Effects of Transcranial Direct Current Stimulation on Working Memory in Children with Mild Intellectual Disabilities

Working Memory (WM) plays a key role in learning and contributes to developing cognitive functioning. WM supports temporary storing and manipulating the required data for complex cognitive tasks, such as language comprehension, reasoning, and learning. Most studies indicated that children with Intellectual Disability (ID) poorly perform in most WM tasks, compared to their typically-developing counterparts, regardless of the etiology of ID or Intelligence Quotient (IQ). Transcranial Direct Current Stimulation (tDCS), as a noninvasive brain stimulation was represented to increment the cortical excitability in the Dorsolateral Prefrontal Cortex (DLPFC) and improve WM. This is contrary to other brain stimulation approaches, normally involving the active triggering. WM training is especially important for children with ID. Therefore, this study aimed to determine the effects of tDCS on WM in children aged 7 to 14 years with mild ID.

This was a quasi-experimental study with pretest-posttest and a control group design. The statistical population included children in the age range of 7-14 years referring to Atieh clinic in Tehran City, Iran, in 2018. Among the clients, children whose IQ test scores were between 70 and 85 based on the results of the New Version of the Tehran-Stanford-Binet ‎Intelligence Scale (TSBIS; Afrooz & Kamkari, 2006) were diagnosed with mild ID (borderline) and according to a clinical interview. In total, 32 eligible volunteers were selected as the study sample by convenience and purposive methods. In the process of conducting the research, 12 subjects stopped cooperating; eventually, the data of 20 individuals were analyzed (experiential group=15 & control group=15 subjects). The inclusion criteria included children aged 7-14 years, and the IQ test score to range between 70-85 based on the TSBIS. Any comorbid biopsychological disorders (epilepsy, autism, etc.), drug use, head trauma, the presence of any foreign object in the head, like cerebral shunt, and the occurrence of any unwanted adverse effects, like headache during the meeting were among the exclusion criteria of the study. Only the experimental group received 5 consecutive sessions of tDCS in the DLPFC with an intensity of 1 mA for 30 minutes. At the end of the sessions, for both groups, the working memory subtest was re-performed based on the same test. Descriptive statistics, including mean and standard deviation as well as inferential statistical methods, such as Independent Samples t-test and univariate Analysis of Covariance (ANCOVA) were used to analyze the obtained data in SPSS and SmartPLS. A significance level of 0.01 was used for all statistical tests.
 

The present study results suggested that after eliminating the pretest effects, the between-group difference was significant in the posttest; thus, tDCS increased WM performance in the intervention group, compared to the controls (p</em><0.001). Furthermore, the obtained Eta coefficient (0.56) highlighted the effects of the intervention.

Based on the present research findings, tDCS impacts WM in children with mild ID (borderline) .