A total of 70 papers were identified from databases of which 15 duplicates were eliminated. Fifty-five papers were evaluated based on titles and abstracts. Of these, 32 reports were excluded due to not being related to the area of our review. Twenty-three full-text papers and 10 gray literature papers were evaluated. A final total of 17 papers were included: 8 from PubMed, 2 from PsycINFO, 3 from Embase, 1 from WorldCat, and 3 from Google Scholar, not counting duplicates (see Table 1). The selected study years range from 1981 to 2020. From them, 6 music-producing studies were found with instruments playing (active music), 10 papers when music was listened to (receptive music), and 1 study in which an interactive metronome was used to work on rhythm (interactive music). The possibility of performing a meta-analysis was discarded due to the heterogeneity in study designs and outcomes. Instead, we extracted one or more statistical outcomes from each study. Results are shown in Table 1 in order to synthesize and integrate them [25,29]. For the outcome integration, we particularly focused on the music modality (active, passive, and interactive) on one side and the ADHD symptomatology treated through music on the other side, grouping the studies for this analysis rather than treating each one of them individually [30].

Effects of active and passive music therapy on people with ADHD are summarized in Textbox 1. Active music therapy was effective in facilitating hemispheric synchrony [11], self-esteem [31], improving social skills [32], and decreasing aggressive behavior [33]. Music therapists perceive music therapy as an effective method in ADHD treatment in combination with other interventions such as medication or psychotherapy [34]. Regarding passive music therapy, we found 4 studies using music as a distractor; the results indicate that music did not produce higher distraction in any case [35-37]. Six studies found that listening to music can help in specific abilities such as arithmetic skills [35], reading comprehension [38], balance performance [36], attention [39], or reduced disruptive behavior [37,40]. In regard to the style of music, we found 2 studies showing reductions in motor activity when children with ADHD were listening to rock music [37,40]. Two other studies used relaxing music [36,38], another one used jazz music [41], and 3 other ones used classical music [39,42,43]. Heart rate variability (HRV) in ADHD was significantly lower when listening to calm music, while HRV was not significantly affected in the control group [38]. However, negative effects were also found by listening to silence and binaural sound. Vorster [44] found that binaural sound reduced attention in people with ADHD, while Zimmermann et al [43] explored the effect of silence on increased arousal and decreased positive mood.

No scientific literature on video games and music applied in ADHD was found in PubMed, Embase, PsycINFO, and Cochrane. Most of the video game studies found do not focus on timing skills or musical abilities [23], and music applied in ADHD was not used in video games but in other contexts (Table 1). Only one video game was found in Google Scholar, “ADDventurous Rhythmical Planet” [47], which combines video games and music for children with ADHD aged 8 to 12 years. This video game works on social skills through rhythm. Music creation through technology has also been explored in other disorders such as autism spectrum disorder and severe intellectual disabilities facilitating social communication skills [48].

Video games are generally effective and well-accepted tools for cognitive skills training in ADHD. The immediate feedback from video games provides direct performance support for individuals with ADHD; they tend to have engagement rates with low numbers of dropouts and increased participation, motivation, and sense of agency [23]. Repetition of tasks through game mechanics is done by increasing difficulty with therapeutic goals [49]. Despite the potential therapeutic impact that specific game mechanics involving rhythmic skills can provide [19], we have not found any video game that considers timing skills in ADHD. However, music video games have been used in some fields of health promotion [50]. In other disorders, namely, Parkinson disease, video games such as “Rhythm Workers” focused on beat perception via finger tapping [49,51]; “GenVirtual” uses a musical game with augmented reality to aid motor and cognitive rehabilitation [52].

Therefore, we focused on applied music therapy methods (active, passive, and interactive) to design specific video games to study timing deficits in ADHD. Audio may improve interactivity in video games in two ways, active and passive: a player causes sounds with his actions (active) and music can modulate the focus and emotional state of the player (passive) [53]. Due to the relevance of timing skills, therapeutic video games may include the training of music in specific tasks (active). For doing so, it is essential to rely on paradigms that have demonstrated validity in training related to timing deficits in ADHD. Based on Noreika et al [18], there are specific tasks for measuring related aspects. Timing deficits could be reduced through training or assessed through motor timing (eg, free tapping, sensorimotor synchronization, or rhythm reproduction task), perceptual timing (eg, with a discrimination task, verbal duration estimation, or a duration production or reproduction task), and temporal foresight (eg, with major difficulty to work it through delay or temporal discounting or gambling paradigms) [18]. These timing tasks can be combined with some ideas for commercial games with specific treatment goals for people with ADHD [50,54].

Regarding the passive way of using music in serious video games, the effect of listening to music in video games is unclear. A study suggests that music reduces the experienced duration of playing with or manipulating time perception [55]. However, this study emphasizes that the effect of music on video games is a complex phenomenon. Immersion and flow state also could depend on the music chosen in the game (whether we like it or not) or could be due to other factors related to game mechanics or enjoyment of the game. An effective sound design can serve to improve the realism and emotional depth of the player experience (eg, terror genre video games) [53]. Overall, music can provide an aesthetic in the game and reduces the possibility of boredom [55]. Moreover, white noise in a bandwidth of 65-80 dB or binaural sound could be incorporated into video games to enhance beneficial effects in ADHD [56]. Other aspects to consider are that effective sound design can facilitate the gamers’ experience (eg, victory songs associated with achievement or sounds indicating error can guide the player in achieving gaming goals). Finally, every player action should receive sound feedback (eg, select from the menu or reach a target, among others).

Music is a complex cognitive challenge that apparently has not provided us an evolutionary advantage. However, music provides us great pleasure [13], and brain activation is associated with plenty of complex demands [9,11].

It is a commonly accepted statement that children with ADHD need reduced environmental distraction and silence to perform better. On the contrary, some studies indicate that listening to music before or during a task could improve attentional performance better than silence [35,37,38]. In the case of children with neurotypical development, they also improved their performance with background music [12] except in reading comprehension tasks, where calm music distracted them, while people with ADHD improved their performance [38]. However, the effect of background music on attentional improvement could be modulated by the arousal state of participants, musical elements (eg, tempo, loudness, or the presence or absence of lyrics) [12,42], or difficult task outcome [35,36,43]. In one study [42], the tempo of a song was distorted (faster vs slower), finding performance improvements in slower tempo or silent conditions in children with ADHD, depending on the type of task. Regarding tasks, results suggest that in the case of ADHD, external stimulation with music can help adjust arousal to an optimal level in monotonous tasks. However, when the task is complex, external stimulation may worsen performance [35]. In hard tasks, some studies suggest that white noise may be effective in specific tasks like speech recognition, reading, writing speed, and working memory [36,56]. According to the cognitive-energetic model of ADHD arousal [57] or optimal stimulation theory [58], the distractibility of children with ADHD is a functional attempt to modulate themselves. Silence could generate a major seeking of novelty or stimulation [35] or even provoke distressing mood states during wait times in adults with ADHD [43]. Regarding style of music, the rhythmic and intense beat of rock music stimulate greater brain arousal that overrides environmental distractions [40], while relaxing music decreases HRV, affected in ADHD [38], and decreases negative mood [43].

Regarding active music performance, regularly playing a musical instrument can promote interhemispheric synchronization in people with ADHD, although this higher neural efficiency does not reach the musically trained controls. The choice of musical instrument can help increase practice frequency. Drums or guitars were preferred by people with ADHD, while piano was the preferred choice in children with dyslexia [11]. Moreover, active music can provide significant improvements in social functioning [32] and impulsivity [45]. Another aspect that has not usually been considered is whether it is the musical practice of a solo instrument or of an orchestral instrument which, a priori, could have a beneficial effect on social relations. Furthermore, rhythm gives temporal structure to the music. Following the beat involves sensorimotor integration between predictive (top-down) and reactive (bottom-up) processing where attentional control is involved; regular beat is supported by one’s internal clock, which seems to be faster in ADHD [10]. We tend to move following the rhythm of the music [46], and sounds that follow a pattern can induce stabilization of breathing or movement [59].

As mentioned earlier, people with ADHD have timing deficits, which can range from milliseconds and seconds (eg, perceptual timing and motor timing) to days or years (eg, temporal foresight) [18]. Moreover, patients with dyslexia, ADHD, or ADD have abnormal interhemispheric asynchrony (10-40 ms) of the primary auditory evoked P1 response and oversized left planum temporale as compared with controls [11]. Children with ADHD may also show difficulties in timing-based rhythm [19] or lower scores in rhythmic and melodic tasks [11]. Timing deficits are negatively correlated with impulsivity and delay aversion [60]. Rhythm training may be one more area to integrate into cognitive training in ADHD; some results suggest that after a 12-session rhythm training, children with ADHD reduced impulsivity and improved visual-spatial working memory [61]. A rhythm-based video game for ADHD named “ADDventurous Rhythmical Planet” [47] is associated with improvements in social skills in children aged 8 to 12 years [48]. Training based on rhythm could be integrated into video game mechanics [19]. However, available musical games on the market are not satisfying it because the task in these video games consists only of reacting to visual stimulation while music is presented (eg, games consisting of catching objects at a precise moment or dancing games where players execute movements mimicking a model) without therapeutic goals or with insufficient feedback [54]. Designing specific games that train in timing skills may be especially beneficial in ADHD treatment.

This study has limitations. Few studies were found while searching the databases, so we opted for broadening the search using gray literature from Google Scholar and WorldCat. In addition, the studies were quite heterogeneous, so a meta-analysis could not be carried out. Instead, we qualitatively integrated outcomes. We structured the outcomes considering 2 main variables, music therapy modality (active, passive, and interactive) and reduction in symptomatology, but qualitative reviews are nonetheless more prone to biases in their conclusions [29]. These results should then be interpreted with caution. More specific research is needed to investigate the effects of music on motor and neurodevelopmental disorders such as ADHD. Despite the relevance of timing skills in daily life, it is not sufficiently considered in research and clinical practice [18]. Timing skills could be affected in daily life activities such as predicting the precise moment when a vehicle approaches, particularly in some specific ADHD profiles (because timing deficits may not be present in all of them) [19]. Assessing whether timing skills are affected in people with ADHD may bring us closer to a clearer profile [11,18]. Some authors such as Serrallach et al [11] suggest that auditory cortex patterns could be a differentiating biomarker in ADHD with great relevance in the clinic. The treatment of the 3 pathways mentioned by Sonuga-Barke et al [14] related to executive functions, delay aversion, and timing skills should be considered in order to approach an adequate treatment profile and understand the heterogeneity of ADHD.

The benefits of music as a part of serious video games for people with ADHD have not been directly assessed. In general terms, this systematic review shows that active (playing music), passive (listening to music), and interactive (music-making with feedback) music therapy are beneficial in reducing ADHD symptomatology and increasing task performance in people with ADHD. Rather than being a distraction, music can help to modulate emotional and cognitive states. Rhythm seems to be a music component particularly benefitted by improved timing perception and regulation. We propose integrating music into the mechanical design of ADHD serious video games, especially related to rhythm-based mechanics.