In this stage, the multidisciplinary team builds the game’s prototype, considering the game components highlighted by Kinross [19]: game mechanics balanced with elements of game design to improve engagement, motivation, and reinforcement.

Since video games intrinsically have a motivational design [29], a serious video game should be similarly designed. In line with the IDEAL-Games framework, we suggest the user-centered design (UCD) as a design methodology. This iterative methodology is oriented to take into account what the users themselves expect from the game [37]. Finally, compared with commercial games, serious video games must be scientifically tested to check that the usability and efficiency are adequate.

The UCD methodology also allows for recording user performance; collecting this preliminary data with real users allows for closer adjustment to the patient’s needs. In the development of TSTM [21,22], there were two events that emerged in user testing. Enigma had to be refined to improve the impact on working memory, and for Smasher, including a bug representing a distractor (a bird flying in a strange way), psychologists suggested that this incoherent stimulus could be more distracting and was incorporated as an advanced distractor. UCD methodology allowed us to redesign the game mechanics considering how people with ADHD behaved during the game.

The initial TSTM design was modified following suggestions from patients with ADHD about esthetics, game difficulty, or rewards [22]. The UCD methodology fulfilled two additional objectives: the designers checked the correct functioning of the game and the ADHD patients felt involved in the development process.

Treatment with therapeutic video games may help to improve adherence to multimodal treatment in ADHD by bringing patients closer to treatment.

The game’s theme is important in the design process, as player preferences may differ from one player to another. Games such as EndeavorRx [38] or Plan-it-Commander [39] are inspired by space and astronauts. Regarding TSTM, each mini game takes place in the forest, as regular interaction with nature can be beneficial for people with ADHD [40]. Furthermore, it is possible to design exercises that are compatible with different game environments. Thus, users could choose the setting of the game without modifying its mechanics. By doing so, motivation for the task may be enhanced along with an increase in the adherence to treatment. Although this guide is oriented to the usually chosen age range of 6 to 18 years, generating different appearances depending on the patient’s age would likely be a good approach to treat ADHD during an individual’s growth and development. The same mechanics could even be used for adults by adopting more mature and sophisticated themes.

A serious video game also has the following requirements: (1) easy to understand, (2) require a minimal cognitive load (baseline), and (3) gradual level of difficulty. The baseline can be designed as an in-game tutorial. It is important to ensure that the person has understood the task and how to perform it. All of these elements were examined in the previously mentioned usability study for TSTM [22]. The aforementioned user-centered model [37] can help to adjust design guidelines.

Sampayo-Vargas et al [41] stressed that in order to make educational computer games intrinsically motivational, it is critical to provide an optimal level of challenge. The difficulty curve should be neither too boring nor too frustrating. Users play the game and try to learn the correct pattern to follow to pass the given task and master it. Initially, they are entertained; however, when the game is no longer a challenge, users tend to become bored and give up [42]. For people with ADHD, novel challenges may improve adherence to treatment by increasing motivation. Furthermore, the game must automatically adjust to the individual’s performance level [37,41]. Games such as EndeavorRx [38] change the level of difficulty until the player is performing at an 80% rate of accuracy. Here, it is important to stress that artificial intelligence (AI) may help to personalize and update the needs of each individual in a specific context and time by adjusting at that precise moment (eg, type of game, intensity).

To avoid boredom, game design components should be included in any serious video game related to objects, mechanics, dynamics, and emotions [43]. “Juiciness” is a novel concept that refers to adding visual embellishments, sounds, and other types of nonfunctional elements, thus allowing developers to improve players’ experience in their games [29]. Juiciness should not affect the game mechanics but rather only focus on the extra elements (eg, menus, transitions, feedback), pursuing the development of a simple, minimalistic, intuitive, and easy-to-understand interface.

People with ADHD have limited self-perception and objectivity in task performance [4]. Therefore, providing simple and direct feedback as the player progresses consistently is a fundamental requirement in the design of the proposed serious video games [42]. To avoid too much frustration that may lead to abandoning the video game, points, badges, leaderboards, and other game components can be used. Moreover, introducing small doses of challenge could be helpful to improve frustration tolerance. Achieving engagement through the attractiveness of the task may benefit emotional dysregulation. Indeed, a recent review [44] concluded that regular, nonexcessive video game use can help to enhance emotional regulation in children. A good example is EmoGalaxy [45], which is a video game that improved emotional regulation in children diagnosed with ADHD.

Although video games are usually visually based, music and sounds are also fundamental elements that facilitate flow in video games alongside other elements. However, these elements should not be overwhelming, because people with ADHD are more likely to be distracted. Thus, audio-visual integration can be used to vary the intensity of emotional experience states [46]. To facilitate the sense of progress, missions can be completed. Performance can be quantified and made visible in changes in the user interface, counters, and score, among other elements [42]. This feedback can be provided by using as many senses as possible, including visual stimulus (ie, colors to show the level of performance), auditory stimulus (ie, sounds to point at a hit, error, task change), or nociceptive stimulus (ie, the vibration of the game controller can indicate that a mistake has been made).

People with ADHD present broad clinical and cognitive variability [3]. Accordingly, any serious therapeutic intervention should ideally provide a tailored treatment targeting each patient’s specific difficulties and circumstances. In other words, serious video games should be developed within a personalized medicine framework [47].

Dovis et al [48] suggested that children with ADHD are less stimulated by reinforcement than typically developing children (likely due to a dopaminergic deficit), and therefore require higher amounts and frequencies of reward to perform optimally. These rewards should be more frequent at the beginning of the game (extrinsic motivation) and should decrease in frequency as the sessions progress, rewarding good performance with larger rewards (intrinsic motivation). However, people with ADHD may have delay aversion: a preference for small, immediate over larger, but delayed rewards [49]. Thus, it may be interesting to work on impulsivity through the reinforcement system [50]. Good task execution can also be rewarded through juicy embellishments and in-game achievements.

As previously discussed, people with ADHD may be more dependent on extrinsic motivation [51,52]. Children with ADHD are also more at risk of addiction to video games [13]. To avoid this, the focus should be on limiting the time spent playing. Furthermore, these time constraints may positively impact engagement by avoiding fatigue.

Research estimates that the average student's attention span is 10-20 minutes [53]; however, one of the symptoms of ADHD is sustained attention. Children with ADHD frequently experience hyper focus (the experience of focusing on something to the extent that all other stimuli are almost completely excluded) when playing video games [54]. Despite this, hyper focus cannot ensure that patients will continue to receive benefits, as illustrated in theories of deliberate practice [55].

Consequently, we have to reach a compromise between reinforcing the patient's attention beyond their own attention span and reducing gaming time to avoid addiction. Following the state of the art [15-17], treatments of 20-40 minutes 4-5 days a week seem to achieve more positive effects.

King et al [56] proposed five characteristics and their potential for game abuse: social features, manipulation and control features, narrative and identity features, reward and punishment features, and presentation features (see Table 1).

Sessions should last approximately 20-30 minutes depending on the patient’s age. We recommend dividing the session into a variety of tasks [52]. Every 10 minutes, there can be a change of task (greater variability) or short breaks [4]. Furthermore, it is important to plan a sufficient number of sessions to consolidate learning. For instance, each TSTM training session lasts 25-30 minutes divided into three blocks. Two blocks of 10 minutes each are used for training with two game mechanics (Smasher, Kuburi, Tekateki, or Enigma), along with a third block of 5 minutes of chess-based game mechanics. The sessions are counterbalanced, randomizing the order of the blocks to avoid being repetitive or boring (see [23]).

According to Koster’s [57] definition in the Theory of Fun of Game Design, game mechanics are “rule-based systems/simulations that facilitate and encourage the user to explore and learn the properties of their possibility space through the use of feedback mechanisms.”

In a serious video game, game mechanics are the main intervention components through which patients acquire metacognitive strategies and skills to improve their cognitive abilities [18]. Game mechanics promote learning and repetition of the task, which can help to establish generalization.

Analyzing the serious video games included in several recent reviews [15-17] led us to the conclusion that almost all current ADHD treatments are structured in a compilation of mini games that generally tend to be similar to a regular video game (with respect to narrative, lore, esthetics, etc). In contrast, serious games for ADHD assessment are rarely more than one mini game, inspired by single tests. This is in keeping with a general recommendation of developing short and discrete tasks for people with ADHD, among others [43]. The use of mini games provides a clear advantage: the combination of mini games helps to personalize treatment for each patient by being able to distinguish which cognitive processes require more training.

Mini games can be created by adapting a cognitive task using game-up and mapping techniques [43] or from scratch. To attract attention, we can introduce design strategies such as gamifying the cognitive task stimuli to try to produce positive sensations. However, this process is specific to the disorder for which the treatment is intended. Khaleghi et al [43] proposed following the Objects, Mechanics, Dynamics, Emotions (OMDE) design guidelines. When designing game mechanics, it is critical to validate the cognitive aspects of the game. For example, in people with ADHD, it is necessary to control distractions, the cognitive load, and the time spent. Indeed, there are three main cognitive aspects that affect people with ADHD: attention, executive functions, and daily skills.

Inattention is one of the core symptoms of ADHD [2]. In traditional attention-training tasks, children with ADHD perform worse than typically developing children [52]. New mini games can be created to treat different types of attention. Additionally, ADHD presents great comorbidity with executive dysfunction [3]. Some games can help to train executive functions such as inhibition control, working memory, planning, or reasoning.

Lastly, one of the major criticisms of the use of serious video games is the lack of ecological validity [39,40]. Thus, proposed video games should induce the transfer of cognitive skills to domains of daily life functioning (transferability). Transferability is based in the improvement of different executive functions, which may help with scholarly development and motivation. Cognitive training is effective if the patient applies it in their daily life. For example, children with ADHD show impairments in social skills, which can have serious long-term consequences. Games that explore cooperation skills, such as “Space Club” [39], may improve social benefits. For example, TSTM incorporated chess mechanics, because chess can improve math performance, thus demonstrating transference into the educational domain [21].