Introduction
Intellectual disability is a disorder that begins during the developmental period and refers to deficiencies in adaptive functioning and intelligence in learning, social and conceptual domains [1]. People with mild intellectual disability accounts for 85% of all people diagnosed with intellectual disability [6, 7]. Deficiencies in working memory are related to the increase in the degree of intelligence impairment, and the general dysfunction in working memory is related to the level of intelligence [9, 10, 11]. 
Play therapy is an emerging method through which therapists can make significant progress in the treatment of children with intellectual disabilities [20]. Child-centered play therapy follows the constructs of client-centered therapy developed by Carl Rogers (1951). Virginia Axline, a colleague of Rogers, applied this principle to children and called it child-centered play therapy [28, 29]. The child-centered play therapy used in this study was Adlerian play therapy which was developed by Dr. Terry Kottman [33, 34].
Method
his study was registered as a clinical trial (IRCT20190412043243N1). Also, after submitting the proposal to the ethics committee and obtaining an ethics code (IR.USWR.REC.1398.043), and obtaining a letter of introduction from the Tehran University of Social Welfare and Rehabilitation Sciences to coordinate with the Qom Education Organization, research permission was issued. After holding a briefing meeting and obtaining parental consent, 60 eligible students were selected, of whom 32 cases were excluded from the intervention due to no participation in all sessions. Using the formula for calculating the sample size and counting the possible attrition, the number of people reached 28. In this stage, the booklet related to the informed consent form and demographic information was delivered to the parents of the selected students. Then, the samples were divided into blocks based on age and educational stage, and randomization was performed in each block separately by assigning cards A and B, and students were randomly assigned to one of the experimental and control groups. Finally, the first evaluation was performed for both groups.
It should be noted that this was a single-blind study. The study evaluator was a person with a master’s degree in psychometrics and when measuring response in the pre-test, post-test, and follow-up stages, this person was not aware of the implementation process and the groups. In the next stage, the experimental group received the child-centered Adlerian play therapy for ten weeks (three sessions per week and 45 minutes per session). After the interventions and data collection, a second evaluation was performed for both groups. The data were analyzed and the final evaluation (follow-up) was performed by the research team one month after the intervention without any mediation.
Data analyzing method
Descriptive statistics were used to report the mean, frequency, and standard deviation of the data. The distribution of data related to the Wechsler Intelligence Scale for Children, Fourth Edition (WISC-IV) did not follow the normal distribution according to the Shapiro-Wilk test (P>0.05). Therefore, nonparametric tests, including Friedman, Mann-Whitney, and Bonferroni tests and Generalized Estimation Equation (GEE) were used to analyze the data. A significance level of 0.05 was considered and SPSS v. 22 was used for data analysis.
Results
The subjects included 28 female students with mild intellectual disabilities ranging in age from 7 to 12 years. Most subjects in each group were aged seven years (first grade) with six people (43%), whereas the lowest frequency in each group was related to the age of nine years (third grade) with two people (14%). According to Table 1, the working memory was examined in three stages: pre-test, post-test, and follow-up for both experimental and control groups.


According to Table 2, the mean score of working memory of the experimental and control groups was not significantly different in the pre-test (p>0.05).


The mean score of working memory in the experimental group in the pre-test was 6.28 and in the control group was 5.92, which showed no significant difference. The P of the mean test of working memory of post-test in the experimental and control groups was significantly different (p<0.05). The mean score of working memory in the post-test was ten in the experimental group and 5.64 in the control group, which indicates the effectiveness of the intervention. Also, the mean score of working memory in the follow-up stage between the experimental and control groups was significantly different (p<0.05). The mean of working memory in the follow-up stage is 12.64 in the experimental group and 5.85 in the control group.
According to the P of the Friedman test (which was used to examine the difference between the intragroup mean in the three measurement steps), it is observed that the mean of working memory pre-test, post-test, and follow-up in the experimental group was significantly different (P>0.05). In the control group, the P of the Friedman test is 0.582, that is, the mean of working memory in the control group, pre-test, post-test, and follow-up was not significantly different (p>0.05).
According to the chart showing the average working memory in the experimental and control groups; in the control group, the mean of working memory did not change during the three stages of the test, but in the experimental group, the mean of working memory changed significantly post-test and the follow-up.
Due to the abnormality of the changes, the GEE model was used to examine the trend of changes in the responses.
According to Table 2, the Bonferroni test showed that in the control group, the mean score of working memory was not significantly different in any of the measurement steps (P>0.05). In the experimental group, the mean score of working memory in all stages of measurement was significantly different (P<0.05). In the pre-test, the mean score of working memory in the experimental and control groups was not significantly different (P>0.05). In the post-test stage, because the P was very close to 0.05, we can ignore it and consider it significant (P= 0.054). In the follow-up stage, the mean score of working memory in the experimental and control groups was significantly different (P<0.05) (Figure 1). Discussion
This study aimed to investigate the effect of Adlerian play therapy on the working memory of students with mild intellectual disabilities. The results of this study indicated that the child-centered Adlerian play therapy improved the working memory of students with mild intellectual disabilities. Data analysis showed that after the intervention, the mean score of working memory was significantly different between the experimental and control groups and this score was significantly higher in the experimental group. This finding is consistent with the results of the following studies. Kalkhran and Shariati [20] investigated the effect of play therapy on the short-term memory of children with teachable intellectual disabilities. Taghipour [22] also conducted a study on the effectiveness of rhythmic games on attention and working memory in children with mild intellectual disability. In both studies, the effect of play therapy on improving short-term and active memory was mentioned. Asghari Nikah [39] also emphasized the effect of play therapy on children’s working memory (inattentive/hyperactive).
Similar to our study using the child-centered method, Karamalian et al. [40] also found the effectiveness of child-centered play therapy on the working memory of children with learning disabilities. According to Landreth [26], there is a rule in child-centered play therapy suggesting that “to understand the child and his behaviors, the child’s perception of reality must be understood.” Given the “general law of seeing with the child’s eyes”, the therapist should avoid judging or evaluating even the simplest behaviors of the child and try to understand the child’s internal judgment framework. Ray et al. [14] reported strong evidence for the effectiveness of the child-centered play therapy intervention in primary schools. Perryman et al. [46] also found that child-centered play therapy had significant results in students’ educational achievement.
Mirzaie et al. [42] compared the effect of play therapy with filial therapy and Adlerian methods on attention and hyperactivity in children with attention-deficit / hyperactivity disorder and parenting stress of their parents. They emphasized the effectiveness of Adlerian play therapy. According to Blanco’s [19] research, the game itself has a motivational aspect and is full of positive vitality and excitement and can be a factor encouraging the child to participate in learning sessions with more power.
According to the Kottman child-therapist play therapy method, play is a tool to establish a friendly relationship with children; therefore, the child is helped to show feelings, thoughts, and emotions that he is unable to express. This method provides a way for the children to explore their desires and goals. Ultimately, this method provides a space for the child to gain insight into his behavior and motivation [28]. 
According to Szatkowska’s research on the effect of emotions and motivations on working memory capacity, it can be said that Adlerian child-centered play therapy increased students’ working memory capacity by raising their motivation level. As we know, the nature of play is to increase children’s self-efficacy [26, 44, 45] and in Adlerian play therapy, the child understands a new experience of relationships during activity (play) and without being judged, he feels valued, grown, and matured. Also, by expressing his emotional needs, while playing and meeting them properly and optimally, he feels worthy, and because the child willingly participates in this intervention [28], the Adlerian child-centered play therapy was effective in this research.
Because in this study, the research team created play therapy sessions in the school environment, it caused the student to get out of the difficulty and fatigue. He was informed about his lessons and homework and entered an atmosphere, where willingly attends classrooms and learning sessions [46]. Regarding how Adlerian child-centered play therapy focuses on working memory in children with mild intellectual disability, the relationship between enhancing or improving social communication through Adlerian play therapy has a positive effect on a child’s working memory. McQuade [47] emphasized the relationship between working memory and social communication and reported a relation between the deterioration of executive functions and poor social performance [47, 48, 49]. It can be concluded that in the present study, play therapy sessions were able to improve communication and social behaviors leading to the improvement in working memory.
The effectiveness of play therapy can be evaluated in several ways. The play has many therapeutic capacities, which helps a child with an intellectual disability to identify and solve his problems [49]. A comparison of the mean score of working memory of children in the intervention group in the pre-test and post-test showed a significant difference. It seems that subjects felt satisfied by attending play therapy sessions so that in the initial sessions of play therapy, conditions were provided for the close emotional relationship between the child and the therapist and to increase the child’s sense of self-confidence. Along with these sessions, the child was able to increase information manipulation in the final sessions that took place for one month. Thus, the reflection of children’s working memory can be seen well in the evaluations performed.
Isanejads also reported the effect of play therapy on working memory [21] and emphasized the effect of computer games on the attention and working memory of students with attention-deficit/hyperactivity disorder. Jadidian and Sharifi [25] studied the effect of computer games on time of selective reactions and working memory. Also, Azizi et al. [50] in a clinical trial concluded that cognitive-behavioral play therapy was effective in sustained attention of children with learning disabilities but had no effect on the working and short-term memory of these children, which is not consistent with our study. 
Conclusion
Due to the effectiveness of Adlerian child-centered play therapy on the working memory of students with mild intellectual disabilities, it can be helpful as a complementary method along with other common therapies to increase the working memory capacity of female students aged 7 to 12 years with mild intellectual disability. Briefly, Adlerian child-centered play therapy indirectly enhanced general cognitive functions, such as working memory. However, a longer study period or more subjects may be able to provide more information in rejecting or confirming the results obtained.

Ethical Considerations
Compliance with ethical guidelines
This study was approved by the ethics committee of the University of Social Welfare and Rehabilitation Sciences (Code: IR.USWR.REC.1398.043). An informed consent was obtained from the participants, after explaining the study objectives to them and assuring them of the confidentiality of their information. They were free to leave the study at any time.

Funding
This study was extracted from the master thesis of Navida Javaheri.

Authors' contributions
Conceptualization: Hooshang Mirzai and Navida Johari; methodology, resources: Hojatollah Haghgo and Navida Javaheri; validation, initial draft prepation and editing & review: Hooshang Mirzai, Hojat Allah Haghgoo, and Navida Johari; Data analysis, investigation: All authors; supervision: Hooshang Mirzai, Hojat Allah Haghgoo, and Samaneh Hosseinzadeh; Project administration: Hoshang Mirazei.

Conflict of interest
The authors declare no conflict of interest.

Acknowledgments
The authors declare no conflict of interest.


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