## 4 Discussion  
The aim of this study was to provide a state-of-the-art literature review in the gamification of cognitive training and especially in the training of Working Memory, in order to provide an updated overview of the existing research and evidence for the GEs being utilized in the training and assessment of Working Memory specifically. Following PRISMA guidelines [60], a systematic literature review was conducted, which identified 44 studies published from 2015 to 2020. The risk of bias assessment, which was conducted according to the Cochrane's RoB tool, showed optimal quality of the included studies. The only domains that presented a higher risk of bias were the randomization process, for which some studies demonstrated poor or no randomization, and the missing of outcome data, which was caused by some dropouts during the intervention process. During the search process, we also identified several systematic reviews and meta-analyses, such as [49], which presented 33 studies published between 2007 and 2015. Another study identified 49 studies between 2008 and 2017 [55], Ref. [53] included 91 papers from 2006 to 2018 and Ref. [56] identified 24 studies between 2006 to 2018. This study identified 8 more studies after 2018, which were eligible to be included. For the inclusion of GEs, we searched the literature and previous studies, and we decided to include 9 GEs that were the most interesting to examine and that simultaneously had a greater impact on the training process, as far as we were concerned [48; 49; 50; 51; 52; 53; 55]. A major problem that we faced was the absence of any detailed information and descriptions about the gamified tasks/games that were used from the reported studies; thus, it was necessary to check any additional sources of information that were publicly available (web pages, videos, etc. ), in order to extract the required information. Regarding the use of GEs, there was a variety of the selected elements, which were scarce over the included studies. Overall, according to the results, Conflict was the most  
Figure 2: Risk of bias assessment using Cochrane’s RoB2 tool. commonly used game element (N = 39), which, considering the demands of WM (which requires an amount of information to be held for a specific time frame), looks to be ideal and the most significant game element that can be incorporated to gamification tasks. Along with Feedback (N = 37), which may contain a scoring system, right or wrong answers or assessments of performed actions, they are the two most prevalent GEs that were used in the studies. This lies in contrast to the findings provided by [53], and Ref. [49] reported Action Points (as a scoring system, points, etc.) as the most frequently used game element. A possible yet reasonable explanation is that, for 8 studies, there was no information or evidence regarding the presence of GEs acting as Action Points; thus, it is very possible that the total number may be higher than the reported 31. As mentioned before, the second most commonly used game element was Feedback, which is a key element in behavior changes, and it acts as an indication of performance and can be delivered through visual or auditory stimuli. In addition, Progression and Difficulty Adjustment were also used in most of the cases (N = 33 and N = 32, respectively). Participants can highly appreciate the fact that they always have an overview of their progress, as the training tasks can be lengthy in time for completion. Difficulty Adjustment is critical in building personalized training tasks and is a key element for training success. Since participants can vary in measures of cognitive performance, the delivered training should reflect these measures. Extremely easy or difficult tasks may jeopardize adherence and engagement, either by causing boredom or disappointment in cases of uncompleted tasks. On the other side, the most underused tasks were associated with socialization context (Competition and Cooperation) and the use of avatars.