Robby Van Delden
email: r.w.vandelden@utwente.nl

Pauline Aarts
email: p.aarts@maartenskliniek.nl

Betsy Van Dijk

Design of Tangible Games for Children Undergoing Occupational and Physical Therapy

Keywords: Participatory design, games for therapy, tangible games, special user groups, children, cerebral palsy

Games can offer an entertaining alternative to repetitive tasks. In this paper, we propose the use of tangible interactive games for the repetitive training of upper limbs in the therapy of children with Cerebral Palsy (CP). We obtained promising results. The total of four created games succeeded in triggering all the to-be-trained movements properly and in a motivating and entertaining way. A physical quiz game was especially successful as children kept on playing the game making the proper movements without additional encouragement or instructions of the therapists or researchers. These results indicate that in this kind of occupational or physical therapy, there is additional value in using tangible interactive games. Furthermore, the research shows the importance of including the therapists in the design of games and we report on several ways to achieve that.

Introduction

Hitting a button for almost a thousand times does not sound like much fun. However, over the last three decades children have had fun doing this in the form of computer games. Most forms of physical therapy for children are also repetitive in nature, especially for children with unilateral Cerebral Palsy (CP). Unilateral CP is a disorder of posture and/or motor function due to brain damage in the first year of life. In the unilateral type of CP, the right or left side of the body is affected. Therapy for CP children can be effective but can also be boring at times because of this repetitive nature [START_REF] Aarts | Effectiveness of modified constraint-induced movement therapy in children with unilateral spastic cerebral palsy: a randomized controlled trial[END_REF]. Using interactive games could reasonably be expected to help in bringing additional motivation and so more enjoyable therapy.

The TagTiles platform can be used for creating such interactive games with tangibles; it is a system that can automatically localise multiple objects on its surface by using RFID tags. This system gives feedback with full colour LEDs and with playing sounds. In previous research, TagTiles games were created for therapy for CP patients [START_REF] Li | A tangible tabletop game supporting therapy of children with cerebral palsy[END_REF]. However, these games were not implemented as the proper movements were not triggered enough and compensating movements were observed too often. Nonetheless, the use of entertaining interactive tangible games built for the TagTiles system was promising according to the therapists. That new games for the TagTiles system were not developed earlier by therapists themselves is probably due to a lack of time, effort and know-how from therapists to develop games that are suitable [START_REF] Kierkegaard | From top to bottom: End user development, motivation, creativity and organisational support[END_REF].

Children with unilateral CP are inclined to make compensating movements. In other words, they prevent certain movements that are hard for them and thereby further diminish their capability of performing these movements. Therapy especially focuses on training the fine motor skills of the upper body. Therapists have a good insight in how to trigger the correct movements and what the preferences are of the children. We thus chose to use a participatory design approach. At the same time, we had to keep in mind that there is limited time available for non-therapy activities by the therapists and children [START_REF] Kierkegaard | From top to bottom: End user development, motivation, creativity and organisational support[END_REF].

These children may also suffer from forms of mental retardation and low selfesteem. Although several guidelines for developing motivating games exist (e.g. [START_REF] Malone | Making learning fun: A taxonomy of intrinsic motivations for learning[END_REF]), they do not explicitly take into account the special needs of these children. Thus, during this research we looked into the use of sounds and evaluated and compared different versions of tangible games for therapy along the dimensions of challenge and interpersonal competition.

In related work, several attempts of using interactive games in therapy are reported and results seem promising [START_REF] Annema | Videogames in therapy: a therapist's perspective[END_REF][START_REF] Chen | Use of virtual reality to improve upper-extremity control in children with cerebral palsy: a single-subject design[END_REF][START_REF] Deutsch | Use of a low-cost, commercially available gaming console (Wii) for rehabilitation of an adolescent with cerebral palsy[END_REF][START_REF] Griffiths | The therapeutic use of videogames in childhood and adolescence[END_REF]. However, to our knowledge there are no successful and well-tested implementations of interactive tangible games for physical therapy of CP patients focusing on the upper limbs. This was a main motivator for our research.

Over one and a half years of research, several design tools and techniques were used to deal effectively with this challenge. We used a participatory design approach, incorporating the therapists knowledge and the children's interests. This was done in two main rounds of development leading to four basic games. The games were evaluated in a mainly qualitative way with a total of seven therapists and eighteen different children undergoing therapy.

The remainder of this paper is structured as follows. In the following section, we summarise related work on motivating games and games for therapy. Section 3 describes the methodology and test setup. Section 4 describes the games developed. This is followed in Section 5 with the results. The paper concludes with a discussion and proposals for future work.

Motivation and Games for Physical Therapy

Adding motivation and turning cumbersome activities into fun and engaging tasks for children is challenging. Malone and Lepper pioneered research in this field [START_REF] Malone | Making learning fun: A taxonomy of intrinsic motivations for learning[END_REF]. They worked on fun and motivation in educational computer games.

We will only mention some of their findings as their work has already been analysed, rewritten and complemented several times (e.g. [START_REF] Kannetis | Towards adapting fantasy, curiosity and challenge in multimodal dialogue systems for preschoolers[END_REF][START_REF] Moon | Exploring variables affecting player's intrinsic motivation in educational games[END_REF]).

Their analysis resulted in four individual criteria: Challenge, Curiosity, Fantasy and Control. Three interpersonal criteria were also stated: Cooperation, Competition and Recognition (meaning appreciation). These aspects still seem to cover the most important aspects of what makes games fun and can be found in one way or another in most of the recently developed frameworks (e.g. [START_REF] Korhonen | Understanding playful user experience through digital games[END_REF]).

TagTiles has been targeted as a platform for therapy before by Li et al. [START_REF] Li | A tangible tabletop game supporting therapy of children with cerebral palsy[END_REF]. They developed three games for TagTiles. Results were promising but they overestimated the physical abilities of the children and the games lacked the capability to withstand possible compensations by the children to circumvent the to-be-trained movements. This mainly involves circumventing difficult movements. For example, using their non-affected arm to grab something or bending their affected wrist downward (palmar) to an extreme extent. However, "All participants said they enjoyed playing the game" and "Therapists were happy to observe children engage in the desired movements spontaneously, something that they said was not common in their standard therapy sessions." [15, p190].

Getting games to be used in therapy also requires incorporating the context and therapist's view. Applicable requirements and effective methodology for this context can be found in [START_REF] Annema | Videogames in therapy: a therapist's perspective[END_REF]. Based on observation of therapy and a small-scale implementation, the following was emphasised: the importance of limited preparation and start up time, adaptability and adaptivity, simplicity and (where possible) automated logging.

Several studies underline that using interactive video games for therapy and rehabilitation can be useful [START_REF] Griffiths | The therapeutic use of videogames in childhood and adolescence[END_REF]. For instance, playing the basic games on the Wii had a positive result in motor function of the legs, such as increased gait distance and increased postural control of a 13-year old CP patient suffering from double leg impairment [START_REF] Deutsch | Use of a low-cost, commercially available gaming console (Wii) for rehabilitation of an adolescent with cerebral palsy[END_REF]. Another small-scale study of four children using a camera-based input virtual reality application for two weeks, showed it is also possible to improve "reaching kinematics" of children with CP by using games [START_REF] Chen | Use of virtual reality to improve upper-extremity control in children with cerebral palsy: a single-subject design[END_REF]. The therapy of our target group mainly focused on training fine motor skills, such as improving their grasps and learning new grasps. Thus, instead of using a camera-based input they might benefit more from using tangibles.

Test Setup and Methodology

A case study was done in collaboration with the Sint Maartenskliniek (Nijmegen, the Netherlands), a hospital that specialises in posture and movement. This hospital offers specialised therapy for children between two and a half and eight years old with mild forms of unilateral CP. The therapy focuses on improving their affected hand and arm coordination and functioning. It consists of playful exercises such as playing board games and singing songs. During these exercises, they repetitively move their hands and arms. An example of such a therapy is the one called Pirate Group, in which the context of pirates is used to engage the children in the therapy. This is a type of modified Constraint Induced Movement Therapy combined with Bimanual Training intervention (mCIMT-BiT); children are forced to use their affected arm in the first few weeks, but later on they are allowed to use both hands in order to train their coordination. For further details of the therapy, please refer to [START_REF] Aarts | Effectiveness of modified constraint-induced movement therapy in children with unilateral spastic cerebral palsy: a randomized controlled trial[END_REF].

As understanding both the therapy and the target group requires detailed knowledge, we chose to use a participatory design method. We implemented this throughout the design process: in gathering requirements, during the generation and development of concepts and finally in the evaluations. This was done in an iterative fashion in two main design rounds. The first round contained five subiterations and we made small changes to the games in each sub-iteration. In this first round of development we investigated several game dynamics using three games, whereas the second round focused more on therapists' and children's experiences.

We included spoken instructions in all games. The games made use of logging functionality to automatically record the time needed to finish a game. After the children finished the games, we asked them some questions, e.g. "Which game would you like to play again?". We also asked therapists for their feedback, focusing on movements and applicability. Below we will further explain the methodology used, and describe both the participants and the setting. In the next section, the games created will be described.

First Round

In the first sub-iteration we started with listening closely to the therapists to get to know the therapy, the target group and to assemble the requirements for the games. To this end, we held several interviews and informal discussions with four different therapists. In addition, an observational study was done by actively participating in two therapy sessions of one single group of five children with CP. To aid understanding of the therapists and to make communication easier, we did a literature review on CP. During the observational sessions, we witnessed the interests of the children closely and recorded these by taking notes. These interests were then used as basis for the first three games. These games were designed using a mixture and adaptation of design techniques involving physically acting out movements, combined with the use of objects lying around [START_REF] Burns | Actors, hairdos & videotapeinformance design[END_REF][START_REF] Buur | Hands-only scenarios and video action walls: novel methods for tangible user interaction design[END_REF][START_REF] Djajadiningrat | Interaction relabelling and extreme characters: methods for exploring aesthetic interactions[END_REF]: we were able to generate three games by repetitively acting out the tobe-trained movements. By using body movements, we tried to reduce the gap between creating games and triggering the proper movements. For further details about this methodology, please refer to [START_REF] Van Delden | Design of therapeutic TagTile games for children with unilateral spastic cerebral paresis[END_REF]. We programmed the games using the SDK for TagTiles [START_REF] Van Herk | Espranto sdk: an adaptive programming environment for tangible applications[END_REF]. We finished the first sub-iteration by showing a video prototype to one of the therapists.

In the second sub-iteration we tested the games informally on ten non-CP participants, mainly to find bugs and to improve game play. These tests were done in a home setting.

The third sub-iteration consisted of an evaluation with end users to test the suitability of the games and the triggered movements. We did the tests of the third sub-iteration in a classroom-like environment, containing one or two therapists, one researcher and one or two participants. Six children participated: three boys and three girls, all between four and six years old.

The fourth sub-iteration consisted of an in-subject test with four children. We used this evaluation to gain more insight into three game variables: sound, challenge and inter-personal competition, with a focus on the latter two. Because of the target group's reduced self-image it is important to choose the proper level of challenge which could help to increase their feeling of competence. Competition and cooperation are also relevant to that feeling. However, using one TagTiles system with two children at a time is impractical. In that case, the interactive space is too small to effectively train movements and using a turn-taking approach will reduce effectiveness of therapy as intensity is an important factor. Competition is also stimulated by adding a scoring system [START_REF] Malone | Making learning fun: A taxonomy of intrinsic motivations for learning[END_REF]. The children played two variants of two games to analyse the effect of challenge and competition; these versions and the results will be described in Section 5. They played the "A-versions" in the first session and the "B-versions" in the second session or vice versa. The participants of this fourth sub-iteration did their tests in a small room with one participant, one therapist per child and a researcher present. The children were one 8 year old boy and three girls, two of 9 and one of 8.

In the fifth sub-iteration, another group of children tested the games. The participants of the fifth sub-iteration used the same setting. The five children were an 8 year old girl, a 5 year old boy and girl, a 2 1 /2 and a 3 year old girl. They played only one version of the game.

Second round

We started the second round with a more thorough cooperative design session, to better include the therapists also during the creation process. During this session, two therapists developed several ideas using two design techniques in less than an hour. This design session was done with a physiotherapist and an occupational therapist in one of their offices.

The first design technique used is interaction relabelling [START_REF] Djajadiningrat | Interaction relabelling and extreme characters: methods for exploring aesthetic interactions[END_REF]. In this technique, interactions of an object are translated to a new object, or in our case translated to a tangible game. Based on a first test session, with three other participants (non-therapists), it was decided to make use of objects with a mechanical character to efficiently trigger creativity. We used a stapler, a multi-head screwdriver, a Rubik's Cube and a small plastic box with a magnetic fastening. Participants were told to take the first prop in their hands and explain how they would use this. Based on this explanation they had to come up with ways to alter this into a game, training the wanted movements.

The second technique was a shortened version of Systematic Inventive Thinking (SIT). SIT is a way to alter a product, or to overcome a problem, by using effective often reoccurring thinking patterns [START_REF] Horowitz | Creative design methodology and the sit method[END_REF]. SIT consists of the following steps:

identifying the components of the idea/product, applying one of the thinking patterns, namely division, multiplication, subtraction, (task) unification or (change of) attribute dependency, visualising this solution, foreseeing whether it will fulfil the (new) needs.

For both techniques, a small explanatory presentation of 5 minutes was given to the therapists just before using the technique. Interaction relabelling was used for 20 minutes to create ideas followed by 25 minutes of SIT to further develop some of those ideas. Based on the outcome of this session, one game was created, incorporating the input of the therapists.

This game was tested by observing children playing the game. Three children participated: two 8 year old boys with reasonable hand and arm functionality and one 8 year old girl with more limited capabilities. The classroom setting of the second sub-iteration in the first round was used again. Children had to play the game for 15 to 30 minutes until a good image of the interaction was formed by direct observations. Following the gameplay, two therapists were interviewed. This included one therapist that observed all the three participants playing. The other therapist, who also participated in the design session, observed only two of the participants.

Games

Three games were created in the first round: a hitting game, an animal game and a boat shooting game. In the second round, a quiz game was developed. The games, the movements they trigger and the origin of the used themes will be described in this section.

Hitting Game

The hitting game is a simple game based on triggering rotation of the hand backwards. Children have to wear two hook and loop fastener strips for sensing purposes, one fastened around the wrist and one around the palm of the hand; see Fig. 1. One of the squares is lit yellow for about two seconds and then 'jumps' to the next square. While keeping their arm on the board they have to raise their palm and then hit the lighted square in time. On hitting the square, one of ten 'funny' sounds is played and a point is awarded. If the wrist is raised while hitting, this is indicated by playing a spoken text. Once 16 points are achieved the game is won and the border of the board flashes green for several seconds.

Animal Game

During the observations, children were seen to enjoy making animal sounds and listening to how others made animal sounds. This was used as the inspiration for the animal game; see Fig. 2. In this game, the name of a colour is played, e.g. "Brown", then the child has to pick a miniature animal and place it on the board at the square lit in this colour. On placing the proper animal, either an indication that the child performed well is played, for example "Well done, a dog" or an animal sound is played. All the animals have to be placed and removed from the board to finish a round; the game consists of two rounds. If a child takes too long to place the animal, e.g. when losing concentration, a reminder is given "Which animal is brown?". This is followed by indicating the right animal after some seconds with "The doggy is brown". When a child places the wrong animal, this is also stated: "That is not the right animal". The game requires extension of the elbow and trains several grasps. We tried to pick the right variety of shapes and weight to trigger several grasps and to reduce the possibility of compensating movements.

Boat Shooting Game

Fitting the topic of the Pirate group, this game involves rotating a boat to shoot incoming targets. In the game created, targets slowly move down and a pirate boat has to be turned into the direction of the target; see Fig. 3. Aiming in the right direction results in destroying the target and one of two explosion sounds being played. The rotation of the boat requires rotating the hand backwards. To make this boat game playable for children with very limited ability to move their hand backwards, the board was rotated between 0 and 45 degrees, depending on the maximum angle the child showed to be able to manage.

Quiz Game

This game is based on answering multiple choice questions. It is implemented on a newer version of TagTiles; see Fig. 4. To answer the questions, several actions have to be performed, that trigger several movements. The game consists of: picking a category, getting a random question, hearing the options, choosing an answer, loading this answer and then hearing if it is correct or not. With the selection of actions and objects we tried to minimise compensating movements. The group from which three participants were selected in the evaluation of this game participated in (mCIMT-BiT) therapy with "Ik hou van Holland" as a metaphor. This is a Dutch TV-show in which Dutch TV personalities answer questions about Dutch habits, songs and personalities.

The game created starts with the intro tune and shows a tulip symbol. A category of questions has to be selected using a small wooden cube; see Fig. 5. This action requires a correct grasp of a small object triggering the pincher grasp; see Fig. 5.

An instruction is played to tip a question tag onto the board from the question box. The tipping movement triggers rotating the arm inwards and triggers outward rotation when placing it on the table again.

Four multiple choice answers can be heard by rotating a block over the board. This triggers rotating the hand backwards; see Fig. 4. An instruction is given to select the correct answer by placing an answer disc onto the board. This action also triggers the pincher grasp.

The user is instructed to load the answer. The answer has to be put into a tube. The user is then instructed to throw the 'answer' on the board. If the answer is wrong, the game proceeds again from hearing the question. If a question has been answered incorrectly for the second time or the correct answer is given, then the game starts again with selecting the category.

When all questions from a category are answered and this category is chosen, the user is instructed to select another category. If this happens, then the score is shown on the board. At the start of the game, the therapist or user can select the number of questions that have to be answered ranging from 5-12. When all answers from one category and the total number of questions are answered, the user hears that he or she did very well to answer all these questions correctly. This is followed by 20 seconds of the Queen song "We are the champions".

Evaluation & Results

Several evaluations have been done throughout the research. The most important of these will be described in this section. We have little to say about the use of a video prototype of a preliminary version in the first iteration. It was useful as it did not require fitting in a co-located meeting in a tight schedule. Also, it gave an early indication of whether the games would work. As for the second sub-iteration: testing with non-CP participants we found bugs and made improvements without 'bothering' children or therapists. Nine changes were incorporated as a result of these tests.

Game Design Variables

We researched three game design variables: sound, competition and challenge. We used an in-subject test in the fourth sub-iteration for the latter two.

Sound In the third sub-iteration, we let some of the children play the hitting game without sound. We used direct observations of a researcher and therapists. Not having sound reduced the clarity of feedback and the entertainment drastically, thus in the forth and fifth sub-iterations we only used the version with sound. Analysis of videos taken in the fourth sub-iteration showed that three of the four children grinned upon hearing the first or second sound in the hitting game; see Fig. 1. Including the video analysis of the fifth sub-iteration of the first round as well, six out of nine children grinned or smiled as soon as they heard the first or second sound.

Competition We compared two versions of the animal game to evaluate influence of competition. In the "A-version" of the animal game we introduced a competitive objective in two ways: [START_REF] Aarts | Effectiveness of modified constraint-induced movement therapy in children with unilateral spastic cerebral palsy: a randomized controlled trial[END_REF] we instructed the children to finish as quickly as possible and to make as few mistakes as possible, (2) we introduced feedback on high scores after finishing the first and the second round of a game. In the "B-version" we excluded both forms of competition. The participants of the fourth sub-iteration did not give clear responses after hearing their results. The first participant sighed deeply and moved with her chair from left to right. The third participant stated he found the animal game too simplistic. We had expected that the encouragement would result in significantly faster finishing of the second round of the animal game. However, such an effect could not be observed.

Challenge We compared two versions of the boat game. The "A-version" with a higher level of challenge by targets coming down faster after each hit. The game also goes into a game over state when a target reaches the bottom of the board. The "B-version" did not have such an increase of speed or game over state. Two of the girls of the fourth sub-iteration smiled after losing the "A-version". In the interview, they both stated they disliked the game (both versions), they disliked that it was so difficult to aim in the required direction. One girl laughed after winning the pirate boat game after having lost the altered version during her previous session. One boy had some difficulty playing the game mainly due to the version of the tangible boat used. He could not finish it on his own and showed no clear reaction after losing the game. The challenge in the boat game was high, both handling the tangible object and aiming it in time, so all participants of the fourth sub-iteration lost the altered game. No clear indications of difference in perceived entertainment value was seen between the two versions of the game.

Use of SIT and Interaction Relabelling

Using interaction relabelling resulted in generating eight ideas. From these ideas, four were chosen in conjunction with the therapists to further develop, each using one or two randomly selected thinking patterns. Applying and understanding the thinking patterns was found to be more difficult than using interaction relabelling. The SIT-session nonetheless resulted in ten simple alterations. We implemented these proposed game elements and movement-object combinations in the quiz game.

Age Dependency

Based on the fourth sub-iteration (in-subject tests) and the fifth sub-iteration combined, taking everyone's first attempt and using an independent samples one-tailed t-test we could see a clear age difference in completion time for the animal game (p < 0.001) and the hitting game (p < 0.001) between 2-3 year olds(n=2) and 5-9 year olds (n=7). For the hitting game, there was also a significant difference between 5 and 8-9 year olds and between 2-5 and 8-9 year olds(both p < 0.05)).

The second youngest child needed help in hitting the first target in the hitting game. In the animal game, someone had to put the correct animal in her hand and she needed additional instructions from the therapists. The pirate game was played together with the therapist but even in this way she could not understand it enough to play the game. Similar problems occurred with the youngest child. She often had to be directed towards where the target was in order to keep her attention on the game. During the animal game, the animals either had to be pointed out or handed to her. Several times she had a difficult time placing the animal upright on its pedestal.

The second youngest child showed clear use of the unwanted extreme (palmar) flexing of the wrist when placing some of the animals. However, the youngest did perform a proper grasp and placement by herself, see Fig. 2.

Liking, Triggered Movements and Compensating Movements

Responses from the children were quite positive. In the first round, one of the children of the third sub-iteration even spontaneously mentioned she would like to have the game at home. In the fourth and fifth sub-iterations, the hitting game and animal game were the most preferred (both 4 times). The youngest participant preferred the boat game. After being encourage by a therapist, she acknowledged this was because she liked the attractive tangible object. The therapists especially liked the animal game.

The children in the second round were positive about the quiz game. One of the boys, even without encouragement, kept on playing. Both boys replied that they would like to play the game again. The girl stated she did like the game more than other exercises she had done in therapy thus far. One of the participants found the questions a little too hard, though another did not mind that he did not know all the answers.

According to the three therapists present during tests in the first round, all targeted movements with the exception of rotating the arm outward were triggered. Turning the arm outward was triggered later on in the quiz game, for instance by putting the answer tube next to the board. In the discussion, both therapists concluded that especially the children with better arm-functions made the proper movements. Some forms of compensation were seen. For example, in the first sub-iteration a child moved the boat to the targets instead of turning it. In the boat game, extreme (palmar) flexing of the wrist was also seen. We fixed the boat to an axis in the latest version of the boat game to prevent these things from happening; see Fig. 3. With loosening or tightening the axis we could also adjust to the strength of the child. The animal game was mainly used as intended with a minimum of compensation, except for the second youngest child. In the quiz game, another girl -being slightly less capable-made more compensating movements than the two boys. For instance, she had to be reminded several times to perform the actions with her affected arm. Also, instead of turning the answer block she tumbled it by gently hitting it.

One of the therapists liked the feature that multiple movements were triggered in one game, especially if the game could be adapted to the player's personal functioning, for example by skipping parts of the game. The newer version of TagTiles was also liked, as it did not need an external laptop. This made it more mobile and reduced preparation time.

Discussion

Based on our research findings, we can draw several conclusions and make proposals for future research. Our findings can be very useful for future research in the field of tangible games, serious gaming for physical therapy and in general for designing interactive systems for people with specific needs.

Methodology

We found that design techniques with acting out of movements were very applicable for designing physical therapy games. It certainly seemed to reduce the gap between training movements and developing a game. The relabelling in the participatory design session of the second round seemed useful and also very applicable. However, SIT seemed to need some more time to comprehend and use efficiently. Children, especially those with special needs such as children with CP, form a diverse and hard to reach target group. This makes it difficult to do proper research. We found that one should be very careful in generalising for instance the capabilities of the children.

Utilisation of Traditional Game Variables for CP Therapy

Sound brings additional fun to games. Increasing challenge to the extend that one sometimes loses a game, might add extra motivation and feelings of fulfilment and capability when in the next attempt one does win. For instance, one girl showed a very enthusiastic response winning after losing first. Losing a game did not lead to extreme reactions like crying. Our addition of an inter/intra-personal competition feature by using high scores did not lead to noticeable effects in our tests.

Using Tangibles for Occupational and Physical Therapy

We found significant differences in the target group, for instance between age groups, hence we think it is essential to make games adaptable. We did some tests in which therapists could change settings in a GUI, such as timings and which animals to use, but the results were inconclusive. Based on interviews we came to the conclusion that if one instead uses a purely adaptive system, it might fail on some occasions. A system will have a hard time recognising the difference between lost attention (so a possible lack of challenge) and low capabilities (so too challenging). One solution is an adaptable adaptive system: one that can automatically change the game within the limits of a specific user.

We succeeded in creating an entertaining set of games that can bring additional motivation and fun in the context of occupational and/or physical therapy for children with CP. The games succeeded in training the correct movements related to goal-directed activities. Especially the fourth game, being a physical quiz created for the latest version of TagTiles, seems very promising. The children kept on playing the game, mainly making the proper movements without additional encouragement by or instructions from the therapists or researcher. However, some compensation still occurred, such as the tendency to grab objects with the non-affected hand during the quiz game and (palmar) flexing of the wrist too far in both the animal game and the boat game.

This kind of compensating movements remains a problem especially if one prefers partially unsupervised therapy. According to one of the therapists, a more mechanical device strictly guiding the movements might be more suitable for children with more limited physical capabilities, as compensating movements can be inhibited in such a device to a larger extent. On the downside such approaches usually require additional setup time and have higher costs. Using other less or non-tangible interaction techniques, such as the Kinect or Wii, might be more suitable for detecting and training some movements [START_REF] Chen | Use of virtual reality to improve upper-extremity control in children with cerebral palsy: a single-subject design[END_REF], for instance for the turning of the underarm outwards. Even if a child cannot grasp an object, it can still benefit from training other kinds of movements but might not manage with tangibles alone. However, using tangibles can trigger training of grasps quite well -as demonstrated in our research.

Future Work

Ongoing research by the consortium called Wikitherapist will further investigate the opportunities and limitations of using the TagTiles platform in combination with end-user development for occupational and physical therapy purposes. More quantitative measures about improvement in motor function using tangible games in therapy could help to encourage more widespread use. An interesting observation was the enjoyment in destroying virtual things. This was both stated in interviews and seen in observations. Although much research has been done on the effects of violence in games, we only found a limited amount of research investigating the link between fun and destroying virtual things. During several iterations, we found and solved both major and minor problems. However, we succeeded in triggering the correct movements and increasing motivation. It is considered worthwhile to further improve the quiz game with adaptability, adaptivity and incorporating additional game development principles, so as to make it an even more suitable and entertaining activity to use in occupational and physical therapy.

Fig. 1 .

 1 Fig. 1. A nine year old girl laughing while playing the hitting game.

Fig. 2 .

 2 Fig. 2. A young girl playing the animal game.

Fig. 3 .

 3 Fig. 3. The final version of the tangibles used for the boat shooting game.

Fig. 4 .

 4 Fig. 4. Hearing the answers in the "Ik hou van Holland" game: tumbling a block, targets a passive form of rotating the hand backwards.

Fig. 5 .

 5 Fig. 5. The start of the "Ik hou van Holland" game. Placement of a wooden cube to select a category which triggers the shown "pincher grasp".

Acknowledgments. This publication was supported by the Dutch national program COMMIT. Intellident sponsored us with several RFID tags. Many thanks go out to: Julia Garde and Ronald Poppe for the pleasant discussions and their useful input, to the therapists of the Sint Maartenskliniek and to Serious Toys for providing information, help and the TagTiles used for this research.


Anastasiia Beznosyk

Peter Quax

Wim Lamotte

Karin Coninx

The Effect of Closely-coupled Interaction on Player Experience in Casual Games

Keywords: casual games, cooperative game patterns, closely-coupled interaction, loosely-coupled interaction

This paper presents a study investigating cooperation between players in casual games. Although widely used in co-located or asynchronous settings, cooperative gameplay elements are not popular in networked synchronous casual games. In our study, we have analyzed different types of cooperation between players in casual games. Each of these is based on a certain cooperative game design pattern, and can be classified as either closely-or loosely-coupled. Six game patterns have been selected and an equal number of games developed, each targeting one pattern. By means of a user experiment we have investigated which cooperation types fit most of the criteria that define casual games. More specifically, we have focused on the applicability of close coupling between players. Based on the games used in the experiment, most patterns with closely-coupled interaction have shown an overall higher user evaluation than loosely-coupled, satisfying criteria of casual games. These results indicate that introducing close coupling in the casual games under consideration is a potential way to increase the player experience.

Introduction and Related Work

Over the last decade, casual games have become one of the most popular game type played over the Internet. These video games, targeted at a mass audience of casual gamers, attract because of their gameplay simplicity, a short play time and a minimum of required commitments to progress in a game [START_REF] Nielsen | Insights on casual games: analysis of casual games for the PC[END_REF]. Additional popularity of casual games is caused by the rise of social networks (e.g. Facebook) and the availability of various game consoles (e.g. Wii) and mobile devices (e.g. smartphones, tablets) that allow competition and cooperation among friends [START_REF] Li | Exploring social interactions and attributes of casual multiplayer mobile gaming[END_REF][START_REF] Mueller | Evaluating a distributed physical leisure game for three players[END_REF]. Studies on digital games have shown that social characteristics of play settings have a strong impact on players' in-game experience [START_REF] Gajadhar | Let's sweep some mines together: Social interaction & competition in casual games[END_REF].

Leaving competitive casual games out of the scope of this paper, our work focuses on cooperation in casual games. While intensively addressed in hardcore gaming (where gamers prefer to take significant time and develop their skill level, like in role-playing games), cooperation has not been widely investigated in the context of casual games. Here, most existing works focus on asynchronous interaction [START_REF] Bogost | Asynchronous multiplay: Futures for casual multiplayer experience[END_REF], where simultaneous play is not required. This type of collaboration is widely used for games on social networks (FarmVille 1 or CityVille 2 ). With the increased use of social games the game producers started to utilize this type of play [START_REF] Di Loreto | Social casual games success is not so casual[END_REF], allowing players to play a game in sequence and break whenever is needed to "accommodate real life necessities". Casual games with synchronous collaboration appear to be mainly co-located. Players share the same screen and can naturally communicate and see each other. Games like Rock Band 3 and Mario Bros. Wii 4 are successful examples of synchronous cooperative casual games.

In most cases, games played over distance, which employ synchronous features, are limited to communication such as different forms of chats [START_REF] Ricchetti | What makes social games social?[END_REF]. Existing cooperative games that can be played remotely, are mainly based on looselycoupled interaction (e.g. achieving a shared goal) without direct players' influence on each other. To our knowledge, closely-coupled collaboration, where one player's actions are directly influenced by the other(s), remains limited in existing casual games. One of the possible reasons for that can be a necessity to actively communicate and coordinate actions when tightly collaborating. Casual games are characterized by short game sessions oppositely to hardcore games, which take a substantial amount of time. Therefore, providing rich voice communication in casual games may become cumbersome for game developers or players might not want to be heard (e.g. playing during breaks at work).

We investigate the application of synchronous cooperation in remote casual games where no communication exists between players. By analyzing different types of cooperation (based on cooperative game patterns [START_REF] El-Nasr | Understanding and evaluating cooperative games[END_REF]), we aim to study the effect of closely-coupled collaboration on player experience in casual games through a comparison with loosely-coupled interaction.

Cooperative Patterns

Patterns in game design have been widely investigated [START_REF] Mcgee | Patterns and computer game design innovation[END_REF][START_REF] Kreimeier | The case for game design patterns[END_REF][START_REF] Björk | Patterns in Game Design (Game Development Series)[END_REF]. Game patterns are descriptions of reoccurring interaction, that depict how game components are used by players to affect various aspects of gameplay [START_REF] Björk | Patterns in Game Design (Game Development Series)[END_REF]. Since cooperation became an integral part of many multiplayer games and virtual communities, defining and analyzing effective cooperative patterns are the primarily goals in game design studies [START_REF] El-Nasr | Understanding and evaluating cooperative games[END_REF][START_REF] Rocha | Game mechanics for cooperative games[END_REF].

The authors of [START_REF] Schmeil | Knowledge sharing and collaborative learning in Second Life: a classification of virtual 3D group interaction scripts[END_REF] define collaboration patterns for knowledge sharing in Second Life 5 based on the purpose of interaction (e.g. virtual meeting, design studio). Interaction patterns in massive multiplayer games (based on the example of Star War Galaxies6 ) are investigated by Ducheneaut et al. in [START_REF] Ducheneaut | The social side of gaming: a study of interaction patterns in a massively multiplayer online game[END_REF]. Here the authors focus on the social aspects of interaction (verbal and non-verbal communication between players). Such cooperative patterns as turn-taking and enforced collaboration are studied by Goh et al. [START_REF] Goh | Potential challenges in collaborative game design for inclusive settings[END_REF].

Studies presented in [START_REF] El-Nasr | Understanding and evaluating cooperative games[END_REF][START_REF] Rocha | Game mechanics for cooperative games[END_REF][START_REF] Zagal | Collaborative games: lessons learned from board games[END_REF] investigate cooperative patterns based on existing games. In [START_REF] Zagal | Collaborative games: lessons learned from board games[END_REF], Zagal et al. explore cooperative patterns within traditional board games, and summarize observations that can be applied to computer games. Work presented by Rocha et al. [START_REF] Rocha | Game mechanics for cooperative games[END_REF] introduces a list of cooperative patterns based on the analysis of numerous commercial computer games available on the market. This list is considerably extended by El-Nasr et al. [START_REF] El-Nasr | Understanding and evaluating cooperative games[END_REF] by analyzing more recent games that support some form of collaboration between players. Here, the authors also investigate which pattern triggers which event (e.g. laughing, discussions, waiting for other players, etc.) during the shared gameplay.

Although cooperative patterns have been widely analyzed [START_REF] Zagal | Collaborative games: lessons learned from board games[END_REF][START_REF] Rocha | Game mechanics for cooperative games[END_REF][START_REF] El-Nasr | Understanding and evaluating cooperative games[END_REF], these works always assume presence of communication between players or even an ability to see each other. Oppositely to the existing studies, our work focuses on collaboration in games where communication is not supported and players are remotely located. When considering interaction over distance, especially when no communication is allowed, additional challenges are present when trying to maintain the same level of player experience. Games based on the same cooperative patterns can result in an entirely different experience when considered in a non-collocated setup. Moreover, for our research a consistent game design is utilized to provide players with an equal perception of the game world while highlighting the impact of each interaction pattern. Most previous works are based on the existing games, where the same patterns are evaluated in different games that may influence player experience. Therefore, we find it crucial to investigate how interaction between players can be improved based on introducing different cooperative patterns in order to compensate a possible negative effect due to a lack of communication. We believe, knowing this may help game developers improve player interaction in cooperative multiplayer casual games.

Experiment

In order to analyze synchronous cooperation in remote casual games, an experiment was conducted. During the experiment pairs of participants had to evaluate six games, each based on one of the cooperative patterns. A selection of patterns (selected from the list presented in [START_REF] El-Nasr | Understanding and evaluating cooperative games[END_REF]) was made based on their popularity and the frequency of appearance in existing multiplayer games. Based on the coupling between players, we grouped the selected patterns either in closely-or loosely-coupled type of interaction. Our previous studies have shown a positive player reaction towards closely-coupled collaboration in comparison with looselycoupled when accomplishing game-like tasks in a 3D virtual environment [START_REF] Beznosyk | The influence of cooperative game design patterns for remote play on player experience[END_REF]. In our current work, we investigate whether closely-coupled interaction can be beneficial for casual game design by providing players with a better experience than loose coupling. In order to decide upon the suitability of the cooperation type in the context of casual games, each game was analyzed based on five criteria: excitement, engagement, challenge, understandability and replay value (further referred as replayability).

Participants

We recruited thirty-six unpaid volunteers (thirty-one males and five females) to participate in the experiment. The average age of participants was 28 years old, from 21 to 38 years. Most of them had a computer science background and were recruited among university staff and students. According to self evaluation, the average player experience with any type of casual games was 3.19 on a scale from 1 (never played) to 5 (played a lot).

Apparatus

During the experiment two players were located in neighboring rooms separated by a hallway. Each player used a 15.4" laptop connected over a LAN. One of the laptops was a HP Compaq 8510p (Intel Core 2 Duo T8100, 2.1 GHz, 3 GB with ATI Mobile Radeon HD2600 graphic adapter) and the other was a Dell Latitude E6510 (Intel Core i3 M370, 2.4 GHz, 2 GB with NVIDIA NVS 3100M). A separate external keyboard was attached to each laptop for a more comfortable input. No communication was possible between the two players. One observer was present in each room and sat beside the participant.

Developed Cooperative Casual Games

Six custom games were created for the experiment, each adopting one of the selected cooperative game patterns. Based on the coupling between players, we classified each game into one of two categories: closely-or loosely-coupled. If a game requires a lot of waiting or if the actions of one player directly affect the other player, it was categorized as the first type. The games that do not require tight collaboration between players and allow more independent performance were assigned to the second type.

For every game, a similar 3D virtual environment (Fig. 1) was developed, which consists of several islands (a rectangular area, on which all game elements are located). Players are represented by alien-like avatars used from Unity 3D tutorial 7 . To distinguish the two avatars in the virtual environment, one is colored in a light blue color, while the other avatar is brown. Players are able to navigate freely in the environment and are not forced to stay together. They have to collect different objects by running over them. Some of the objects are located on higher platforms not directly reachable by the players. Therefore, they have to use the jumping pads that help a player to jump higher to collect certain items. In order to get on a different island players need to jump across the abyss. If one of the players falls off the island the team loses one life. In all games players have five lives. After a fall, the player reappears at one of the respawn points. A group score is calculated and analyzed to measure the successfulness of the game completion. All closely-coupled games have equal conditions: players need to pick up 75 objects within 7 minutes. Loosely-coupled games are not designed equally and their play conditions are described further individually. The duration of loosely-coupled games is limited to 5 minutes. The game continues until one of the following conditions is met: (1) players collect all required objects; (2) players lose all their lives; (3) the time runs out. A limited resources pattern is adopted for closely-coupled Game 1. Two players have to collect items, but are able to store a maximum of 10 items at the same time. Once both players reach the maximum amount of items, they can collect the following 10 objects. If one of the players collects 10 objects he/she has to wait for the other player and cannot pick up new items in the meantime.

A complementary pattern is used in closely-coupled Game 2, which implies that players have a different role to complement each others' activities within the game. During this game two roles are introduced. One player moves the jumping pad around the island while not being able to jump, and the other player uses it for jumping to reach objects located on higher platforms. There is only one jumping pad on each island. The roles are assigned randomly when players start the game.

An interaction with the same object pattern is followed in closely-coupled Game 3. In this game players have to move the jumping pad simultaneously. As soon as one of the players selects the jumping pad to move, the other player receives a message on his/her screen that the pad is selected and he/she is needed to help moving it. However, it does not indicate the location where the player has to be in order to help his/her teammate. When selected by two players, the jumping pad can be moved when both players walk in the same direction. Both players can use it for jumping. Similar to game 2, there is only one jumping pad on each island.

Loosely-coupled Game 4 utilizes a shared puzzles pattern. Here, the focus is to collect 10 special objects: each contains a heart with a letter on one side. Once all 10 special objects and therefore 10 letters are found, players need to use them to formulate a word containing all the letters, and put them in a designated window. The game succeeds when the word is entered correctly. Players do not see what words are entered by their partners while guessing. Once the correct solution is given by one player, the other one can also see it in his/her window.

An abilities that can be used on other players pattern is used in looselycoupled Game 5. In this game, players have to collect two types of objects: hearts and weapons. Each one is assigned to one player. They can see only one type which is randomly assigned on starting the game. Every time a player collects his/her 10 items, he/she gets the ability to see the partner's objects for about twenty seconds, and is able to collect them as well. The goal of the game is to collect 150 objects together, where every player has at least 50 objects of own type, and 15 objects of the partner's type.

A shared goals pattern is utilized in loosely-coupled Game 6. The collaboration is reduced to a shared goal of collecting 115 objects while acting independent from the partner.

Procedure

During the experiment, participant pairs completed consequentially six sessions, each corresponded to one of the casual games described earlier. In each game the player had to collaborate with his/her partner who was located in a different room. Like in many online games, players were coupled anonymously and, therefore, did not know who their partner was. Any form of communication (voice chat, text chat, pop-up messages, etc.) was avoided. Pop-up windows were used only in one game to support some basic level of awareness between two players. Before the actual experiment, a pilot test was performed to check the playability of every game.

Before the experiment, participants read a brief introduction and conducted a five-minute trial to familiarize themselves with the gaming environment and controls. In addition, written rules were given, in which both the goal and the way of interacting with the partner were explained. After each game, players were asked to evaluate the subjective perception of their experience. Based on the way they interact with the partner, they were asked to quantify the level of their excitement and engagement. Additionally, they evaluated how challenging and easy to understand each game was. Finally, they provided information regarding game replayability. For evaluation purposes, a visual analogue scale (VAS) was used. The participants marked on the 10 cm line the point that they felt represented their perception of the current state from not at all to very much. Also, the behavior of each player was observed and analyzed afterwards. It took approximately 60 minutes for each pair to complete the actual experiment.

Design

During the experiment, a within-subject design was used. The independent variable was the game type with six conditions. All participants, in pairs, had to complete six sessions testing every game type. The order of the conditions was counterbalanced using a balanced Latin square design. The dependent variables were excitement, engagement, challenge, understandability and replayability, so different components of a subjective evaluation of each game type. These were collected after each session through a questionnaire.

Hypotheses

To analyze six cooperative games from the perspective of their applicability for casual gaming, and more specifically to investigate the potential of closelycoupled games, we compare them based on five different criteria. While some of these criteria are chosen based on general requirements for computer games (excitement, engagement, challenge), the others represent characteristics more specific for casual games (understandability and replayability).

We state five hypotheses that apply to the games evaluated in our study. We want to see whether closely-coupled interaction, integrated in our games, can be of any advantage for casual gaming when compared to loose coupling. To check this assumption the following hypotheses are formulated with respect to the casual games criteria: H1: each closely-coupled game provides higher excitement than each of the looselycoupled games evaluated in the experiment; H2: each closely-coupled game provides higher engagement than each of the loosely-coupled games evaluated in the experiment; H3: each closely-coupled game provides more challenges than each of the looselycoupled games evaluated in the experiment without any negative impact on player excitement; H4: each closely-coupled game does not provide additional difficulty to understand the interaction between players in the game when compared to each of the loosely-coupled games evaluated in the experiment; H5: each closely-coupled game provides a higher level of replayability than each of the loosely-coupled games evaluated in the experiment.

Results

This section presents the results of our study. Firstly, analysis of the six games is reported based on five criteria: excitement, engagement, challenge, understandability and replayability. Secondly, we examine player experience based on the observations done during the experiment.

Subjective Player Evaluation

During the experiment the six games were compared based on five criteria: excitement, engagement, challenge, understandability and replayability. Fig. 2 represents the averages and standard deviations of each criterion for every game based on the subjective player evaluation.

Although the games evaluated in the experiment share a lot of similar features like the graphics and in-game tasks (e.g. navigation, jumping, collecting objects), the interaction between players has been designed differently. Therefore, we check the hypotheses, presented in the previous section, separately for each closely-coupled pattern. By doing so we identify the closely-coupled patterns that improve casual gaming experience when compared with the loosely-coupled patterns. Table 1 summarizes the decision upon each hypothesis (accepted or rejected).

Fig. 2. Subjective player evaluation of the games

First of all, we analyzed player excitement in every game. From Fig. 2 we observe that closely-coupled games support a higher level of excitement. Repeated measures ANOVA with a Greenhouse-Geisser correction has shown a significant difference between the six games (F (3.72, 130.34) = 28.72, p < 0.01). A Bonferroni post-hoc test revealed that games 2 and 3 were found significantly more exciting (p < 0.01) than any of loosely-coupled games. Although game 1 showed an increase of player excitement when compared to the loosely-coupled games, the difference was not significant.

We have also discovered that not all closely-coupled games used in the experiment were equally exciting, as a significant difference existed among them. In particular, game 1 was found to be significantly less exiting (p < 0.01 ) than the other two closely-coupled games. One of the possible explanations for this could be the different nature of the in-game activities. Games 2 and 3 were the only ones where players had to move the jumping pad, which was not necessary in the other games.

The second important characteristic is a high level of player engagement with a game. We asked players to evaluate how engaging they found cooperation in every game. Performed repeated measures ANOVA with a Greenhouse-Geisser correction has shown that not all games were similarly engaging (F (3.75, 131.22) = 36.54, p < 0.01). Again we observed that games 2 and 3 obtained the highest points based on the players ratings. These two games were significantly more engaging than all loosely-coupled games (p < 0.01). Though being evaluated higher than the loosely-coupled games, game 1 did not significantly differ from them.

The next step was to analyze how challenging interaction between players in each game was. All games, except games 2 and 3, received a relatively low rating (on average for each game below 4 on the 0 to 10 scale). We found a significant difference among the six games (F (5,175) = 13.54, p < 0.01). Both games 2 and 3 have shown to be more challenging than other games considered in the study (p < 0.01).

As additional challenges may decrease player enjoyment while playing a game, we investigated whether any negative influence existed in our case. This was particularly important for closely-coupled games, as this type of interaction was found to be more challenging. For each game, we checked the correlation between level of challenge and player excitement. Every of the six games has shown a significant positive correlation between these two parameters, indicating that in fact additional challenges, caused by the interaction between players in cooperative games, result in a higher enjoyment. This confirmed that more challenging closely-coupled games did not negatively impact player excitement.

The following criterion evaluated by players was understandability. As casual games imply easy understandable rules that do not require much experience, it is important that the way of interaction introduced in cooperative gaming does not decrease this understandability. All games have shown a high level of comprehension, without a significant difference across the six games (F (4.11, 143.87) = 2.18, p > 0.05).

The last characteristic rated by players was replay value or replayability of the game. For casual games, it is a paramount characteristic that guarantees that people will be willing to play the same game again in the future without getting bored very quickly. Repeated measures ANOVA with a Greenhouse-Geisser correction has shown that the six games were not equal in this characteristic (F (3.45, 120.72) = 9.97, p < 0.01). Based on a Bonferroni post-hoc test, the games of the closely-coupled group have shown a higher player preference to replay the games again with a similar type of interaction between players (p < 0.01). The highest values were obtained for games 2 and 3, showing a significantly higher level of replay value in comparison with all loosely-coupled games.

The obtained results have shown the difference among closely-coupled games, explained earlier in this section. As can be seen from Table 1, only one hypothesis was confirmed for game 1. At the same time, games 2 and 3 were evaluated similarly based on all criteria due to the design resemblance between these two games. Every hypothesis was confirmed for games 2 and 3, indicating their high potential for casual game design.

Observations of Players' Behaviour

Besides the subjective player evaluation, we analyzed player experience based on the observations done during the experiment. We present these observations under three categories: players' emotions, performance and communication.For the latter, we consider different remarks that the participants gave about the communication component in cooperative games, as well as their attempts to communicate with the other player through the in-game actions.

Players' emotions. Although several players were quite concentrated while playing, almost all of them showed a lot of positive emotions (smiling and laughing). In particular, this occurred when the player encountered his/her partner's avatar. Players were free to navigate in the environment, and therefore, could lose each other in the 3D world. This happened mainly during loosely-coupled games, where players did not stay together most of the time and rarely met each other. The positive reaction appeared also when the players were in each others' way, making the game a bit competitive by preventing the partner to pick up his/her objects directly. When being alone in the scene, players showed to be less emotional and were more focused on the task. They indicated that the looselycoupled games appeared to be more competitive than cooperative, with the goal of collecting more objects than the partner. Therefore, participants were focused on the task of getting more objects. Because most positive emotions were caused by (successful or not) interaction with another game character, loosely-coupled games resulted in less smiling or laughing than closely-coupled ones. One of the most 'emotional' games was game 3, as it required almost constant interaction with a shared object and, thus, with the other player. Due to its nature, this game caused a lot of excitement among players. Positive emotions were not only caused by a successful play, but also, for example, while trying to move the same object in different directions.

Performance. Observing player performance revealed the following aspects of cooperative interaction. In case of closely-coupled interaction, players tended to follow their partners all the time, in case help would be necessary during the game (mainly in games 2 and 3; but this also occasionally occurred in other games). If, for some reason, they lost each other in the environment, we observed players trying to find their partner as soon as possible, especially when they were waiting and could not proceed further without their help. As there was no communication, players tried to take into account all visual information in order to adjust their actions to partner's performance and help the partner. For example in game 2, once a player saw that his/her partner experienced difficulties in reaching objects on higher platforms, he/she immediately adjusted the position of the jumping pad. In loosely-coupled games players were free to decide whether or not they preferred to be alone or stay together with the other player. In contrast with this situation, closely-coupled games forced both players to be together or to wait for each other most of the time. We observed that a player acting extremely slow, made the experience of the other player less enjoyable.

Communication. During the experiment players were not allowed to communicate in any form. They learned from the game rules what they had to do, but could not discuss strategy or ask for help. Nevertheless, some of the players indicated that they were not affected too much by the absence of communication. In fact, they pointed out that the absence of communication made the game more challenging, and thus, more interesting and entertaining. They stated that information, they obtained from rules and what they observed on the screen, was sufficient for a successful gameplay. Based on the observations, we concluded that this was mainly typical for the evaluated closely-coupled games, as players were together most of the time and were aware of each others' actions. Here, it was not required to search for another player, making the need of communication less strong. In loosely-coupled games, players were not always sharing the same area of the virtual world and, therefore, they were not always aware about the partner's actions.

While observing participants, we noticed frequent occurrence of "communication with the screen". Players tried to explain their partners what they had to do, yet knowing that their messages could not be heard. Being not able to talk, players tried to find out ways in which they could assist their partners when it was obvious that he/she experienced difficulties. One of the solutions, we observed, was an attempt to 'communicate' an advice by moving his/her own avatar in front of the other player. By his/her own action, a 'better' player showed where the other one had to be for an easier accomplishment of the task.

Most games were played successfully under the given conditions of the remote setup without communication. However, half of the participants stated that the presence of communication would increase their performance. Although it was not proven by letting them play with communication, players pointed out that an ability to talk (either via text messages or voice) would make them more efficient. In particular, they mentioned an advantage of communication in order to divide areas for objects search. It was often the case that players lost a lot of time by going to the areas, where their partner had already harvested all objects. In general, participants expressed a strong wish for being able to talk with the other player even when it was not absolutely necessary for the game. For instance, if the strategy or in-game tasks were clear without actual communication, players still wanted to talk in order to make fun of each other and joke together.

Conclusion

In this paper we presented a study investigating synchronous cooperation in remote casual games where no communication existed between players. We analyzed six different types of cooperation (based on cooperative game patterns), each being either closely-or loosely-coupled. It has been shown that introducing closely-coupled interaction in casual games leads to a better player experience. We have observed that two of the three evaluated closely-coupled games introduced a higher level of player excitement, engagement and replayability without additional learning difficulties. In particular, games 2 (complementary roles pattern) and 3 (interaction with the same object pattern) satisfied all 5 criteria considered in the study, showing the advantage over the loosely-coupled games evaluated in the experiment.

Although we have seen player preference towards closely-coupled interaction in games, not all closely-coupled games were evaluated equally. Due to the possibility to design closely-coupled tasks differently, we realize the impact of game design on player evaluation. From the results, we observed that the design of games 2 and 3 differs more from the one of game 1 (limited resources pattern) than from each other.

With this study we have made the first attempt to evaluate cooperative game patterns in remote setup where any form of communication was not supported. Comparing results of our study with the existing works allows us to draw conclusions about an impact of the setup on the player experience [START_REF] Beznosyk | User enjoyment and performance in collaborative and cooperative games in shared 3d virtual environments[END_REF]. In particular, findings reported in [START_REF] El-Nasr | Understanding and evaluating cooperative games[END_REF] showed a higher contribution of several loosely-coupled patterns to the overall player experience (e.g. shared puzzles), while in our case close coupling was more preferred among players. We believe that such difference between remote and co-located setups is caused by the inability to talk.

Based on these findings, we can conclude that integration of closely-coupled interaction together with an appropriate game design has great potential for casual gaming. This triggers further research to confirm the obtained findings and generalize them to other games. By doing so it is possible define ways of making closely-coupled interaction more interesting for cooperative casual games among distributed players.

Fig. 1 .

 1 Fig. 1. An example of the game scene

Table 1 .

 1 Hypotheses check for the closely-coupled games evaluated in the experiment.

	Pattern			Hypothesis confirmed?
	type	H1	H2	H3	H4	H5
		excitement engagement challenge understandability replayability
	Limited	No	No	No	Yes	No
	resources
	Complemen-	Yes	Yes	Yes	Yes	Yes
	tary roles
	Interaction	Yes	Yes	Yes	Yes	Yes
	with same
	object

www.soe.com

available online at http://unity3d.com/support/resources/tutorials/3d-platformgame

Acknowledgments. The research described in this paper is directly funded by Hasselt University through the BOF framework. The authors would like to thank Tom De Weyer for his assistance with the implementation and all participants who contributed to this research by taking part in the experiment.


Joris Dormans

The Effectiveness and Efficiency of Model Driven Game Design

In order for techniques from Model Driven Engineering to be accepted at large by the game industry, it is critical that the effectiveness and efficiency of these techniques are proven for game development.

There is no lack of game design models, but there is no model that has surfaced as an industry standard. Game designers are often reluctant to work with models: they argue these models do not help them design games and actually restrict their creativity. At the same time, the flexibility that model driven engineering allows seems a good fit for the fluidity of the game design process, while clearly defined, generic models can be used to develop automated design tools that increase the development's efficiency.

Introduction

Games are hard to design and develop. Game audiences expect a higher level of quality year after year. For contemporary triple-A console titles this means that development teams easily consists of more than a hundred designers, programmers, and artist, and a production period that spans multiple years. Even for casual games, which up until a few years ago could be developed within a couple of months by a team of under five people, we see changes. The current generation of mobile and social games is already developed by experienced studios that assign twenty to thirty developers to the task and year-long development times are no longer uncommon. In order to keep producing better quality for less money, the games industry needs to find ways to either increase revenues, or improve the efficiency and effectiveness of the development process. This paper investigates how modeling techniques can be used to do the latter, but also at the obstacles that need to be overcome to get model driven techniques accepted by the game development community.

Abstract Game Development Tools

For a while, within the game design community there has been a careful push for the development of abstract tools and methods for game. In a 1999 article Doug Church called for the development of 'formal abstract design tools' [START_REF] Church | Formal abstract design tools[END_REF]. Since then, a number of frameworks and tools have sprung up. Many of them are primarily design vocabularies, created to help understand and identify common structures in games, and avoiding to be prescriptive in their description of games. 1 Berndt Kreimeier suggested to apply the design pattern approach from architecture and software engineering to game design [START_REF] Kreimeier | The case for game design patterns[END_REF]. In contrast to vocabularies, design patterns are prescriptive; they describe 'good', generic solutions to common problems. However, the most prominent work on design patterns within the domain of game design to date [START_REF] Björk | Patterns in Game Design[END_REF], is explicitly distanced from a prescriptive approach, creating a hybrid approach that is closer to a design vocabulary than a pattern language. At the same time, Raph Koster experimented with a graphical grammar to express game mechanics [START_REF] Koster | A grammar of gameplay: game atoms: can games be diagrammed? Presentation at the Game Developers Conference[END_REF], this approach was followed by [START_REF] Bura | A game grammar[END_REF][START_REF] Cook | The chemistry of game design[END_REF][START_REF] Dormans | Machinations: Elemental feedback structures for game design[END_REF]. In contrast to the design vocabulary approach, the focus of these grammars has not been the collections of descriptions they allow, but the design lore that they capture. For example, the Machinations framework [START_REF] Dormans | Machinations: Elemental feedback structures for game design[END_REF] sets out to visualize the structures in game mechanics that create emergent gameplay; in other words, it departs from theoretical vision on quality in games, and constructs a tool set that allow game designers to interact with the structures that contribute to that quality more directly.

In the Machinations framework the diagrams expressing game mechanics act as a domain specific language (DSL) for a subset of game development; in this case for a game's "internal economy" [START_REF] Adams | Fundamentals of Game Design[END_REF]. Using similar DSLs for level design opens up the possibility of applying techniques and ideas from Model Driven Engineering (MDE) [START_REF] Brown | An introduction to model driven architecture[END_REF] to game design. Graph transformations might be used to transform machinations diagrams to graphs outlining interactive missions for level design or vice versa [START_REF] Dormans | Level design as model transformation: A strategy for automated content generation[END_REF]. Similar ideas have been explored by Reyno and Carsí Cubel from a more technical perspective focusing on automatically generating code for games [START_REF] Reyno | Model-driven game development: 2d platform game prototyping[END_REF][START_REF] Reyno | Automatic prototyping in model-driven game development[END_REF]. The techniques typically used in a MDE approach to game development (transformational grammars, UML, Petri nets, and so on) require a considerable effort to use for game designers that do not have a background in software engineering. In fact, one of the biggest challenges to introduce any abstract game development tool, is to convince game designers of its value in the first place.

Design versus Engineering

Leaving aside the much longer history of board games, game development is a very young field. The current generation of prominent game developers got to the place they are today because of hard work, entrepreneurship, and bravura. For the early pioneers of the field there were no abstract tools to guide them. Nonetheless, they have created an industry. As a result there is a certain level of animosity towards abstract design tools. A fair number of game designers dismiss design tools because they do not think the tools are effective enough, they fear the tools might actually harm the creative process, or both [START_REF] Guttemberg | An academic approach to game design: Is it worth it? Gamasutra[END_REF][START_REF] Sheffield | Defining games: Raph koster's game grammar[END_REF].

The lack of proven, effective design methods is a serious concern. The current DSLs and design vocabularies for games tend to have a steep learning curve, and are anything but widespread. Although many are designed for actual design work, few have found frequent use outside universities. However, effectiveness is not an argument against design tools, it is a requirement: the effectiveness of a game design tool versus the effort required to master it should be apparent. Typically, this means that using a design tool should lead to better games built in less time, and the time reduction should be more than the time required to learn the tool.

The negative effects on the creative process are trickier to deal with. Many advocates of game design tools argue that they are designed to support the creative process, while many designers experience tools as rigid and restrictive instruments. Although, it can be reasoned that this argument against abstract, theoretical methods for game design stems from a naïve perspective on art and creativity. After all, practitioners of any form of art, from painting and sculpture to cinema and performance, use abstract tools such as the theory of perspective, composition, and editing techniques, to teach and improve their form. However, it is equally important to make clear in what ways game design tools support rather than restrict game design.

By nature game design is a flexible process. It is impossible to plan and design a game on paper and expect good results from simply building the game as per specification. Too often, what looks good on paper does not work as expected, or simply lacks fun. Gameplay is the emergent result of players interacting with a dynamic system. Until that system exists in some prototype form, it is impossible to say whether or not it works. This means that an important task of the game designer is to spot opportunities as they arise during the development of a game. A famous illustration of this effect is the development of SimCity from the editor for another game (Raid on Bungeling Bay). While working with the editor, game designer Will Wright discovered that it was much more fun to build the urban environment than it was to fly around it and shoot enemies as the original game intended.

MDE is a suitable approach to deal with the dynamic process of designing games. By capturing different aspects of games in different models and describing how one model can be transformed into a different model allows for a sufficient level of agility: for one game mechanics might be the natural starting point, while for another game interactive missions might be. At the same time, using models forces designers to think about their game at an abstract level. In addition, different models foreground different structures in the game's design; in a way, each model acts as a different lens. Machinations diagrams foreground feedback loops that operate in a game's economy, by building such a diagram, the designer will have to deal with these structures consciously. Daniel Cook's skill chains [START_REF] Cook | The chemistry of game design[END_REF] visualize the dependencies between the skills players must master to successfully play the game: it is a useful lens that should be used to investigate tutorial stages and learning curves in any game.

However, model driven engineering has not been applied to game design extensively. As discussed above, within the game design community there are no widely accepted abstract models for game development, not even for specialized tasks such as level design or interactive storytelling. Without clearly defined, widely accepted models that have a proven positive impact on the development process, it will be hard to convince the games industry to start applying model driven engineering techniques on a larger scale. Yet, it is the argument of this paper that the game development community would do good to investigate the opportunities presented by MDE, as these opportunities could lead to a more efficiently designed games which in turn leaves more room to elevate creative game design to the next level.

Applying Models Effectively

Looking at the common problems encountered during game development, problems regarding design and managing game features are the most prominent [START_REF] Petrillo | What went wrong? a survey of problems in game development[END_REF]. Traditionally, the content and scope of a game's design is recorded using a game design document. However, these documents have a poor track record: they are considered to be a burden to create while they are hardly consulted by the development team.

Using generic, abstract models can increase the effectiveness of game development far beyond the current practice of writing game design documents. Clearly defined, generic models are less open to different interpretations by different team members. In the most ideal case, the model's syntax is completely unambiguous. When done right it is even possible to use formal models such as Petri nets [START_REF] Brom | Petri nets for game plot[END_REF][START_REF] Araújo | Modeling games with petri nets[END_REF] to identify structural strengths and weaknesses of a design in an early stage. Although similar effects can be reach with relative simple and informal models such as the skill chain diagrams (see above). These diagrams expose a number of structural characteristics such as the number of connections between individual skill nodes and the relative width and depth of the diagram, which are indications of the relative steepness and length of the learning curve, respectively.

The Machinations framework is a good example a generic, abstract model that foregrounds structural qualities of a game's design. The number of nodes, connections, but also the number of feedback loops in a Machination diagram, are all important indications of the complexity of the game and the quality of the gameplay that emerges from it. The pattern library that is part of the framework catalogues common structures that are indicative of particular dynamic effects. The use of these patterns helps designers to understand and improve the dynamic gameplay. At the same time the patterns are flexible enough to enhance creativity. There are many ways to implement each single pattern, and the number of ways to combine patterns is infinite.

The wide range of levels of abstraction the Machinations diagrams are able to express can be leveraged to create a design strategy. A game designer can start out with a fairly simple model of the game and elaborate on it by slowly increasing the complexity by replacing simple constructions with more complex ones. The design patterns can be used as a guide for this process. Relative simple patterns can be replaced by more complex ones. Within the Machinations framework this process is referred to as elaboration, and each pattern in the library indicates what patterns it elaborates and by what patterns it is elaborated.

Applying Models Efficiently

The application of model driven engineering techniques also creates opportunities to increase the efficiency of the game development process. Generic, abstract models can act as quick, early prototypes that would normally require more time and effort to build. They can be used to simulate games and collect gameplay data at a very early stage. In addition, these models can be used to develop automated design tools that can speed up the process and freeing designers from manual work to focus on those aspects of the development process that benefit most from their creative labor.

These days, all game developers are convinced that prototyping is a critical aspect of developing games. Because gameplay is an emergent result of the game as a dynamic system, the best way to find out whether or not a game works is to build the system and set it in motion. Visual representations such as Machinations diagrams can help identify important structural features that create the emergent gameplay and help designers to shape the gameplay towards a target form, they can never fully replace a play test session using a playable prototype. Fortunately, Machinations diagrams can be executed and allow user interaction when running. This means that they can actually serve as abstract, low fidelity prototypes. At the same time, the diagrams allows the designer to define artificial players and quickly run many simulated play sessions to collect data. This helps designers to balance games in a very early stage. Although, there remains a gap between the model and the real game. Data collected in this way primes designers for gameplay effects that might occur, and already allows them to find out what measures can be taken to counter-act certain unwanted effects.

With sufficient sophisticated models and tools, MDE for game design can be taken much further. Recent experiments with mixed initiative procedural content generation (PCG) [START_REF] Smith | Tanagra: A mixed-initiative level design tool[END_REF][START_REF] Smelik | Inegrating procedural generation and manual editing of virtual worlds[END_REF] sketch the possibility of creating design tools that automate certain tasks to speed up the process of design. Procedural content generation is already being used in commercial games, in most cases to generate levels either during design time, or every time the player starts a new session. The mixed initiative approach to PCG has the computer algorithm collaborate with a human designer in order to get better results. However, PCG techniques are typically tailored towards a specific game. Using generic and abstract models, the same techniques can be applied to multiple games, and hopefully will lead to the development of intelligent, generic design tools. MDE and model transformations seem to be a perfect match for this development as they offer enough flexibility to deal with the agility of the game development process while they are defined clearly enough to be automated [START_REF] Dormans | Level design as model transformation: A strategy for automated content generation[END_REF]. Ideally a game designer will be able to go back and forth through and make changes to different, connected models of the same game, although it is a considerable technical challenge to create a system that allows a designer to step back to an early model of a game, make changes to it, and automatically reapply all transformations that generated the current state of the game.

Discussion

However promising the prospect of applying MDE to game design is, the current state of the art of MDE in game design still faces many challenges. A lack of widely accepted models to represent different aspects of games means that currently no one can expect game designers understand and apply these models. Successful models need to be expressive enough to be able to deal with an infinite variety of games, while still be intuitive to game designers. In addition, they should not require too much effort to master. At this moment is remains unclear whether or not models like this are going to surface soon. No one should expect one single model for the entire process of game development, or every aspect of a game, to emerge. It is better to focus on different models for particular aspects of games and game design. Using multiple models creates a more flexible frame work that and allows designers to adopt MDE techniques one step at a time. However, multiple models also present a problem. It is not always clear how transformations can be defined that allow a model of a certain type to be transformed into a model of another type. For example, a graph based model might be used to express mechanics or missions, but there is no transformational grammar that specifies how a graph can be transformed into a spatial map representing a game level.

MDE can be used to develop generic, automated design tools. These should be build to support designers in their task. Ideally a design tool manages trivial, repetitive task and foregrounds potential structural strengths and weakness of a design. No tool should require trivial, repetitive task from the designer. This allows the designer to focus on the creative tasks. It is critical that the designer stays in control of the creative aspect of the design process at all times. Model transformations are a good way of codifying design strategies for a particular game or a game genre. Using automated design tools, development teams should expect a considerable effort to get the tool up and running. For this reason it is unlikely that the entire process can be automated at once. Small steps should be taken for each new development cycle, starting with steps are more generic and can be easily reused in subsequent projects.

Finally, we have very little experience of applying these techniques to actual games. Currently, many game development studios have settled into particular ways to develop games that are not necessarily suited to MDE. They have set up content pipelines and follow milestone plans. Further research on how to integrate MDE techniques and automated design tools into this process is needed.

Most notably among these are the Game Ontology Project (see www.gameontology. com and the 400 Project (see www.theinspiracy.com/400_project)