was performed, just that the outputs of each user's EEG processing pipeline (i.e., each user's LDA output) were merged in the game. This decided which mental state was dominant and deduced the commands that produced the feedback.

F. Provisional conclusion

We designed a complete BCI system for multi-user interaction with a video game. We provided a high level architecture and a first implementation using 3 general purpose desktop computers (dual cores / 4 GB RAM). Fully operationnal, this system worked in real time: acquisition, EEG signal processing, classification and feedback rendering. We achieved with our setup a constant frame rate of 60 frames per second. The different components were interconnected on a local network, supporting easily the connection and reconnection of different video game programs. We proposed three different mode for our game: Solo, Collaborative and Competitive.

More versions could be easily developed and connected to the BCI system. Finally, this setup also had monitoring and recording capabilities, thus can be used in an experimental environment for evaluation purposes.

The following sections describe two experimental evaluations we conducted to evaluate the BrainArena BCI video game, and the influence of a multi-user gaming experience on the BCI performances and user preferences.

IV. EXPERIMENT 1: COMPARING SINGLE-USER VERSUS MULTI-USER CONDITIONS

This experiment evaluated the multi-user and the singleuser interaction paradigm. We aimed to study the influence of a multi-user situation on the performances and user experience of two users connected through BCI to the same video game program. Only two conditions were selected for this first evaluation: single-user condition (SOLO) versus a 2-user collaborative condition (COLLAB).

The classification accuracy during the online sessions was used as a performance metric. The reported performances are the maximal classification accuracy over the trial duration, as done by Graz [START_REF] Schlogl | Characterization of four-class motor imagery eeg data for the bci-competition 2005[END_REF] [START_REF] Ramoser | Optimal spatial filtering of single trial eeg during imagined hand movement[END_REF]. Through subjective questionnaires we also compared the user acceptance and enjoyment regarding the two interaction techniques. The questionnaire was divided into two parts. In the first part of the questionnaire, participants rated the two paradigms on a 7-point Likert scale, over several criteria. A second part of the questionnaire intended to gather subjective answers to various questions regarding the tasks and how they handled them, and their impressions on the collaborative task and feedback.

A. Population

The population consisted of 20 volunteers, all of them naive users of BCI technologies. The subjects age ranged from 23 to 52 years old (mean 31.1), 15 males and 5 females. From this group 10 pairs were formed. In 8 pairs the participants knew each other, and volunteered to participate together. The 2 other pairs were formed randomly with the 4 remaining volunteers.

B. EEG Configuration

The setup for this experiment used the apparatus previously described, with the following configuration. EEG signals were sampled at 512 Hz. We used 8 EEG channels located around the right and left motor cortices: C3, FC3, CP3, C1 and C4, FC4, CP4, C2. We assumed that CSP spatial filters can be reliably computed on this number of sensors since Ang et al. successfully used it with even less sensors (3 bipolar channels only) in their winning entry of BCI competition IV, data set 2b [START_REF] Ang | Filter bank common spatial pattern algorithm on bci competition iv datasets 2a and 2b[END_REF].

C. Procedure

The procedure was inspired by the Graz BCI [START_REF] Pfurtscheller | Graz-bci: state of the art and clinical applications[END_REF]. One session consisted of 40 trials, 20 left and 20 right, in a random order. At time t = 0 a cross was displayed on screen, marking the start of trial. At t = 3s the instruction was displayed as a left or right arrow, instructing the user to perform left or right motor imagery, respectively. At time t = 4.250s, the feedback began to be displayed (only in the online phase, not in the training phase). At time t = 8s the feedback phase ended. The pause between each trial was randomly chosen between 1.5 and 3.5 seconds. Each session lasted approximately 8 minutes. Participants had a 3 minutes break between sessions.

The experiment consisted of 5 sessions. The first session was the acquisition of a training set for the CSP filters and LDA classifier. During this first session no feedback was displayed, only the cross and instructions. The following sessions were either in the SOLO or COLLAB condition (2 SOLO, 2 COLLAB). Preliminary testers reported a better understanding of the instruction if they started in the SOLO condition. Therefore, the first session was always in the SOLO condition, the condition order for the other sessions being randomly chosen.

Before starting the experiment, we instructed the user on how to perform hand motor imagery. As recommended in [START_REF] Neuper | Imagery of motor actions: Differential effects of kinesthetic and visualmotor mode of imagery in single-trial EEG[END_REF] we asked the user to perform kinesthetic imagery (feeling the sensations of movement, first person process) rather than visual imagery (seeing yourself doing the movement, third person process).

The whole experiment, installation and explanations included, lasted approximately 90 minutes.

D. Results

The mean classification accuracies obtained in each condition are displayed in Table I. The mean accuracy for the SOLO condition was 71.3% while it was 73.9% for the COLLAB condition. This difference was not found to be significant with a paired t-test although it did show a trend (p = 0.06). Looking at the course of performance over sessions do not suggest that a learning effect occured. The average performance curves are indeed rather flat. This is consistent with observation in [START_REF] Kübler | How much learning is involved in BCI-control?[END_REF] that suggest that there is no learning on such a small number of sessions.

We divided the subject pool into 2 subgroups, according to their performance levels. The Winner subgroup consisted of the dominant participants of each pair (best mean overall accuracy). The Loser subgroup was the other half, with the worst mean accuracy of each pair. As shown in Table I, in the Winner group the mean classification accuracy in SOLO condition was 75.0%, and 80.0% in the COLLAB condition. This difference was found to be significant with a paired t-test (p = 0.01). The Loser group showed no significant differences between the two conditions (SOLO: 67.5%, COLLAB: 67.8%).

From the quantitative questionnaires we extracted the different grades given by the 20 participants. Figure 9 presents the mean and standard deviation for each criteria:

• Difficulty to achieve the task (1: very difficult, 7: very easy) correction). The differences for the other criteria were not found to be significant. The subjective questionnaire addressed 5 themes: 1) Motor Imagery Strategy: What was your strategy to successfully achieve the task? Was it different in the two conditions? 2) Motivation: Did you find different motivations while performing the experience alone or with a partner? 3) Impressions on the Collaborative Feedback: Did the feedback of your partner influence you? If so, how? 4) Self-Evaluation of the Performances: Did you find yourself better than your partner? Three choices were available better / equal / worse. Participants were also asked to give details on how and why they felt such a difference. 5) Preference: What is your preferred condition, alone or with a partner?

Table II lists the different categories and number of subjects per category. The 2 major strategies used by the participants involved thinking of simple hand or arm movements. The users reported different motivations when playing the game: "not to impede the other's performance" or "find a better mental state control" incorporated most participants (5/20 participants for both), while others chose more competition-oriented answers (score goals, be better than the other). When asked for their impressions on the collaborative feedback, participants reported negative (7/20 users), neutral (8/20 users) or positive feelings (5/20 users) on whether they found the feedback disturbing, purely informative or helpful respectively. While 7/20 participants found themselves better than their partner, 8/20 found no differences and 5/20 felt worse performances. Finally, the preference question showed that 10/20 participants preferred the single-user condition. 6/20 participants preferred the collaborative gaming context.

The user impressions toward the collaborative interaction divided the population into three subgroups: positive perception (76.5% mean classification performance during collaborative sessions), neutral perception (76.7%) or negative perception (68.9%). The performance differences between negative perception and neutral or positive perceptions were found to be significant with a paired t-test (p < 0.05). When looking at the classification performance of these three subgroups during single-user sessions, we also found to be significant differences between negative perception (67% accuracy) and neutral (74.7% accuracy) or positive perception (76.5% accuracy).

The mean classification accuracies for the participants that preferred SOLO over COLLAB condition were 70.1% in SOLO sessions and 75.9% in COLLAB session. The difference was found to be significant (paired t-test, p < 0.01). The subgroup that preferred the COLLAB condition achieved a mean classification performance of 70.1% in SOLO condition and 69.4% in COLLAB, the difference not being significant.

V. EXPERIMENT 2: COMPARING COLLABORATIVE VERSUS COMPETITIVE CONDITIONS

As observed in Experiment 1, the introduction of a multiuser BCI paradigm could influence the motivation and behaviour of the participants. In this second experiment, our objective was to qualify more precisely the multi-user experience, and study how the users handle a competitive situation compared to a collaborative one. We kept the best participants from the first experiment and introduced the competitive condition COMPET. We monitored the same metrics as before, i.e. performance and participants' answers to a subjective questionnaire.

A. Population

We selected the 8 best performing subjects based on the classification accuracy obtained from experiment 1. Two pairs were already formed during the first evaluation, the two other pairs were randomly arranged.

B. EEG Configuration

We used the same apparatus, with the following configuration for the EEG acquisition. The amplifiers acquired data at a 512 Hz sampling frequency. In order to try and improve the quality of the CSP filters, we raised the channel number to 16 for each user: C3, C4, FC3, FC4, C1, C2, CP3, CP4, C5, C6, Fz, Cz, FCz, CPz, Pz and POz.

C. Procedure

This experiment was divided into 7 sessions. The first session was the training session, made of 40 trials (20 left, 20 right in a random order). The 6 following sessions consisted of 2 random sequences of 3 sessions, from each of the 3 conditions {SOLO, COLLAB, COMPET}. These 6 sessions were shorter than in the previous evaluation (30 trials, 15 left and 15 right in a random order) to limit the overall duration of the experiment. The complete experiment lasted approximately 105 minutes.

D. Results

The classifier accuracy was computed for each user using the same procedure as in the first evaluation. The mean classification accuracy values of the 3 conditions for the 8 subjects can be found in Table III.

There was no significant difference between the collaborative and competitive conditions (COLLAB: 75,4% ; COMPET: 74,6%). This suggests that using a competitive context rather than a collaborative one may not lower the performance achieved by the users. We separated each pair to form the Winner and Loser subgroups as we did in section IV-D. The Winner groups achieved classification accuracies of 77.5% in SOLO, 81.67% in COLLAB and 79.58% in COMPET, the Loser group showed inverse tendency with 74.17% in SOLO, 69.17% in COLLAB and 69.58% in COMPET. However none of these differences were significant, which can be partially explained by the small number of subjects.

The means and standard deviations of the questionnaire grades are presented in Figure 10. Once again Fun and Motivation showed better results with the 2 multi-user conditions than with single-user interaction. There was no significant difference after Bonferroni correction. Finally, the subjective questionnaires were analyzed to classify the user on the same thematics as in experiment 1: strategy, motivations, impressions toward multi-user feedback, self evaluation of performances and preferences.

The strategies used were separated in two categories (4/8 participants per category): self-centered (focus on the mental imagery) or context-based (analyse the situation and adapt the mental state). The motivations were either competitive (3/8 users) or focus on achieving the task and score goals (2/8 users). 4/8 users found the multi-user feedback helpful, 3/8 users were disturbed by it. Most of the users (5/8 participants) found their performances equal to that of their partner. Finally, the users equally prefered the competitive and collaborative conditions. None prefered the single-user condition.

This second experiment represents a reliable basis to ac-knowledge trends, however it is done on a rather small number of subjects. In order to confirm these trends, experiments on more subjects are necessary.

VI. DISCUSSION

The BrainArena BCI game was functional in experimental conditions. The system managed to handle EEG signal processing and feedback rendering in real-time. The mean classification performances over all users were above 70% in all conditions. The two experiments we conducted on the BrainArena BCI game allows us to evaluate the influence of the multi-user interaction on users performance and subjective impressions.

The first evaluation on 20 naive subjects compared the single-user situation with the multi-user situation using the collaborative condition. Although the mean classification performance was not significantly better in collaborative condition, it showed a tendency (p = 0.06), which will have to be confirmed in further studies. However, when analyzing separately the best performing users and the worst performing ones from each pair, we found a significant difference between collaborative and single-user for the best performing user only. This means that operating a BCI in a multi-user context is possible without any performance drop, and may even increase the classification performances of the best performing users. The 7/20 participants that found the collaborative feedback disturbing reported for example: "Too much information to handle..." or "it's moving too fast, it disturbs my concentration". As this user group performed poorly in both conditions (67% in single-user, 68.9% in collaborative) we can assume that they could not handle the visual stimuli even in the single-user condition. The users' self-evaluation of performances showed unbalanced results. More participants found themselves better than their partner, compared to those who found themselves worse (7 and 5 participants respectively). They were right in both cases if we look at their placement in the Winner/Loser classification. However 5 participants could not perceive the difference of performance, even when this difference was high. For example the user 1 of pair 6 perceived equal performances while he outclass his partner by 10%. This can be interpreted as another evidence of the difficulty to understand and evaluate the collaborative feedback. These results offer several ideas of improvement in order to make the BrainArena BCI game more user-friendly: e.g. smoothing the real-time feedback to lower the disturbance, or extending the trial duration to give users the time to understand what is going on.

As the quantitative questionnaires revealed, motivation and enjoyment showed significant differences in favor of the collaborative mode. Difficulty and global appreciation were not statistically different. Thus we can assume that a multi-user interaction can be controlled with the same level of difficulty as a single-user interaction. However the user preferences showed surprising results: 10 subjects preferred the singleuser condition and only 6 the collaborative one. Additionally, those who preferred playing in a single-user condition performed better during the collaborative task, while the subjects that prefered the collaborative task showed no differences in performance between the two conditions. This can be interpreted in different ways. First, we could simply assume that user preferences are not only dependent on the performances achieved, nor on the enjoyment and motivation to do the task. Secondly, several users prefered the single-user condition just because they were in that case the only cause of success or failure. The motivation that was the most raised from the questionnaire was "I don't want to impede my partner" or "Understanding and finding a better control strategy" (5 users in both categories). Even if they performed better in the collaborative mode, they still prefered a condition they fully controlled. A third interpretation could be related to the link between task complexity and efforts required to achieve the task. As the collaborative task is found more complex, the user must focus more intensively which therefore ensure higher performance. However when asking for a preference, the users turned to the simplest task: the single-user game mode. This also applies to the second group who prefer the collaborative condition: as they handle well the collaborative task complexity, their efforts are on a similar level in both conditions, which leads to similar performances. This first evaluation highlights the complex relation between user impressions, difficulty of the task, feedback understanding and performances. The user preferences are not only related to their performances nor motivations to do the task. The psychology plays an important role, and even more when two users are interacting within the same context. As the user impressions show, the effects of multi-user may be negative (e.g. "I felt very frustrated when I didnt manage to help my partner", "I was totally discouraged when I saw the other going on the other side") or positive (e.g. "good to have someone to help when we don't manage one side, "it's better to laugh together when we make mistakes than being frustrated when we fail alone"). We see a high variablity in the users behavior and subjective impressions.

The second evaluation was conducted on a smaller subset made of the best participants. As they only participated in 4 sessions in the first evaluation, we cannot qualify them as trained users, but we assume their first experience made it easier to handle the task in one more multi-user condition : the competitive mode.

When asked for their strategy to perform the mental imagery task, all participants naturally took a higher level of abstraction: they tried either to "focus on the mental imagery" or "analyse the context and adapt their mental state". In the previous evaluation, they were mostly focused on doing the hand mental imagery or arm, or manipulating an object. This group of subjects is characterized by a competitive spirit, revealed in the questionnaires: the users only reported to be motivated by "scoring goals" or "being better than the other".

The classification results in the three conditions showed no significant difference (75.83% in single-user, 75.42% in collaborative, and 74.58% in competitive). All these results have to be interpreted with care, as the limited number of subjects questions the relevance of further statistical analysis: additional experiments should be evaluated to further confirm our results. The quantitative questionnaire results also revealed no differences between the three conditions. This result sug-gests that using a competitive condition over a collaborative one does not affect the motivation and enjoyment of the users. While performances remain the same in the three conditions, the preferences go clearly in the multi-user direction: half the subjects prefered the collaborative task, and the other half the competitive condition. None of them prefered the single-user condition. This further reveals that users' preferences are not only related to the performance achieved.

As the second experiment uses a reduced number of subjects (8 subjects, 4 pairs), future work should include larger experiments to evaluate the collaborative versus competitive conditions in order to make stronger conclusions.

The user impressions towards the multi-user feedback was very variable and contrasted. Further research could be focused on finding the best feedback concept. Such feedback should be understood easily by all users, allowing them to accurately evaluate their performance without giving them too much disturbance. The tradeoff may be very dependant on users, and finding new adaptation techniques, supervised or not, would be useful.

The BrainArena multi-user BCI game gives every user equal possibility at controlling the application. Thus disabled people could play such a game together with healthy users with less or identical frustration. Such multi-user BCI systems could provide a new way of communication and interaction between a patient and his/her relatives, based on entertainment.

VII. CONCLUSION

In this paper we have studied the design and evaluation of a multi-user BCI video game called BrainArena. This video game application is based on hand motor imagery, and allows two users to simultaneously play a simple football game. The players can use left (or right) imagined hand movements to push a ball towards the left (or right) goals. A collaborative mode enables players to push together in the same direction, whereas a competitive mode enables users to play a duel and try to push in opposite directions. The BrainArena video game was evaluated on 20 subjects in a first experiment that compared single-user and multi-user collaborative conditions. The 8 best performers participated in a second experiment where we added the multi-user competitive condition. Our results show first that our multi-user video game is fully operational and that our different scenarios can be rapidly handled by participants. Motivation and fun were strongly increased in multi-user conditions. Interestingly, the performance of the "winners" (people who performed better than their partner) was found to increase in multi-user versus solo conditions, suggesting a potential benefit of a multi-user approach. Some interesting psychological factors or behaviors are also exhibited. When two users are playing the same BCI game, specific feelings of frustration, hesitation, shyness, or irritability could occur. This opens possibility for further work, where behavioral studies could benefit to BCI game design.

In these two experiments, most of the participants (16/20 subjects) knew each other and volunteered together. This proportion does not make possible any statistical analysis, however this would be interesting to study if and to what extent this could influence the motivations and performances of the users in the different conditions. On a broader scale, multiuser BCI design, for video gaming purpose or not, could also benefits from sociological studies on team design, collaboration and motivation among teams, and how it can influence task performance. For example, Harrison et.al. addressed how deep-level relationship between team members can influence the performances [START_REF] Harrison | Time, teams, and task performance: Changing effects of surface-and deep-level diversity on group functioning[END_REF]. For a review on team performances and especially on the factors that influence team performance, the interested reader can refer to [START_REF] Salas | On teams, teamwork, and team performance: Discoveries and developments[END_REF].

We focused our study on the classification performances and the user experience. In future work it would be interesting to conduct further analysis from a neuroscience perspective, e.g. by comparing the event-related synchronization and desynchronization (ERS/ERD) [START_REF] Pfurtscheller | Motor imagery and direct braincomputer communication[END_REF] between the different conditions.

Taken together our results pave the ways to new designs and new evaluations of multi-user BCI applications.

Fig. 1 .

 1 Fig. 1.Single-user BCI interaction loop, illustrated by an example (in red/italic font) inspired by the use of motor imagery paradigm. Two bipolar channels are acquired on users scalps during a hand motor imagery trial. The signal processing outputs a class label with a probability (e.g. class left with 75% probability). The decision making concludes that the probability is high enough to decide the user mental state: a left hand motor imagery. In the interaction technique this decision is translated into a command for the application: move left. From this command the application produces a visual feedback: the ball on screen is moving left.

Fig. 2 .

 2 Fig. 2. Four different scenarios of multi-user BCI interactions depending on the user connection level, illustrated by examples in red/italic font. Only the connection level inputs and outputs are described. (1): The EEG signals from 2 users performing hand motor imagery are merged in the same signal processing, that produces a unique classification with a probability (class Left with 80% probability). (2): Two hand motor imagery classifications are merged in the decision making, which here simply chooses the most probable mental state between two classes. (3): Two decisions, one hand motor imagery state and one focused target a BCI paradigm such as SSVEP, are merged to issue a unique command for the application: move the selected target to the left. (4): Two independant commands issued by 2 separated motor imagery BCI pipelines, are merged into the application to produce a complex feedback where two different objects (a ball and a goal keeper) are moved on a screen.

Fig. 3 .

 3 Fig. 3. Architecture of the BrainArena multi-user BCI application.

Fig. 4 .

 4 Fig. 4. Experiment apparatus.

Fig. 5 .

 5 Fig. 5. Two users playing BrainArena in a competitive trial.

Fig. 9 .

 9 Fig. 9. Means and standard deviations of the quantitative questionnaire grades. A star * indicates a significant difference between the two conditions (p < 0.05, Wilcoxon signed rank test with Bonferroni correction for multiple comparisons).

Fig. 10 . 4 Motivations 3 4 Self

 10434 Fig. 10. Means and standard deviations of the quantitative questionnaire grades.

ACKNOWLEDGEMENT

This work was supported by the French National Research Agency within the OpenViBE2 project.